IMAT Biology Fundamentals: The Key
Concepts
Make sure you are using the latest version of this book
August 6th, 2023
Contents
Preface
. . . . . . . . . . . . . . . . . . . . . . .
1, p. 9
EnterMedSchool.com
. . . . . . . . . . . . . . . . . 1.1, p. 9
License and Copyrights
. . . . . . . . . . . . . . . . 1.2, p. 9
IMAT Study Planner
. . . . . . . . . . . . . . . . .
1.3, p. 10
Study Techniques . . . . . . . . . . . . . . . . . .
1.4, p. 10
Suggestion: Chemistry
. . . . . . . . . . . . . . .
1.5, p. 12
Achieving Optimal Results . . . . . . . . . . . . . .
1.6, p. 13
Practice Questions
1.7, p. 13
. . . . . . . . . . . . . . . . .
Where Are The Numbers?
. . . . . . . . . . . . . .
1.8, p. 14
Introduction to Biology . . . . . . . . . . . . . . . . 2, p. 15
The Properties of Life
. . . . . . . . . . . . . . . .
2.1, p. 16
The Levels of Biological Organization . . . . . . . . .
2.2, p. 18
Chemical Bonds
. . . . . . . . . . . . . . . . . . 2.3, p. 20
The Role of Water in Life . . . . . . . . . . . . . . . 2.4, p. 25
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Acid and Bases
. . . . . . . . . . . . . . . . . . .
Biological Macromolecules
2.5, p. 33
. . . . . . . . . . . . . . 3, p. 39
Dehydration Synthesis and Hydrolysis
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3.1, p. 39
Carbohydrates
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3.2, p. 41
Amino Acids
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3.3, p. 47
Lipids
. . . . . . . . . . . . . . . . . . . . . . . 3.4, p. 64
Nucleic Acids
. . . . . . . . . . . . . . . . . . . . 3.5, p. 79
Cell Biology
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Membrane-bound Organelles
Prokaryotic Cells
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4, p. 89
4.1, p. 92
. . . . . . . . . . . . . . . . . . 4.2, p. 93
Eukaryotic Cells . . . . . . . . . . . . . . . . . . . 4.3, p. 102
The Endomembrane System . . . . . . . . . . . . . 4.4, p. 128
Cytoskeleton
. . . . . . . . . . . . . . . . . . . . 4.5, p. 136
Comparing Animal, Plant and Bacteria Cells
The Plasma Membrane
. . . . . 4.6, p. 152
. . . . . . . . . . . . . . .
The Concentration Gradient
5, p. 155
. . . . . . . . . . . . . 5.1, p. 156
Membrane Proteins . . . . . . . . . . . . . . . . . 5.2, p. 158
Cell Membrane Fluidity
. . . . . . . . . . . . . . . 5.3, p. 181
Active and Passive Transport . . . . . . . . . . . . . 5.4, p. 183
Energy and Metabolism
Coupling Reactions
. . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 6.1, p. 194
Catabolism and Anabolism
Energy
. . . . . . . . . . . . . 6.2, p. 195
. . . . . . . . . . . . . . . . . . . . . . . 6.3, p. 196
The Laws of Thermodynamics
Enzymes
6, p. 192
. . . . . . . . . . . . 6.4, p. 197
. . . . . . . . . . . . . . . . . . . . .
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6.5, p. 204
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Why ATP?
. . . . . . . . . . . . . . . . . . . . . 6.6, p. 216
Cellular Respiration
. . . . . . . . . . . . . . . . . 6.7, p. 217
Energy in Biology
. . . . . . . . . . . . . . . . . . 6.8, p. 218
Glucose
. . . . . . . . . . . . . . . . . . . . .
6.9, p. 222
Glycolysis . . . . . . . . . . . . . . . . . . . . .
6.10, p. 226
The Krebs Cycle . . . . . . . . . . . . . . . . . .
6.11, p. 234
The Electron Transport Chain
. . . . . . . . . . . 6.12, p. 240
Oxidative Phosphorylation . . . . . . . . . . . . . 6.13, p. 244
Summary of the Yields of the Different Biochemical Pathways .
. . . . . . . . . . . . . . . . . . . . . . . . . . 6.14, p. 245
Photosynthesis . . . . . . . . . . . . . . . . . . . 7, p. 247
Light-Dependent Reactions
. . . . . . . . . . . . . 7.1, p. 249
Light Independent Reaction . . . . . . . . . . . . . 7.2, p. 252
The Interplay between Light-dependent and Light-independent
Reactions . . . . . . . . . . . . . . . . . . . . . . 7.3, p. 255
The Cell Reproduction Cycle
. . . . . . . . . . . . 8, p. 257
The Cell Cycle . . . . . . . . . . . . . . . . . . . . 8.1, p. 258
Mitosis
. . . . . . . . . . . . . . . . . . . . . .
8.2, p. 268
Meiosis . . . . . . . . . . . . . . . . . . . . . . . 8.3, p. 271
The Main Differences Between Mitosis and Meiosis
Mendel’s Experiments and Heredity
Phenotypes and Genotypes
Sex-Linked Disorders
.
8.4, p. 277
. . . . . . . . 9, p. 280
. . . . . . . . . . . . . 9.1, p. 280
. . . . . . . . . . . . . . .
9.2, p. 282
Mendelian Genetics . . . . . . . . . . . . . . . .
9.3, p. 285
Genetic Pedigrees . . . . . . . . . . . . . . . . .
9.4, p. 293
The Central Dogma . . . . . . . . . . . . . . . . . 10, p. 296
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Coding and Template Strands
Replication
. . . . . . . . . . .
10.1, p. 298
. . . . . . . . . . . . . . . . . . . . 10.2, p. 300
Eukaryotic and Prokaryotic Transcription
. . . . . . 10.3, p. 305
Translation . . . . . . . . . . . . . . . . . . . . 10.4, p. 309
Answering Questions on Complementary DNA strands . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 10.5, p. 313
Biotechnology
. . . . . . . . . . . . . . . . . . .
Common Subjects
Evolution
11, p. 317
. . . . . . . . . . . . . . . . . 11.1, p. 317
. . . . . . . . . . . . . . . . . . . . . 12, p. 324
Mechanism . . . . . . . . . . . . . . . . . . . .
12.1, p. 324
Formation of New Species . . . . . . . . . . . . .
12.2, p. 332
Population Evolution
. . . . . . . . . . . . . . . 12.3, p. 340
The Hardy-Weinberg equilibrium . . . . . . . . . .
12.4, p. 343
Viruses . . . . . . . . . . . . . . . . . . . . . . . 13, p. 345
Main Characteristics of Viruses . . . . . . . . . . .
13.1, p. 345
Structure . . . . . . . . . . . . . . . . . . . . .
13.2, p. 347
Viral Life Cycles
13.3, p. 353
. . . . . . . . . . . . . . . . . .
Viruses in the Field of Medicine
. . . . . . . . . .
13.4, p. 361
Viral Transmission . . . . . . . . . . . . . . . . .
13.5, p. 364
Bacteria and Archaea
. . . . . . . . . . . . . . . 14, p. 366
Main Characteristics of Bacteria
Gram Positive and Negative
Prokaryotic Metabolism
Bacterial Infections
14.1, p. 367
. . . . . . . . . . . . 14.2, p. 369
. . . . . . . . . . . . . .
14.3, p. 372
. . . . . . . . . . . . . . . . 14.4, p. 376
Bacteria and Biotechnology
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14.5, p. 377
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Human’s Physiology - The Basics
Homeostasis
. . . . . . . . . . 15, p. 380
. . . . . . . . . . . . . . . . . . .
15.1, p. 380
Epithelial Tissue . . . . . . . . . . . . . . . . . .
15.2, p. 384
Glandular Epithelium
15.3, p. 386
. . . . . . . . . . . . . . .
Connective Tissue . . . . . . . . . . . . . . . . . 15.4, p. 389
Muscle Tissues
. . . . . . . . . . . . . . . . . .
15.5, p. 396
Nervous Tissues . . . . . . . . . . . . . . . . . .
15.6, p. 397
The Digestive System . . . . . . . . . . . . . . .
16, p. 402
Digestive Enzymes
16.1, p. 410
. . . . . . . . . . . . . . . .
The Nervous System
Parts of a Neuron
. . . . . . . . . . . . . . . . 17, p. 421
. . . . . . . . . . . . . . . . . . 17.1, p. 421
Neural Communication
Signal Propagation
. . . . . . . . . . . . . .
17.2, p. 423
. . . . . . . . . . . . . . . .
17.3, p. 426
Action Potential . . . . . . . . . . . . . . . . . . 17.4, p. 429
Channels . . . . . . . . . . . . . . . . . . . . .
17.5, p. 435
The Central Nervous System
17.6, p. 439
. . . . . . . . . . . .
The Peripheral Nervous System
. . . . . . . . . .
17.7, p. 456
. . . . . . . . . . . . . . . . .
18, p. 465
General and Special Senses in Humans . . . . . . .
18.1, p. 465
Sensory Systems
Taste and Smell . . . . . . . . . . . . . . . . . . 18.2, p. 469
Hearing and Vestibular Sensation
Vision
. . . . . . . . .
18.3, p. 472
. . . . . . . . . . . . . . . . . . . . . . 18.4, p. 479
The Endocrine System
. . . . . . . . . . . . . .
Regulation of Hormone Production
Types of Hormones
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19, p. 485
19.1, p. 487
. . . . . . . . . . . . . . . . 19.2, p. 488
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Endocrine Glands . . . . . . . . . . . . . . . . . 19.3, p. 490
Hypothalamic-Pituitary Axis
. . . . . . . . . . . .
19.4, p. 491
Thyroid Gland . . . . . . . . . . . . . . . . . . .
19.5, p. 493
Parathyroid Glands
. . . . . . . . . . . . . . . . 19.6, p. 495
Calcium Level Regulation
The Adrenal Glands
Pineal Gland
. . . . . . . . . . . . . 19.7, p. 496
. . . . . . . . . . . . . . . . 19.8, p. 498
. . . . . . . . . . . . . . . . . . . 19.9, p. 500
The Gonads . . . . . . . . . . . . . . . . . . . . 19.10, p. 501
Secondary Endocrine Function
Pancreatic Hormones
Osmoregulation
. . . . . . . . . . 19.11, p. 504
. . . . . . . . . . . . . . . 19.12, p. 505
. . . . . . . . . . . . . . . . .
Reproductive System
19.13, p. 512
. . . . . . . . . . . . . . . 19.14, p. 516
Hormonal Regulation of Stress . . . . . . . . . . .
19.15, p. 517
The Musculoskeletal System
. . . . . . . . . . .
20, p. 520
. . . . . . . . . . . . . . . . .
20.1, p. 520
Skeletal Muscles
ATP and Muscle Contraction
. . . . . . . . . . . . 20.2, p. 526
Excitation–Contraction Coupling
The Respiratory System
. . . . . . . . . .
20.3, p. 531
. . . . . . . . . . . . . . 21, p. 533
The Basic Principles of Gas Exchange . . . . . . . .
21.1, p. 533
Lung Volumes and Capacities
. . . . . . . . . . .
21.2, p. 536
Human Breathing Mechanism . . . . . . . . . . .
21.3, p. 538
Transport of Gases in Blood
. . . . . . . . . . . .
21.4, p. 541
The Circulatory System . . . . . . . . . . . . . .
22, p. 549
The Circulatory System
22.1, p. 549
. . . . . . . . . . . . . .
Blood and its Components
. . . . . . . . . . . . 22.2, p. 550
The Heart . . . . . . . . . . . . . . . . . . . . . 22.3, p. 552
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The Cardiac Cycle
. . . . . . . . . . . . . . . . . 22.4, p. 559
Arteries, Veins, and Capillaries
. . . . . . . . . . .
Blood Flow and Blood Pressure Regulation
Osmotic Regulation and Excretion
The Lymphatic System
Osmotic Balance
. . . . . 22.6, p. 564
. . . . . . . .
23, p. 568
. . . . . . . . . . . . . .
23.1, p. 568
. . . . . . . . . . . . . . . . . 23.2, p. 570
The Structure and Function of the Kidneys
Anatomy
22.5, p. 561
. . . . . 23.3, p. 574
. . . . . . . . . . . . . . . . . . . . . 23.4, p. 575
The Nephron
. . . . . . . . . . . . . . . . . . . 23.5, p. 577
The Urea Cycle
. . . . . . . . . . . . . . . . . . 23.6, p. 587
The Immune System
. . . . . . . . . . . . . . . . 24, p. 591
The Innate Immune Response
. . . . . . . . . . .
24.1, p. 591
The Adaptive Immune Response . . . . . . . . . . 24.2, p. 597
Primary and Secondary Immune Response . . . . . 24.3, p. 607
Human Reproduction
. . . . . . . . . . . . . .
25, p. 609
Human Reproductive Anatomy and Gametogenesis
25.1, p. 609
Spermatogenesis
. . . . . . . . . . . . . . . . .
25.2, p. 614
. . . . . . . . . . . . . . . . . . . .
25.3, p. 619
Oogenesis
The Menstrual Cycle . . . . . . . . . . . . . . . . 25.4, p. 624
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1 Preface
1.1 EnterMedSchool.com
Founded in 2019 by Ari Horesh, a medical student at the University of
Pavia, Italy, EnterMedSchool is dedicated to providing high-quality
admission exam preparation at an affordable price. The revenue
generated from paid classes, courses and materials is used to
fund the creation of free resources that are accessible to all
students. The materials you are currently using are an example
of this commitment to accessible education.
1.2 License and Copyrights
This book is for personal and educational use only. It is strictly
forbidden to print this book. We encourage the use of the digital
version of our books as they are interactive and can be accessed on
various devices. By using the digital version of our books, we can help
fight climate change by reducing the amount of paper used in the
publishing process. Please respect the intellectual property rights of
the authors and publishers of this book and refrain from distributing
or reproducing it in any form. Thank you for your cooperation.
Please note that this book may contain errors or inaccuracies. If you
think you have encountered an error in the book, we encourage you
to contact us immediately so that we can address the issue.
Furthermore, please be aware that this book is provided ”as is” without any warranty of any kind, either express or implied. This means
that the authors and publishers of this book do not make any guarantees about the accuracy, completeness, or reliability of the inforEnterMedSchool.com
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mation contained within. Use of this book is at your own risk and
you assume full responsibility for any consequences that may result
from your reliance on the information contained within.
We hope that this book will be a useful resource for your personal
and educational purposes.
1.3 IMAT Study Planner
This book comprehensively covers all the biology topics for the IMAT
exam, making it an excellent study resource for those starting from
scratch. Although there may be a few minor variations, it covers the
vast majority of topics. However, we recommend that you use our
study planner and the ”Pearson’s Biology Global Approach” book
at least once before using this book for IMAT preparation. The
most effective way to utilize this book is to use it as a second revision
after acquiring knowledge of the material. This book is specifically
designed to help you focus on the essential concepts required to
excel in the IMAT exam.
1.4 Study Techniques
Keep This in Mind
Our team is dedicatedly working on developing an online
course for premeds and medical students that focuses on study
techniques. We will keep everyone informed about its release.
The EnterMedSchool.com website will release a comprehensive
course in June 2023 on techniques for improving memory and
retention. These techniques are specifically tailored for premed and
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medical students and can save valuable time. Focusing on retaining
information for longer periods and understanding concepts is key
to success in biology. While flashcards can be useful for memorizing definitions and values, techniques like visualization and mind
mapping can help you connect ideas and fully understand complex
concepts, which is especially important when you practice for the
IMAT exam.
1. Spaced repetition: This involves reviewing information at increasingly longer intervals over time. This helps to reinforce the
information in your memory and helps it stay there for longer.
2. Elaborative rehearsal: This involves actively thinking about
and trying to understand the information you are trying to remember, rather than just passively reading or repeating it. You
can use techniques such as mnemonic devices, or try to create
connections between the new information and something you
already know.
3. Practice testing: Testing yourself on the information you are
trying to learn can be a very effective way to improve your memory for that information. This can involve creating flashcards or
quizzing yourself, for example.
4. Visualization: Creating a mental image of the information you
are trying to remember can be a helpful way to commit it to
memory. You can also try drawing or creating diagrams to represent the information.
5. Chunking: This involves breaking down large amounts of information into smaller, more manageable chunks. This can make
the information easier to remember and recall.
It’s important to note that no one technique will work for everyone,
and the best approach may vary depending on the individual and
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the type of information they are trying to remember. It may be
helpful to experiment with different techniques to see which one
works best for you. For further information about these researchbased study techniques go on EnterMedSchool.com
1.5 Suggestion: Chemistry
Chemistry is a fundamental science that is essential for understanding the properties and behavior of matter, including the substances
that make up living organisms. By studying chemistry before approaching this book, you can gain a deeper understanding of the
basic principles that govern the behavior of atoms and molecules,
which is essential for understanding many of the processes that
occur within living cells. This understanding can also help you to
better understand the mechanisms of chemical reactions and how
they can be used to produce useful products, such as drugs and
other chemicals.
Furthermore, chemistry is closely related to other scientific disciplines, such as biology, physics, and environmental science, and a
strong foundation in chemistry can provide a valuable basis for further study in these areas. For example, an understanding of chemical reactions and the properties of different molecules is essential
for understanding how cells and other biological systems function,
and chemistry is also crucial for understanding the environmental
impacts of different chemical processes and substances.
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Key Takeaways
While we strongly suggest that you familiarize yourself with
IMAT Chemistry using our other complimentary resources before embarking on this particular material, it is okay to use this
book before learning Chemistry
1.6 Achieving Optimal Results
Biology for the IMAT exam is not just about memorization, but rather
about understanding the concepts and principles that underlie biological processes. While flashcards and notes can be helpful tools
for organizing and reviewing material, it is more important to truly
grasp the underlying ideas. In the book that we offer at EnterMedSchool.com, we present the material in a unique way, using analogies and real-life examples to help readers fully comprehend the
concepts. We recommend using our book as a final review resource
in the months leading up to the exam, to ensure that you have a
deep understanding of the material. Don’t just rely on memorization techniques, but take the time to really understand the material,
and you will be well-prepared for the IMAT exam.
1.7 Practice Questions
It is crucial to practice biology questions in preparation for the IMAT
exam. Our IMAT practice books, which contain the same content as
the textbooks and follow a similar structure, can be found for free on
the EnterMedSchool.com website. These books are an excellent resource for understanding the concepts and improving your chances
of success on the exam. Don’t miss this opportunity to hone your
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skills and increase your confidence on exam day.
1.8 Where Are The Numbers?
Are you tired of biology textbooks that are filled with complicated
equations and numbers that are hard to understand? Do you struggle to grasp important biological concepts because you feel like
you’re just memorizing a bunch of facts without really understanding what they mean? If so, then I have some great news for you!
Our biology textbook is designed to help you truly understand the
concepts in a way that is both engaging and meaningful. Instead
of just throwing numbers and equations at you, our textbook uses
clear, concise language and helpful analogies to explain complex
ideas in a way that is easy to understand.
With this textbook, you’ll be able to grasp the underlying principles
of biology and see how they apply to real-world situations. You’ll also
have access to a wealth of interactive resources and activities that
will help you reinforce your understanding and retain the material
long-term.
So if you’re ready to truly understand biology, rather than just memorizing facts, then this is the textbook for you. Let’s dive in and explore
the fascinating world of biology together!
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2
Introduction to Biology
Welcome to the world of IMAT biology! In this textbook, we will be
exploring the fascinating world of living things and how they work.
This book is specifically designed to help students prepare for the
IMAT and understand the important concepts that will be tested
on the exam.
Did You Know?
As you progress through this textbook, you’ll come across various pop-ups like this one that highlight important points to
remember and provide memory aids to help you solidify the
study material in your mind.
In this textbook, we will learn about the basic concepts of biology,
including cells, DNA, and the different processes that keep living
organisms functioning. In the second part of this book, we will also
delve into the world of physiology, which is the study of how living
things function. This will include topics such as how the body’s
systems work together to maintain homeostasis, and how different
organs and tissues work together to keep us healthy.
It is important to not just read the material in this book, but to also
put what you have learned into practice. This means actively engaging with the questions and activities provided, as well as conducting
your own research and experiments by watching YouTube videos
of real life experiment and tutorials on the different applications of
Biology. By doing this, you will not only improve your understanding
of the material, but you will also develop important critical thinking
and problem-solving skills that will be useful on the IMAT.
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Additionally, we recommend that you supplement your study of
this textbook with other available biology resources recommended
by EnterMedSchool.com. By using various study materials, you can
ensure that you have a well-rounded understanding of the essential
concepts covered on the IMAT.
So let’s get started on this exciting journey into the world of biology!
Whether you are a student just starting out in the field, or a lifelong
learner looking to expand your knowledge, this textbook has something for everyone. Let’s dive in and discover the incredible world of
living things!
2.1
The Properties of Life
Life is a complex and diverse phenomenon that is characterized by
certain unique properties. These properties include metabolism,
growth and development, response to stimuli, homeostasis, reproduction, and evolution. Metabolism refers to the process by which
living things transform energy and matter in order to sustain themselves. Growth and development involve the change and growth
that living things undergo over time. Response to stimuli refers to
the ability of living things to sense and respond to changes in their
environment. Homeostasis involves the maintenance of a stable internal environment, even when the external environment changes.
Reproduction is the ability of living things to produce offspring, either sexually or asexually. Finally, evolution is the process by which
living things can change and adapt over time through natural selection. Understanding these properties of life can help us better
understand the complex and fascinating world of living organisms.
1. Order: Living things are highly organized and have a complex
structure that is specific to their species. For example, cells
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are the basic unit of life, and different types of cells have different structures and functions that enable them to carry out
specialized tasks. At higher levels of organization, living things
are composed of tissues, organs, and organ systems that work
together to maintain the functions of the organism as a whole.
2. Energy processing: Living things require energy to carry out
their life processes, such as metabolism, growth, and movement.
They obtain energy from their environment in the form of food,
and then use cellular processes such as respiration and photosynthesis to convert that energy into a form that can be used to
power cellular processes.
3. Growth and development: Living things grow and develop
over time, often following a specific pattern or sequence. This
growth and development is controlled by genes, which provide
the instructions for the synthesis of proteins and other cellular
components necessary for growth and development. For example, a fertilized egg develops into a multicellular organism
through a series of developmental stages, ultimately becoming
a fully formed adult organism.
4. Evolutionary adaptation: Living things are capable of adapting
to their environment over time through the process of evolution.
This allows them to better survive and reproduce in changing
environmental conditions. Adaptations can be structural, such
as the development of wings in birds for flight, or behavioral,
such as the migration of animals to avoid harsh environmental
conditions.
5. Regulation: Living things have mechanisms for maintaining
internal conditions, or homeostasis. This helps them maintain
a stable internal environment despite changes in the external
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environment. For example, animals can regulate their body
temperature, pH balance, and other physiological variables to
keep their internal environment within a narrow range of values.
6. Response to the environment: Living things can sense and
respond to changes in their environment. This allows them to
survive and reproduce in their environment. For example, plants
can sense the presence of light and grow towards it, while animals can move towards sources of food or away from predators.
7. Reproduction: Living things are capable of producing offspring
through the process of reproduction. This ensures the continuation of their species over time. Reproduction can occur through
sexual or asexual means. Sexual reproduction involves the fusion of gametes (sperm and egg cells) from two individuals to
produce genetically diverse offspring. Asexual reproduction involves the production of genetically identical offspring from a
single individual.
2.2 The Levels of Biological Organization
The levels of biological organization refer to the various levels of complexity at which living things can be organized, from the smallest
level of molecules to the largest level of the biosphere. Each level of
organization builds upon the one before it and has its own unique
properties and characteristics. Understanding the levels of biological organization is important in studying the structure and function
of living things, as well as how they interact with each other and their
environment. It allows us to understand the complexity of life on
Earth and how different levels of organization are interconnected.
1. Molecules: The smallest level of biological organization, molecules
are the building blocks of life. They include DNA, proteins, and
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carbohydrates, among others.
2. Organelles: Organelles are specialized structures within cells
that carry out specific functions. Examples include the mitochondria, which generate energy for the cell, and the nucleus,
which contains the cell’s genetic material.
3. Cells: Cells are the basic unit of life and are capable of carrying
out all of the processes necessary for life. They can be either
prokaryotic (without a nucleus) or eukaryotic (with a nucleus).
4. Tissues: Tissues are groups of cells that work together to perform a specific function. Examples include epithelial tissue,
which covers and lines surfaces, and muscle tissue, which enables movement.
5. Organs: Organs are composed of two or more tissues and are
specialized for a specific function. Examples include the heart,
lungs, and liver.
6. Organisms: Organisms can be individuals, such as a human, or
they can be part of a larger group, such as a colony of ants or a
flock of birds.
7. Populations: Populations are groups of individuals of the same
species living in the same area.
8. Communities: Communities are groups of different species
living in the same area.
9. Ecosystems: Ecosystems are communities of living organisms
together with their physical and chemical environment.
10. The biosphere: The biosphere encompasses all living things and
their environments on Earth. It includes all of the ecosystems
on the planet and extends from the depths of the oceans to the
highest mountains.
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Key Takeaways
Skin is an organ because it is a group of tissues that work together to perform specific functions within the body. As the
largest organ in the human body, it serves many vital roles,
such as providing a protective barrier against environmental
factors, regulating body temperature, and sensing touch.
To understand the difference between tissue and organ, it is
essential to grasp the hierarchy of biological organization. Tissues are composed of similar cells that work together to carry
out a specific function. There are four primary types of tissue in
the human body: epithelial, connective, muscle, and nervous
tissue.
Organs, on the other hand, are composed of multiple types of
tissues that are structurally and functionally integrated. They
work together to achieve a common goal, such as the heart
pumping blood or the lungs facilitating gas exchange. In the
case of the skin, it is composed of various tissue types, including
epithelial, connective, and nervous tissue, which collaborate to
perform its diverse functions.
2.3 Chemical Bonds
Chemical bonds are the forces that stick atoms together to make
molecules and compounds. Some bonds, called covalent bonds,
happen when atoms share electrons. Other bonds, called ionic
bonds, happen when electrons are transferred from one atom to
another. There are also weaker bonds, like hydrogen bonds and van
der Waals forces, that are important in shaping the 3D structure
of molecules and biological molecules. Understanding chemical
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bonds is important for figuring out how molecules behave and interact with each other in fields like chemistry, biology, and materials
science.
2.3.1
Covalent Bonds
Covalent bonds are a type of chemical bond formed by the sharing
of a pair of valence electrons between two atoms. In a covalent
bond, the atoms are held together by the shared electrons, which
creates a stable molecule or compound. For example, in a hydrogen
molecule (H2), two hydrogen atoms share a pair of electrons to form
a covalent bond.
Lightbulb Moments
An analogy to understand covalent bonds is to imagine two
people holding a rope. Each person holds one end of the rope,
and the rope represents the shared pair of electrons in a covalent bond. The two people can pull the rope together, just
as two atoms share electrons in a covalent bond to hold the
molecule together.
There are also polar and nonpolar covalent bonds. In a nonpolar
covalent bond, the shared electrons are equally shared between
the two atoms, and the molecule has no partial charges. An example of a nonpolar covalent bond is the bond between two oxygen
atoms (O2) in an oxygen molecule.
In a polar covalent bond, the shared electrons are not equally
shared between the two atoms, and the molecule has partial
charges. An example of a polar covalent bond is the bond between
hydrogen and oxygen atoms in a water molecule (H2O). In this
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molecule, the oxygen atom has a partial negative charge, and the
hydrogen atoms have a partial positive charge.
Overall, covalent bonds are important in holding molecules and
compounds together in many biological and chemical systems.
Lightbulb Moments
Imagine a game of tug-of-war, where two people are pulling on
opposite ends of a rope. The rope represents a covalent bond
between two atoms, and the people symbolize the sharing of
electrons in that bond.
In a non-polar covalent bond, the two people have equal
strength, resulting in a balanced tug-of-war where neither person gains an advantage. This balance reflects the equal sharing
of electrons between atoms with similar electronegativity, such
as two hydrogen atoms bonded together (H2).
In a polar covalent bond, however, one person is stronger than
the other, causing an imbalance in the tug-of-war game. This
unevenness illustrates the unequal sharing of electrons between atoms with different electronegativities, such as hydrogen and oxygen in a water molecule (H2O). The stronger person
(oxygen) pulls the rope (electrons) towards themselves, creating an uneven distribution of electron density and resulting
in regions of partial positive and negative charges within the
molecule.
2.3.2 Ionic Bonds
Ionic bonds are a type of chemical bond that form when two oppositely charged ions attract each other. Ions are formed when an
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tive charge. For example, when a sodium atom loses an electron, it
becomes a positively charged ion (Na+), and when a chlorine atom
gains an electron, it becomes a negatively charged ion (Cl-). These
ions can then attract each other to form an ionic bond, as in the
case of sodium chloride (NaCl), commonly known as table salt.
Lightbulb Moments
An analogy to understand ionic bonds is to imagine two magnets with opposite charges. The magnets attract each other,
just as positively and negatively charged ions attract each other
to form an ionic bond.
In an ionic bond, the electron transfer is not shared equally between
the atoms, as in covalent bonds. Instead, one atom completely loses
electrons, and the other atom completely gains electrons, resulting
in a complete transfer of electrons from one atom to the other. This
creates two charged ions that attract each other to form an ionic
bond.
Ionic bonds are common in many biological and chemical systems,
such as the formation of salts and minerals in the body. They are
also important in the function of nerve and muscle cells, where the
exchange of ions helps to generate electrical signals.
2.3.3 Weak Chemical Interactions
Weak chemical interactions are a type of bond that occur between
molecules or within a molecule, which are weaker than covalent and
ionic bonds. They are important in many biological systems, where
they help to stabilize and shape molecules, such as proteins and
DNA. Weak chemical interactions can be divided into three main
types: hydrogen bonds, van der Waals interactions, and hydrophobic
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interactions. These types of interactions play a critical role in the
structure and function of biomolecules, as well as in the interactions
between molecules in chemical reactions.
1. Hydrogen bonds - a type of bond that occurs when a hydrogen
atom is shared between two electronegative atoms, such as
nitrogen or oxygen. Hydrogen bonds are weaker than covalent
bonds but stronger than van der Waals interactions. They are important in stabilizing the structure of biological molecules, such
as the double helix structure of DNA. An example of hydrogen
bonding can be seen in the bonding between water molecules,
where hydrogen bonds form between the hydrogen atoms of
one water molecule and the oxygen atoms of neighboring water
molecules.
2. Van der Waals interactions - a type of interaction that occurs between molecules due to fluctuations in electron density. Van der
Waals interactions are weak and occur between all molecules,
but are particularly important in large molecules, such as proteins, where they contribute to the three-dimensional structure
of the molecule. An example of van der Waals interactions can
be seen in the attraction between nonpolar molecules, such
as in the interactions between the carbon-hydrogen bonds in
hydrocarbons.
3. Hydrophobic interactions - a type of interaction that occurs
between nonpolar molecules in the presence of water. Hydrophobic interactions are the result of the tendency of nonpolar
molecules to exclude water molecules. In biological systems,
they play a role in the folding and stability of proteins, as well as
in the formation of cell membranes. An example of hydrophobic
interactions can be seen in the aggregation of oil droplets in
water, where the oil molecules aggregate to minimize contact
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with water.
2.4 The Role of Water in Life
The water molecule has a special shape that allows it to form hydrogen bonds with other water molecules. This is what gives water
some amazing properties that are super important for living things.
Water is a great solvent, which means that it can dissolve lots of
different things. This is because the hydrogen bonds in water allow
ions and other polar molecules to dissolve easily. Water is also really
good at holding onto heat, which helps to keep the temperature of
living things stable.
Key Takeaways
”Like dissolves like” is a straightforward saying that refers to
the ability of substances to dissolve in one another. It means
that substances with similar chemical properties or structures
are more likely to dissolve in each other. In simpler terms, it
indicates that a substance will dissolve more easily in a liquid
that shares its characteristics.
For example, polar substances, which have an uneven distribution of electrical charge, will generally dissolve better in polar
solvents, like water. On the other hand, non-polar substances,
which have an even distribution of electrical charge, will dissolve more readily in non-polar solvents, like oil. This concept is
often used to explain why oil and water don’t mix; they have
different properties, so they don’t dissolve in each other.
Water has some other unique properties, too, like the fact that it’s
less dense as a solid than as a liquid. This is why ice floats on top
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of water and doesn’t sink to the bottom. Water also has cohesive
properties, which means that it sticks to itself really well. This is why
water can travel up really tall plants from the roots to the leaves.
Water is so important for life that about 60-70% of the human body
is made up of it! Without water, life just wouldn’t be the same. That’s
why scientists are always looking for water on other planets - they
want to see if there could be life there too.
Water is a polar molecule, meaning that it has a positive charge at
one end (the hydrogen atoms) and a negative charge at the other
end (the oxygen atom). This polarity gives water some important
properties that are essential for life.
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Key Takeaways
Water is essential for life because it plays a critical role in various
biological processes and serves as the foundation for all living
organisms. Here are a few reasons why water is so important
for life:
1. Universal solvent: Water is known as the ”universal solvent”
due to its ability to dissolve a wide range of substances. This
characteristic facilitates the transport of nutrients, waste,
and other molecules within living organisms, enabling essential chemical reactions to take place.
2. Medium for chemical reactions: Most biochemical reactions that sustain life occur in an aqueous environment.
Water provides the necessary medium for these reactions,
allowing molecules to interact, break down, or combine to
form new compounds.
3. Temperature regulation: Water has a high heat capacity,
meaning it can absorb and release large amounts of heat
without experiencing significant temperature changes.
This property helps maintain a stable temperature within
living organisms, ensuring optimal conditions for biological
processes.
4. Cellular structure and function: Water is a primary component of cells, making up about 70 percent of their mass.
It provides the structure and shape of cells, supports vital functions such as osmosis and diffusion, and aids in
maintaining cellular homeostasis.
5. Lubrication and protection: Water acts as a lubricant for
joints and various tissues, reducing friction and providing
cushioning. It also protects organs from shock and damage
by surrounding them in a liquid environment.
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One of the most important properties of water is its ability to dissolve a wide variety of substances. This is because the positive and
negative ends of the water molecule can attract and bind to the
ions of other molecules, allowing them to dissolve and be carried
throughout the body. For example, water helps to transport nutrients, electrolytes, and oxygen throughout the body and helps to
regulate body temperature by dissipating heat.
Water’s polarity also makes it a good solvent for many biological
molecules. This is because the positive and negative ends of the
water molecule can interact with the charged or polar groups on
biological molecules, allowing them to dissolve and interact with one
another. This is important for many biological processes, including
chemical reactions, enzyme function, and the folding and stability
of proteins.
Water’s polarity also gives it a high specific heat capacity, which
means that it can absorb or release a lot of heat without changing
temperature. This property allows water to act as a thermal buffer,
helping to regulate the temperature of living organisms and the
environment.
Water’s polarity is crucial for many aspects of biology and the function of living things. It plays a vital role in many biological processes
and is essential for life.
2.4.1
High Heat Capacity
Water has the ability to absorb or release large amounts of heat
without experiencing a significant change in temperature, a property known as high heat capacity. This characteristic is due to the
presence of hydrogen bonds between water molecules. Water’s
specific heat capacity, which refers to the amount of heat required
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to raise the temperature of one gram of a substance by one degree
Celsius, is higher than that of any other liquid. For water, this value
is one calorie. As a result, it takes a longer time for water to heat up
or cool down compared to other substances. In fact, the specific
heat capacity of water is about five times higher than that of sand.
This is one reason why the land cools faster than the water in the
sea.
The high heat capacity of water also plays a role in the regulation
of body temperature in warm-blooded animals. Water acts as a
thermal buffer, helping to evenly distribute heat throughout the
body and maintain a more stable temperature. It functions similarly
to a car’s cooling system, which transports heat from warm areas
to cooler areas. By doing so, water helps to prevent overheating or
rapid cooling of the body.
2.4.2 Heat of Vaporization
Water has a high heat of vaporization, meaning that it requires a
large amount of energy to change from a liquid to a gas (The heat
of vaporization is defined as the amount of heat needed to turn 1
g of a liquid into a vapor). It takes about 540 calories of energy to
vaporize one gram of water. This process occurs on the surface of
water and is facilitated by the breaking of hydrogen bonds between
water molecules. These bonds make it difficult for water molecules
to separate from each other, which is necessary for the water to
enter its gaseous phase (steam). This is why water requires more
heat to boil than other liquids, such as ethanol, which forms one
less hydrogen bond between its molecules compared to water.
Once the water reaches its boiling point of 100◦ Celsius (212◦ Fahrenheit), the heat is finally able to break the hydrogen bonds between
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the water molecules, and the kinetic energy between the water
molecules allows them to escape from the liquid as a gas (with an
increased temperature, the particles gain kinetic energy and move
faster). Even when the water is below its boiling point, some of the
surface water molecules can gain enough energy from other water
molecules to escape and vaporize, a process known as evaporation.
The fact that hydrogen bonds need to be broken for water to evaporate means that a significant amount of energy is used in the
process. As the water evaporates, energy is absorbed, causing the
environment where the evaporation is taking place to cool. In many
living organisms, including humans, the evaporation of sweat, which
is 90% water, helps to regulate body temperature by cooling the
surface of the body.
2.4.3 Solvent Properties
As mentioned earlier, water is a polar molecule, meaning that it has
a positive charge on one end (the hydrogen atoms) and a negative
charge on the other end (the oxygen atom). This polarity allows
water to dissolve other polar molecules and ionic compounds, making it a good solvent. When ions or polar molecules are added to
water, the charges on these molecules interact with the positive
and negative ends of the water molecule, forming hydrogen bonds.
This creates a ”sphere of hydration,” or a hydration shell, around the
particle, which helps to keep it separated or dispersed in the water.
One common example is when ionic compounds such as table salt
(N aCl) are added to water, the molecules of N aCl dissociate into N a+
and Cl– ions. The positive sodium ion is surrounded by the negative
charge of the oxygen on the water molecule while the negative
chloride ion is surrounded by the positive charge of the hydrogen
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on the water molecule. This process, known as dissociation, occurs
when atoms or groups of atoms break off from molecules and form
ions.
Figure 1: When table salt (NaCl) is mixed in water, spheres of hydration are formed around the ions. Credit: openstax.org
2.4.4 The Cohesive and Adhesive Properties of Water
Water has two special properties that help it stick to things: Cohesion
and Adhesion.
Cohesion and adhesion are two important properties of water that
arise due to the unique properties of the water molecule and its
ability to form hydrogen bonds. Cohesion refers to the ability of water
molecules to stick together, which is why water droplets can form
and also why it can flow. Hydrogen bonds between water molecules
create an attractive force that keeps the molecules together, which
is why water has a high surface tension and can form droplets.
Adhesion, on the other hand, refers to the ability of water to stick
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to other surfaces. This is because the hydrogen bonds that hold
water molecules together can also form between water molecules
and other substances, such as the surfaces of plant cells. Adhesion
of water to the walls of plant cells allows water to be transported
upwards from the roots to the leaves, in a process known as capillary
action.
Cohesion and adhesion are important properties of water that have
many biological implications. For example, the ability of water to be
transported against gravity through the vessels of plants is due to
the cohesive and adhesive properties of water. The surface tension
of water also allows certain insects, such as water striders, to walk
on water without breaking the surface..
Figure 2: Water’s cohesive and adhesive properties allow this water
strider (Gerris sp.) to stay afloat. (credit: Tim Vickers)
Water’s cohesive and adhesive properties are also important for
many other things, like how water moves through the earth and
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how it helps to keep our bodies hydrated. Without these unique
properties, life as we know it would be very different!
2.5 Acid and Bases
Acids and bases are important chemical concepts that have many
applications in biology. In general, acids are substances that release
hydrogen ions (H+) when dissolved in water, while bases are substances that release hydroxide ions (OH-) when dissolved in water.
The strength of an acid or base is determined by its dissociation
constant, or the extent to which it releases these ions.
In biological systems, many important molecules are either acidic
or basic. For example, amino acids (the building blocks of proteins)
have both acidic and basic functional groups. The pH of a solution (which is a measure of its acidity or basicity) can also affect
the structure and function of biological molecules. For example,
enzymes (which are proteins that catalyze chemical reactions) have
an optimal pH range in which they are most active. If the pH of the
environment surrounding an enzyme is too acidic or too basic, the
enzyme may denature (lose its shape and function).
The pH of biological systems is tightly regulated through a number
of mechanisms. One of the most important of these is the buffering
capacity of biological fluids. Buffers are substances that can accept
or donate hydrogen ions, depending on the pH of the surrounding
environment. This helps to maintain a relatively constant pH in
biological systems, which is important for the proper functioning of
many biological processes.
In summary, acids and bases are important chemical concepts that
have many applications in biology. In biological systems, many important molecules are either acidic or basic, and the pH of biological
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fluids is tightly regulated to maintain proper function.
2.5.1
Acids
Acids are an important class of chemical compounds that have a
sour taste and can be corrosive to certain materials. In terms of their
chemistry, acids are defined as substances that release hydrogen
ions (H+) when dissolved in water. The strength of an acid is determined by its dissociation constant, or the extent to which it releases
these hydrogen ions.
One of the most common acids found in biological systems is hydrochloric acid (HCl), which is produced by cells in the stomach to
aid in the digestion of food. Another important acid is carbonic acid
(H2CO3), which is formed when carbon dioxide (CO2) dissolves in
water. Carbonic acid is an important component of the bicarbonate
buffering system, which helps to regulate the pH of blood and other
biological fluids.
Acids can also play important roles in chemical reactions in biological systems. For example, many enzymes (which are proteins
that catalyze chemical reactions) require an acidic environment to
function properly. In addition, some reactions in metabolism involve
the release of protons (H+) as a way of transferring energy from one
molecule to another.
Despite their importance, acids can also be harmful to biological
systems if their concentration becomes too high. This can cause
a decrease in pH, which can lead to the denaturation of proteins
and other biomolecules. To prevent this, biological systems have a
number of mechanisms in place to regulate the concentration of
acids and maintain a relatively constant pH.
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2.5.2 Bases
Bases, also known as alkaline substances, are another important
class of chemical compounds that have a bitter taste and a slippery
feel. In terms of their chemistry, bases are defined as substances that
release hydroxide ions (OH-) when dissolved in water. The strength
of a base is determined by its dissociation constant, or the extent to
which it releases these hydroxide ions.
One of the most common bases found in biological systems is
sodium hydroxide (NaOH), which is used by cells to regulate the pH
of the extracellular fluid. Another important base is ammonia (NH3),
which is produced by cells in the kidneys to help remove excess
hydrogen ions from the body.
Bases can also play important roles in chemical reactions in biological systems. For example, many enzymes require a basic environment to function properly. In addition, some reactions in metabolism
involve the removal of protons (H+) from molecules, which can be
facilitated by the presence of a base.
Similar to acids, bases can be harmful to biological systems if their
concentration becomes too high. This can cause an increase in
pH, which can also lead to the denaturation of proteins and other
biomolecules. To prevent this, biological systems have a number
of mechanisms in place to regulate the concentration of bases and
maintain a relatively constant pH.
In summary, bases are an important class of chemical compounds
that have many applications in biology. In biological systems, many
important molecules are either acidic or basic, and the pH of biological fluids is tightly regulated to maintain proper function.
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2.5.3 Biological Buffers
A pH buffer is a special substance that helps to keep the pH (a measure we use to know how acidic or basic a substance is by counting
the amount of H + or OH − in the solution) at a stable level. In the
human body, pH buffers are vital because they help to maintain the
proper pH levels in different parts of the body.
For example, the pH of your blood should be kept between 7.35 and
7.45, a slightly basic pH. This is important because if the pH of your
blood gets too high or too low, it can be harmful to your body. The
pH buffer in your blood helps to keep the pH at the right level by
absorbing or releasing extra hydrogen ions or hydroxide ions, which
can change the pH.
There are many types of pH buffers in the human body, and they
are found in different parts of the body, like the blood, the stomach,
and the urine. These buffers help to maintain the proper pH levels
in these different parts of the body, which is essential for processes
such as digestion and metabolism.
The main buffer system in our body is the bicarbonate buffer system,
which involves the equilibrium reaction:
H2 CO3 + H2 O *
) HCO3− + H3 O+
This system works by regulating the concentration of bicarbonate
ions (HCO3-) and hydrogen ions (H+) in the blood. When the pH
of the blood starts to become too acidic (below 7.35), the bicarbonate ions react with the excess hydrogen ions to form carbonic acid
(H2 CO3 ), which can then be broken down into water (H2 O) and carbon
dioxide (CO2 ). This process helps to remove the excess hydrogen ions
and restore the pH of the blood to the proper level.
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On the other hand, when the pH of the blood starts to become too
basic (above 7.45), the reverse reaction occurs. The carbonic acid
breaks down into bicarbonate ions and hydrogen ions, releasing the
excess bicarbonate ions and restoring the pH of the blood to the
proper level.
Figure 3: This diagram shows the body’s buffering of blood pH levels.
The blue arrows show the process of raising pH as more CO2 is made.
The purple arrows indicate the reverse process: the lowering of pH
as more bicarbonate is created. (credit: openstax.org)
Imagine that you are in charge of maintaining the pH of a swimming
pool. The pool’s pH should be around 7.4, which is slightly basic.
However, sometimes the pH of the pool can become too acidic
(below 7.4) or too basic (above 7.4).
One way to maintain the pH of the pool is to use a buffer, just like
the bicarbonate buffer system in the blood. In this case, let’s say
that the buffer is a special chemical that you can add to the pool to
help keep the pH at the right level.
When the pH of the pool becomes too acidic, you can add the buffer
to the pool. The buffer will react with the excess hydrogen ions (H + )
in the water, which causes the pH to become too low. This reaction
will remove the excess hydrogen ions and restore the pH of the pool
to the proper level.
testchapterimage.png
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Figure 4: The pH scale measures the concentration of hydrogen ions
(H+) in a solution. (credit: modification of work by Edward Stevens)
(Modification by openstax.org)
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3 Biological Macromolecules
In this section, we will be exploring the essential role that large
molecules play in the functioning and health of living organisms.
From providing the building blocks for our bodies to serving as
sources of energy and genetic information, macromolecules are
vital to life as we know it.
But while macromolecules are necessary for proper function, an
imbalance of any of them can negatively affect our health. For example, too much fat can lead to cardiovascular problems, while excess
protein can strain the kidneys. Maintaining a balanced diet is important to ensure that we get the right amount of each macromolecule
for optimal health.
It is also important to pay attention to the amount of sugar consumed. While small amounts of sugar can be beneficial for providing
quick energy, consuming too much of it can lead to weight gain
and an increased risk of type 2 diabetes. On the other hand, cutting
out sugar entirely can also be harmful, as it can lead to a lack of
energy and nutrient deficiencies. Striking the right balance is key
to maintaining good health.
As we delve deeper into this chapter, we will examine the different
types of macromolecules and their specific functions in the body.
So let’s get started on our journey to understanding these essential
macromolecules!
3.1 Dehydration Synthesis and Hydrolysis
Dehydration reactions, also known as condensation reactions, are
anabolic chemical reactions that involve the removal of a single
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water molecule from two or more molecules, which are then combined to form a larger molecule. The reaction yields a single water
molecule as a byproduct and connects the two molecules together.
Dehydration reactions are important in biology because they help
cells to create new macromolecules, such as proteins, nucleic acids,
and carbohydrates.
Figure 5: In the dehydration synthesis reaction depicted above, two
molecules of glucose are linked together to form the disaccharide
maltose. In the process, a water molecule is formed. (Credit: openstax.org)
For example, when two glucose molecules are linked together through
a dehydration reaction, a disaccharide called maltose is formed, as
shown in the figure above. Similarly, when two amino acids are
joined together, a dehydration reaction occurs, and a bond called a
peptide bond is formed between them to create a new molecule
called a dipeptide. Peptide bonds can be strung together to form a
long chain of amino acids, called a protein.
Hydrolysis reactions, on the other hand, are catabolic chemical reactions that involve the addition of water molecules to break down
larger molecules into smaller ones. Hydrolysis reactions are important in biology because they help cells to break down and use the
energy stored in macromolecules such as sugars, proteins, and fats.
Hydrolysis reactions are also important for breaking down other
macromolecules such as nucleic acids and proteins. Nucleic acids,
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vidual nucleotides through hydrolysis reactions. Similarly, proteins
can be broken down into individual amino acids through hydrolysis
reactions.
Figure 6: In the hydrolysis reaction shown here, the disaccharide
maltose is broken down to form two glucose monomers with the
addition of a water molecule. Note (Credit: openstax.org)
When our bodies digest food, they use hydrolysis reactions to break
down carbohydrates, proteins, and fats into smaller molecules that
can be used for energy. For example, the hydrolysis of maltose,
shown in the figure above, breaks the disaccharide down into two
glucose monomers with the addition of a water molecule.
In summary, dehydration and hydrolysis reactions are important
chemical processes in biology. Dehydration reactions involve the
removal of a single water molecule to create a larger molecule, while
hydrolysis reactions involve the addition of water molecules to break
down larger molecules into smaller ones. These reactions are crucial for the synthesis and breakdown of macromolecules in living
organisms.
3.2
Carbohydrates
Carbohydrates are one of the most important biomolecules found
in living organisms. They are the primary source of energy for our
bodies and play a critical role in providing structural support. CarEnterMedSchool.com
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bohydrates are made up of carbon, hydrogen, and oxygen atoms,
and they are classified into three main types based on the number
of monomers they contain: monosaccharides, disaccharides, and
polysaccharides.
Monosaccharides, such as glucose, fructose, and galactose, are simple sugars that cannot be broken down into smaller sugars. They
are joined together through dehydration synthesis, a process we
discussed earlier, to form more complex carbohydrates.
Disaccharides, like sucrose (table sugar), are made up of two monosaccharides that are linked together by a glycosidic bond. Disaccharides
are an important source of energy for our bodies and are often found
in sweet foods like candy and soda.
Polysaccharides, such as starch, glycogen, and cellulose, are complex
carbohydrates made up of many monosaccharide units. They are
used for energy storage and provide structural support to cells and
tissues. Plants store energy in the form of starch, while animals
store energy in the form of glycogen. Cellulose, on the other hand,
provides structural support to the cell walls of plants.
Insects have a hard outer skeleton made of chitin, a polysaccharide that contains nitrogen. Chitin provides structural support and
protection to insects, making it an important component of their
exoskeletons.
In summary, carbohydrates are an essential biomolecule found in living organisms, providing energy and structural support. They come
in different forms, from simple sugars to complex polysaccharides,
and play important roles in the functioning of our bodies and the
environment around us.
Carbohydrates are organic molecules that contain carbon, hydrogen, and oxygen atoms in the general formula Cn (H2 O)n . The simplest
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Figure 7: Monosaccharides are classified based on the position of
their carbonyl group and the number of carbons in the backbone.
Aldoses have a carbonyl group (indicated in green) at the end of the
carbon chain, and ketoses have a carbonyl group in the middle of
the carbon chain. Trioses, pentoses, and hexoses have three-, five-,
and six-carbon backbones, respectively. Note (Credit: openstax.org)
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carbohydrates are monosaccharides, which are single sugars that
cannot be broken down into simpler sugars. Monosaccharides have
a general formula of Cn H2n On , where n is typically 3 to 7. Monosaccharides can be classified into three categories based on the number of
carbon atoms they contain: trioses (3 carbons), tetroses (4 carbons),
and pentoses (5 carbons). The most common monosaccharides are
hexoses, which have six carbon atoms.
Disaccharides are formed when two monosaccharides are joined
together through a glycosidic bond, which is formed by a dehydration reaction between two monosaccharides. During this reaction,
a molecule of water is removed as two monosaccharides are linked
together to form a single disaccharide.
A glycosidic bond is a covalent bond that links two monosaccharides together. This bond is formed when a hydroxyl group (-OH) on
one monosaccharide reacts with the anomeric carbon of another
monosaccharide, which is a carbon atom that is part of a cyclic
hemiacetal structure. The reaction involves the removal of a water
molecule, which creates a covalent bond between the two monosaccharides. The resulting molecule is a disaccharide, which is a sugar
that consists of two monosaccharides linked together.
Common disaccharides include sucrose, lactose, and maltose. Sucrose, also known as table sugar, is composed of glucose and fructose, while lactose, found in milk, is composed of glucose and galactose. Maltose, found in germinating grains, is composed of two
glucose molecules linked together. Remember: To find a chemical
formula of a disaccharide we need to add both molecules together
and deduce a single water molecule.
Polysaccharides are long chains of monosaccharides that are joined
together by glycosidic bonds. They can be classified into two cateEnterMedSchool.com
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Figure 8: Sucrose is formed when a monomer of glucose and a
monomer of fructose are joined in a dehydration reaction to form a
glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from
the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2
in fructose. (Credit: openstax.org)
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Figure 9: Common disaccharides include maltose (grain sugar),
lactose (milk sugar), and sucrose (table sugar). Note (Credit: openstax.org)
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gories: homopolysaccharides and heteropolysaccharides. Homopolysaccharides are composed of a single type of monosaccharide, while
heteropolysaccharides are composed of multiple types of monosaccharides. Polysaccharides are important for energy storage and
structural support in living organisms. The most common kinds of
polysaccharides are glycogen, cellulose, amylose, and amylopectin.
Glycogen is like a storage unit for glucose in our body that’s made up
of individual glucose molecules. The glucose monomers are linked
via α-1,4 linkages and there is branching linked via α-1,6 linkages. It’s
found in the liver and muscles and acts like starch but in animals.
When our blood sugar gets too low, glycogen is broken down to
release glucose in a process called glycogenolysis.
Cellulose is a natural material found in plant cell walls that gives
them structure and support. It’s the most common type of biopolymer in nature and makes up things like wood and paper. Cellulose
is made up of glucose molecules that are linked together via by β 1-4
glycosidic linkages. In contrast, starch, a plant storage carbohydrate
made of amylose and amylopectin, is made with glucose linked via
α-1,4 or α-1,6 glycosidic linkages. Amylose consists of unbranched
chains (only α-1,4), while amylopectin has additional α-1,6 linkages
that are a result of branching (so it has both α-1,4 and α-1,6 linkages).
3.3
Amino Acids
Amino acids are small molecules that serve as the building blocks of
proteins, which are essential for life. Proteins have many functions in
our bodies, such as helping our muscles move and supporting our
immune system. Amino acids play a crucial role in determining the
structure and function of proteins, and their unique properties allow
them to form different types of proteins with specific functions. A
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Figure 10: In cellulose, glucose monomers are linked in unbranched
chains by β 1-4 glycosidic linkages. Because of the way the glucose
subunits are joined, every glucose monomer is flipped relative to the
next one resulting in a linear, fibrous structure.(Credit: openstax.org)
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Figure 11: Amylose and amylopectin are two different forms of starch.
Amylose is composed of unbranched chains of glucose monomers
connected by α-1,4 glycosidic linkages. Amylopectin is composed
of branched chains of glucose monomers connected by α-1,4 and
α-1,6 glycosidic linkages. Because of the way the subunits are joined,
the glucose chains have a helical structure. Glycogen (not shown) is
similar in structure to amylopectin but more highly branched. Note
(Credit: openstax.org)
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balanced and varied diet is important for obtaining the different
types of amino acids needed for protein synthesis in our bodies..
Amino acids are small molecules that serve as the building blocks of
proteins, which are essential for life. Each amino acid has a specific
structure consisting of a central chiral carbon atom, called alpha
carbon. A chiral carbon is a carbon atom that is bonded to four
different substituents.
In addition to the alpha carbon, amino acids have an amino group
(-NH2) this is the ”Amino” part in ”Amino Acid”. The second group is
a carboxyl group (-COOH) which is made up of carbon, oxygen, and
hydrogen atoms and serves as the acidic part of the amino acid. The
third group is the variable side chain (-R). The unique structure of
each amino acid is determined by the nature of its side chain. The
side chain can be a simple hydrogen atom, or it can be a complex
structure that contains functional groups such as hydroxyl (-OH),
sulfhydryl (-SH), or amino (-NH2). These functional groups give the
amino acids their unique chemical and physical properties, which
play a crucial role in the formation of proteins and their functions in
our bodies. The last substituent bonded to the alpha carbon is just
a single hydrogen atom, which completes the four bonds carbon
loves to have.
An important exemption to the rule is the amino acid Glycine. Glycine
has a single Hydrogen atom as its R group, so Glycine’s carbon isn’t
a chiral carbon.
Great! Now that we understand the basic building block, let’s talk
about the four structural levels of proteins.
The primary structure of a protein is the linear sequence of amino
acids that make up the protein chain (the order of amino acids). The
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Figure 12: Amino acids have a central asymmetric carbon to which
an amino group, a carboxyl group, a hydrogen atom, and a side
chain (R group) are attached. (Credit: openstax.org)
specific sequence of amino acids determines the unique characteristics and function of the protein.
The secondary structure of a protein is the way the protein folds up
into a regular pattern or shape, such as an alpha helix or beta sheet.
These shapes are stabilized by hydrogen bonds between the amino
acids.
The tertiary structure of a protein is the way the protein folds up
even further to create a three-dimensional shape. This structure is
critical for the protein’s function, as it determines the way that the
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Figure 13: There are typically 20 common amino acids commonly
found in proteins, each with a different R group (variant group) that
determines its chemical nature.(Credit: openstax.org)
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protein interacts with other molecules in the cell. Tertiary structure is
stabilized by various interactions such as hydrogen bonds, disulfide
bridges, van der Waals interactions, and hydrophobic interactions.
Finally, the quaternary structure of a protein is the way multiple
protein subunits come together to form a larger complex. This
structure is found in proteins that have more than one polypeptide
chain. Quaternary structure is important for the function of many
proteins, such as enzymes, which often require multiple subunits to
work properly.
3.3.1 Primary Structure
The primary structure of a protein is the linear sequence of amino
acids that make up the protein. It’s called the primary structure
because it’s the first level of organization of the protein.
Amino acids are joined together in a protein by a type of chemical
bond called a peptide bond. When an amino acid is joined to another amino acid, the carboxyl group of one amino acid reacts with
the amino group of the other amino acid, and a water molecule is
released. This reaction is another example of a dehydration reaction
which we discussed earlier. The reaction creates a bond between
the two amino acids, and the result is a long chain of amino acids.
The primary structure of a protein is important because it will eventually determine the three-dimensional shape of the protein, which
in turn determines the protein’s function. Different sequences of
amino acids can fold into different shapes, and each shape is suited
for a specific function. For example, some proteins are enzymes
that help chemical reactions happen more quickly, while others are
structural proteins that help hold tissues together. It is important to
mention that each subsequent protein level relies on the previous
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Figure 14: Peptide bond formation is a dehydration synthesis reaction. The carboxyl group of one amino acid is linked to the amino
group of the incoming amino acid. In the process, a molecule of
water is released. (Credit: openstax.org)
levels.
To understand the primary structure better, imagine that a protein
is like a necklace made up of small beads. Each bead represents an
amino acid, and the beads are joined together by peptide bonds,
which are like the string that holds the beads in place. The sequence
of beads on the necklace represents the primary structure of the
protein, just like the sequence of amino acids in a protein represents
the primary structure of the protein.
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Just like a necklace can be made up of different kinds of beads,
a protein can be made up of different kinds of amino acids. And
just like a necklace can be made in different ways, with the beads
arranged in different patterns, a protein can have a different primary
structure depending on the sequence of amino acids.
Figure 15: Bovine serum insulin is a protein hormone made of two
peptide chains, A (21 amino acids long) and B (30 amino acids long).
In each chain, primary structure is indicated by three-letter abbreviations that represent the names of the amino acids in the order
they are present. The amino acid cysteine (cys) has a sulfhydryl
(SH) group as a side chain. Two sulfhydryl groups can react in the
presence of oxygen to form a disulfide (S-S) bond. Two disulfide
bonds connect the A and B chains together, and a third helps the A
chain fold into the correct shape. Note that all disulfide bonds are
the same length, but are drawn different sizes for clarity. (Credit:
openstax.org)
In sickle cell anemia, a change in just one amino acid out of 600 in
the hemoglobin protein causes a major difference in structure and
function. This single amino acid change is caused by a mutation in
the gene that codes for the hemoglobin protein, which alters the
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primary structure of the protein. This change in primary structure
causes the hemoglobin protein to clump together, resulting in the
characteristic sickle shape of red blood cells. This can lead to a variety
of health problems, including reduced oxygen delivery to tissues
and organ damage.
Figure 16: The beta chain of hemoglobin is 147 residues in length,
yet a single amino acid substitution leads to sickle cell anemia. In
normal hemoglobin, the amino acid at position seven is glutamate.
In sickle cell hemoglobin, this glutamate is replaced by a valine.
(Credit: openstax.org)
3.3.2
Secondary Structure
The secondary structure of a protein is determined by the way that
the chain of amino and carboxyl groups, known as the backbone
of the peptide, interact with each other and form hydrogen bonds.
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There are two main types of secondary structures: Alpha helices
and Beta sheets. An alpha helix is a long, spiral structure that’s held
together by hydrogen bonds between the amino acids. It looks a
bit like a twisted ladder. Beta sheets are flat structures made up of
several parallel strands of amino acids held together by hydrogen
bonds. They can be either parallel, with the strands running in the
same direction, or anti-parallel, with the strands running in opposite
directions.
Let’s go back to our necklace analogy: Imagine that you want to fold
the necklace into a specific shape. You can do this by twisting the
necklace into a spiral, like an alpha helix, or by laying the necklace
flat and arranging the beads in parallel or antiparallel strands, like
a beta-sheet. The way that you fold the necklace represents the
secondary structure of the protein, just like the way that a protein
folds into an alpha helix or a beta sheet represents the secondary
structure of the protein.
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Figure 17: The α-helix and β -pleated sheet are secondary structures of
proteins that form because of hydrogen bonding between carbonyl
and amino groups in the peptide backbone. Certain amino acids
have a propensity to form an α-helix, while others have a propensity
to form a β -pleated sheet. (Credit: openstax.org)
3.3.3
Tertiary Structure
The tertiary structure of a protein is determined by the interactions between the side chains (R groups), of the amino acids. Side
chains are the groups of atoms that are attached to the alpha carbon of each amino acid, and they can be different for each amino
acid. Some side chains are attracted to water and are said to be
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hydrophilic, while others are repelled by water and are said to be
hydrophobic.
Key Takeaways
Remember: The Tertiary structure of a protein is important because it helps determine the overall three-dimensional shape
of the protein, which in turn determines the protein’s function.
Different Tertiary structures can fold into different shapes, and
each shape is suited for a specific function.
Now, imagine that each bead has a small group of atoms attached
to it, representing the side chains of the amino acids. Some of these
groups are hydrophilic, meaning that they’re attracted to water,
while others are hydrophobic, meaning that they’re repelled by
water.
As the protein folds up into its three-dimensional shape, the hydrophilic side chains will tend to be on the outside of the protein,
where they can interact with water, while the hydrophobic side
chains will tend to be on the inside of the protein, where they’re
shielded from water. This helps stabilize the protein’s tertiary structure and gives the protein its overall three-dimensional shape.
There are several types of side chains in amino acids. Disulfide
bridges, also known as disulfide bonds, are covalent bonds that
form between two cysteine residues in a protein. These bonds are
a type of chemical bond that forms when two sulfur atoms are
bonded together by a covalent bond. Disulfide bonds play a key role
in stabilizing the tertiary structure of proteins, which refers to the
three-dimensional shape of the protein.
It’s important not to confuse the terms ”cysteine” and ”cystine”. Cysteine is an amino acid, while cystine is formed when two cysteine
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amino acids are joined together by a disulfide bond.
Figure 18: The tertiary structure of proteins is determined by a variety
of chemical interactions. These include hydrophobic interactions,
ionic bonding, hydrogen bonding and disulfide linkages. (Credit:
openstax.org)
3.3.4
Quaternary Structure
The quaternary structure of a protein is the way that multiple proteins come together to form a larger complex. It’s called the Quaternary structure because it’s the fourth level of organization of the
protein, after the primary structure, the secondary structure, and
the tertiary structure.
Quaternary structures can be very complex and can involve many
different proteins coming together in different ways and shapes.
For example, some Quaternary structures are made up of multiple
copies of the same protein, while others are made up of different
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types of proteins.
The Quaternary structure of a protein is important because it determines the overall function of the protein complex. Different Quaternary structures can perform different functions, and each function
is important for the proper functioning of the cell or organism.
Hemoglobin is a protein that is made out of four different proteins
attached together by hydrophobic interactions, hydrogen bonding,
and salt bridges. Hemoglobin is found in red blood cells and is
mainly responsible for carrying oxygen from the lungs to the rest
of the body. Hemoglobin is a good example of a protein with a
Quaternary structure.
Hemoglobin is made up of four subunits, each of which is a separate
protein. These subunits are called alpha, beta, gamma, and delta,
and they’re arranged in a specific way to form the overall structure
of hemoglobin.
There are several types of hemoglobin, including fetal hemoglobin
and adult hemoglobin. Fetal hemoglobin has a higher affinity for
oxygen than adult hemoglobin, which allows it to bind more readily
to oxygen and transport it to the developing fetus. After birth, the
body switches to producing adult hemoglobin, which has a lower
affinity for oxygen but is more efficient at releasing oxygen to the
tissues.
3.3.5
Hemoglobin Cooperative Binding
Hemoglobin cooperative binding refers to the way that hemoglobin
binds to oxygen. When oxygen binds to one subunit of hemoglobin,
it changes the shape of the protein and makes it easier for the
other subunits to bind to oxygen as well. This process is known as
cooperative binding, because the binding of one oxygen molecule to
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Figure 19: The four levels of protein structure can be observed in
these illustrations. (credit: modification of work by National Human
Genome Research Institute) (Modified by: openstax.org)
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a hemoglobin subunit cooperates with the binding of other oxygen
molecules to the other subunits.
The cooperative binding of oxygen to hemoglobin allows the protein
to bind more oxygen at lower partial pressures of oxygen. This is
important because it enables the hemoglobin to extract oxygen
from the air in the lungs and deliver it to the tissues where it is
needed, even when the partial pressure of oxygen is low.
Cooperative binding is a key feature of hemoglobin and is important for the efficient transport of oxygen throughout the body. It
allows the hemoglobin to bind and release oxygen in a way that
is responsive to the changing oxygen needs of the body’s tissues,
ensuring that they receive a constant supply of oxygen.
Therefore, cooperative binding is the result of the Quartanery structure of the Hemoglobin protein!
To understand how hemoglobin is structured, imagine that you
are a chef in a busy kitchen. Each ingredient has a specific role
to play in the recipe. One ingredient might provide flavor, while
another might add texture. But while each ingredient can be used
independently, they also work together to create a finished dish,
and if there is any change in one of the ingredients, it will affect
how much of the other ingredients you use as well. Similarly, the
subunits of hemoglobin work together to perform the vital task of
transporting oxygen throughout the body, just like how ingredients
work together to create a tasty dish.
3.3.6 Globular and Fibrous Proteins
There are two main categories of protein structure: Globular proteins
and Fibrous proteins.
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Globular proteins are spherical in shape and are usually soluble in
water. They are found in the cytoplasm of cells and perform various
functions, including acting as enzymes and transporting molecules
within the body. Examples of globular proteins include hemoglobin,
and lactoglobulin, which is found in milk.
Fibrous proteins, on the other hand, are long and slender and are
usually insoluble in water. They are found in tissues such as hair,
nails, and skin and provide structural support. Examples of fibrous
proteins include collagen, which is found in connective tissue, and
keratin, which is found in hair and nails.
3.4
Lipids
Lipids are a diverse group of organic molecules that are important
for a variety of biological functions. They include fats, waxes, sterols,
and certain types of oils. Lipids are characterized by their ability to
dissolve in nonpolar solvents, such as ether or olive oil, but not in
water.
Remember: The saying ”like-dissolves-like” refers to the idea that a
substance will dissolve more readily in a solvent that is similar to itself
in terms of polarity. Polar substances have an uneven distribution
of charge and are more likely to dissolve in polar solvents, while
nonpolar substances have an even distribution of charge and are
more likely to dissolve in nonpolar solvents. Here are two examples
of polar and nonpolar substances: Polar substance: Water (H2O) is
a polar substance because it has a partially positive charge on the
hydrogen atoms and a partially negative charge on the oxygen atom.
This uneven distribution of charge makes it more likely to dissolve
polar substances. Nonpolar substance: Hexane (C6H14) is a nonpolar
substance because it has an even distribution of charge. It is more
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likely to dissolve nonpolar substances. In general, polar substances
tend to dissolve polar substances and nonpolar substances tend to
dissolve nonpolar substances, following the ”like-dissolves-like” rule.
One of the most important roles of lipids is as an energy store. Fats
and oils are composed of glycerol and fatty acids, which can be
broken down by the body to release large amounts of energy. Lipids
are also important structural components of cell membranes, where
they help to maintain the integrity and fluidity of the membrane.
Sterols, such as cholesterol, are important components of cell membranes and are also precursors to hormones and other signaling
molecules. Waxes are used for waterproofing and protection in
plants and animals, and some lipids, such as the pigment carotenoids,
are important for pigmentation and protection from UV radiation.
Some common types of lipids include:
Triglycerides (fats and oils)
Phospholipids (components of cell membranes)
Sterols (such as cholesterol)
Waxes (such as beeswax and plant cutin)
Carotenoids (pigments found in plants and animals)
3.4.1 Saturated and Unsaturated Fats
Before we dive into the different concepts related to lipids, it would
be best to understand the concept of saturation and unsaturation.
When all of the carbon-carbon bonds in the fatty acid chain are
single bonds, the fatty acid is said to be ”saturated.” Saturated fatty
acids have the maximum number of hydrogen atoms attached to
each carbon in the chain and are therefore solid at room temperEnterMedSchool.com
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ature. Examples of saturated fatty acids include palmitic acid and
stearic acid, which are found in animal fats. In contrast, when one
or more carbon-carbon double bonds are present in the fatty acid
chain, the fatty acid is said to be ”unsaturated.” Unsaturated fatty
acids have fewer hydrogen atoms attached to each carbon in the
chain and are therefore liquid at room temperature. Examples of
unsaturated fatty acids include oleic acid and linoleic acid, which
are found in vegetable oils.
The degree of saturation in lipids affects their melting point, which
is the temperature at which they change from a solid to a liquid.
Saturated lipids have higher melting points than unsaturated lipids,
because the strong interactions between the saturated fatty acid
chains make it more difficult for the molecules to move and become
liquid. In contrast, the presence of double bonds in unsaturated
fatty acids creates a kink in the fatty acid chain, which prevents the
molecules from packing closely together and lowers the melting
point of the lipid. This is why unsaturated fats are usually liquid at
room temperature, while saturated fats are solid.
To understand the concept better, imagine a group of people standing in a crowded room. The people are like the fatty acid molecules
in a triglyceride molecule, and the room is like the triglyceride
molecule.
If the people are tightly packed together, it will be harder for them
to move and break free from each other. This is like a triglyceride
molecule with saturated fatty acids, which have a higher melting
point and boiling point because the fatty acid molecules are more
tightly packed together and have fewer double bonds.
On the other hand, if the people are not as tightly packed together
and there is more space between them, it will be easier for them
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to move and break free from each other. This is like a triglyceride
molecule with unsaturated fatty acids, which have a lower melting
point and boiling point because the fatty acid molecules are not as
tightly packed together and have more double bonds.
Lipids can be classified as saturated, monounsaturated, or polyunsaturated depending on the number of double bonds in the fatty
acid chains. Saturated lipids contain only saturated fatty acids, while
unsaturated lipids contain one or more unsaturated fatty acids. Monounsaturated lipids have one double bond in the fatty acid chain,
while polyunsaturated lipids have two or more double bonds.
The degree of saturation in lipids is important for their function in
the body. Saturated fats, for example, are typically solid at room
temperature and tend to be associated with an increased risk of
heart disease. In contrast, unsaturated fats are usually liquid at
room temperature and are associated with a reduced risk of heart
disease. The degree of saturation also affects the fluidity of the cell
membrane, as lipids with more double bonds in their fatty acid
chains are more flexible and fluid than lipids with fewer double
bonds.
3.4.2
Triglycerides
Triglycerides, also known as triacylglycerols or neutral fats, are a type
of lipid that plays an important role in the body. Triglycerides are
called triglycerides because they are composed of three fatty acid
molecules bonded to a single glycerol molecule. The prefix ”tri-”
means three, and ”glyceride” refers to the glycerol molecule. The
fatty acids are attached to the glycerol molecule by ester bonds,
creating a molecule that has three fatty acid chains hanging off it.
Triglycerides can be either saturated or unsaturated, and the degree
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Figure 20: Saturated fatty acids have hydrocarbon chains connected
by single bonds only. Unsaturated fatty acids have one or more double bonds. Each double bond may be in a cis or trans configuration.
In the cis configuration, both hydrogens are on the same side of the
hydrocarbon chain. In the trans configuration, the hydrogens are on
opposite sides. A cis double bond causes a kink in the chain. (Credit:
openstax.org)
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of saturation affects their physical properties.
Triglycerides are the main form of fat stored in the body, and they
are also found in foods such as vegetable oils, butter, and lard. In the
body, triglycerides serve as an important source of energy, and they
are broken down into their component parts and used by cells for
energy production. The glycerol molecule is converted to glucose,
which can be used by cells for energy, while the fatty acid molecules
are broken down into smaller molecules called ketone bodies, which
can also be used for energy.
Excessive consumption of triglycerides can lead to obesity and other
health problems, such as heart disease and diabetes. However,
triglycerides also play a role in the structure and function of cell
membranes. They are one of the main components of the phospholipid bilayer, which forms the basic structure of the cell membrane.
The fatty acid chains in the triglycerides help to make the membrane more fluid and flexible, allowing the cell to carry out its many
functions.
Triglycerides can be found in various parts of the human body, such
as adipose tissue, which is the tissue that stores fat. Adipose tissue
is found throughout the body, but it is especially concentrated in
areas such as the abdomen, hips, and thighs. Triglycerides are also
present in the bloodstream and can be measured using a blood test.
High levels of triglycerides in the blood can be an indication of various health problems, such as metabolic disorders or cardiovascular
disease.
In summary, triglycerides are an essential component of the human body, serving as an important source of energy and playing a
role in the structure and function of cell membranes. They can be
found in various parts of the body, including adipose tissue and the
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bloodstream, and are present in many common foods.
Figure 21: Triacylglycerol is formed by the joining of three fatty acids
to a glycerol backbone in a dehydration reaction. Three molecules
of water are released in the process. (Credit: openstax.org)
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3.4.3
Phospholipids
Phospholipids are a type of lipid that is essential for the structure
and function of cell membranes. They are composed of a glycerol
backbone covalently bonded to two fatty acid chains and a phosphate group attached to the glycerol backbone. The phosphate
group is negatively charged, which gives phospholipids their unique
properties.
The importance of phospholipids in the human body cannot be
overstated. They are the main building blocks of cell membranes
and help to regulate what goes in and out of cells. In addition, phospholipids also play a role in cell signaling and are involved in various
biological processes such as blood clotting and inflammation.
Phospholipids are found in many different parts of the body, including cell membranes, nerve tissue, and the brain. They are also
present in foods such as egg yolks and soybeans. Deficiencies in
phospholipids have been linked to various health conditions, such
as neurological disorders and liver disease. Therefore, ensuring an
adequate intake of phospholipids is important for overall health and
well-being..
As mentioned earlier, the general structure of a phospholipid consists of a glycerol molecule with two fatty acid chains and a phosphate group attached to the third carbon of the glycerol. The phosphate group is negatively charged, which gives phospholipids their
unique properties.
The fatty acid chains are long, nonpolar hydrocarbon chains that
are hydrophobic (water-fearing). They consist of a carboxylic acid
group (-COOH) at one end and a methyl group (-CH3) at the other
end. The fatty acid chains vary in length and degree of saturation,
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Figure 22: A phospholipid is a molecule with two fatty acids and a
modified phosphate group attached to a glycerol backbone. The
phosphate may be modified by the addition of charged or polar
chemical groups. (Credit: openstax.org)
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which affects the fluidity and permeability of the cell membrane.
In contrast, the phosphate group and the glycerol molecule are
hydrophilic (water-loving) due to their polar nature. The phosphate
group contains a negatively charged phosphate ion (-PO4) and is often attached to another polar or charged molecule, such as choline
or serine, forming a phosphatidylcholine or phosphatidylserine, respectively. This polar head group faces outward and interacts with
the aqueous environment, while the hydrophobic fatty acid tails
face inward, away from the water.
The combination of the hydrophilic head group and hydrophobic
tails makes phospholipids amphipathic, meaning they have both hydrophilic and hydrophobic properties. This unique property allows
phospholipids to spontaneously form bilayers, with the hydrophobic tails facing inward and the hydrophilic heads facing outward.
The phospholipid bilayer forms the basis of all cell membranes and
organelles, providing a barrier between the inside and outside of
the cell while allowing for selective transport of molecules..
The phospholipid bilayer serves several functions, including acting
as a barrier to prevent unwanted molecules from entering or exiting
the cell, as well as facilitating the transport of specific molecules
across the membrane. It also plays a role in cell signaling and cell
recognition. The bilayer’s fluid nature allows it to be flexible, allowing the cell to change shape, and its selective permeability allows
specific molecules to pass through. This selective permeability is
essential for regulating the transport of ions and molecules into and
out of the cell, which is crucial for maintaining the cell’s internal
environment.
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Figure 23: The phospholipid bilayer is the major component of all cellular membranes. The hydrophilic head groups of the phospholipids
face the aqueous solution. The hydrophobic tails are sequestered in
the middle of the bilayer. (Credit: openstax.org)
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brane. The proteins can act as transport channels, enzymes, or
receptors, among other functions. The lipid composition of the bilayer varies among different cell types and organelles, and the ratio
of different lipid species can affect membrane fluidity and stability.
For example, the presence of cholesterol in the bilayer can affect
the fluidity of the membrane, making it more or less permeable to
different molecules.
Phospholipid bilayers are found in various parts of the human body,
as mentioned earlier, and their functions vary depending on the
cell type and location. In the plasma membrane of cells, the bilayer
acts as a barrier to protect the cell and regulate the transport of
molecules. In the myelin sheath, the bilayer acts as an insulating
layer that allows for the rapid conduction of nerve impulses.
Overall, the phospholipid bilayer is a critical component of all cell
membranes, serving various functions that are essential for the
survival of the cell and the organism as a whole.
Lipid bilayers can form vesicles through a process called self-assembly.
When lipids are placed in an aqueous solution, they will spontaneously arrange themselves into a bilayer structure. This is because
the hydrophobic tails of the lipids are attracted to one another,
while the hydrophilic heads interact with the surrounding water
molecules. The resulting structure is a stable, dynamic membrane
that can enclose a small volume of water or other solution.
The phospholipid bilayer acts like a wall in that it separates the inside
of the cell from the outside environment, much like a wall separates
the inside of a house from the outside. The proteins and structures
on the surface of the phospholipid bilayer can be thought of as
plants growing on the wall’s surface. The roots of the plants are
anchored in the hydrophobic interior of the wall, while the plants
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Figure 24: Fats are molecules that have both hydrophobic and hydrophilic properties, meaning they are amphiphilic. The long hydrocarbon tail of a fat molecule repels water, while the glycerol part
of the molecule is attracted to water. When fats are in water, they
form a structure called a micelle, where the hydrophilic heads of
the fat molecules are on the outer surface and the hydrophobic tails
are on the inside. This arrangement protects the hydrophobic tails
from the water and allows the hydrophilic heads to interact with the
surrounding water molecules. (Credit: openstax.org)
themselves are on the outside, exposed to the environment. These
proteins and structures perform various functions, such as transporting molecules across the membrane, communicating with other
cells, and interacting with the environment. Sometimes these plants
don’t have roots, and they just climb and hold to the wall itself, while
many others are anchored in the wall and sometimes the roots even
go all the way to the other side!
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3.4.4
Sterols
Sterols are a unique class of lipids that play important structural and
biological roles in cells. They have a fused ring structure composed
of four rings, three of which are six-membered and one of which
is a five-membered ring. The structure of the fused rings is crucial to their function in cell membranes, where they help regulate
membrane fluidity and permeability.
Cholesterol, the most well-known sterol, is an essential component
of the human body and is found in animal cells. Its structure is similar
to other sterols, with a fused ring structure and a hydrocarbon tail.
The hydroxyl (-OH) group on the steroid ring makes cholesterol
amphipathic, meaning it has both hydrophilic and hydrophobic
regions.
The unique structure of cholesterol allows it to interact with the
hydrophobic tails of phospholipids in cell membranes, playing an
important role in maintaining membrane fluidity and rigidity. In
addition to its structural role, cholesterol is also a precursor for the
synthesis of steroid hormones and bile acids.
Cholesterol is synthesized in the liver and is also obtained from
the diet. It plays a number of important roles in the body, including serving as a precursor for the synthesis of hormones, such as
testosterone and estrogen, and bile acids as mentioned, which are
necessary for the digestion and absorption of fats.
Other important sterols include ergosterol, which is found in fungi
and plant cells, and stigmasterol, which is found in plants. These
sterols have different biological functions, but they share a similar
chemical structure to cholesterol.
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volved in the synthesis of hormones and other biomolecules, they
help to regulate cell membrane fluidity and structure, and they
play a role in the absorption and metabolism of fats. They are also
important for maintaining the overall health of cells and tissues.
3.4.5
Waxes
Waxes are complex lipids that have unique physical and chemical
properties due to their intricate structure. Their high molecular
weight and hydrophobic nature make them water-resistant, which is
important for their function in waterproofing and protection. Waxes
are composed of a long-chain fatty acid and a long-chain alcohol
that are esterified through an ester bond. The length and degree
of saturation of these chains can vary depending on the organism
and the specific function of the wax. For example, plant waxes often
contain very long-chain fatty acids and alcohols, which contribute
to their water-repelling properties. In contrast, animal waxes such
as beeswax contain shorter-chain fatty acids and alcohols. Waxes
can be found in a variety of biological structures such as the cuticle
of plants, the feathers of birds, and the exoskeletons of insects.
One of the most important functions of waxes is to provide a protective barrier for the surface of an organism. In plants, waxes are
found on the surface of leaves, stems, and other plant organs, where
they help to reduce water loss and protect against insect damage
and fungal infections. In animals, waxes are found on the surface of
the skin and in the ear canal, where they help to keep the skin and
ears moist and prevent them from becoming dry and irritated.
Waxes also play a role in the regulation of body temperature in
animals. In birds and mammals, waxes are found in the feathers and
fur, respectively, and they help to insulate the body and maintain
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a constant body temperature. In insects, waxes are found on the
surface of the exoskeleton, where they help to prevent desiccation
and regulate body temperature.
In addition to their protective and regulatory functions, waxes also
have a number of other biological roles. For example, some waxes
are used as signaling molecules, and others are involved in the
synthesis of hormones and other biomolecules. Overall, waxes are
important for the function and survival of a wide range of organisms.
3.5
Nucleic Acids
Nucleic acids are some of the most important biomolecules in living
organisms. They play a critical role in the transmission and expression of genetic information, allowing cells to pass on their genetic
instructions to the next generation of cells. This genetic information not only determines an organism’s physical traits, but also the
functions and behaviors of its cells. In addition to their role in genetic transmission, nucleic acids are also involved in many other
cellular processes, including DNA repair, RNA synthesis, and protein
synthesis.
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two
main types of nucleic acids. DNA is found primarily in the nucleus
of cells, and it contains the genetic information that is passed down
from one generation to the next. RNA, on the other hand, is synthesized from DNA and plays a crucial role in the production of proteins,
which are essential for the structure and function of cells. The study
of nucleic acids has revolutionized our understanding of genetics
and has led to numerous discoveries in medicine and biotechnology.
In the cell, DNA can be found in the nucleus and in the organelles
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(chloroplasts, and mitochondria) of eukaryotes. In prokaryotic cells,
the DNA is free, it is not enclosed in a membranous envelope (can
be found most commonly as singular bacterial chromosomes or as
plasmids).
3.5.1 Purines and Pyrimidines
Purines and pyrimidines are two types of nitrogenous bases that
are found in nucleotides, the building blocks of nucleic acids such
as DNA and RNA. Purines are larger molecules than pyrimidines
and consist of a six-membered ring fused to a five-membered ring.
The purine bases that are found in nucleic acids are adenine (A) and
guanine (G). Pyrimidines, on the other hand, are smaller molecules
and consist of a single six-membered ring. The pyrimidine bases
that are found in nucleic acids are cytosine (C), thymine (T), and
uracil (U). In DNA, thymine is present instead of uracil.
The difference in size between purines and pyrimidines has important implications for the structure of DNA and RNA. Because purines
are larger than pyrimidines, they pair more easily with pyrimidines,
which helps to maintain the structural stability of DNA and RNA. In
DNA, A always pairs with T, and G always pairs with C, while in RNA,
A pairs with U, and G pairs with C. This base pairing is crucial for the
proper functioning of DNA and RNA as genetic information carriers.
Understanding the differences between purines and pyrimidines
is essential for understanding the molecular basis of genetics and
the processes that underlie the transmission of genetic information
from one generation to the next.
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3.5.2 DNA and RNA
DNA, or deoxyribonucleic acid, is the genetic material that carries
the hereditary information in all living organisms. It is a doublestranded molecule that consists of a sugar-phosphate backbone
and four nitrogenous bases: adenine (A), cytosine (C), guanine (G),
and thymine (T). These four bases pair up with each other through
hydrogen bonds: A pairs with T, and C pairs with G. The sequence
of these bases carries the genetic information that is passed down
from one generation to the next.
DNA is wrapped around proteins called histones to form chromatin.
The chromatin can be further compacted into chromosomes during
cell division. When the DNA is tightly packed, it is called heterochromatin, and when it is more open and accessible, it is called euchromatin. Heterochromatin is usually not actively transcribed, while
euchromatin is more accessible to enzymes involved in transcription
and is usually actively transcribed.
The information encoded in DNA is used to synthesize proteins and
other molecules that are essential for life processes. The process
of transcription involves the conversion of the DNA code into RNA,
and translation involves the conversion of RNA into proteins. These
processes are tightly regulated to ensure the correct expression
of genes and the production of the necessary molecules at the
appropriate times and in the appropriate amounts. DNA replication,
or the process of copying the DNA, is also essential for cell division
and the transmission of genetic information to daughter cells..
RNA, or ribonucleic acid, is a single-stranded molecule made up of
a ribose sugar-phosphate backbone and four nitrogenous bases:
adenine (A), cytosine (C), guanine (G), and uracil (U). The nitrogenous bases in RNA are the same as those in DNA, except that uracil
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replaces thymine in RNA.
RNA plays a central role in the transfer of genetic information from
DNA to the ribosomes, where proteins are synthesized. This process
is known as the central dogma of molecular biology. In the first step
of the central dogma, DNA is transcribed into RNA by an enzyme
called RNA polymerase. The RNA molecule then carries the genetic
information from the nucleus to the ribosomes, where it serves as a
template for the synthesis of proteins in a process called translation.
There are four main types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and microRNA (miRNA).
mRNA carries genetic information from DNA to ribosomes, which
are responsible for synthesizing proteins in a cell. When a cell requires a particular protein to be made, the gene that codes for it is
”turned on” and mRNA is synthesized in the nucleus. The base sequence of mRNA is similar to the coding sequence of the DNA it was
transcribed from, except that the RNA contains uracil (U) instead of
thymine (T). In the cytoplasm, mRNA interacts with ribosomes and
other cellular machinery to translate the genetic code into a protein.
rRNA, which is a component of ribosomes, helps to catalyze the
chemical reactions involved in protein synthesis. tRNA molecules
bring amino acids to the ribosome, where they are assembled into
a protein chain.
Finally, miRNA is a small, non-coding RNA molecule that can regulate gene expression by binding to mRNA and preventing its translation into protein.
In addition to its role in protein synthesis, RNA also plays a catalytic
role in certain chemical reactions. These specialized RNA molecules,
known as ribozymes, have been shown to catalyze reactions such as
peptide bond formation and RNA splicing. Overall, RNA is a versatile
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and essential molecule in the functioning of cells and the expression
of genetic information.
DNA and RNA are nucleic acids composed of nucleotides that differ
in their sugar component. The sugar in DNA is deoxyribose, while
in RNA, it is ribose. The primary difference between these sugars is
the presence (or absence) of a hydroxyl group on the second carbon
atom. The carbon atoms of the sugar molecule are designated as 1′,
2′, 3′, 4′, and 5′, with 1′ being read as “one prime”.
Each nucleotide contains a phosphate residue connected to the
hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group
of the 3′ carbon of the next sugar in the nucleotide sequence. This
linkage is called a 5′-3′ phosphodiester linkage, and its formation
involves the removal of two phosphate groups. It is not formed by
simple dehydration reactions like other linkages in macromolecules
mentioned earlier.
A polynucleotide is a chain of nucleotides linked together by phosphodiester bonds. DNA and RNA are polynucleotides that contain
many thousands of these phosphodiester linkages. The sequence of
the nitrogenous bases in DNA and RNA carries genetic information
that encodes the instructions for the synthesis of proteins and other
molecules. The central dogma of molecular biology describes the
flow of genetic information from DNA to RNA to protein.
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Figure 25: Steroids such as cholesterol and cortisol are composed of
four fused hydrocarbon rings. (Credit: openstax.org)
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Figure 26: A ribosome has two parts: a large subunit and a small subunit. The mRNA sits in between the two subunits. A tRNA molecule
recognizes a codon on the mRNA, binds to it by complementary
base pairing, and adds the correct amino acid to the growing peptide chain. (Credit: openstax.org)
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Figure 27: In a double stranded DNA molecule, the two strands run
antiparallel to one another so that one strand runs 5′ to 3′ and the
other 3′ to 5′. The phosphate backbone is located on the outside,
and the bases are in the middle. Adenine forms hydrogen bonds
(or base pairs) with thymine, and guanine base pairs with cytosine.
(Credit: openstax.org)
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Figure 28: Native DNA is an antiparallel double helix. The phosphate
backbone (indicated by the curvy lines) is on the outside, and the
bases are on the inside. Each base from one strand interacts via
hydrogen bonding with a base from the opposing strand. (credit:
Jerome Walker/Dennis Myts)
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Figure 29: A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups.
Carbon residues in the pentose are numbered 1′ through 5′ (the
prime distinguishes these residues from those in the base, which
are numbered without using a prime notation). The base is attached
to the 1′ position of the ribose, and the phosphate is attached to the
5′ position. When a polynucleotide is formed, the 5′ phosphate of
the incoming nucleotide attaches to the 3′ hydroxyl group at the
end of the growing chain. Two types of pentose are found in nucleotides, deoxyribose (found in DNA) and ribose (found in RNA).
Deoxyribose is similar in structure to ribose, but it has an H instead
of an OH at the 2′ position. Bases can be divided into two categories:
purines and pyrimidines. Purines have a double ring structure, and
pyrimidines have a single ring.. (Credit: openstax.org)
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4 Cell Biology
As you may already know, cells are the fundamental unit of life, and
they come in a wide variety of shapes and sizes, each with its own
unique functions and roles. In this chapter, we will embark on a
journey through the complex and intricate world of cells, exploring
their structure, function, and role in the overall functioning of living
organisms.
As you delve into this chapter, you will learn about the different
types of cells found in the natural world, including prokaryotic cells
and eukaryotic cells. You will also discover the intricate structure
of cells, including their various organelles and their functions, such
as the mitochondria, which produce energy for the cell, and the
endoplasmic reticulum and Golgi apparatus, which are involved in
protein synthesis and transport.
In addition to learning about the structure and function of cells,
we will also delve into the process of cell division, which is essential
for the growth and repair of living organisms. You will learn about
mitosis and meiosis, the two types of cell division, and how they
allow cells to replicate and pass on genetic information to the next
generation.
But cells are not just isolated units; they are also highly interactive and communicate with each other in a variety of ways. In this
chapter, we will discuss the various mechanisms by which cells communicate, including through chemical signaling, cell-cell adhesion,
and extracellular matrix.
”Cell Biology” is the study of cells, which are the basic unit of life.
All living things are made up of cells, and they perform all of the
functions necessary for life, such as growth, metabolism, and reproEnterMedSchool.com
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duction.
Cells come in many different shapes and sizes, and they can be found
in every living organism on Earth, from tiny bacteria to complex
animals like humans. Cells are the building blocks of life, and they
are responsible for all of the functions necessary for an organism to
survive.
One of the most exciting things about cells is that they have their
own internal structures and organelles, which are specialized parts
that perform specific functions. For example, the mitochondria are
the ”powerhouses” of the cell, responsible for generating energy in
the form of a molecule called ”ATP”. The nucleus is the ”brain” of
the cell, containing the genetic material that controls all of the cell’s
activities.
Prokaryotic cells, are usually smaller and simpler in structure than
eukaryotic cells. They are found in single-celled organisms, such as
bacteria. Prokaryotic cells do not have a nucleus or other membranebound organelles. Instead, their genetic material is found in a single
loop of DNA floating in the cytoplasm. Bacteria also have loops of
genetic material called ”Plasmids”, they have a similar role, and allow
the bacteria to gain different characteristics rather quickly, such as
antibiotic resistance, by transferring the plasmids to the current, or
the next generation of bacteria.
Eukaryotic cells are found in more complex organisms, such as
plants and animals. They are larger and more complex than prokaryotic cells and have a defined nucleus and membrane-bound organelles.
Membrane-bound organelles are subcellular structures that are
enclosed within their own lipid bilayer membrane. They are found
in eukaryotic cells, which are cells that have a true nucleus and
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other membrane-bound organelles. Membrane-bound organelles
are specialized structures that perform specific functions within the
cell.
The lipid bilayer membrane that encloses these organelles serves
several important functions. It acts as a barrier, separating the contents of the organelle from the rest of the cell. It also helps maintain
the organelle’s structural integrity and regulates the flow of materials in and out of it.
Some other important organelles in eukaryotic cells include the
endoplasmic reticulum, which is involved in the synthesis and transport of proteins and lipids, and the Golgi apparatus, which is involved
in the modification, sorting, and delivery of proteins and lipids to
their final destination. Don’t worry! We will soon learn about all of
these organelles one by one!
Note (will be in a specific section): We recommend viewing the 1987
children’s series ”Once Upon a Time... Life” to enhance comprehension of basic biological principles.
The cell membrane separates the inside of the cell from the outside environment and helps to maintain the cell’s shape. The cell
membrane is mostly made up of lipids and proteins, and it is selectively permeable, allowing certain substances to pass through while
blocking others.
In addition to the organelles mentioned earlier, a few other important structures are found in cells that are worth discussing.
One of these important structures is the cytoskeleton, a network of
protein fibers that gives the cell its shape and helps it maintain its
structure. The cytoskeleton comprises three types of protein fibers:
microtubules, microfilaments, and intermediate filaments. These
fibers can be found throughout the cell. They play a vital role in
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many of the cell’s activities, including movement, division, and the
transport of materials within the cell.
Some protist, plant, and fungi cells have a structure called the cell
wall, a different structure than the cell membrane, which serves as
a protective layer. The cell wall helps to give the cell its shape and
provides extra support and protection.
4.1 Membrane-bound Organelles
Membrane-bound organelles are specialized structures within a
cell enclosed by a membrane. These organelles perform specific
functions necessary for the cell to carry out its various activities. On
the IMAT exam, there’s a list of essential organelles worth learning.
Let’s review all of the organelles we will cover in-depth later on:
The nucleus: The nucleus is the central organelle of the cell and
is responsible for storing and processing genetic information.
It is surrounded by a double lipid bilayer membrane called the
nuclear envelope, which separates the contents of the nucleus
from the rest of the cell.
The endoplasmic reticulum (ER): The endoplasmic reticulum (ER)
is a network of flattened sacs and tubes that is involved in the
synthesis, modification, and transport of proteins and lipids. The
ER is divided into two main types: the smooth endoplasmic
reticulum (SER) and the rough endoplasmic reticulum (RER).
The SER is involved in the synthesis of lipids, while the RER is
involved in the synthesis of proteins.
The Golgi apparatus: The Golgi apparatus is a stack of flattened
membrane-bound sacs that is involved in the sorting, modification, and transport of proteins and lipids. It receives newly
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synthesized molecules from the endoplasmic reticulum and
sorts them into vesicles for transport to their final destination.
The mitochondria: The mitochondria are the cell’s powerhouses,
responsible for producing the energy that the cell needs to
carry out its various functions. They are surrounded by a double
membrane and contain their own DNA and ribosomes.
The lysosomes: Lysosomes are small, membrane-bound organelles
that contain hydrolytic enzymes that are responsible for breaking down waste materials and cellular debris. They help to maintain the cell’s internal environment by removing unnecessary or
damaged materials.
The peroxisomes: Peroxisomes are small, membrane-bound organelles that contain enzymes that are involved in the breakdown of fatty acids and the detoxification of harmful substances.
They are similar to lysosomes in function, but they do not contain
the same types of hydrolytic enzymes.
The vacuoles: Vacuoles are large, membrane-bound organelles
that are involved in the storage of various substances, including
water, salts, and pigments. Plant cells typically have one or more
large central vacuoles that are used to store water and other
substances, while animal cells have smaller vacuoles that are
involved in the storage of specific substances.
4.2 Prokaryotic Cells
Prokaryotic cells are the simplest and most ancient type of cells.
Prokaryotes are single-celled organisms that lack a nucleus and
other membrane-bound organelles. They are found in a variety of
environments, including soil, water, and the human body. Examples
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of prokaryotes include bacteria and archaea.
Despite their simple structure, prokaryotes play an important role in
the biological ecosystem. They are involved in nutrient cycling and
play a crucial role in the decomposition of organic matter. They are
also important in the production of certain foods such as cheese,
yogurt, and vinegar. Some types of bacteria can cause disease, while
others are beneficial to human health, such as those found in the
human gut microbiome.
They are characterized by the absence of a true nucleus and membranebound organelles. Instead, their genetic material is found in a single,
circular chromosome that is located in the cytoplasm, in an area
called ”nucleoid”. Prokaryotes are usually much smaller than eukaryotic cells as most common bacteria are about 1 to 2 microns in
diameter and 5 to 10 microns long, on average.
Prokaryotes are classified into two main groups: Bacteria and Archaea. Bacteria are the most well-known type of prokaryotes and
are found in almost every environment on Earth. They are important
for a variety of ecological functions, including the decomposition of
organic matter, the nitrogen cycle, and the production of various
substances that are used by other organisms. Archaea are a group
of prokaryotes that are adapted to extreme environments, such as
hot springs, salt flats, and the deep sea.
Prokaryotes are generally classified based on their shape, which
can be rod-shaped (bacilli), spherical (cocci), or spiral (spirilla). They
can also be classified based on their method of obtaining energy,
which can be through photosynthesis (phototrophs) or through
the breakdown of organic matter (heterotrophs). Prokaryotes are
adapted for survival in a variety of environments and have a range of
mechanisms for obtaining nutrients, such as the ability to take up
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nutrients from the environment or to produce their own nutrients
through photosynthesis. They also have a range of mechanisms for
defending against predators and environmental stresses, such as
the production of antibiotics or the ability to form endospores.
Endospores allows the bacterium to produce a dormant and highly
resistant cell to protect the cell’s genetic material in times of extreme
stress, such as high UV radiation and temperatures.
Prokaryotes are important in many different fields, including medicine,
agriculture, and environmental science. They are also important
models for understanding basic cellular processes, as they are much
simpler and easier to study than eukaryotic cells.
In past IMAT exams, it was not necessary to memorize the specific
names of bacteria. Instead, it was expected that you have a general
understanding of the concept of bacteria and the differences between different types of cells, such as animal, plant, bacteria, and
archaea. It is important to understand the concepts and general
characteristics of these different types of cells rather than memorizing specific names and types.
4.2.1 Prokaryotic Structure
Prokaryotic cells are the simplest and most ancient type of cells.
They are characterized by the absence of a true nucleus and membranebound organelles. Instead, their genetic material is found in a single,
circular chromosome that is located in the cytoplasm.
The basic structure of a prokaryotic cell consists of the following
components:
Cell wall: The cell wall is a rigid, protective layer that surrounds
the cell and gives it its shape. It is made up of peptidoglycan,
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a complex carbohydrate-protein compound that is unique to
prokaryotes.
Plasma membrane: The plasma membrane is a thin, flexible barrier
that surrounds the cell and separates the inside from the outside.
It is made up of a phospholipid bilayer and is responsible for
maintaining the cell’s internal environment and regulating the
movement of substances in and out of the cell.
One major difference is that the phospholipid bilayers of eukaryotic cells are typically thicker than those of bacterial cells. This
is due to the presence of additional lipids and proteins within
the bilayer of eukaryotic cells, which help to form specialized
structures which are involved in various processes such as endocytosis, exocytosis, and signaling.
Another difference is that the phospholipid bilayers of eukaryotic
cells are more complex and dynamic, with a greater variety of
lipids and proteins present. In contrast, bacterial phospholipid
bilayers are relatively simple, with only a few types of lipids and
proteins present.
Finally, the phospholipid bilayers of eukaryotic cells are more
tightly packed and oriented in a specific direction, while those of
bacterial cells are more disordered and fluid. This difference in
packing and orientation is due to the presence of proteins and
lipids with specific shapes and properties within the eukaryotic
bilayer.
Cytoplasm: The cytoplasm is the gel-like substance that fills the
cell and surrounds the genetic material. It contains all of the
cell’s metabolic machinery, including enzymes, ribosomes, and
other cellular structures.
There are several differences between the cytoplasm of bacteria
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and eukaryotes crucial to remember to the IMAT exam.
One major difference is the presence of a cytoskeleton in eukaryotes. The cytoskeleton is a network of protein fibers that
provides structural support and helps to maintain the shape
of eukaryotic cells. It is composed of three types of fibers: microtubules, microfilaments, and intermediate filaments. Bacteria do not have the cytoskeleton, therefore do not have microtubules, microfilaments, or intermediate filaments. Research
recently showed that some bacteria have its own ”cytoskeleton”,
which comprised of different proteins with similar function.
Another difference is the presence of membrane-bound organelles in eukaryotes. Membrane-bound organelles are specialized subcellular structures that perform specific functions within
the cell. Examples of organelles found in eukaryotes include
the nucleus, mitochondria, Golgi apparatus, and endoplasmic
reticulum. Bacteria do not have membrane-bound organelles,
but they may have non-membrane bounds structures, such as
ribosomes and nucleoids.
A third difference is the presence of a nuclear envelope in eukaryotes. The nuclear envelope is a double-membraned structure
that surrounds the nucleus and separates it from the cytoplasm.
It is involved in controlling the movement of materials in and
out of the nucleus. Bacteria do not have a nuclear envelope, but
they may have a nucleoid, which is a region within the cell that
contains the genetic material.
Overall, the cytoplasm of eukaryotes is more complex and organized than that of bacteria, due to the presence of organelles,
a cytoskeleton, and a nuclear envelope. These structures allow
eukaryotes to perform a wider range of functions and to have
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more specialized and compartmentalized cell biology.
Genetic material: The genetic material of a prokaryotic cell is found
in a single, circular chromosome that is located in the cytoplasm.
It contains all of the genetic information needed for the cell to
carry out its functions.
Plasmids are small, circular pieces of DNA that are found in
some bacterial cells. They are separate from the bacterial chromosome and can replicate independently of it. Plasmids are
usually present in small numbers in a cell, but under certain
conditions, their population can increase.
Plasmids are often used as tools in molecular biology because
they can be easily manipulated and introduced into bacterial
cells. They can carry genetic material from one organism to another, which is why they are sometimes referred to as ”vehicles”
for genetic information. Plasmids can also be used to express
proteins or to modify the properties of bacterial cells.
Plasmids are typically found in bacteria that live in environments
with changing conditions, such as soil or the human gut. They
allow bacteria to adapt to their environment by providing them
with additional functions, such as the ability to break down
certain compounds or to resist antibiotics.
Bacterial Ribosomes: Ribosomes are small, spherical structures
that are found in the cytoplasm and are involved in the synthesis
of proteins.
Flagella: Flagella are long, whip-like appendages that are used
by some prokaryotes for movement. They are composed of a
protein called flagellin and are attached to the cell wall.
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face of some prokaryotes. They are used for a variety of purposes,
including attachment to surfaces, transfer of genetic material,
and communication with other cells.
Figure 30: Generalized Prokaryotic Structure. This diagram illustrates the fundamental components and organization of prokaryotic cells, including the plasma membrane, cytoplasm, nucleoid,
ribosomes, and cell wall. (Credit: openstax.org)
4.2.2 Bacterial Reproduction
Bacteria are prokaryotic cells that can reproduce through several
methods. The most common method of bacterial reproduction is
binary fission. In binary fission, a single cell divides into two identical
daughter cells. The process of binary fission starts when the bacterial
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cell’s genetic material replicates, or copies itself. Once the genetic
material has been replicated, the cell begins to grow and elongate,
and a new cell wall begins to form down the center of the cell,
creating two separate compartments. The two copies of the genetic
material are then separated and moved into the two compartments,
and the cell eventually splits into two separate daughter cells.
Another method of bacterial reproduction is budding. Some bacteria are capable of budding, in which a new cell grows as an outgrowth from the parent cell and eventually detaches to form a separate cell. This is different from binary fission, where the cell splits into
two identical daughter cells. In budding, the new cell is not identical to the parent cell, as it may have received different amounts of
cytoplasm or genetic material.
Fragmentation is another method of bacterial reproduction. This
occurs when a parent cell breaks into multiple fragments, each
of which can develop into a new cell. Fragmentation can occur
spontaneously, or it can be induced by environmental factors such
as nutrient depletion or physical trauma.
Each of these methods of bacterial reproduction has its advantages
and disadvantages. Binary fission is the most efficient method for
producing large populations of bacteria quickly, while budding and
fragmentation may allow bacteria to adapt to changing environmental conditions. Genetic recombination, on the other hand, allows
bacteria to acquire new traits that may be advantageous in certain
environments. Overall, the ability of bacteria to reproduce rapidly
and adapt to changing conditions has made them important players in many ecological, industrial, and medical contexts.
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4.2.3
Horizontal Gene Transfer
Horizontal gene transfer is the transfer of genetic material from
one organism to another, without the involvement of reproduction.
This process is particularly common in prokaryotes, which can exchange genetic material through several mechanisms, including
transformation, transduction, and conjugation. These mechanisms
are different from traditional reproduction, which involves the passing of genetic material from one generation to the next.
Transformation is the process by which bacteria take up foreign DNA
from their surroundings and incorporate it into their own genome.
This foreign DNA can come from other bacteria, viruses, or even the
environment. Once incorporated into the bacterial genome, the
new genetic material can be expressed and inherited by subsequent
generations of bacteria. For example, Streptococcus pneumoniae
bacteria can take up DNA fragments from dead bacterial cells in
their environment, allowing them to acquire antibiotic resistance
genes.
Transduction occurs when a virus transfers genetic material from
one bacterial cell to another. In this process, a virus infects a bacterial cell and incorporates a fragment of the bacterial genome into
its own DNA. When the virus infects another bacterial cell, it transfers the incorporated genetic material along with its own DNA. This
can result in the transfer of genes between bacteria that are not
closely related. For example, bacteriophages, viruses that infect bacteria, can transfer antibiotic resistance genes from one bacterium
to another.
Conjugation is a more complex process in which bacterial cells transfer genetic material directly to each other through a specialized appendage called a pilus. In this process, a donor cell forms a pilus that
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attaches to a recipient cell, and a DNA molecule called a plasmid is
transferred from the donor cell to the recipient cell. The plasmid can
contain genes for antibiotic resistance, virulence factors, or other
traits that are advantageous to the bacterial cell. For example, some
strains of Escherichia coli can transfer a plasmid that contains genes
for toxin production to other E. coli cells.
Linear transformation is a specific type of horizontal gene transfer in which a bacterial cell takes up and incorporates linear DNA
molecules from the environment. These DNA molecules can come
from other bacteria, viruses, or even eukaryotes. Linear transformation allows bacteria to acquire new genetic material and to adapt
to new environments. For example, the bacterium Acinetobacter
baylyi is known for its ability to take up DNA fragments from the
environment and incorporate them into its genome, allowing it to
adapt to a variety of environmental conditions.
In conclusion, horizontal gene transfer is an important mechanism
that allows prokaryotes to exchange genetic information and to
rapidly adapt to changing environments. Transformation, transduction, and conjugation are three different ways in which bacteria can
transfer genetic material between cells. Linear transformation is a
specific type of horizontal gene transfer that involves the uptake of
linear DNA molecules from the environment. These mechanisms
allow prokaryotes to acquire new genetic material and to evolve
new traits, contributing to their diversity and adaptability.
4.3 Eukaryotic Cells
Have you ever wondered how complex organisms like plants, animals, and fungi are made up of so many different cells, each with its
own specialized functions? The answer lies in the incredible world
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Figure 31: In prokaryotes, there exist three mechanisms for exchanging DNA. Transformation (a) involves the uptake of prokaryotic DNA
directly from the environment, which may remain as plasmid DNA
or become part of the host genome. Transduction (b) occurs when
a bacteriophage injects DNA into a cell containing a small fragment
of DNA from a different prokaryote. Conjugation (c) involves the
transfer of DNA from one cell to another through a mating bridge
or pilus connecting the two cells, which is formed after the sex pilus
brings the two bacteria in close proximity. (Credit: openstax.org)
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of eukaryotic cells. Eukaryotic cells are the building blocks of life as
we know it, and they are responsible for the incredible diversity and
complexity of the living world.
Eukaryotic cells are larger and more complex than their simpler,
smaller counterparts, the prokaryotic cells. They have a true nucleus,
which contains the cell’s genetic material, or DNA, and is surrounded
by a double membrane. This genetic material is organized into
chromosomes, which carry the instructions that control the cell’s
functions and characteristics.
In addition to the nucleus, eukaryotic cells have a variety of other
membrane-bound organelles, including the endoplasmic reticulum (ER), Golgi apparatus, mitochondria, and lysosomes. These
organelles work together to carry out the many functions necessary
for the cell to survive and thrive.
One of the most important features of eukaryotic cells is their ability
to form tissues and organs, which work together to carry out the
functions of the organism as a whole. For example, the cells that
make up a plant’s leaves work together to photosynthesize and
produce energy for the plant, while the cells that make up a human’s
heart work together to pump blood throughout the body.
So, while eukaryotic cells may seem complex and mysterious, they
are truly the building blocks of life. By studying these incredible cells,
we can begin to understand the incredible diversity and complexity
of the living world around us.
4.3.1
The Cytoplasm
The cytoplasm is a complex, gel-like substance that fills the cell and
surrounds its organelles. It is composed of water, ions, and a variety
of organic molecules, including enzymes, nucleic acids, and small
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organic molecules such as amino acids and sugars. The cytoplasm
is a dynamic environment that is constantly changing and adapting
to the needs of the cell.
One of the main functions of the cytoplasm is to provide a medium in
which chemical reactions can occur. The cytoplasm contains a wide
variety of enzymes that catalyze the chemical reactions necessary for
the cell to function. These enzymes are involved in processes such
as metabolism, energy production, and the synthesis of proteins,
lipids, and other molecules. For example, the cytoplasm of a plant
cell contains chloroplasts, which are responsible for photosynthesis
and the production of energy-rich molecules like glucose.
The cytoplasm also plays a crucial role in the movement of materials
within the cell. It contains a network of protein fibers called the
cytoskeleton, which helps the cell maintain its shape and structure
and also helps move organelles and other materials within the cell.
For example, during cell division, the cytoskeleton helps to move
the chromosomes to opposite ends of the cell.
The viscosity of the cytoplasm can vary depending on the chemical composition and concentration of the molecules it contains. It
can also change in response to various stimuli, such as changes in
temperature or the presence of certain molecules. The viscosity of
the cytoplasm can be compared to the consistency of Jell-O: it is
fluid enough to allow movement within the cell, but solid enough
to provide support to the cell’s structures.
The cytoplasm can be divided into two primary parts: the endoplasm and the ectoplasm. The endoplasm is the central area of
the cytoplasm that contains the organelles. The ectoplasm is the
more gel-like peripheral portion of the cytoplasm of a cell. This division can be compared to the division of a jelly doughnut, where
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the filling (endoplasm) is surrounded by a more gel-like outer layer
(ectoplasm).
The endoplasm contains the cell’s organelles, which are specialized
structures that perform specific functions within the cell. These organelles include the nucleus, the endoplasmic reticulum, the Golgi
apparatus, mitochondria, peroxisomes, lysosomes, and vacuoles.
Each of these organelles has its own unique structure and function
that contribute to the overall functioning of the cell.
The ectoplasm is a more diffuse, gel-like region of the cytoplasm
that surrounds the endoplasm and surrounds the cell’s organelles.
It is composed of water, ions, and various organic molecules, such as
enzymes and structural proteins. The ectoplasm provides support
to the cell’s structures and helps to maintain the shape of the cell.
Overall, the cytoplasm is a complex and dynamic environment that
is essential to the functioning of the cell. It provides a medium for
chemical reactions, supports the cell’s structures, and facilitates
movement of materials within the cell. The division of the cytoplasm into the endoplasm and ectoplasm, along with the presence
of specialized organelles, allows for the efficient and coordinated
functioning of the cell.
4.3.2
The Nucleus
The nucleus can be compared to the control center of a factory,
where important decisions are made and instructions are sent out
to the rest of the organization. Similarly, the nucleus is the control
center of the cell, where genetic information is stored and regulated,
and important decisions are made about the cell’s functions and
activities.
One of the key features of the nucleus is its double membrane,
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which is composed of two lipid bilayers. This double membrane acts
as a barrier, separating the genetic material within the nucleus from
the rest of the cell. This can be compared to a fortress wall, which
protects the valuable contents within from external threats.
Inside the nucleus, the genetic material is organized into chromosomes, which can be thought of as the cell’s instruction manual.
This manual contains all the information needed to carry out the
cell’s functions and activities, and it is carefully regulated to ensure
that the cell operates efficiently and effectively.
Inside the nucleus, there are a variety of other structures and organelles. These include the nucleolus, which is a non-membranebound organelle involved in the synthesis of ribosomes. The nucleolus is the site of rRNA synthesis and ribosome assembly, and it can
be compared to a factory where parts are assembled and packaged
before being sent out for use.
The nucleus also contains the nucleoplasm, which is the gel-like
substance that fills the nucleus and surrounds the chromatin. The
nucleoplasm can be compared to the fluid inside a water balloon,
providing a medium for the many processes that occur within the
nucleus.
The chromatin is visible when the cell is viewed under a microscope,
and it contains the cell’s genetic material. The chromatin can be
compared to a tangled ball of yarn, with the genetic material wound
tightly around proteins called histones. The chromatin helps to
protect the DNA and regulate gene expression.
The nucleus also contains numerous small organelles called ribosomes, which are responsible for synthesizing proteins. Ribosomes
can be found both in the cytoplasm and attached to the endoplasmic reticulum. They can be compared to workers in a factory,
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assembling the components needed to carry out the cell’s functions.
Overall, the nucleus is an important organelle that plays a central
role in the regulation of the cell’s functions and the expression of its
genetic information. The nucleolus and nucleoplasm, along with
the chromatin and ribosomes, work together to ensure that the cell
operates efficiently and effectively.
Figure 32: Within the nucleus of a cell, chromatin (DNA plus proteins)
is stored in the nucleoplasm, a gel-like substance. The nucleolus,
which is a condensed region of chromatin, is where ribosome synthesis takes place. The boundary of the nucleus is known as the
nuclear envelope, and it is composed of two phospholipid bilayers: an inner and an outer membrane. The nuclear membrane is
connected to the endoplasmic reticulum, and nuclear pores are
present to enable the transport of substances in and out of the nucleus. (Credit: openstax.org)
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4.3.3 Whar Are Nuclear Pores?
Nuclear pores are large, complex structures that facilitate the exchange of materials between the nucleus and the cytoplasm in
eukaryotic cells. These pores are composed of multiple protein components and are embedded in the double membrane of the nuclear
envelope.
The actual pore itself is formed by a large protein complex called
the nuclear pore complex (NPC). The NPC is made up of over 30
different proteins, collectively called nucleoporins. These proteins
form a mesh-like structure that spans the nuclear envelope, creating
a passageway for molecules to move in and out of the nucleus.
Each nucleoporin is composed of multiple repeating units called
FG-nucleoporin domains, which contain phenylalanine-glycine (FG)
amino acid repeats. These domains are thought to create a selectively permeable barrier within the nuclear pore, allowing certain
molecules to pass through while preventing others from doing so.
In addition to the NPC, the nuclear envelope itself also plays an
important role in nuclear transport. The envelope is composed
of two lipid bilayers, an inner and an outer membrane, which are
connected by transmembrane proteins called nucleoporins. These
proteins help to anchor the NPC to the nuclear envelope, as well as
regulate the passage of molecules through the pore.
The nuclear lamina, a network of intermediate filaments that provides structural support to the nucleus, also plays a role in nuclear
transport. The lamina is located on the inner side of the nuclear
envelope and is composed of proteins called lamins. These proteins
interact with the nucleoporins and other components of the nuclear
envelope to help regulate the movement of molecules in and out of
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the nucleus.
Overall, nuclear pores are complex structures that allow for the regulated exchange of materials between the nucleus and the cytoplasm.
The nucleoporins and other components of the nuclear envelope
work together to create a selective barrier, while the nuclear lamina
provides structural support and helps to regulate nuclear transport.
Figure 33: (Credit: Figure depicting nuclear pores, which regulate
the transport of molecules between the nucleus and cytoplasm of a
eukaryotic cellopen stax.org)
4.3.4
Chromatin and Chromosomes
Chromatin is a complex of DNA and proteins found within the nucleus of eukaryotic cells, which makes up the chromosomes that
carry the genetic information of the cell. Chromatin is composed
of long strands of DNA wrapped around proteins called histones.
The DNA strands in chromatin are tightly coiled and compacted,
making it possible to fit a large amount of genetic information into
a small space.
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Chromosomes are the highly organized and compact structures
that contain the genetic material of an organism. They are made up
of chromatin and are visible under a microscope during cell division.
Each chromosome is a single, continuous molecule of DNA, and
each species has a characteristic number of chromosomes. For
instance, humans have 46 chromosomes, while fruit flies have only
four.
In addition to histones, chromosomes also contain other proteins
that help to organize and compact the chromatin. One of these proteins is called condensin, which plays a crucial role in the formation
of chromosomes during cell division. Condensin helps to organize
the chromatin into highly compacted and organized structures that
can be easily segregated into daughter cells during cell division.
The structure of chromosomes can vary depending on the stage of
the cell cycle. During interphase, the stage of the cell cycle when the
cell is not actively dividing, chromosomes are in a more relaxed and
extended state, allowing for gene expression and DNA replication.
During cell division, however, the chromosomes become highly
condensed and tightly packed, making it possible for them to be
separated into daughter cells.
It is important to note that while chromosomes are highly organized
and compact structures, they are not the only form of chromatin
in the cell. Chromatin can exist in a range of different structures,
from the loosely packed euchromatin to the highly compacted heterochromatin. These are two different types of chromatin that differ
in their level of compaction and accessibility to enzymes and transcription factors. Euchromatin is loosely packed chromatin that is
more accessible to these factors, allowing for the transcription of
genes and the expression of specific traits or characteristics. Heterochromatin, on the other hand, is highly compacted and tightly
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packed chromatin that is less accessible to these factors, resulting
in genes being less actively transcribed or silenced.
The balance between euchromatin and heterochromatin is crucial
for the proper functioning of cells. During development and differentiation, specific genes need to be turned on or off to allow for the
proper development of tissues and organs. This is accomplished
through changes in chromatin structure, which can create a more
open or closed chromatin state, depending on the gene’s needs.
For example, during the development of the brain, specific genes
involved in neuron development and function are turned on, while
other genes that are not necessary for brain development are turned
off. This process is regulated through changes in chromatin structure, which can create a more open chromatin state for the active
genes and a more closed chromatin state for the inactive genes.
Understanding the organization and regulation of both chromatin
and chromosomes is crucial for understanding the functioning of
cells and the development of diseases such as cancer. Changes in
the structure of chromatin and chromosomes can lead to alterations
in gene expression, resulting in abnormal cell growth and function.
Thus, research into the organization and regulation of chromatin
and chromosomes is an important area of study in molecular biology.
Note: Did you know? If we simply calculate the length of the DNA
on the basis of the distance between each nucleotide, i.e. 0.34nm or
0.34 × 10− 9m. It will come out to be around 2.2 meters!
4.3.5
Histones
Histones are proteins that play a critical role in the structure and
function of DNA in eukaryotic cells (cells that have a true nucleus
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Figure 34: This image (a) shows the various levels of chromatin’s
organization (DNA and protein). The other image (b) shows the
paired chromosomes.(credit (Credit: openstax.org)
and other membrane-bound organelles). They are responsible for
compacting and organizing the DNA of the cell into a structure
called chromatin.
Histones can be modified in various ways, including acetylation,
methylation, and phosphorylation. These modifications can affect
the structure of the chromatin and the accessibility of the DNA to
the cell’s machinery, such as enzymes that are involved in gene expression. For example, acetylation of histones is generally associated
with more open, transcriptionally active chromatin, while methylation of histones is generally associated with more condensed, transcriptionally inactive chromatin.
Each type of histone plays a distinct role in regulating the structure
and function of chromatin. For example, H2A and H2B histones are
involved in stabilizing the nucleosome structure, while H3 and H4
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histones play a crucial role in regulating gene expression.
H3 and H4 histones are subject to various modifications, such as
acetylation, methylation, and phosphorylation, that can either enhance or inhibit gene expression. Acetylation of H3 and H4 histones,
for instance, is associated with increased gene expression, while
methylation can either increase or decrease gene expression depending on the specific location of the modification. Additionally,
phosphorylation of H3 histones is involved in the regulation of cell
division.
Another type of histone, H1, is not part of the nucleosome structure but is involved in organizing and compacting the chromatin
into higher-order structures. H1 histones bind to the linker DNA between nucleosomes and play a role in the formation of higher-order
chromatin structures, such as the 30-nm fiber.
Overall, the different types of histones play important and distinct
roles in the regulation of chromatin structure and function. By modifying the histones and chromatin structure, cells are able to regulate
gene expression and carry out a variety of functions necessary for
cell growth, differentiation, and development.
The compact levels of chromosomes, chromatin, and histones are
related to the organization of the cell’s genetic material. Chromosomes are long strands of DNA that are found in the nucleus of a
cell. They contain the cell’s genetic information and are made up of
chromatin, which is a complex of DNA and histones. The compact
levels of chromosomes, chromatin, and histones are important for
the cell’s function because they allow the cell to fit a large amount
of genetic material into a small space, such as the nucleus of a cell.
Overall, histones play a crucial role in the structure and function of
DNA in eukaryotic cells. They help to compact and organize the DNA
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into a highly condensed structure called chromatin, which is essential for the cell’s function and survival. Modifications to histones,
such as acetylation and methylation, can affect the structure and
accessibility of the DNA, which can have important consequences
for gene expression and other cellular processes.
One way to differentiate between chromatin and chromosomes is
to think about the stage of the cell cycle at which they are visible.
Chromatin is present throughout the cell cycle (the growth and
living phase of the cell), while chromosomes are only visible during cell division (where the cell divides after it grew and lived for a
while), when the chromatin is highly condensed and visible under
a microscope. We will learn more about the cell cycle in a future
chapter.
An analogy that may help to understand the idea of genetic material
is to think of a recipe book. Just as a recipe book contains instructions for making a variety of dishes, the DNA in our cells contains
the instructions for making all of the proteins and other molecules
that are necessary for the cell to function. The recipes in a cookbook
are written in a specific language and order, and the instructions in
DNA are written in the language of nucleotide base pairs. Just as
a cookbook can be passed down from one generation to the next,
the genetic information in DNA is passed from one generation to
the next through the process of reproduction. To protect the book
(DNA), it’s best not to bring it into the kitchen (Cytoplasm) where
it could potentially be damaged (mutations). Instead, you quickly
flip through the book, take a photo of the recipe you need (RNA),
and return the book to a safe location (Nucleus). Then, you bring
the copy of the recipe into the kitchen to use while cooking. This
will ensure that the book stays in good condition while still allowing
you to access the recipe.
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4.3.6 Ribosomes
Ribosomes are small, but mighty structures found in all living cells.
These tiny organelles are responsible for creating the proteins that
make up much of the body’s structure and perform many of its
functions. Think of them like tiny protein factories that work tirelessly
to produce the building blocks of life. Just as a carpenter needs
wood, nails, and tools to build a house, a ribosome needs amino
acids, messenger RNA, and transfer RNA to build a protein. Without
ribosomes, the cell would not be able to create the proteins it needs
to survive and carry out its functions. In this way, ribosomes are
essential to all forms of life, from the tiniest bacterium to the largest
mammal.
The structure of ribosomes varies depending on whether they are
found in eukaryotic or prokaryotic cells. In eukaryotic cells have a
ribosome size of 80S, ribosomes are composed of two subunits, a
larger one called the 60S subunit and a smaller one called the 40S
subunit. The 70S ribosome of prokartyoes is smaller and made up
of a 50S and 30S subunits. The two subunits are held together by
non-covalent bonds and can dissociate from each other when not
in use.
Ribosomes are like chefs in a kitchen, reading a recipe book (mRNA)
and using it to create a dish (protein). Just as a chef follows a recipe
to add ingredients in a specific order, ribosomes read the mRNA to
add amino acids in the correct sequence to build the protein. This
process is called translation, and it is a vital part of gene expression.
Ribosomes can synthesize a wide variety of proteins, from enzymes
that catalyze chemical reactions to structural proteins that make up
the cell’s cytoskeleton. For example, ribosomes in pancreatic cells
synthesize digestive enzymes that break down food in the digestive
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system, while ribosomes in muscle cells synthesize the proteins that
make up muscle fibers. The process of protein synthesis is essential
for the proper functioning of cells and is regulated by various factors,
including hormones, nutrients, and environmental conditions.
Ribosomes are found in the cytoplasm of cells and are also found
attached to the endoplasmic reticulum (ER). In eukaryotic cells,
ribosomes on the surface of the ER are called the rough endoplasmic
reticulum (RER) and are involved in the synthesis of proteins that are
destined for the cell’s membrane or for secretion. Ribosomes that
are not bound to the ER are called free ribosomes and are involved
in the synthesis of proteins that are to be used within the cell.
In summary, ribosomes are important organelles that play a central
role in the synthesis of proteins. The structure and function of ribosomes varies depending on whether they are found in eukaryotic
or prokaryotic cells.
4.3.7
The "S" in Ribosolmal Weight
The ”S” in ribosomal weight refers to the sedimentation coefficient of
ribosomes, which is a measure of how quickly ribosomes sediment
(or settle to the bottom of a tube) in a centrifuge. The sedimentation
coefficient is often used as a measure of the size of ribosomes and
other cellular particles.
Ribosomes can be isolated from cells and then purified by centrifugation. The purified ribosomes are then suspended in a solution
and subjected to a process called ultracentrifugation. During ultracentrifugation, the sample is placed in a tube and spun at very
high speeds in a centrifuge. The ribosomes sediment to the bottom
of the tube based on their size and shape, with larger ribosomes
sedimenting more quickly than smaller ones. The rate at which
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Figure 35: Ribosomes are composed of a large subunit at the top
and a small subunit at the bottom. These subunits work together
to assemble amino acids into proteins during the process of protein
synthesis. Ribosomes play a crucial role in the translation of genetic
information from RNA molecules into functional proteins. (Credit:
openstax.org)
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the ribosomes sediment can be measured using a device called a
spectrophotometer, which measures the amount of light absorbed
by the sample as it is spinning in the centrifuge. The sedimentation
coefficient (S value) is then calculated based on the rate at which
the ribosomes sediment and their size.
Prokaryotic ribosomes are smaller than eukaryotic ribosomes. This
is because prokaryotes are generally smaller and less complex than
eukaryotes, and their ribosomes reflect this difference in size. The
ribosomes of mitochondria, which are the energy-producing organelles found in eukaryotic cells, are similar in size to prokaryotic
ribosomes. This is thought to be due to the fact that mitochondria
have a prokaryotic ancestry and have retained some characteristics
of prokaryotes, including the size of their ribosomes.
4.3.8 The Mitochondria
To learn about the Mitochondria we, first of all, have to learn about
the Endosymbiotic theory. The endosymbiotic theory is a scientific
theory that explains the origin of mitochondria, which are small
organelles found in the cells of most eukaryotes, including animals,
plants, and fungi. According to this theory, mitochondria are the
descendants of ancient bacteria that were engulfed by a larger host
cell millions of years ago.
The story of the endosymbiotic theory begins billions of years ago,
when the Earth was still a young planet. At this time, life on Earth
consisted of simple, single-celled organisms known as prokaryotes.
These organisms were small and relatively simple in structure, with
no defined nucleus or other specialized organelles.
One day, a prokaryotic cell known as a host cell encountered a
smaller prokaryotic cell known as a bacterium. The host cell was
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much larger than the bacterium, and it had the ability to engulf
and incorporate other cells into its own structure. So, the host cell
engulfed the bacterium, trapping it within its own membrane.
At first, the host cell saw the bacterium as a threat, and it attempted
to destroy it using its own cellular machinery. However, the bacterium was able to resist these attempts and survive inside the host
cell. Over time, the host cell began to recognize the bacterium as a
beneficial symbiont, and it began to allow the bacterium to live and
reproduce within its own membrane.
As the bacterium reproduced and passed its genetic material down
to its offspring, it began to evolve and adapt to its new environment
inside the host cell. It developed new functions and abilities that
helped it to survive and thrive within the host cell. Eventually, the
bacterium became so integrated into the host cell’s functions that
it became an essential part of the host cell’s own structure and
metabolism.
Today, the descendants of these ancient bacteria can be found in
the cells of most eukaryotes, where they are known as mitochondria.
They are responsible for producing energy for the cell through a
process called cellular respiration, and they play a crucial role in the
overall health and function of the cell.
So, the endosymbiotic theory is a fascinating story of how two simple,
single-celled organisms came together in a symbiotic relationship
that would ultimately shape the course of evolution on Earth. It is a
testament to the incredible adaptability and resilience of life, and it
serves as a reminder of the complex and interconnected nature of
the living world. Now, let’s talk about modern Mitochondria:
Mitochondria are specialized organelles found in both plant and
animal cells. They are commonly known as the ”powerhouses” or
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”energy factories” of the cell due to their role in producing the cell’s
main energy currency, adenosine triphosphate (ATP). Mitochondria
are responsible for the process of cellular respiration, which involves
converting nutrients, such as glucose, into ATP. This process uses
oxygen and produces carbon dioxide as a waste product. The ATP
produced is then used to fuel a wide range of cellular processes,
including muscle contraction, cellular communication, and cellular
maintenance. In addition to their role in energy production, mitochondria also play a role in other processes such as apoptosis
(programmed cell death) and the synthesis of certain lipids and
hormones.
The structure of mitochondria is complex, and they are composed of
a variety of proteins, lipids, and nucleic acids. Mitochondria are ovalshaped, double-membrane organelles with their own ribosomes
and DNA. They have a double membrane, with the inner membrane
being more highly folded than the outer membrane. The outer
membrane is smooth, while the inner membrane is highly folded
and contains numerous infoldings called cristae. The inner membrane of the mitochondria is where most of the ATP production
occurs, with the cristae providing a large surface area for enzymes
involved in the process. The area surrounded by the cristae is known
as the mitochondrial matrix, which contains enzymes involved in
the citric acid cycle and other biochemical pathways that produce
ATP.
The two membranes of the mitochondria are composed of phospholipids and embedded with proteins. The outer membrane is
porous and allows for the passage of small molecules, while the inner
membrane is highly selective and contains transport proteins that
regulate the passage of molecules, such as ions and metabolites, in
and out of the matrix. The selective nature of the inner membrane
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allows for the establishment of an electrochemical gradient that
drives the synthesis of ATP.
The inner membrane is responsible for several important functions
in the mitochondria. It is the site of the electron transport chain,
which is responsible for generating most of the cell’s ATP (the main
energy currency of the cell). The electron transport chain involves
a series of proteins that transfer electrons from one molecule to
another, generating a proton gradient that is used to produce ATP.
This process requires a large surface area in order to accommodate
all of the proteins that are involved in the electron transport chain.
In addition to the electron transport chain, the inner membrane is
also the site of the citric acid cycle, also known as the Krebs cycle.
This is a series of reactions that takes place in the mitochondria,
in which glucose and other nutrients are broken down to produce
energy in the form of ATP. The citric acid cycle also requires a large
surface area in order to accommodate all of the enzymes that are
involved in the reactions.
The outer membrane of the mitochondria does not need to be
folded because it does not have as many functions as the inner
membrane. Its main role is to act as a barrier, separating the inside
of the mitochondria from the rest of the cell. It does not need to
have a large surface area for carrying out reactions, so it does not
need to be folded.
Mitochondria are found in different numbers and concentrations in
different cell types, depending on their energy requirements. For
example, muscle cells have a high concentration of mitochondria to
support the energy demands of muscle contraction, while other cells
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producing capacity as needed.
Mitochondria are found in most eukaryotic cells, with the exception
of red blood cells, which do not have mitochondria. They are particularly abundant in cells that have high energy demands, such as
muscle cells and nerve cells.
In conclusion, the function of mitochondria in the cell is to produce
ATP, the cell’s main energy currency, through cellular respiration.
The structure of mitochondria includes a double membrane with the
inner membrane containing cristae that provide a large surface area
for ATP production, transport proteins, and the mitochondrial matrix
containing enzymes for the biochemical pathways that produce
ATP. The number and concentration of mitochondria in different cell
types reflect their energy requirements, with muscle cells having a
higher concentration of mitochondria. Understanding the structure
and function of mitochondria is crucial for understanding cellular
energy metabolism and the development of diseases related to
mitochondrial dysfunction.
4.3.9
Peroxisomes
Peroxisomes are small, single-membrane-bound organelles found
in the cytoplasm of eukaryotic cells. They are similar in size and
shape to lysosomes and are also involved in the breakdown of biomolecules.
Peroxisomes are characterized by the presence of enzymes that use
oxygen to break down certain biomolecules, such as fatty acids and
amino acids. One of the main functions of peroxisomes is to detoxify
harmful substances, such as alcohol, by converting them into more
harmless compounds. They also play a role in the synthesis of certain
lipids, such as plasmalogens, which are important for the proper
functioning of cell membranes.
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Figure 36: The electron micrograph displays a mitochondrion captured by an electron microscope, highlighting its structure. This
organelle consists of an outer and inner membrane. The inner membrane contains cristae, which are folds that amplify the surface area
for more efficient ATP synthesis. The intermembrane space is the
area between the two membranes, while the mitochondrial matrix
is the space inside the inner membrane. ATP synthesis occurs on
the inner membrane. (Credit: openstax.org)
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The structure of peroxisomes is relatively simple, and they are composed of a single membrane that encloses the enzymes and other
molecules that are involved in their functions. Peroxisomes are
formed from the endoplasmic reticulum (ER) and are similar in size
and shape to lysosomes. However, unlike lysosomes, which are
formed from the Golgi apparatus, peroxisomes are not involved in
the degradation of large molecules such as proteins and polysaccharides.
Peroxisomes are found in most eukaryotic cells, and their functions
vary depending on the type of cell and the specific needs of the
organism. They are particularly important in the liver, where they
play a role in the detoxification of harmful substances.
4.3.10
Animal vs. Plant Cells
Plant and animal cells are both eukaryotic cells, meaning they have
a nucleus and membrane-bound organelles. However, there are
several significant differences between the two cell types in terms
of structure and function.
• Cell wall: One of the most noticeable differences between plant
and animal cells is the presence of a rigid cell wall in plant cells.
The cell wall is made primarily of cellulose, a complex carbohydrate, and provides structural support and protection for the
plant cell. Animal cells, on the other hand, lack a cell wall and
are surrounded only by a plasma membrane.
• Vacuoles: Both plant and animal cells contain vacuoles, which
are membrane-bound sacs used for storage and waste disposal.
However, plant cells generally have a large central vacuole that
occupies a significant portion of the cell’s volume. This central
vacuole stores water, nutrients, and waste products and also
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helps maintain turgor pressure, which gives the plant cell its
shape and rigidity. In contrast, animal cells have smaller vacuoles that are more dispersed throughout the cell.
• Plastids: Plant cells contain plastids, specialized organelles involved in the synthesis and storage of pigments, starch, and
lipids. The most well-known plastid is the chloroplast, which
contains the green pigment chlorophyll and is responsible for
photosynthesis, the process by which plants convert sunlight
into chemical energy. Animal cells do not have plastids, as they
do not perform photosynthesis.
• Centrioles: Animal cells contain centrioles, which are cylindrical
structures that play a crucial role in cell division by organizing
the microtubules that form the spindle fibers. Plant cells do not
have centrioles; instead, they rely on other structures to organize
the spindle fibers during cell division.
• Lysosomes: Lysosomes are membrane-bound organelles that
contain enzymes responsible for breaking down waste materials and cellular debris. While animal cells typically contain
lysosomes, plant cells often rely on other organelles, such as
vacuoles and peroxisomes, for similar functions.
• Cytoskeleton: Both plant and animal cells have a cytoskeleton, a
network of protein fibers that provide structural support and play
a role in cell movement and division. However, the cytoskeleton
in plant cells is generally less extensive and more rigid than in
animal cells, which reflects the different requirements for cell
shape and movement.
• Shape and size: Plant cells are typically more rigid and rectangular or square in shape due to the presence of the cell wall,
while animal cells are more flexible and can adopt a variety of
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shapes. Plant cells are generally larger than animal cells, with
the central vacuole occupying a significant portion of the cell’s
volume.
In summary, plant and animal cells share many similarities as eukaryotic cells, but there are key differences in their structure and
organelles, such as the presence of a cell wall, plastids, and a large
central vacuole in plant cells, and centrioles and lysosomes in animal
cells. These differences reflect the unique functions and requirements of each cell type.
4.3.11 The Chloroplast
The chloroplast is a specialized organelle found in plant cells and
some other types of organisms, such as algae. It is responsible for
photosynthesis, the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose.
Chloroplasts contain pigments, such as chlorophyll, that absorb light
energy and use it to convert water and carbon dioxide into glucose
and oxygen through a process called the Calvin cycle. Chloroplasts
are found in the cells of plants and algae and are typically located in
the mesophyll tissue of leaves, where they are exposed to light. They
are shaped like discs and are surrounded by a double membrane.
Inside the chloroplast, there are thylakoid membranes, where the
light-dependent reactions of photosynthesis occur, and a stroma,
where the light-independent reactions of photosynthesis take place.
The structure of a chloroplast can vary depending on the type of
plant or algae it is found in, but all chloroplasts have a few key
components:
The outer membrane encloses the chloroplast and is made up of
lipids.
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The inner membrane surrounds the stroma, which is the fluidfilled space inside the chloroplast. The inner membrane is also
made up of lipids and contains proteins that are involved in the
photosynthetic process.
The thylakoid membrane is found inside the inner membrane and
is made up of proteins and pigments such as chlorophyll. Thylakoids are arranged in stacks called grana.
The thylakoid lumen is the space inside the thylakoid membrane
and contains water.
The stroma contains enzymes that are involved in the synthesis of
glucose from carbon dioxide and water.
During photosynthesis, light energy is absorbed by pigments such
as chlorophyll, which are located in the thylakoids. This energy is
used to convert water and carbon dioxide into glucose and oxygen.
The glucose is then used by the plant for energy, and the oxygen is
released into the atmosphere as a byproduct of the reaction.
4.4 The Endomembrane System
The endomembrane system is a group of interconnected structures
within the cell that are involved in the synthesis, modification, and
transport of various molecules. It includes the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and the plasma
membrane. These structures work together to create, modify, and
transport proteins and lipids within the cell, as well as between cells.
The endoplasmic reticulum (ER) is a network of flattened membranebound sacs and tubes that is involved in the synthesis and modification of proteins and lipids. There are two types of ER: smooth
endoplasmic reticulum (SER) and rough endoplasmic reticulum
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Figure 37: Within the chloroplast, there exist an outer membrane,
an inner membrane, and thylakoid membranes arranged in stacked
structures called grana. The thylakoid space refers to the space
enclosed by the thylakoid membranes. Light harvesting reactions
occur on the thylakoid membranes, while sugar synthesis takes
place in the fluid within the inner membrane, known as the stroma.
Furthermore, the chloroplast has its own genome, which is a single
circular chromosome. (Credit: openstax.org)
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(RER).
The smooth endoplasmic reticulum (SER) lacks ribosomes and appears smooth under a microscope. It is involved in the synthesis
and modification of lipids, including phospholipids and cholesterol,
which are important components of cell membranes. The SER also
plays a role in the detoxification of drugs and other harmful substances in liver cells.
On the other hand, the rough endoplasmic reticulum (RER) is involved in protein synthesis and modification. It is called ”rough”
because it is studded with ribosomes on its outer surface, giving it a
bumpy appearance under a microscope. The ribosomes attached to
the RER are the site of protein synthesis, and the proteins are then
transported into the lumen of the RER, where they undergo further
modification and folding.
The structural differences between the two types of ER are due to
their different functions. The ribosomes attached to the RER are
responsible for the synthesis of proteins, while the lack of ribosomes
on the SER allows it to carry out its lipid synthesis and detoxification
functions without interference.
An analogy for the difference between the two types of ER could
be a factory with two different assembly lines. The RER would be
the assembly line that produces products (proteins) with the help
of specialized workers (ribosomes), while the SER would be the
assembly line that produces the materials (lipids) needed for the
products to be made, without the need for specialized workers.
Overall, the endoplasmic reticulum is a crucial part of the endomembrane system that plays a vital role in protein and lipid synthesis,
modification, and transport within the cell.
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4.4.1 The ER Signal Sequence
The ER signal sequence is a sequence of amino acids that is found
in the N-terminus (beginning) of proteins that are destined for the
endoplasmic reticulum (ER). This sequence serves as a signal for
the protein to be transported into the ER by a process called protein
targeting or protein sorting.
The main purpose of the ER signal sequence is to ensure that the
protein is properly folded and modified within the ER before it is
transported to its final destination, either within the cell or to the
cell surface. This is important because many proteins require proper
folding and modification to function correctly. For example, some
proteins require the addition of carbohydrate groups to function
properly, while others require disulfide bonds to stabilize their structure.
The ER signal sequence is typically composed of a stretch of hydrophobic amino acids, such as leucine, valine, and methionine.
These hydrophobic amino acids interact with the lipid bilayer of the
ER membrane and help the protein to be inserted into the membrane. The signal sequence also contains positively charged amino
acids, such as lysine and arginine, which interact with the negatively
charged phospholipids in the ER membrane.
Once the protein with the ER signal sequence is transported into
the ER, the signal sequence is cleaved off by a signal peptidase
enzyme. The remaining protein is then folded and modified in the
ER, including the addition of carbohydrate groups and disulfide
bonds, before it is transported to its final destination.
Defects in the ER signal sequence and protein folding process can
lead to a variety of diseases, including cancer and neurodegenerative
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disorders. For example, misfolded proteins in the ER can trigger
a response called the unfolded protein response (UPR), which can
lead to cell death if not properly regulated.
In summary, the ER signal sequence plays a critical role in ensuring
that proteins are properly folded and modified within the ER before
they are transported to their final destination, and defects in this
process can lead to a variety of diseases.
4.4.2
The Golgi Apparatus
The Golgi apparatus is a stack of flattened membrane-bound sacs
that is involved in the modification, sorting, and packaging of proteins and lipids. It is a crucial part of the secretory pathway, which is
responsible for the transport of proteins and lipids from the ER to
their final destinations within the cell or to the cell surface.
The Golgi apparatus is made up of flattened stacks of membranebound sacs called cisternae. The cisternae are arranged in a linear
fashion, with the cis face (entry face) at one end and the trans face
(exit face) at the other end. The Golgi is therefore often referred to
as having a ”cis-to-trans” orientation. The cis face is where vesicles
from the ER enter the Golgi, while the trans face is where vesicles
carrying modified and packaged proteins and lipids exit the Golgi.
The Golgi is responsible for sorting, modifying, and packaging proteins and lipids that are produced in the endoplasmic reticulum
(ER) and transported to the Golgi by vesicles. The Golgi apparatus
performs a number of chemical modifications on these molecules,
including:
1. Glycosylation: the addition of carbohydrate groups to proteins
and lipids. This is one of the most common modifications that
occurs in the Golgi, and it is important for the proper folding
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and function of many proteins. For example, the ABO blood
group antigens are determined by the addition of different carbohydrate groups to a protein called H antigen in the Golgi.
2. Phosphorylation: the addition of phosphate groups to proteins
and lipids. This modification can regulate protein activity and
localization within the cell. For example, the addition of a phosphate group to a protein called tau in the Golgi can promote its
binding to microtubules, which are important for maintaining
the structure of nerve cells.
3. Sulfation: the addition of sulfate groups to proteins and lipids.
This modification can influence the ability of proteins to interact
with other molecules within the cell. For example, sulfation of
the carbohydrate groups on a protein called heparin in the Golgi
is important for its anticoagulant activity.
4. Proteolysis: the cleavage of proteins into smaller peptides or
amino acids. This modification can regulate protein activity and
function. For example, the processing of the precursor protein
proinsulin in the Golgi produces the mature hormone insulin,
which regulates glucose metabolism in the body.
Once the proteins and lipids have been modified in the Golgi, they
are packaged into vesicles and transported to their final destination,
either within the cell or to the cell surface. For example, proteins that
are destined for secretion from the cell are packaged into vesicles
that fuse with the plasma membrane and release their contents
outside the cell.
In summary, the Golgi apparatus is a complex organelle that performs a variety of chemical modifications on proteins and lipids,
including glycosylation, phosphorylation, sulfation, and proteolysis.
These modifications are essential for the proper folding, function,
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and localization of these molecules within the cell. The Golgi also
plays a critical role in sorting and packaging these molecules into
vesicles for transport to their final destination. The Golgi apparatus
can be thought of as a shipping company that receives packages
(proteins and lipids) from the endoplasmic reticulum (ER) and processes them for their final destination.
Just like a shipping company has sorting and packaging facilities,
the Golgi apparatus has machinery that sorts and packages the proteins and lipids. The Golgi also has workers (enzymes) that perform
various chemical modifications on the packages, similar to how a
shipping company might add postage or customs labels.
Once the packages have been modified and packaged, they are
shipped out to their final destination, either within the cell or to the
cell surface.
Just like a shipping company is an important part of the supply chain
for getting goods to their final destination, the Golgi apparatus is
an important part of the cellular machinery for getting proteins
and lipids to where they need to go. Defects in the Golgi apparatus
can lead to problems with protein sorting and modification, just as
problems with a shipping company can lead to delays or problems
with getting goods to their final destination.
4.4.3 Lysosomes and Vesicles
Lysosomes are small, membrane-bound organelles that contain
hydrolytic enzymes that can break down a variety of biomolecules,
including proteins, carbohydrates, and lipids. They are involved in
the degradation of material taken up by the cell through endocytosis,
as well as in the degradation of aged or damaged cell organelles.
The structure of lysosomes is unique in that they are formed by the
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fusion of vesicles produced by the Golgi apparatus with endosomes
containing material that has been internalized by the cell. Once the
lysosome is formed, it contains hydrolytic enzymes that can break
down the material inside into smaller molecules that can be reused
by the cell.
Lysosomes are essential for maintaining cellular homeostasis, as
they are responsible for breaking down and recycling cellular waste
products. Defects in lysosomal function can lead to a variety of
lysosomal storage disorders, such as Tay-Sachs disease and Gaucher
disease, where the accumulation of undegraded biomolecules in
lysosomes can cause cellular damage and organ dysfunction.
Vesicles, on the other hand, are small, membrane-bound structures
that transport materials within the cell. They can be formed by the
Golgi apparatus, the RER, or by the plasma membrane through
processes such as endocytosis or exocytosis. Vesicles can transport
materials to other parts of the cell, or they can fuse with the plasma
membrane to release their contents outside of the cell.
The structure of vesicles can vary depending on their function. For
example, synaptic vesicles in nerve cells are specialized vesicles that
store neurotransmitters, while secretory vesicles in glandular cells
store and secrete hormones.
Vesicles are medically relevant because they are involved in a variety
of cellular processes that can impact human health. For example,
defects in vesicle transport can lead to neurodegenerative disorders
such as Parkinson’s and Alzheimer’s disease. Additionally, viral infections can hijack the vesicle transport system to facilitate their own
replication and spread within the body.
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posal, breaking down and recycling waste products within the cell.
Vesicles act like a mail carrier, transporting materials to different
parts of the cell or outside of the cell, like a mail carrier delivering
packages to different addresses.
Figure 38: After phagocytizing a potentially pathogenic bacterium,
a macrophage fuses with lysosomes within the cell to eliminate the
pathogen. (Credit: openstax.org)
4.5 Cytoskeleton
The cytoskeleton is a network of protein filaments within the cell that
provides structural support, helps to maintain cell shape, and plays
a role in cell division and movement. There are three main types
of protein filaments that make up the cytoskeleton: microtubules,
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microfilaments, and intermediate filaments.
Microtubules are long, thin, tube-like structures made up of protofilaments of the protein tubulin. They are found throughout the cell,
but are particularly abundant near the nucleus and in the periphery
of the cell. Microtubules are involved in the maintenance of cell
shape, the movement of organelles within the cell, and the separation of chromosomes during cell division. Microtubules are also
involved in the formation of the mitotic spindle, which is essential
for the proper segregation of chromosomes during cell division.
Microtubules have a variety of specific uses within the cell. For
example, they form the axonemes of cilia and flagella, which are
involved in the movement of cells and the movement of fluids over
the surfaces of cells. They also provide tracks for the transport of
materials within the cell, including the transport of neurotransmittercontaining vesicles in nerve cells.
Microfilaments are thin, rope-like structures made up of the protein
actin. They are found throughout the cell, but are particularly abundant near the plasma membrane. Microfilaments are involved in
cell movement, such as in the movement of cilia and flagella, and
in the contraction of muscle cells. Microfilaments are also involved
in cell division and in the formation of the contractile ring during
cytokinesis, which helps to divide the cell into two daughter cells.
Microfilaments have a variety of specific uses within the cell. For
example, they form the contractile ring in the cleavage furrow during
cell division, which helps to separate the cell into two daughter
cells. They also form the structural framework for microvilli, which
are finger-like projections on the surface of cells that increase the
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ments and microtubules. They are made up of a variety of proteins
and are found throughout the cell, but are particularly abundant in
cells that are subjected to mechanical stress, such as epithelial cells
and muscle cells. Intermediate filaments are involved in the maintenance of cell shape, as well as in the attachment of organelles to
the cytoskeleton.
Intermediate filaments have a variety of specific uses within the cell.
For example, they provide the structural support for the nuclear
envelope, which surrounds and protects the genetic material of the
cell. They also provide the structural support for desmosomes, which
are specialized cell junctions that help to anchor cells together in
tissues that are subjected to mechanical stress, such as the skin and
heart tissue.
In summary, the cytoskeleton is a complex network of protein filaments that provides structural support, maintains cell shape, and
plays a critical role in cell division and movement. The three main
types of protein filaments in the cytoskeleton - microtubules, microfilaments, and intermediate filaments - differ in their structure,
function, and location within the cell, and each has specific uses
within the cell.
4.5.1 Microtubules
Microtubules are long, thin, tube-like structures that are an important component of the cytoskeleton in eukaryotic cells. They are
composed of protein subunits called tubulin, which polymerize to
form the microtubule.
Microtubules have a number of important functions in the cell, including:
1. Maintaining the shape and structure of the cell: Microtubules
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Figure 39: Microfilaments play a vital role in strengthening the cortex around the inner edge of the cell and function like rubber bands
to resist tension. Within the cell’s interior, microtubules help maintain cell shape by withstanding compressive forces. Furthermore,
intermediate filaments are distributed throughout the cell and serve
to anchor organelles in place (Credit: openstax.org)
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help to maintain the overall shape of the cell and provide structural support for organelles and other structures within the cell.
For example, microtubules help to maintain the shape of neurons, which have long extensions called axons and dendrites
that require a stable framework to support them.
2. Participating in cell division: During cell division, microtubules
help to separate the duplicated chromosomes and distribute
them to the two daughter cells. They do this by forming the
mitotic spindle, a structure that helps to pull the chromosomes
apart. Microtubules also play a role in cytokinesis, the process
of dividing the cell into two daughter cells.
3. Transporting materials within the cell: Microtubules can also act
as tracks for motor proteins, which transport vesicles, organelles,
and other materials within the cell. For example, kinesin and
dynein are motor proteins that move along microtubules and
transport vesicles containing neurotransmitters in nerve cells.
4. Regulating gene expression: Microtubules can also play a role
in regulating gene expression by helping to position the chromosomes within the cell. This positioning can affect how genes
are expressed and can influence cell fate and differentiation.
In addition to these functions, microtubules are involved in a variety
of cellular processes, such as the maintenance of cell polarity, the
formation of cilia and flagella, and the movement of cells. Defects
in microtubule function can lead to a variety of diseases, including
cancer, neurodegenerative disorders, and ciliopathies, which are
disorders caused by defects in cilia function. An analogy for microtubules could be a scaffold or support beam in a building. Just like a
scaffold provides support and maintains the structure of a building,
microtubules provide support and maintain the structure of the
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cell.
Another way to understand the structure and function of microtubules, is you can think of them as the ”highways” of the cell. Just
as highways help to transport people and goods from one place to
another, microtubules help to transport materials within the cell.
They also help to maintain the overall structure of the cell and play
a role in important processes such as cell division.
There are several different types of motor proteins that can move
along microtubules, including kinesin and dynein. Kinesin moves
towards the plus end of the microtubule, while dynein moves towards the minus end. Motor proteins are important for a variety of
processes, including the transport of vesicles and organelles within
the cell, and the movement of cilia and flagella.
Figure 40: The walls of microtubules consist of 13 polymerized
dimers of �-tubulin and �-tubulin (right image). The left image
shows the corresponding molecular structure. (Credit: openstax.org)
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4.5.2
Motor Proteins
Kinesin and dynein are two types of motor proteins that are responsible for transporting cargo within cells along microtubules. Kinesin
moves towards the plus end of the microtubule, while dynein moves
towards the minus end, allowing them to move cargo in opposite
directions.
An analogy for kinesin and dynein could be like trains on a railway system. Just like trains travel in opposite directions on a railway system,
kinesin and dynein move in opposite directions along microtubules
to transport cargo within the cell.
Kinesin is composed of two globular heads and a long, flexible stalk
that connects them. The heads of kinesin bind to the microtubule
and use energy from ATP hydrolysis to ”walk” towards the plus end
of the microtubule, pulling cargo along with them. An example of
kinesin in action is the transport of synaptic vesicles containing neurotransmitters in nerve cells. Kinesin moves these vesicles from the
cell body to the axon terminal, where they are released to transmit
signals to other neurons or to muscles.
Dynein is similar to kinesin in that it is composed of two globular
heads and a long, flexible stalk, but it moves in the opposite direction
towards the minus end of the microtubule. Dynein is responsible
for moving cargo such as vesicles and organelles towards the center
of the cell or towards the microtubule-organizing center (MTOC).
An example of dynein in action is the movement of chromosomes
during cell division. Dynein helps to position the mitotic spindle and
move the chromosomes towards the MTOC for proper segregation.
Kinesin and dynein are also involved in the movement of cilia and
flagella, which are hair-like structures that extend from the surface
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of cells and are involved in movement and sensing. Defects in kinesin, dynein, or microtubules can lead to problems with ciliary
or flagellar movement, resulting in diseases known as ciliopathies,
which can affect various organs and tissues in the body. For example, defects in the cilia of the respiratory tract can lead to chronic
respiratory infections, while defects in the cilia of the kidneys can
cause polycystic kidney disease.
In addition, defects in the function of kinesin, dynein, or microtubules can also contribute to neurodegenerative disorders such
as Alzheimer’s disease and Parkinson’s disease, which involve the
abnormal accumulation of proteins in the brain. Understanding the
functions of kinesin, dynein, and microtubules is therefore crucial
for understanding the underlying mechanisms of these diseases
and developing potential treatments.
4.5.3 Cilia and Flagella
Cilia and flagella are fascinating structures with important functions in movement, sensing, and signaling in both prokaryotic and
eukaryotic cells. To better understand these structures, it helps to
break down their individual components and functions.
Cilia are hair-like structures that protrude from the surface of cells
and are involved in various physiological processes. They consist
of microtubules arranged in a 9 + 2 pattern, anchored by proteins
called dyneins. Kinesin and dynein motor proteins move along the
microtubules, with kinesin moving towards the plus end and dynein
moving towards the minus end. This coordinated movement results
in the cilium bending and moving in a waving motion. Cilia can be
found in many different tissues and organs, including the respiratory
and reproductive tracts and the inner ear.
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Flagella, on the other hand, are whip-like structures that are longer
than cilia and used for propulsion. They are also composed of microtubules arranged in a 9 + 2 pattern, anchored by dyneins. Flagella
are generally found only in sperm cells, where they help the sperm
swim towards the egg.
The bacterial flagellum is similar in structure to eukaryotic cilia and
flagella, but it is composed of a single, long filament of flagellin
anchored to the cell membrane and surrounded by a sheath. The
flagellum is powered by a motor protein called the flagellar motor,
which uses the energy from ATP to rotate the filament.
Primary cilia, on the other hand, are small, hair-like structures that
are found on the surface of many different types of cells in the body.
They are similar in structure to other cilia, but they are generally
shorter and have a different function. Primary cilia consist of microtubules arranged in a 9 + 0 pattern and are involved in sensory
functions, signal transduction, and development. They are found on
many different types of cells, including epithelial cells, nerve cells,
and muscle cells.
It is important to note that defects in cilia and flagella function
can lead to a variety of medical conditions, including ciliopathies,
neurodegenerative disorders, and male infertility. Therefore, understanding the structure and function of these fascinating structures
can have important medical implications.
Cilia are found in a number of different tissues and organs, including:
1. The respiratory tract: Cilia lining the respiratory tract help to
move mucus and trapped particles out of the lungs.
2. The reproductive tract: Cilia in the female reproductive tract
help to move eggs and sperm towards the uterus and Fallopian
tubes.
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3. The inner ear: Cilia in the inner ear help to detect sound and
maintain balance.
Flagella on the other hand can mainly be found on the head of the
sperm, it helps it propel towards the egg.
Figure 41: The electron micrograph illustrates two flagella and reveals the 9 + 2 arrangement of microtubules. Specifically, one microtubule doublet is surrounded by nine additional microtubule
doublets. (Credit: openstax.org)
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4.5.4
The Microtubule-Organizing Center
The microtubule-organizing center (MTOC) is a structure found in
eukaryotic cells that is responsible for organizing and maintaining
the microtubules in the cell. The MTOC is typically located near the
center of the cell and is composed of a group of proteins called the
centrosome, which is made up of two centrioles.
Each centriole is made up of nine triplets of microtubules arranged
in a circular fashion. The triplets are connected by proteins called
nexins, which help to hold the microtubules in place. The centrioles
are surrounded by a layer of protein called the pericentriolar material
(PCM), which helps to anchor the centrioles to the cell membrane
and is involved in the nucleation of microtubules.
The centrosome is the primary site for the nucleation of microtubules in the cell, and the centrioles are responsible for the synthesis of the microtubules. The centrosomes replicate during the cell
cycle and produce daughter centrioles, which are used to form the
mitotic spindle during cell division. The mitotic spindle is a structure made up of microtubules that helps to separate the duplicated
chromosomes and distribute them to the daughter cells.
Defects in the function of the MTOC can result in a number of diseases, including ciliopathies, which are caused by defects in the
structure or function of cilia and flagella, as well as neurodegenerative disorders. Additionally, the MTOC is a target for cancer therapies,
as the mitotic spindle is essential for cell division and proliferation,
and disrupting its function can lead to cell death.
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4.5.5
Microfilaments
Microfilaments are thin, rod-like structures that are an important
component of the cytoskeleton in eukaryotic cells. They are composed of protein subunits called actin, which polymerize to form
the microfilament.
Microfilaments have a number of important functions in the cell,
including:
1. Maintaining the shape and structure of the cell: Microfilaments,
also known as actin filaments, are one of the main components
of the cytoskeleton, which is a network of protein filaments
within the cell that provides structural support and helps to
maintain cell shape. Microfilaments are responsible for maintaining the overall shape of the cell and for providing structural
support for organelles and other structures within the cell. For
example, microfilaments help to maintain the shape of red blood
cells, which are able to squeeze through narrow capillaries due
to their flexible shape.
2. Microfilaments are also important for the movement of cells.
They are involved in processes such as cell division, muscle contraction, and the movement of organelles within the cell. During
cell division, microfilaments help to form the contractile ring,
which helps to divide the cell into two daughter cells. In muscle
cells, microfilaments work together with myosin motor proteins
to generate the force needed for muscle contraction. Microfilaments also help to transport organelles within the cell, such as
vesicles and mitochondria
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mit information within the cell. For example, microfilaments
can interact with other proteins to regulate the activity of enzymes involved in signaling pathways. Microfilaments can also
be involved in the formation of membrane protrusions, such as
filopodia and lamellipodia, which are important for cell movement and for interactions between cells.
To understand the structure and function of microfilaments, you
can think of them as the ”scaffolding” of the cell. Just as scaffolding
helps to support the structure of a building, microfilaments help
to support the structure of the cell and play a role in important
processes such as cell movement.
Actin and microfilaments are important components of the cytoskeleton, a network of fibers that helps to maintain the shape
and structure of eukaryotic cells. They are made up of protein filaments that are arranged in a linear fashion and are essential for
many different functions in the cell.
Actin is a protein that is found in all eukaryotic cells and is an important component of the cytoskeleton. It is composed of a chain of
monomeric subunits called actin monomers, which are arranged in
a helical fashion. There are two main types of actin found in cells:
filamentous actin (F-actin) and globular actin (G-actin).
F-actin is the filamentous form of actin and is made up of long
chains of actin monomers. It is an important component of the
cytoskeleton and is involved in many different functions in the cell,
including:
1. Cell shape: F-actin, or filamentous actin, is a major component
of the cytoskeleton and is responsible for maintaining the shape
of the cell. It forms a dynamic scaffold that provides support and
structure to the cell, allowing other cell components to attach
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to it. F-actin also helps to maintain the integrity of the plasma
membrane by anchoring it to the cytoskeleton.
2. Movement: F-actin plays a critical role in the movement of cells
and organelles within the cell. It is involved in the formation of
contractile structures, such as the contractile ring during cytokinesis and the actin-myosin complex in muscle cells. F-actin is
also involved in the movement of cilia and flagella, which are
hair-like structures that protrude from the surface of some cells
and are used for locomotion or to move fluids over the surface
of the cell. In addition, F-actin is involved in the movement of
organelles and vesicles within the cell, through interactions with
motor proteins such as myosin.
3. Signal transduction: F-actin is involved in signaling pathways
that control various cellular processes. For example, F-actin is
involved in the activation of Rho GTPases, which are signaling
molecules that regulate actin dynamics and are involved in processes such as cell division and migration. F-actin is also involved
in the activation of transcription factors, which are proteins that
control the expression of genes in response to specific signals. In
addition, F-actin is involved in the formation of specialized structures within the cell, such as filopodia and lamellipodia, which
are involved in processes such as cell migration and adhesion.
G-actin is a small, globular protein that is made up of individual
actin monomers. It is typically found in the cytosol of eukaryotic
cells, where it can exist in equilibrium with F-actin, the filamentous
form of actin.
G-actin plays a crucial role in the formation of F-actin filaments, as it
is the building block for the filaments. Actin nucleation factors, such
as Arp2/3 and formins, bind to G-actin and promote the formation
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of F-actin filaments by polymerizing the individual actin monomers
into long chains.
In addition to its role in filament formation, G-actin also plays a role
in various cellular processes, such as cell migration, cell division,
and intracellular transport. For example, G-actin is involved in the
formation of pseudopodia, which are temporary protrusions that
allow cells to move in a particular direction. G-actin is also involved
in the formation of contractile rings during cytokinesis, the process
by which a cell divides into two daughter cells.
The concentration of G-actin in the cytosol is tightly regulated, as it
is important for maintaining the balance between G-actin and Factin. Alterations in the balance can lead to changes in the structure
and function of the cell. For example, changes in the concentration
of G-actin have been associated with various diseases, including
cancer and heart disease.
Overall, G-actin is a crucial protein in the cytosol of eukaryotic cells,
where it serves as a building block for F-actin filaments and plays a
role in various cellular processes. Its location and function are tightly
regulated, and alterations in its concentration can have significant
consequences for the cell.
4.5.6 Intermediate filaments
Intermediate filaments are a diverse group of fibrous proteins that
are an integral part of the cytoskeleton in eukaryotic cells. They
are so named because of their intermediate size between microtubules and microfilaments. Intermediate filaments are found in
the cells of animals, plants, and fungi, where they play a vital role in
maintaining cellular structure and integrity. Unlike microtubules
and microfilaments, intermediate filaments are not involved in cell
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division or movement but provide structural support to cells and
tissues subjected to mechanical stress. Intermediate filaments are
made up of a family of proteins that differ in their composition and
structure depending on the cell type and tissue. They are organized
into bundles, networks, and sheets, where they act as anchors to
hold cells and tissues together. Intermediate filaments are present
in a wide variety of cell types, including epithelial cells, muscle cells,
and nerve cells. Their diverse structural properties allow them to
fulfill a variety of roles in different cell types and tissues. Understanding the structure and function of intermediate filaments is crucial
to understanding the behavior of cells and tissues under various
physiological and pathological conditions.
Intermediate filaments have a number of important functions in
the cell, including:
1. Maintaining the shape and structure of the cell: Intermediate
filaments provide mechanical support to cells and help to maintain their overall shape and structure. They form a network of
filaments that is resistant to tensile forces, which can help cells
to resist deformation and maintain their shape. Different types
of intermediate filaments are found in different cell types, with
some being particularly important for maintaining the shape of
cells that are subjected to mechanical stress, such as skin cells
or muscle cells.
2. Participating in cell movement: While intermediate filaments
are not as directly involved in cell movement as microtubules
or microfilaments, they can still play a role in this process. For
example, intermediate filaments have been shown to be important for the migration of some types of cells, such as fibroblasts.
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volved in cell movement, such as the lamellipodia and filopodia
that extend from the leading edge of migrating cells.
3. Protecting the cell from mechanical stress: Intermediate filaments can help to protect cells from mechanical stress by
providing a tough, flexible network that can withstand forces
without breaking. This is particularly important in cells that are
subjected to constant mechanical stress, such as skin cells or
muscle cells. For example, intermediate filaments in muscle
cells help to maintain the integrity of the sarcomere structure
during muscle contraction, preventing damage to the muscle
fibers.
To understand the structure and function of intermediate filaments,
you can think of them as the ”reinforcing bars” of the cell. Just as
reinforcing bars help to strengthen a structure and protect it from
stress, intermediate filaments help to maintain the overall shape of
the cell and protect it from mechanical stress.
Figure 42: Comprised of fibrous proteins, intermediate filaments are
composed of multiple strands that are intertwined with each other.
(Credit: openstax.org)
4.6
Comparing Animal, Plant and Bacteria Cells
Animal cells:
1. Lack a cell wall
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2. Have smaller cell size compared to plant cells
3. Have centrioles, which are involved in cell division
4. Have lysosomes, which contain enzymes that break down waste
materials and cell debris
5. Have mitochondria, which are responsible for generating energy
in the form of ATP
6. Do not have chloroplasts, which are responsible for photosynthesis in plant cells
Plant cells:
1. Have a cell wall made of cellulose, which provides support and
protection
2. Have larger cell size compared to animal cells
3. Do not have centrioles
4. Have a central vacuole, which stores water and other substances
5. Have chloroplasts, which are responsible for photosynthesis
6. Do not have lysosomes
Bacteria cells:
1. Have a cell wall made of peptidoglycan, which provides support
and protection
2. Can have various shapes, including spherical, rod-shaped, and
spiral-shaped
3. Do not have a defined nucleus; instead, their genetic material is
found in a region called the nucleoid
4. Do not have organelles like mitochondria or lysosomes
5. Can have flagella, which are used for movement
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6. Can have pili, which are used for attachment and exchange of
genetic material
Despite these differences, all cells have certain features in common:
1. All cells are surrounded by a cell membrane, which separates
the inside of the cell from the outside environment and helps
to maintain homeostasis
2. All cells contain genetic material in the form of DNA, which
carries the instructions for all of the cell’s functions
3. All cells contain ribosomes, which are responsible for synthesizing proteins, but Bacterial and mitochondrial ribosomes are
smaller than the ribosomes of eukaryotes.
4. All cells carry out metabolism, the chemical reactions that sustain life.
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5
The Plasma Membrane
Imagine a city surrounded by a protective wall that separates the
bustling activity inside from the outside world. In the same way, the
plasma membrane is a barrier that encloses the busy and complex
inner workings of the cell from the surrounding environment. The
plasma membrane, also known as the cell membrane, is a thin and
flexible barrier that surrounds the cell and separates the interior of
the cell from the external environment. It is made up of a phospholipid bilayer, which is a double layer of phospholipid molecules.
The phospholipid bilayer is composed of two layers of phospholipid
molecules arranged with their hydrophobic tails facing inward and
their hydrophilic heads facing outward.
But the plasma membrane is not just a static wall - it is a dynamic
structure constantly in motion, like a dance floor with different types
of dancers moving and changing positions. This is where the fluid
mosaic model comes in, describing the plasma membrane as a fluidlike mosaic of different types of molecules, like a painting made up
of colorful tiles. The fluid mosaic model is a model that suggests
that the plasma membrane is a dynamic structure that is composed
of a mosaic of different types of molecules, including phospholipids,
proteins, and carbohydrates. These molecules are constantly moving and rearranging themselves within the membrane, giving it a
fluid-like quality.
And just like how the city wall has gates for people and goods to enter
and exit, the plasma membrane has proteins that act as gatekeepers,
allowing molecules to pass through and communicating with other
cells. These proteins can be found either embedded within the
membrane or attached to its surface, like players on a sports team
working together for the benefit of the whole. The fluid mosaic
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model also proposes that proteins in the plasma membrane play
important roles in the transport of molecules across the membrane,
as well as in cell-cell communication and signaling. The proteins
can be either integral proteins, which are embedded within the
phospholipid bilayer, or peripheral proteins, which are attached to
the surface of the membrane.
Figure 43: Eukaryotic plasma membranes are composed of a phospholipid bilayer that contains embedded proteins and cholesterol.
(Credit: openstax.org)
5.1 The Concentration Gradient
Concentration gradient is a term used to describe the difference
in the concentration of a substance between two regions. In the
context of cell biology, it refers to the difference in the concentration
of a substance on either side of the cell membrane. This difference
in concentration is what drives the movement of substances across
the membrane.
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Before delving into the various channels involved in the plasma
membrane, it is important to have a basic understanding of concentration gradients. The concept of concentration gradient is fundamental to understanding how substances move across the membrane and the role of different channels in this process.
Substances move across the membrane from areas of high concentration to areas of low concentration, in order to reach a state of
equilibrium. This movement is driven by the concentration gradient,
which acts like a force that pushes the substance from an area of
high concentration to an area of low concentration.
The concentration gradient is an important concept to be aware of
because it is a driving force for many physiological processes. For
example, the movement of ions across the membrane is essential
for the generation and propagation of action potentials in neurons
and muscle cells. The concentration gradient of ions across the
membrane plays a critical role in establishing the resting potential
of cells, which is necessary for the transmission of electrical signals.
Additionally, understanding concentration gradients is important
for understanding the role of different channels in the plasma membrane. Channels are specialized proteins that allow the movement
of substances across the membrane. The specific types of substances that can pass through these channels are determined by
the properties of the channels themselves, as well as the concentration gradient of the substance across the membrane.
For example, ion channels allow the movement of ions across the
membrane in response to changes in the concentration gradient
of the ion. Gated channels open or close in response to specific
stimuli, such as changes in voltage or the binding of a specific
molecule. Pore-forming channels, on the other hand, are formed by
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the oligomerization of multiple channel proteins and can be either
selective or non-selective in terms of the types of substances that
can pass through.
In summary, understanding the concept of concentration gradient
is essential for understanding how substances move across the
plasma membrane and the role of different channels in this process.
It is an important topic to be aware of before delving into the details
of the different types of channels involved in the plasma membrane.
5.2 Membrane Proteins
Integral proteins and peripheral proteins are two types of proteins
found in the plasma membrane.
5.2.1
Integral Proteins
Integral proteins are embedded within the phospholipid bilayer of
the plasma membrane and are an integral part of the membrane
structure. They are also known as trans-membrane proteins because
they span the entire thickness of the membrane, with one end inside
the cell and the other end outside the cell. Integral proteins have
hydrophobic regions that interact with the hydrophobic tails of the
phospholipid molecules in the membrane, allowing them to be
integrated into the membrane structure.
There are several types of integral proteins, including:
Channel proteins: These proteins create channels through the
membrane that allow specific ions or small molecules to pass through.
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Figure 44: Electrochemical gradients result from the combined influence of concentration gradients and electrical gradients. Outside
the cell, Na+ ions have a higher concentration, while inside the cell,
K+ ions are more concentrated. Despite this, the cell’s interior has a
negative net charge compared to the other side of the membrane.
This occurs because of the presence of K+ binding proteins and
other negatively charged molecules. The difference in electrical
charges draws the positively charged Na ions towards the cell’s interior, forming the electrical gradient. In contrast, K ions tend to move
through K channels towards the cell’s exterior due to the concentration difference, forming the concentration gradient. The structures
labeled A represent proteins. (Credit: openstax.org)
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5.2.2 Channel Proteins
Channel proteins are integral membrane proteins that allow ions
and small molecules to pass through the membrane by creating
a channel through which the substances can pass. These proteins
are important in the movement of substances across the membrane, which is essential for maintaining homeostasis within cells
and tissues.
There are several types of channel proteins, including:
5.2.3 Ion Channels
Ion channels: These proteins allow ions to pass through the membrane. Ion channels can be either selective or non-selective, meaning that they can allow specific ions to pass through or any ion to
pass through, respectively.
Ion channels are a type of channel protein that allow ions to pass
through the membrane. Ion channels are integral membrane proteins, meaning that they are embedded within the lipid bilayer of
the cell membrane. They are composed of proteins called subunits,
which form a channel through which ions can pass.
Ion channels can be either selective or non-selective. Selective ion
channels allow specific ions to pass through the channel, while nonselective ion channels allow any ion to pass through. Selective ion
channels are specific to certain ions, such as sodium, potassium,
calcium, or chloride ions.
Ion channels can also be classified based on their activation properties. Some ion channels are always open, while others are gated,
meaning that they can open or close in response to specific stimuli.
For example, some ion channels are activated by changes in the
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voltage across the membrane, while others are activated by the
binding of a specific molecule.
One example of an ion channel is the sodium-potassium pump,
which is involved in maintaining the resting membrane potential
of nerve cells. Another example is the calcium channel, which is involved in muscle contraction and neurotransmitter release. Chloride
channels are found in the kidneys and are involved in the regulation
of salt and water balance.
Ion channels are found in many different locations within the body,
including nerve cells, muscle cells, and epithelial cells. In nerve cells,
ion channels play a crucial role in the generation and transmission
of action potentials. In muscle cells, ion channels are involved in
muscle contraction. In epithelial cells, ion channels are involved in
the transport of ions and water across cell membranes.
5.2.4 Gated Channels
Gated channels are important proteins that control the flow of ions
across the cell membrane in response to specific stimuli. They act
like tiny gates, opening and closing to allow ions to pass through in
a regulated manner.
One example of gated channels is voltage-gated channels. These
channels are activated by changes in the voltage across the membrane, and are important in the generation and propagation of
action potentials in neurons and muscle cells. An analogy to explain how voltage-gated channels work is to imagine a gate that
opens when a certain amount of pressure is applied to it. In the case
of voltage-gated channels, the gate opens when a certain voltage
threshold is reached.
Another type of gated channel is ligand-gated channels. These
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Figure 45: Ion channel proteins are regulated by gates, which can
either permit or prevent ion transport through them. The channels remain closed when the gates are shut, and no ions can pass
through. In contrast, when the channels open, specific ions diffuse
through them, with channel proteins exhibiting a high degree of
specificity, allowing only certain ions or a subset of ions to pass
through. (Credit: openstax.org)
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channels are activated by the binding of a specific molecule, called a
ligand, to the channel protein. Ligand-gated channels are important
in the transmission of signals between cells and in the regulation of
enzyme activity. An analogy to explain how ligand-gated channels
work is to imagine a gate that opens when a specific key is inserted
into the lock. In the case of ligand-gated channels, the key is the
ligand that binds to the channel protein, causing the gate to open.
Mechanically-gated channels are another type of gated channel
that are activated by physical stimuli, such as pressure or stretch.
Mechanically-gated channels are important in the regulation of
blood pressure and in the detection of sensory stimuli. An analogy
to explain how mechanically-gated channels work is to imagine a
gate that opens when a certain amount of force is applied to it. In
the case of mechanically-gated channels, the gate opens when the
membrane is stretched or compressed by physical stimuli.
Gated channels play a crucial role in many physiological processes,
including the transmission of signals within and between cells, the
regulation of enzyme activity, and the generation and propagation
of action potentials. Without gated channels, the proper functioning
of many biological processes would be disrupted.
5.2.5 Leak Channels
Leak channels are present in many types of cells, including neurons,
muscle cells, and epithelial cells. In neurons, leak channels help to
maintain the resting potential of the cell, which is essential for the
transmission of action potentials. The resting potential of a neuron
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tion gradients, which helps to maintain the resting potential. The
pore of the channel is lined with amino acids that interact with ions
as they pass through the channel. The selectivity of the channel for
specific ions is determined by the size and charge of these amino
acids. While leak channels are non-selective, they do exhibit some
preference for certain ions based on their size and charge.
Leak channels also play a role in regulating cell volume. For example,
in kidney cells, leak channels help to regulate the osmotic pressure
inside the cell by allowing the passive movement of ions across the
membrane. This helps to prevent the cell from swelling or shrinking
in response to changes in the extracellular environment.
An analogy for leak channels is a leaky faucet. Just as a leaky faucet
allows water to pass through continuously, leak channels allow ions
to pass through the membrane continuously. However, unlike a
leaky faucet, leak channels are essential for maintaining the normal
functioning of cells and tissues.
In summary, leak channels are always open and allow ions to pass
through the membrane continuously, which helps to maintain the
resting potential of cells and regulate cell volume. While leak channels may seem insignificant compared to gated channels, they play
an important role in many physiological processes and are essential
for the normal functioning of cells and tissues.
5.2.6
Pore-forming Channels
Pore-forming channels are a type of channel protein that are formed
by the oligomerization, or the grouping together, of multiple channel proteins. Pore-forming channels can be either selective or nonselective, depending on the specific proteins that form the channel.
These channels are characterized by a pore or channel that is formed
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by the oligomerization of the protein subunits. The channel is typically lined with hydrophilic residues, which allows it to interact with
and transport ions or other small molecules.
Pore-forming channels play an essential role in many physiological processes, including the transport of ions and small molecules
across the membrane, the regulation of cell volume, and the maintenance of ion gradients across the membrane. Some examples of
pore-forming channels include:
1. Voltage-gated potassium channels: These channels are activated by changes in the voltage across the membrane and allow the movement of potassium ions through the membrane.
Voltage-gated potassium channels are important in the regulation of the resting potential of cells and in the repolarization
phase of action potentials.
2. Connexons: These channels are formed by the oligomerization
of six connexin proteins and allow the movement of ions and
small molecules between cells. Connexons are important in the
communication between cells and in the regulation of enzyme
activity.
3. Aquaporins: These channels are selective for the movement of
water molecules and allow the rapid movement of water across
the membrane. Aquaporins are important in the regulation of
water balance in the body.
4. Porins: These channels are formed by the oligomerization of
multiple porin proteins and allow the movement of small molecules
through the outer membrane of bacteria. Porins are important
in the uptake of nutrients and the elimination of waste products
in bacteria.
An analogy for pore-forming channels could be a pipe or a tunnel
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that allows the movement of water or other substances. Just as a
pipe allows the movement of water from one location to another,
pore-forming channels allow the movement of ions or other small
molecules across the membrane.
In summary, pore-forming channels are a type of channel protein
that form a pore or channel through which ions or small molecules
can move across the membrane. These channels play an essential
role in many physiological processes, including the regulation of the
resting potential of cells, the communication between cells, and the
maintenance of ion gradients across the membrane.
5.2.7
Aquaporins
Aquaporins are highly selective channels that only allow the passage
of water molecules through the membrane, preventing the passage
of other small molecules and ions. They are able to selectively filter
water due to their specific structural features. The narrowest part of
the channel, known as the selectivity filter, is formed by a chain of
amino acids that are arranged in a specific manner to create a pore
that allows only water molecules to pass through. The presence
of hydrophobic residues lining the pore also helps to prevent the
passage of ions and other small molecules.
Aquaporins are important in many physiological processes. For
example, they play a crucial role in the kidney’s ability to reabsorb
water from the urine and return it to the body. In the absence of
aquaporins, the urine produced by the kidneys would be very dilute
and the body would lose water rapidly, leading to dehydration.
An analogy for aquaporins can be a water slide at a water park. Just
as a water slide allows only water to pass through while preventing
the passage of other objects, aquaporins allow only water to pass
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through their channel while excluding other molecules and ions.
There are several types of aquaporins, including:
Aquaporin 0 (AQP0): AQP0 is found in the lens of the eye and is
important in maintaining the transparency of the lens.
Aquaporin 1 (AQP1): AQP1 is found in the endothelial cells that line
the blood vessels and is important in the regulation of blood pressure.
Aquaporin 2 (AQP2): AQP2 is found in the renal tubules of the kidney
and is important in the regulation of water balance in the body.
Aquaporin 4 (AQP4): AQP4 is found in the brain and is important in
the regulation of brain edema.
It is not necessary for the IMAT to memorize the different types and
locations of aquaporins.
Aquaporins are important in maintaining proper hydration and electrolyte balance within cells. Dysfunction of aquaporins can lead
to various diseases and disorders, including diabetes insipidus, a
condition characterized by excessive thirst and the excretion of large
amounts of dilute urine. Therefore, understanding the structure
and function of aquaporins is important for the development of
treatments for these conditions.
In addition to these types of channel proteins, there are also several
subtypes of channel proteins that can be classified based on their
specific functions. For example, there are channels that specifically
allow the movement of calcium ions, potassium ions, or sodium
ions.
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5.2.8 Carrier Proteins
Carrier proteins are integral membrane proteins that bind to specific
molecules and transport them across the membrane. These proteins undergo conformational changes upon binding of the specific
molecule, which allows the molecule to be transported across the
membrane.
Uniporters are carrier proteins that transport a single type of substance across the membrane. Uniporters can be either passive or
active. Passive uniporters transport substances down their concentration gradient, while active uniporters require energy to transport
substances against their concentration gradient. One example of
a uniporter is the glucose transporter, which transports glucose
molecules across the plasma membrane.
Symporters are carrier proteins that transport two types of substances across the membrane in the same direction. Symporters can
be either passive or active. Passive symporters transport substances
down their concentration gradient, while active symporters require
energy to transport substances against their concentration gradient.
One example of a symporter is the sodium-glucose cotransporter,
which transports both sodium ions and glucose molecules across
the plasma membrane.
Antiporters are carrier proteins that transport two types of substances across the membrane in opposite directions. Antiporters
can be either passive or active. Passive antiporters transport substances down their concentration gradient, while active antiporters
require energy to transport substances against their concentration
gradient. One example of an antiporter is the sodium-potassium
pump, which transports sodium ions out of the cell and potassium
ions into the cell.
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Carrier proteins are important in many physiological processes, including the transport of ions and small molecules across the membrane, the regulation of cell volume, and the maintenance of ion
gradients across the membrane. Dysfunctions of carrier proteins
can lead to various diseases and disorders, including cystinuria, a
condition characterized by the formation of kidney stones due to the
impaired transport of cystine molecules. Therefore, understanding
the structure and function of carrier proteins is important for the
development of treatments for these conditions.
5.2.9 Receptor proteins
Receptor proteins are a diverse group of integral membrane proteins
that play critical roles in the communication and response of cells
to external stimuli. Despite their shared function, they have distinct
differences in their structure and function.
G protein-coupled receptors (GPCRs) are the largest family of receptor proteins, with over 800 members. They consist of a single
polypeptide chain that traverses the cell membrane seven times,
forming a helical structure. GPCRs are activated when a ligand
binds to the extracellular portion of the receptor, causing a conformational change that activates a G protein on the intracellular side
of the membrane. G proteins then transmit the signal to effector
proteins that activate or inhibit intracellular signaling pathways.
Ionotropic receptors are another type of receptor protein that directly gate ion channels. They consist of multiple subunits that come
together to form a transmembrane pore. When a ligand binds to
the extracellular domain of the receptor, it causes a conformational
change that opens the pore, allowing ions to pass through the membrane. Ionotropic receptors are important in the transmission of
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Figure 46: With the assistance of carrier proteins, certain substances
can move across the plasma membrane in the direction of their
concentration gradient. These carrier proteins undergo a conformational change to facilitate the transport of molecules across the
membrane. (Credit: openstax.org)
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signals within cells, such as in the nervous system.
Enzyme-linked receptors are a type of receptor protein that have
both an extracellular ligand-binding domain and an intracellular
enzymatic domain. Upon ligand binding, these receptors undergo
a conformational change that activates the enzymatic domain, leading to the production of second messenger molecules that transmit
the signal to the inside of the cell. Enzyme-linked receptors are important in the regulation of enzyme activity and in the transmission
of signals between cells.
An analogy for receptor proteins is a lock and key. Just as a specific
key is needed to open a specific lock, a specific ligand is needed to
bind to a specific receptor protein. The shape of the receptor protein
is like the lock, while the shape of the ligand is like the key. Each
type of receptor protein has a unique lock and key that allows for
specific signaling and responses.
Understanding the different types of receptor proteins and their
mechanisms of action is important in the development of treatments for various diseases and disorders, including cancer, diabetes,
and neurological disorders.
5.2.10 Peripheral Proteins
Peripheral proteins are a type of protein that are not embedded
within the phospholipid bilayer of the cell membrane, but are instead attached to the surface of the membrane. They are held in
place by noncovalent bonds, such as hydrogen bonds, and can be
easily removed from the membrane. Peripheral proteins do not
span the entire thickness of the membrane and are not involved
in the transport of molecules across the membrane. Instead, they
play important roles in cell-cell communication and signaling, and
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in the organization and function of the membrane.
There are two types of peripheral proteins: those attached to the
cytosolic side of the membrane, and those attached to the extracellular side of the membrane. Some examples of peripheral proteins
include:
Enzymes: Some peripheral proteins act as enzymes, catalyzing
chemical reactions within or outside the cell. For example, the
enzyme carbonic anhydrase is a peripheral protein that catalyzes
the conversion of carbon dioxide and water into bicarbonate ions
and protons.
Signal transduction proteins: Some peripheral proteins are involved
in the transmission of signals between cells or within cells. For
example, the peripheral protein G protein-coupled receptor (GPCR)
is involved in the transmission of signals between cells through the
activation of a G protein.
Cell adhesion proteins: Some peripheral proteins are involved in the
attachment of cells to other cells or to extracellular matrix proteins.
For example, integrins are peripheral proteins that are involved in
cell adhesion and the attachment of cells to the extracellular matrix.
Structural support proteins: Some peripheral proteins provide structural support to the cell membrane or to the cytoskeleton. For example, spectrin is a peripheral protein that provides structural support
to the cytoskeleton.
An analogy for peripheral proteins is the glue that holds together a
book cover. Just as the glue is not embedded within the cover, but
instead holds the cover in place, peripheral proteins are attached
to the surface of the membrane and hold the membrane in place.
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tions within the cell.
5.2.11 Signal Cascades
A cellular signal cascade is a series of events that occurs inside a cell
in response to a signal from the outside. It’s like a chain reaction one event triggers the next, which triggers the next, and so on.
Imagine that you’re playing a game of tag with your friends. One
of your friends is ”it” and is trying to tag you. You see your friend
coming and you start running. As you run, you might yell to your
other friends to let them know that you’re being chased. This is
kind of like a cellular signal cascade - the ”tag” is the signal, and the
running and yelling are the events that happen in response to the
signal.
In a cell, a signal might be something like a hormone or a chemical
messenger. When the cell receives the signal, it starts a series of
events inside the cell to respond to the signal. These events might
include things like turning certain genes on or off, or activating
certain proteins.
Cellular signal cascades are important because they allow cells to
communicate with each other and respond to changes in their
environment. They also play a role in many important processes
in the body, such as immune responses, muscle contraction, and
blood clotting.
5.2.12 Cellular Communication
There are several types of cell communication, including:
Direct communication: This occurs when cells come into direct contact with one another and exchange signals through gap junctions
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or tight junctions.
Paracrine signaling: This occurs when a signaling molecule is
released by a cell and diffuses through the extracellular space to
nearby cells, where it can bind to receptors on those cells and trigger
a response.
Paracrine signaling is a type of cell-to-cell communication in which
a signaling molecule (called a paracrine factor) is released by a cell
and diffuses through the extracellular space to nearby cells, where
it can bind to receptors on those cells and trigger a response.
Paracrine signaling is important in a variety of physiological processes, including growth, development, and tissue repair. For example, during development, paracrine signaling plays a role in the
differentiation of cells into specific tissues and organs. In tissue
repair, paracrine signaling can stimulate the proliferation and migration of cells to the site of injury, leading to the formation of new
tissue.
Paracrine signaling is also important in the immune system. For
example, certain immune cells, such as macrophages and T cells,
release paracrine factors that can stimulate the activation and proliferation of other immune cells.
Unlike hormones, which are released into the bloodstream and
travel to distant cells or tissues, paracrine factors are typically only
effective at relatively short distances from the cell that releases them.
This allows for highly specific and localized signaling within tissues.
There are two types of junctions that can be involved in paracrine
signaling: gap junctions and tight junctions.
Gap junctions are small channels that allow ions and small molecules
to pass directly between cells. They are formed by the alignment of
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two connexons, which are protein complexes that span the plasma
membrane of each cell. Gap junctions allow cells to communicate
directly with one another and exchange signals, such as electrical
signals or signaling molecules.
Tight junctions, also known as zonula occludens, are a type of junction that seals off the space between cells and creates a barrier
between different tissues or compartments within the body. Tight
junctions are formed by the alignment of transmembrane proteins
called occludins and claudins, which seal off the space between
cells and prevent the passage of molecules between them. Tight
junctions are important in maintaining the barrier function of tissues and organs, such as the blood-brain barrier and the intestinal
barrier.
Both gap junctions and tight junctions can be involved in paracrine
signaling, depending on the specific context and the signaling pathways involved. For example, gap junctions may allow cells to exchange signaling molecules directly, while tight junctions may regulate the passage of signaling molecules between different tissues
or compartments.
Autocrine signaling: This occurs when a cell releases a signaling
molecule that can bind to receptors on the same cell, thereby regulating its own behavior.
Autocrine signaling is a type of cell-to-cell communication in which
a cell releases a signaling molecule (called an autocrine factor) that
can bind to receptors on the same cell, thereby regulating its own
behavior.
Autocrine signaling is important in a variety of physiological processes, including growth, differentiation, and immune response. For
example, certain growth factors, such as epidermal growth factor
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(EGF) and insulin-like growth factor (IGF), can stimulate the proliferation and differentiation of cells through autocrine signaling.
Autocrine signaling can also be important in the immune system.
For example, certain immune cells, such as macrophages and T cells,
release autocrine factors that can stimulate their own activation and
proliferation.
Autocrine signaling can be a positive feedback loop, in which the
signaling molecule acts on its own cell to stimulate further production of the molecule. This can lead to amplification of the signal
and a rapid response. However, excessive autocrine signaling can
also have negative effects, such as uncontrolled cell growth and
proliferation, which can lead to the development of cancer.
In contrast to paracrine signaling, which involves the release of signaling molecules that act on nearby cells, and endocrine signaling,
which involves the release of signaling molecules into the bloodstream that act on distant cells or tissues, autocrine signaling is
limited to the cell that releases the signaling molecule. This allows
for highly specific and localized signaling within a single cell.
Endocrine signaling: This occurs when a signaling molecule is
secreted into the bloodstream and travels to distant cells or tissues,
where it can bind to receptors and trigger a response.
Endocrine signaling is a type of cell-to-cell communication in which
a signaling molecule (called a hormone) is secreted into the bloodstream by an endocrine gland and travels to distant cells or tissues,
where it can bind to receptors and trigger a response.
Endocrine signaling is important in a variety of physiological processes, including growth, development, metabolism, and the regulation of the body’s internal environment (homeostasis). For example, hormones such as insulin and glucagon regulate glucose
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metabolism, while hormones such as thyroid hormone and cortisol
regulate the body’s response to stress.
Endocrine glands, such as the pituitary gland, the thyroid gland,
and the adrenal gland, secrete hormones into the bloodstream,
which allows them to travel to target cells or tissues throughout the
body. Hormones can bind to specific receptors on the surface of
target cells or inside the cell, and this binding can trigger a variety of
responses, such as the activation of enzymes, the synthesis of new
molecules, or changes in gene expression.
Endocrine signaling is often slower and more long-lasting than other
types of cell communication, such as neurotransmitter signaling or
paracrine signaling. This is because hormones are released into the
bloodstream and must travel to their target cells or tissues, rather
than acting directly on nearby cells.
Neurotransmitter signaling: This occurs when a neuron releases
a signaling molecule (called a neurotransmitter) that can bind to
receptors on a target cell, such as another neuron or a muscle cell.
Immune system signaling: This occurs when cells of the immune
system, such as T cells and B cells, communicate with one another
and with other cells in order to coordinate the immune response.
Quorum sensing: This occurs when bacteria secrete signaling
molecules into their environment and use them to communicate
with one another and coordinate group behavior.
Each of these types of cell communication plays a critical role in
maintaining the function and homeostasis of an organism.
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5.2.13
Ligands
Ligands are molecules that bind to specific proteins, called receptors,
on the surface of cells or inside the cell. The binding of a ligand
to a receptor can initiate a series of events that leads to a cellular
response. Ligands can be classified into several different types based
on their chemical properties and the types of receptors they bind
to. Here are more examples and details of different types of ligands:
Small hydrophobic molecule ligands: These ligands are lipophilic
and can easily pass through the plasma membrane thanks to
their properties. They bind to intracellular receptors, which can
be found in the cytoplasm or the nucleus of the cell. Examples
include steroid hormones such as testosterone and estrogen,
thyroid hormones, and retinoids like vitamin A.
Small hydrophilic molecule ligands: These ligands are water-soluble
and cannot easily cross the plasma membrane. They bind to
receptors on the surface of the cell, such as G protein-coupled
receptors or ionotropic receptors. Examples include neurotransmitters like dopamine and serotonin, and hormones like adrenaline
and insulin.
Peptide ligands: Peptide ligands are chains of amino acids that are
typically longer than small molecule ligands. They can bind to
receptors on the surface of cells or inside the cell if they are small
and hydrophobic enough. Examples include growth factors,
cytokines, and neuropeptides.
Big Protein ligands: Protein ligands are large, complex molecules
that can bind to receptors on the surface of cells but they are
usually too big and polar to pass the membrane. Examples
include antibodies, transferrin, and certain hormones such as
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follicle-stimulating hormone and luteinizing hormone.
Nucleic acid ligands: Nucleic acid ligands are molecules made up
of nucleotides, such as DNA or RNA. They are very small and
can bind to receptors inside the cell, where they can regulate
gene expression and other cellular processes. Examples include
microRNAs, siRNAs, and antisense oligonucleotides.
Ligands play a crucial role in cell signaling and communication.
They allow cells to respond to signals from their environment and
communicate with other cells. Different ligands can bind to specific
receptors and initiate different cellular responses, depending on the
type of receptor and the signaling pathways it activates. By understanding the roles and properties of different ligands, scientists can
gain insights into how cells respond to and interpret signals from
their environment.
5.2.14
Amplification of The Signal
Cellular signaling cascade amplification is a complex process that
involves multiple steps and components. One important aspect
of this process is the specificity of the ligand-receptor interaction,
which determines the downstream signaling pathway that is activated. Different ligands can bind to different receptors and activate
distinct signaling pathways, leading to different cellular responses.
Once the ligand binds to the receptor, it can activate a variety of
signaling molecules, including kinases, second messengers, and
transcription factors. These signaling molecules can then activate
other downstream signaling molecules, leading to a cascade of
events that amplifies the original signal. This amplification is important for ensuring that the cell responds appropriately to the initial
signal and generates a robust cellular response.
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One example of a signaling cascade is the mitogen-activated protein
kinase (MAPK) pathway. This pathway is activated in response to a
variety of signals, including growth factors and stress. The activation
of the MAPK pathway can lead to the phosphorylation and activation
of multiple downstream signaling molecules, ultimately resulting
in changes in gene expression, cell growth, and differentiation.
Another example of cellular signaling cascade amplification is the
production of second messenger molecules, such as cAMP, in response to the activation of G protein-coupled receptors. These second messengers can activate downstream signaling pathways and
amplify the original signal, leading to a robust cellular response.
Overall, cellular signaling cascade amplification is a critical process
for ensuring that cells respond appropriately to signals from their
environment. By understanding the components and mechanisms
of these signaling cascades, scientists can gain insights into the
complex signaling networks that regulate cellular behavior and
function.
5.2.15
On the IMAT
It is true that the IMAT has never specifically asked about the names
of structures involved in cellular signaling, such as in G proteincoupled receptors (GPCRs). However, this does not mean that understanding these concepts is not essential. Think of it this way: if you
were trying to fix a car, you would not just focus on the symptoms
of the problem. You would also want to understand the underlying
mechanisms causing the issue. The same is valid for understanding
cellular signaling. Knowing the names of the specific structures
involved in GPCRs may not be directly tested on the IMAT, but understanding the overall process of how GPCRs and similar receptors
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work and how they transmit signals within cells is crucial for a comprehensive understanding of cellular signaling and function.
In addition, understanding the mechanisms of cellular signaling can
help you to better understand and interpret the effects of drugs and
other therapies you will study during your medical studies. Many
drugs work by targeting specific receptors or signaling pathways,
and understanding how these pathways work can help you to understand the potential benefits and risks of different treatments.
So while the IMAT may not specifically ask about the names of structures involved in cellular signaling, understanding these concepts
is still important for a comprehensive understanding of how cells
work and respond to signals from their environment.
5.3
Cell Membrane Fluidity
The cell membrane is a complex structure composed of lipids, proteins, and carbohydrates, and it plays a critical role in regulating
the exchange of molecules between the cell and its environment.
The fluidity of the cell membrane, or the ease with which the lipid
molecules in the membrane can move and change position, is an
important factor that affects the function of the cell. The fluidity
of the membrane is not constant and can be influenced by several
factors, including temperature, lipid composition, cholesterol, and
the presence of membrane proteins.
5.3.1 Factors Influencing Cell Membrane Fluidity
Temperature: As mentioned, temperature is one of the most important factors that affects cell membrane fluidity. At low temperatures, the lipid molecules in the membrane are more rigid and
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less fluid, which can affect membrane function. For example, in
winter months, animals that live in cold environments, such as
fish, will often have membranes that contain more unsaturated
fatty acids to maintain proper fluidity in colder temperatures.
In contrast, at high temperatures, the lipid molecules become
more fluid and can move more easily, which can also affect membrane function. For example, some bacteria can modify their
membrane lipid composition in response to high temperatures
to maintain proper fluidity.
Lipid composition: The types and ratios of lipid molecules in the
membrane can also affect its fluidity. Membranes containing
a higher proportion of unsaturated fatty acids tend to be more
fluid than those with a higher proportion of saturated fatty acids.
This is because unsaturated fatty acids have kinks in their tails
due to double bonds, which makes it more difficult for them
to pack tightly together. For example, plants and cold-water
fish have membranes with a higher proportion of unsaturated
fatty acids, which helps to maintain proper fluidity in colder
temperatures.
Cholesterol: Cholesterol is a type of lipid found in cell membranes
that can affect the fluidity of the membrane. At low concentrations, cholesterol can help to increase membrane fluidity by
disrupting the packing of the lipid molecules. For example, in
mammalian cells, cholesterol is often found in membranes at a
concentration of around 30
Membrane proteins: The presence of membrane proteins can
also affect the fluidity of the membrane. These proteins can act
as anchors, holding the lipid molecules in place and decreasing the fluidity of the membrane. For example, some integral
membrane proteins, such as transmembrane receptors, have
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large extracellular domains that can interact with other proteins or molecules, which can restrict the movement of the lipid
molecules near the protein. In contrast, some peripheral membrane proteins, such as those involved in signaling pathways,
can interact with lipid molecules and help to maintain proper
fluidity in the membrane.
Overall, the fluidity of the cell membrane is a dynamic property that
is influenced by many factors, and it is important for proper cellular
function.
To understand the concept of fluidity better imagine a sack of marbles. If the marbles are all the same size and shape, they will be
able to fit closely together, resulting in a more ordered and stable
arrangement. This is similar to a membrane with a high proportion
of saturated fatty acids, which tend to be more rigid and have less
fluidity.
On the other hand, if the marbles are different sizes and shapes,
they will not fit as closely together, resulting in a more disordered
and fluid arrangement. This is similar to a membrane with a high
proportion of unsaturated fatty acids, which tend to be more flexible
and have more fluidity.
5.4
Active and Passive Transport
5.4.1 Passive Transport
Passive transport is the movement of molecules across the membrane without the need for energy. This process occurs due to the
random thermal motion of the molecules, which causes them to
move from areas of high concentration to areas of low concentration
until equilibrium is reached. The types of passive transport include
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diffusion, osmosis, and facilitated diffusion.
Passive transport can be compared to a crowd of people moving
through a large room. If there are many people in one area and few
people in another, the crowd will naturally flow from the crowded
area to the less crowded area until the room reaches equilibrium.
Similarly, in passive transport, molecules move from an area of high
concentration to an area of low concentration until equilibrium is
reached.
Diffusion is the movement of small, nonpolar molecules, such as
oxygen and carbon dioxide, across the membrane from an area
of high concentration to an area of low concentration. Osmosis is
the diffusion of water across a selectively permeable membrane
from an area of low solute concentration to an area of high solute
concentration. Facilitated diffusion involves the movement of larger,
polar molecules, such as glucose, across the membrane with the
help of transport proteins. The transport proteins act as channels or
carriers and allow the molecules to move down their concentration
gradient.
Facilitated diffusion can be compared to a revolving door at a busy
entrance. The door allows people to enter and exit the building,
but only one person can enter or exit at a time. Similarly, in facilitated diffusion, transport proteins act as channels or carriers to allow
molecules to enter or exit the cell, but only one molecule can pass
through the protein at a time.
5.4.2 Active Transport
Active transport, on the other hand, requires energy to move molecules
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abling the cell to take up necessary nutrients, such as glucose, ions,
and amino acids. There are two types of active transport: primary
and secondary active transport.
Primary active transport involves the direct use of energy, usually
in the form of ATP, to move molecules against their concentration
gradient. The sodium-potassium pump is an example of primary
active transport, which uses ATP to pump sodium ions out of the
cell and potassium ions into the cell. This process is essential for
maintaining the resting potential of the cell, as well as for the proper
functioning of nerve and muscle cells.
Secondary active transport uses the energy generated from the
movement of one molecule down its concentration gradient to
transport another molecule against its concentration gradient. An
example of secondary active transport is the transport of glucose
into the cell in the small intestine. This process involves the movement of sodium ions down their concentration gradient, which
generates energy that is then used to transport glucose into the
cell.
In short, primary active transport involves the direct use of energy
to move molecules against a concentration gradient. In contrast,
secondary active transport involves the use of energy generated
from the movement of molecules down a concentration gradient
to transport other molecules against their concentration gradient.
Primary active transport can be compared to a person using a handcranked elevator to move a heavy box to a higher floor. The person
must use energy to turn the crank and lift the box against the force
of gravity. Similarly, in primary active transport, the cell uses ATP to
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Figure 47: The sodium-potassium pump serves as an example of
primary active transport, which transports ions, specifically sodium
and potassium ions, across a membrane against their concentration
gradients. Energy for this process is supplied by ATP hydrolysis.
For every three sodium ions moved out of the cell, two potassium
ions are brought into the cell. This generates an electrochemical
gradient that is essential for the functioning of living cells. (Credit:
openstax.org)
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self up the hill. The pulley system represents the transport protein,
which helps to move you up the hill. However, instead of using
energy directly, you are using the energy generated from the movement of another object, such as a heavy bag of rocks, to power
the transport. The bag of rocks represents the molecules moving
down the concentration gradient (the opposite side of the hill) and
generating the energy needed to move you up the hill.
Figure 48: A primary active transport-generated electrochemical
gradient (with Na+ concentration represented in green) stores energy that is used to move other substances against their concentration gradients (such as glucose, depicted in blue). This process is
known as co-transport or secondary active transport. (Credit: openstax.org)
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In summary, passive transport occurs without the need for energy
and moves molecules down their concentration gradient, while active transport requires energy and moves molecules against their
concentration gradient. Primary active transport uses ATP to move
molecules, while secondary active transport uses the energy generated from the movement of one molecule to transport another
molecule.
5.4.3
The Sodium-Glucose Transporter
The Glucose-N a+ Transporter (also known as the Sodium-Glucose CoTransporter, SGLT) is a protein that is responsible for the transport
of glucose and sodium ions across cell membranes. It is vital in
regulating blood sugar levels and electrolyte balance in the body.
There are two main types of SGLT proteins: SGLT1 and SGLT2. SGLT1
is primarily found in the small intestine and is responsible for absorbing glucose and sodium ions from the gut into the bloodstream.
SGLT2 is located in the kidneys and is responsible for the reabsorption of glucose and sodium ions from the renal tubules back into
the bloodstream.
The SGLT proteins use the energy from the concentration gradient of
sodium ions to transport glucose across the cell membrane. When
the concentration of sodium ions is higher outside the cell than
inside, due to the active pump of sodium, the SGLT protein uses
this gradient to transport glucose into the cell. The movement of
Glucose is supported by the energy of secondary active transport,
as the Sodium ions are actively pumped against their concentration
gradient by the ATPase pumps.
In the small intestine, SGLT1 helps to absorb glucose from the diet
and maintain normal blood sugar levels. In the kidneys, SGLT2 helps
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to reabsorb glucose filtered out of the bloodstream by the renal
tubules. This helps to prevent the loss of glucose in the urine and
maintain normal blood sugar levels.
Defects in the SGLT proteins can lead to problems with blood sugar
regulation. For example, mutations in SGLT1 can cause glucosegalactose malabsorption, a condition in which the body cannot
absorb glucose and galactose from the diet. On the other hand,
inhibiting SGLT2 has been shown to be effective in the treatment of
type 2 diabetes, as it helps to lower blood sugar levels by decreasing
the reabsorption of glucose in the kidneys.
5.4.4 Quick Final Review
Diffusion is the movement of molecules from an area of higher
concentration to an area of lower concentration until the concentration is equal on both sides of the membrane. This process occurs
naturally and does not require energy.
Osmosis is the movement of water molecules across a membrane
from an area of high water concentration (high water potential)
to an area of low water concentration (low water potential). This
process also occurs naturally and does not require energy.
Facilitated diffusion is the movement of molecules across a membrane with the help of transport proteins. Like passive transport, facilitated diffusion occurs down a concentration gradient and does not
require energy. However, the transport proteins help the molecules
cross the membrane more efficiently, allowing them to move against
a concentration gradient that would otherwise be too steep for them
to cross on their own.
Active transport requires energy and moves molecules against a
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concentration gradient, while passive transport does not require
energy and moves molecules down a concentration gradient. Both
processes play important roles in maintaining homeostasis within
cells and are essential for the proper functioning of all living organisms.
5.4.5 Bulk Transport
Bulk transport is the process by which cells move large molecules
or particles, such as proteins and organelles, into or out of the cell.
There are four main types of bulk transport: Endocytosis, Phagocytosis, Pinocytosis, and Exocytosis.
Endocytosis is the process by which cells take in molecules or particles by enclosing them in a vesicle or small membrane-bound sac.
There are three types of endocytosis: phagocytosis, pinocytosis, and
receptor-mediated endocytosis.
Phagocytosis is the process by which cells engulf and internalize
solid particles, such as bacteria or dead cells. This process is often
carried out by specialized cells called phagocytes, which are found
in the immune system. Phagocytosis occurs when the cell extends
a portion of its plasma membrane around the particle, forming a
phagosome, which then fuses with a lysosome to form a phagolysosome. Remember that the lysosome contains hydrolytic enzymes
that break down the particle into smaller molecules, which can later
be absorbed by the cell.
Pinocytosis is the process by which cells take in small amounts of
extracellular fluid, along with any dissolved molecules or particles.
This process is often referred to as ”cell drinking” and occurs when
the cell extends small invaginations, or infoldings, in the plasma
membrane, forming a pinocytotic vesicle.
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Receptor-mediated endocytosis is the process by which cells take
in specific molecules or particles by binding them to receptors on
the cell surface. The receptors and their associated molecules or
particles are then internalized into the cell in a vesicle.
Exocytosis is the process by which cells secrete molecules or particles by expelling them from the cell in a vesicle. This process occurs
when a vesicle containing the molecules or particles fuses with the
plasma membrane and releases its contents to the outside of the
cell. Exocytosis is important for the secretion of hormones, enzymes,
and other signaling molecules, as well as for the removal of waste
products from the cell.
In summary, bulk transport involves the movement of large molecules
or particles into or out of the cell. Endocytosis is the process by which
cells take in molecules or particles, while exocytosis is the process by
which cells secrete molecules or particles. Phagocytosis, pinocytosis,
and receptor-mediated endocytosis are three types of endocytosis
that allow cells to internalize specific molecules or particles.
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6
Energy and Metabolism
Metabolism is the sum of all the chemical reactions that take place
in an organism, from the breakdown of nutrients to the synthesis
of macromolecules. These reactions are essential for life and provide the energy and building blocks needed for growth, repair, and
maintenance of all living systems.
Metabolism can be divided into two categories: catabolism and
anabolism. Catabolism involves the breakdown of molecules to
release energy, which is then used to power cellular processes. This
includes the breakdown of carbohydrates, fats, and proteins into
simpler molecules such as glucose, fatty acids, and amino acids.
The energy released during catabolism is stored in the form of ATP
(adenosine triphosphate), the main energy currency of the cell.
Anabolism, on the other hand, involves the synthesis of larger molecules
from smaller ones, using energy in the form of ATP. This includes
the synthesis of macromolecules such as proteins, nucleic acids,
and polysaccharides, as well as the storage of energy in the form of
glycogen and fats.
Together, catabolism and anabolism work to maintain the delicate
balance of energy and nutrients in the body. This balance is essential for the proper functioning of all physiological processes and is
regulated by a complex network of enzymes and hormones.
One of the key molecules involved in metabolism is ATP, or adenosine triphosphate. ATP is a nucleotide that serves as the main energy
source for cells. It is produced through the breakdown of glucose
and other molecules in a process called cellular respiration, which
occurs in the mitochondria of cells.
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ATP works like a ”currency” of energy within cells. When a cell needs
the energy to perform a particular function, it can ”spend” ATP by
breaking it down into ADP (adenosine diphosphate) and releasing
the energy stored in the molecule. This energy can then be used to
power various processes, such as muscle contractions, synthesis of
molecules, and transport of ions across membranes. ADP can also
be broken down into AMP, Adenosine mono-phosphate, but this
process is less common and out was never asked on the IMAT.
To clarify the analogy, you can think of ATP as a battery that stores
energy. When you need to use your phone, you ”spend” the energy
stored in the battery by plugging it into the phone. Similarly, when a
cell needs energy, it ”spends” ATP by breaking it down and releasing
the stored energy.
When you get back home, you can recharge your battery by connecting it to the AC socket, which is equivalent to the process of
cellular respiration, which allows us to take an uncharged battery
(ADP or AMP) and fully charge it again (ATP) for the next time you
need it.
Metabolism is the set of chemical reactions that maintain life in an
organism. ATP is a key molecule involved in metabolism, serving as
the main energy source for cells. It can be ”spent” to power various
processes in the cell.
AT P + H2 O → ADP + Pi + free energy
The equation above demonstrates the hydrolysis of one charged
phosphate group from an ATP molecule. This process is very exothermic, and releases plenty of free energy.
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6.1
Coupling Reactions
Energy coupling is a critical process in biological systems that enables the transfer of energy from one reaction to another. This
process allows cells to perform vital functions, such as synthesizing
complex molecules, moving substances across membranes, and
contracting muscles.
The process of energy coupling involves the use of high-energy
molecules, such as ATP, to provide the energy needed to drive other
cellular processes. ATP is an essential molecule in cells that serves as
the primary source of energy for cellular reactions. It is composed of
a nitrogenous base (adenine), a sugar molecule (ribose), and three
phosphate groups.
The third phosphate group of ATP is connected to the molecule by
a high-energy phosphoanhydride bond, which stores a considerable amount of energy. When this bond is broken by hydrolysis, the
molecule is converted into ADP (adenosine diphosphate), and a significant amount of energy is released. This energy can be harnessed
by the cell to drive other reactions that would otherwise require an
input of energy.
An example of energy coupling is the synthesis of proteins, which
requires the input of energy to form peptide bonds between amino
acids. This energy is provided by the hydrolysis of ATP, which releases
energy that can be used to drive the formation of peptide bonds.
Another example is the transport of ions across cell membranes,
which requires the use of energy to move the ions against their
concentration gradient. This energy is provided by the hydrolysis
of ATP, which powers the ion pumps that move the ions across the
membrane.
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In summary, energy coupling is a critical process in biology that
allows cells to perform work by transferring energy from one reaction to another. ATP serves as a crucial molecule in this process by
providing the energy needed to drive other cellular reactions.
6.2
Catabolism and Anabolism
Catabolism and anabolism are two essential processes that take
place within cells as part of metabolism. Metabolism refers to the
sum of all chemical reactions occurring in an organism that are necessary for life. These reactions either break down complex molecules
into simpler ones (catabolism) or synthesize complex molecules
from simpler ones (anabolism).
Catabolism is the process of breaking down large molecules into
smaller ones, usually to release energy. In catabolism, energy is
often stored in molecules such as adenosine triphosphate (ATP),
which serves as the primary energy currency for cells. Examples of
catabolic processes include cellular respiration and digestion. In
cellular respiration, glucose, a simple sugar, is broken down in a
series of steps to produce ATP, carbon dioxide, and water. During
digestion, complex food molecules are broken down into simpler
nutrients that can be absorbed and used by the body.
Anabolism, on the other hand, is the process of synthesizing complex
molecules from simpler ones, often requiring energy input. Anabolic
processes build and repair the body’s structures, such as tissues and
organs, and are essential for growth and development. Examples of
anabolic processes include protein synthesis, in which amino acids
are linked together to form proteins, and the synthesis of nucleic
acids, such as DNA and RNA, from nucleotides. In both cases, the
energy required for these processes often comes from ATP produced
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during catabolic reactions.
In summary, catabolism and anabolism are two key processes in
metabolism that work together to maintain a balance between
the breakdown and synthesis of molecules within cells. Catabolic
processes release energy by breaking down complex molecules into
simpler ones, while anabolic processes use energy to build complex
molecules from simpler ones. Understanding these processes helps
provide insight into the functioning of living organisms and how
they maintain the energy necessary for life.
Figure 49: Anabolic pathways involve the use of energy to construct
larger molecules, while catabolic pathways release energy by breaking down larger molecules into smaller ones. Both types of pathways
play a crucial role in maintaining the energy equilibrium within cells.
(Credit: openstax.org)
6.3 Energy
Energy is a fundamental concept in science that describes the ability
of things to do work and make changes happen. It is what powers
our bodies, fuels our cars, and generates electricity. But energy
comes in different forms and is constantly changing.
One type of energy is potential energy, which is stored energy that
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is waiting to be used. This can be compared to a ball at the top of a
hill, which has the potential to roll down and push something.
Another type of energy is kinetic energy, which is energy in motion.
This can be compared to a ball rolling down the hill, which has the
energy to push something because it is moving.
Free energy is a type of energy that is available to do work. This can
be compared to a battery, which has stored energy that can be used
to power a device like a flashlight. In biology, the free energy that is
released when ATP is hydrolyzed can be used to drive reactions and
perform work in the cell.
Activation energy is the energy that is needed to start a reaction.
This can be compared to starting a fire, where a match is needed to
provide the activation energy to initiate the combustion reaction. In
biology, enzymes lower the activation energy needed for reactions
to occur, allowing them to happen more efficiently.
In summary, energy is a complex and fascinating concept that drives
the world around us. Understanding the different types of energy
and how they interact is essential to understanding the processes
that occur in our bodies and in the world we live in.
In the next chapter, we will cover all of the different types of energy
that are important to know for the IMAT exam.
6.4 The Laws of Thermodynamics
The laws of thermodynamics are a set of rules that tell us about
energy and how it can be changed. There are multiple rules, but
these are the most important to remember:
The laws of thermodynamics provide a fundamental understanding
of the behavior of energy and how it can be transformed. The first law
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of thermodynamics is also known as the law of energy conservation,
which tells us that energy cannot be created or destroyed, only
transformed from one form to another. This means that the total
amount of energy in the universe remains constant.
An analogy for the first law is a bank account. You can’t create or
destroy money, but you can move it from one account to another
or change its form, such as converting cash into digital currency.
Similarly, energy can be transformed from one form to another, such
as chemical energy being converted to heat or mechanical energy.
The second law of thermodynamics is a fundamental principle in
physics that describes the behavior of energy and entropy in a system. The second law states that in any physical process, the combined entropy (a measure of the disorder or randomness of a system)
of the system and the environment must increase if the process is
irreversible. An analogy for the second law is a messy room. Without
any outside intervention, a room will become more disordered over
time. To clean up the room and restore order, energy must be added
in the form of physical work.
In other words, the final entropy of the system must be greater than
the initial entropy for an irreversible process to occur. This can be
represented by the equation Sf > Si, where Sf is the final entropy and
Si is the initial entropy.
An irreversible process is one that cannot be reversed, such as the
mixing of two gases or the transfer of heat from a hotter body to
a colder body. In these processes, the combined entropy of the
system and the environment increases, because the total amount
of disorder or randomness in the system has increased.
On the other hand, a reversible process is one that can be reversed
without any loss of energy or increase in entropy. For example, a
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gas that is allowed to expand and contract reversibly in a cylinder
with a movable piston can be returned to its original state without
any increase in entropy. In this case, the final entropy of the system
would be equal to the initial entropy, or Sf = Si.
The third law of thermodynamics deals with the behavior of matter
at extremely low temperatures. The law states that as the temperature of a system approaches absolute zero, the entropy of the
system approaches a minimum value. This law helps to explain why
certain materials, such as metals, can become superconducting at
extremely low temperatures, which is useful for various technologies
such as MRI machines.
These laws help us understand how energy works and how it can be
used in different ways. They also help us understand why we can’t
have unlimited energy, and why we always have to be careful about
how we use it.
6.4.1
Entropy
Entropy is a fundamental concept in thermodynamics that is crucial
to understanding the behavior of energy and matter in physical
systems. To put it simply, entropy is a measure of the disorder or
randomness of a system. The greater the disorder or randomness of
the molecules in a system, the higher its entropy.
An analogy to help understand entropy is to think of a room that
is messy and disorganized. The more cluttered and disordered the
room is, the higher its entropy. Conversely, a well-organized and
neat room has a lower entropy. This analogy can be extended to any
physical system, where a higher entropy corresponds to a greater
degree of disorder or randomness.
In thermodynamics, entropy is a measure of the amount of thermal
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energy in a system that is unavailable to do useful work. This is
because work is obtained from ordered molecular motion, so a more
disordered or random system has less ordered motion and thus less
available energy to do useful work. The unit of measurement for
entropy is joules per Kelvin, which represents the amount of thermal
energy per unit temperature that is unavailable to do work.
Entropy plays a crucial role in many physical processes, such as the
transfer of heat and the flow of energy in chemical reactions. In
general, any process that results in an increase in the disorder or
randomness of a system will result in an increase in entropy. This is
known as the second law of thermodynamics, which states that the
total entropy of an isolated system always increases over time.
Understanding entropy is essential for understanding many important phenomena in various fields such as science and engineering,
from the behavior of matter at extremely low temperatures to the
efficiency of engines and power plants.
6.4.2 Enthalpy
Enthalpy is a measure of the energy in a chemical system. It is the
total heat energy measured when the pressure is constant.
Enthalpy is a state function whose change indicates the amount of
heat transferred from a system to its surroundings or vice versa, at
constant pressure. The change in the internal energy of a system is
the sum of the heat transferred and the work done.
Heat energy is the result of the movement of tiny particles called
atoms, molecules or ions in solids, liquids and gases. Heat energy
can be transferred from one object to another. The transfer or flow
due to the difference in temperature between the two objects is
called heat
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Figure 50: Entropy refers to the degree of disorder or randomness
within a system. In general, gases exhibit greater entropy compared
to liquids, while liquids demonstrate higher entropy relative to solids.
(Credit: openstax.org)
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So, in simpler words, ”Heat energy” is the energy that is produced
when two things with different temperatures are brought together.
For example, if you touch a hot pan, you will feel heat because the
heat energy is transferred from the pan to your hand.
Chemical energy is the potential energy that is stored in the bonds
between atoms in a chemical compound. For example, gasoline has
a lot of chemical energy because the atoms in gasoline are bonded
together very strongly. When gasoline is burned, chemical energy
is released as heat energy.
Enthalpy is essential because it helps us understand how energy
is transferred and used in chemical reactions. By measuring the
change in enthalpy of a reaction, we can predict how much energy
will be released or absorbed during the reaction. This helps us understand how different chemicals will react with each other and
how much energy we can get from them.
6.4.3 Activation Energy and Enzymes
Activation energy is the minimum amount of energy that is required
for a chemical reaction to occur. It is like a barrier that must be
overcome before a reaction can proceed.
An analogy to help understand activation energy is to think of a
ball rolling up a hill. The ball represents the reactants in a chemical
reaction, and the hill represents the activation energy. In order for
the ball to reach the top of the hill, it must first be pushed with
enough force to overcome the resistance of the hill. Similarly, in
a chemical reaction, the reactants must be supplied with enough
energy to overcome the activation energy before they can transform
into products.
Activation energy is important because it determines the rate at
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which a chemical reaction will occur. A reaction with a high activation energy will proceed more slowly than a reaction with a lower
activation energy. Enzymes, which are specialized proteins that
catalyze chemical reactions in the body, lower the activation energy
required for a reaction to occur. This allows the reaction to occur
more quickly and efficiently, making it possible for life-sustaining
processes to occur in our bodies.
In summary, activation energy is an essential concept in chemistry
because it determines the rate at which reactions occur. Enzymes
play an important role in lowering activation energy, making chemical reactions in our bodies more efficient and allowing life-sustaining
processes to occur.
There are several factors that can affect the activation energy of a
chemical reaction, including:
Temperature: Increasing the temperature of the reactants will generally increase the rate of the reaction because it increases the average
kinetic energy of the reactants, making it more likely that they will
have the required activation energy. Imagine two people playing
catch with a ball. If they stand close together and throw the ball
slowly, the chance of a successful catch is low. However, if they stand
further apart and throw the ball faster, the chance of a successful
catch is higher. Similarly, if the reactant molecules are moving faster
due to higher temperature, the chance of a successful collision and
reaction is higher.
Catalysts: Catalysts are substances that can reduce the activation
energy of a chemical reaction by providing an alternative pathway
with a lower activation energy. This allows the reaction to occur
more quickly and easily. Catalysts, like enzymes in our bodies, are
like a shortcut or a slide in a playground, just like how a slide helps a
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child to move down faster, a catalyst can help reactant molecules
to overcome the activation energy barrier faster.
Surface area: Increasing the surface area of the reactants can also
increase the rate of the reaction because it increases the number
of reactant molecules that are exposed to each other, increasing
the chance of successful collisions. Imagine trying to dissolve sugar
cubes in water. If the sugar cubes are whole, it will take longer to
dissolve. However, if the sugar cubes are crushed into smaller pieces,
the surface area increases, and the rate of dissolving will be faster.
Concentration: Increasing the concentration of the reactants will
generally increase the rate of the reaction because it increases
the number of reactant molecules that are present, increasing the
chance of successful collisions.
Understanding the concept of activation energy is important in
predicting and controlling the rate of biochemical reactions.
6.5
Enzymes
Enzymes are special proteins that help chemical reactions happen
faster. They do this by lowering the activation energy needed for
the reaction to start.
Imagine you are trying to climb a tall mountain. The mountain is the
chemical reaction and you are the enzyme. The activation energy is
the energy it takes for you to start climbing the mountain.
If the mountain is very steep and rocky, it will be hard for you to start
climbing because it will take a lot of energy. But if the mountain
has a gentle slope and smooth paths, it will be easier for you to start
climbing because it won’t take as much energy.
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ing it much easier for you to start climbing, while also allowing you
to climb faster.
Figure 51: Generalized Prokaryotic Structure. This diagram illustrates
the fundamental components and organization of prokaryotic cells,
including the plasma membrane, cytoplasm, nucleoid, ribosomes,
and cell wall. (Credit: openstax.org)
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6.5.1
Types of Enzymes
As mentioned earlier, enzymes are proteins that catalyze chemical reactions in the cell. They play a critical role in many cellular
processes, including metabolism, DNA replication and repair, and
signaling pathways. There are many different types of enzymes,
each with its own specific function. Here are some examples:
Metabolic enzymes: These catalysts are involved in the biochemical reactions that occur within cells to produce energy, synthesize
new molecules, and break down waste products. Examples include
enzymes that break down sugars for energy, synthesize lipids, and
detoxify harmful substances.
Digestive enzymes: These enzymes are produced by the digestive system to break down food into small molecules that can be
absorbed and used by the body. Examples include amylases that
break down carbohydrates, proteases that break down proteins, and
lipases that break down fats.
DNA-processing enzymes: These enzymes are involved in the synthesis and repair of DNA. Examples include polymerases that synthesize new DNA strands, helicases that unwind DNA strands, and
ligases that seal breaks in the DNA helix.
Signal transduction enzymes: These enzymes are involved in the
signaling pathways that transmit messages within and between
cells. Examples include kinases that transfer phosphate groups to
proteins and phosphatases that remove phosphate groups from
proteins.
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6.5.2 Main Categories of Enzymes
Enzymes are classified into six main categories based on the type of
chemical reaction they catalyze. These categories are oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.
Oxidoreductases: These enzymes catalyze oxidation-reduction reactions, where one molecule is oxidized while another is reduced.
One example of an oxidoreductase is lactate dehydrogenase, which
catalyzes the conversion of lactate to pyruvate during the process
of cellular respiration. An analogy for an oxidoreductase is a seesaw,
where one end goes up as the other goes down.
Transferases: These enzymes catalyze the transfer of a functional
group, such as a phosphate or methyl group, from one molecule to
another. An example of a transferase is hexokinase, which transfers a
phosphate group from ATP to glucose during glycolysis. An analogy
for a transferase is a conveyor belt, where items are passed from one
station to another.
Hydrolases: These enzymes catalyze the hydrolysis of a chemical
bond using water. One example of a hydrolase is lactase, which
breaks down lactose into glucose and galactose. An analogy for a
hydrolase is a pair of scissors, cutting a piece of paper into smaller
pieces.
Lyases: These enzymes catalyze the cleavage of a chemical bond in
a molecule without the addition of water. An example of a lyase is
carbonic anhydrase, which converts carbon dioxide into bicarbonate
and hydrogen ions. An analogy for a lyase is a pair of scissors cutting
a piece of fabric, without any water or other substance involved.
Isomerases: These enzymes catalyze the rearrangement of atoms
within a molecule, converting one isomer into another. One example
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of an isomerase is triose phosphate isomerase, which catalyzes the
interconversion of dihydroxyacetone phosphate and glyceraldehyde3-phosphate during glycolysis. An analogy for an isomerase is a
puzzle piece being rotated or flipped to fit in a different position.
Ligases: These enzymes catalyze the joining of two molecules together, often coupled with the hydrolysis of ATP. One example of
a ligase is DNA ligase, which joins together the Okazaki fragments
during DNA replication. An analogy for a ligase is a glue that binds
two pieces of paper together.
Understanding the different types of enzymes and their functions
is essential in understanding the biochemical reactions that occur
within cells and how they are regulated.
6.5.3 Enzyme Structure
The structure of an enzyme is complex and is essential for its function. Enzymes are made up of one or more polypeptide chains,
which are long chains of amino acids. These chains are folded into
a specific three-dimensional shape, called the enzyme’s active site.
The active site is the region of the enzyme where the chemical
reaction takes place.
The active site is where the substrate, which is the reactant that
the enzyme acts upon, binds to the enzyme. The substrate fits into
the active site like a key fits into a lock. This specific fit between the
substrate and the active site is called the enzyme-substrate complex.
There are also other binding sites on an enzyme that play important
roles in its function. One such site is the allosteric site, which is a
regulatory site on the enzyme that can bind to a molecule called an
allosteric effector. The allosteric effector can either inhibit or activate
the enzyme, depending on the type of effector and the enzyme. The
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allosteric site can be compared to a light switch that can turn a
light on or off. For example, when a cell needs a certain product, an
allosteric activator can bind to the enzyme and increase its activity.
On the other hand, an allosteric inhibitor can bind to the enzyme
and decrease its activity when a product is in excess.
Another important binding site on an enzyme is the cofactor binding
site. A cofactor is a non-protein molecule that is required for the enzyme to function properly. Cofactors can be metal ions, such as zinc
or copper, or organic molecules, such as flavin adenine dinucleotide
(FAD) or nicotinamide adenine dinucleotide (NAD). Cofactors are
essential to enzyme function and can be compared to a chef’s knife.
Just like how a chef needs a knife to chop ingredients properly, an enzyme needs a cofactor to function properly. A metal ion cofactor can
act as a catalyst, stabilizing an intermediate in the reaction, while
an organic molecule cofactor can assist in the transfer of electrons.
In summary, the structure of an enzyme is complex and includes the
active site, where the substrate binds, as well as other binding sites,
such as the allosteric site and the cofactor binding site, which play
important roles in the enzyme’s function. Understanding the structure of enzymes and how they work is essential for understanding
many biological processes and can also have practical applications
in medicine and industry.
6.5.4 Active Site of Enzymes
Enzyme active sites and substrate specificity are fundamental concepts in understanding how enzymes function to catalyze biochemical reactions. These concepts play a significant role in many cellular
processes, and it’s important to grasp them thoroughly.
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cal reactions without being consumed or permanently altered. They
achieve this by lowering the activation energy required for a reaction to take place. Enzymes are proteins with a unique threedimensional structure that allows them to interact specifically with
their target molecules, called substrates.
The active site is a specific region on the enzyme’s surface where
the substrate binds. It is formed by a group of amino acids that
create a pocket or groove, which is complementary in shape and
chemical properties to the substrate. This is what gives enzymes
their remarkable specificity, allowing them to recognize and bind
only specific substrates.
Substrate specificity refers to the enzyme’s ability to selectively recognize and bind to a particular substrate. This is often described
using the lock-and-key analogy: the enzyme (the lock) has a specific shape that allows only the correct substrate (the key) to fit into
the active site. This high level of specificity ensures that enzymes
catalyze only the intended reactions, maintaining order and control
in cellular processes.
There are many examples of enzymes with high substrate specificity.
One example is the enzyme hexokinase, which is responsible for
the first step in glycolysis, the process by which cells break down
glucose to produce energy. Hexokinase specifically catalyzes the
transfer of a phosphate group from ATP (adenosine triphosphate)
to glucose, forming glucose-6-phosphate. The enzyme’s active site
has a precise shape that only accommodates glucose, ensuring that
hexokinase does not mistakenly transfer the phosphate group to
other molecules.
Another example is the enzyme trypsin, which plays a role in digestion by breaking down proteins into smaller fragments. Trypsin’s
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active site is highly specific for peptide bonds that follow positively
charged amino acids, such as lysine or arginine. This specificity enables trypsin to cleave proteins at precise locations, facilitating their
further digestion and absorption.
In summary, enzyme active sites and substrate specificity are key
factors that govern how enzymes interact with their substrates to
catalyze chemical reactions. The unique three-dimensional structure of the active site allows enzymes to recognize and bind to
specific substrates, ensuring that only the intended reactions occur
within cells. Understanding these concepts is crucial for students
studying biochemistry and molecular biology and preparing for the
IMAT.
Figure 52: As per the induced-fit model, enzymes and substrates undergo conformational changes in a dynamic manner upon binding.
The enzyme alters the shape of the substrate into its transition state,
leading to an increase in the reaction rate. (Credit: openstax.org)
6.5.5 Types of Inhibitors
In addition to different enzymes, there are also many different types
of enzyme inhibitors, which are molecules that interfere with the
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activity of enzymes. Enzyme inhibitors can be classified into several
categories:
Figure 53: Competitive and noncompetitive inhibition have distinct
impacts on the rate of a reaction. Competitive inhibitors alter the
initial rate of the reaction but do not affect the maximal rate. In
contrast, noncompetitive inhibitors affect the maximal rate of the
reaction. (Credit: openstax.org)
Competitive inhibitors: A competitive inhibitor is a type of inhibitor
that competes with the substrate for binding to the active site of
an enzyme. This means that the inhibitor and the substrate have a
similar shape and can both bind to the enzyme, but only one can
be converted into a product.
In the case of a competitive inhibitor, the inhibitor binds to the active
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site of the enzyme and blocks the substrate from binding and being
converted into a product. As a result, the rate of the reaction is
decreased and the amount of product produced is also decreased.
If we plot the reaction rate versus the concentration of the substrate,
we can see the effect of a competitive inhibitor on the reaction. Initially, as the concentration of the substrate increases, the reaction
rate also increases, following the curve of the enzyme’s normal catalytic activity. However, as the concentration of the competitive
inhibitor increases, it begins to compete with the substrate for binding to the active site of the enzyme. This causes the reaction rate to
decrease and the curve to flatten out.
Eventually, the reaction reaches a point where the concentration of
the competitive inhibitor is so high that it effectively blocks all substrate from binding to the enzyme and the reaction rate becomes
constant. However, it takes longer for the reaction to reach this point
because of the competitive inhibitor. The total amount of product
produced is the same as it would be without the inhibitor, but it
takes longer to produce because the reaction rate is slowed down.
It is important to note that competitive inhibitors can be overcome
by increasing the concentration of the substrate. Since the inhibitor
is only competing with the substrate for binding to the active site,
increasing the concentration of the substrate allows more substrate
to bind and be converted into product, overcoming the inhibitory
effect of the competitive inhibitor.
Noncompetitive inhibitors: Noncompetitive inhibitors are a type
of inhibitor that bind to a site on the enzyme other than the active
site, and do not directly compete with the substrate for binding.
Noncompetitive inhibitors can still decrease the overall rate of the
reaction, but they do so by altering the shape of the enzyme, rather
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than blocking the substrate from binding.
If we plot the reaction rate versus the concentration of the substrate
in the presence of a noncompetitive inhibitor, we will see a different
pattern compared to a competitive inhibitor. Initially, the reaction
rate will still increase with increasing substrate concentration, following the curve of the enzyme’s normal catalytic activity. However,
as the concentration of the noncompetitive inhibitor increases, it
begins to bind to a site on the enzyme that is not the active site. This
changes the shape of the enzyme and reduces its overall activity. As
a result, the reaction rate begins to decrease, even at high substrate
concentrations.
Unlike with a competitive inhibitor, increasing the concentration of
the substrate will not overcome the inhibitory effect of a noncompetitive inhibitor. This is because the noncompetitive inhibitor is not
directly competing with the substrate for binding to the active site,
and increasing the concentration of the substrate will not change
the shape of the enzyme and remove the inhibitory effect.
In the case of a noncompetitive inhibitor, the maximum reaction rate
(Vmax) will be reduced, but the substrate concentration at which
the maximum reaction rate is reached (Km) will remain unchanged.
This means that while the overall activity of the enzyme is reduced,
the affinity of the enzyme for the substrate is not affected.
As a result, the curve of the reaction rate versus substrate concentration will be shifted to the right, but it will also be flattened, with
a lower maximum reaction rate. This is because the noncompetitive inhibitor reduces the overall activity of the enzyme, causing the
reaction rate to be slowed down at all substrate concentrations.
Uncompetitive inhibitors: An uncompetitive inhibitor is a type
of enzyme inhibitor that specifically targets the enzyme-substrate
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complex, rather than the enzyme alone. This means that the inhibitor only binds to the enzyme-substrate complex and not to the
free enzyme or the free substrate.
Once the enzyme-substrate complex is formed, the uncompetitive
inhibitor binds to a site on the complex, causing a conformational
change that prevents the conversion of the substrate into product. This reduction in the overall activity of the enzyme leads to a
decrease in the reaction rate.
In the case of an uncompetitive inhibitor, the curve of the reaction
rate versus substrate concentration is shifted to the right, similar
to a noncompetitive inhibitor. However, unlike a noncompetitive
inhibitor, the maximum reaction rate (Vmax) and the substrate concentration at which the maximum reaction rate is reached (Km) are
both reduced. This means that the overall activity of the enzyme is
decreased, as well as its affinity for the substrate.
An analogy for an uncompetitive inhibitor is a lock and key. The
enzyme is like a lock and the substrate is like a key that fits perfectly
into the lock’s active site. The uncompetitive inhibitor is like a wedge
that gets stuck between the key and the lock, preventing the lock
from turning and opening. The wedge only fits between the key
and the lock when they are together, so it specifically targets the
lock-key complex and not the lock or the key alone.
Understanding the different types of enzyme inhibitors and their
effects on the enzyme’s activity and substrate affinity is important
in many areas of biology, including drug development and understanding disease mechanisms.
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6.6 Why ATP?
ATP, or adenosine triphosphate, is an essential molecule that serves
as the energy currency of the cell. It plays a crucial role in powering
the various processes and systems within the cell, much like how
gasoline is essential for a car to run.
There are several properties that make ATP well-suited for this role.
First, ATP is a small molecule that is easily transported within cells,
allowing it to be used wherever it is needed. Imagine ATP as a small
container of gasoline that can be easily carried around and used to
fuel different parts of the cell.
Second, ATP is highly reactive, meaning that it can readily release
its energy when needed. This is similar to how gasoline is ignited
to power an engine. ATP’s reactivity allows it to release its energy
quickly and efficiently, making it a suitable source of energy for rapid
processes within the cell.
Finally, ATP can be synthesized and broken down quickly, allowing
the cell to quickly replenish its supply of ATP as needed. This is
similar to how a car’s engine can quickly consume and replenish its
supply of gasoline.
The energy stored in ATP is released through a process called hydrolysis, which involves the addition of a water molecule to the
ATP molecule. This results in the breaking of one of the phosphate
bonds in the ATP molecule, releasing energy in the process. This
energy can then be used to power various cellular processes, such
as the contraction of muscles, the synthesis of proteins, and the
transportation of substances within the cell.
Overall, ATP is an essential molecule that plays a crucial role in the
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ers all of the cell’s various systems and processes, much like how
gasoline is essential for a car to run. Without ATP, the cell would be
unable to perform its various functions, and life as we know it would
not be possible.
Figure 54: ATP serves as the primary energy currency of the cell,
characterized by an adenosine backbone linked with three phosphate groups. (Credit: openstax.org)
6.7
Cellular Respiration
Cellular respiration is the process by which cells convert the energy
stored in organic molecules, such as glucose, into a usable form,
such as ATP. This process occurs in the mitochondria of cells and
involves the breakdown of glucose through a series of chemical
reactions known as the citric acid cycle, or the Krebs cycle.
There are two types of cellular respiration: anaerobic respiration and
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aerobic respiration. Anaerobic respiration occurs in the absence of
oxygen and involves the breakdown of glucose to produce ATP and
lactic acid or ethanol. This type of respiration is less efficient than
aerobic respiration, as it produces fewer ATP molecules per glucose
molecule.
Aerobic respiration, on the other hand, occurs in the presence of
oxygen and involves the breakdown of glucose to produce ATP and
water. This type of respiration is more efficient than anaerobic respiration, as it produces more ATP molecules per glucose molecule.
Aerobic respiration is the most common type of respiration in cells
and is essential for the survival of most organisms.
6.8
Energy in Biology
6.8.1 Redox Reactions
Redox reactions, short for reduction-oxidation reactions, are chemical processes in which the oxidation state of atoms or molecules
changes. These reactions play a vital role in numerous biological processes and are critical to understanding energy production,
metabolism, and cellular respiration. For high school students preparing for the IMAT, grasping the concept of redox reactions and their
importance in biology is essential.
In redox reactions, there is a transfer of electrons between two
species. One species loses electrons (oxidation), and the other
species gains electrons (reduction). These processes always occur
together, as the electrons lost by one species must be gained by
another. The substance that loses electrons is called the reducing agent, while the substance that gains electrons is called the
oxidizing agent.
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To remember the process, you can use the mnemonic OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
Redox reactions are crucial in biological systems, particularly in energy production and metabolism. One of the most well-known
examples is cellular respiration, where glucose is oxidized to release
energy in the form of ATP (adenosine triphosphate). During this
process, oxygen acts as the final electron acceptor and is reduced
to water.
Another important example is photosynthesis, where plants convert
sunlight into chemical energy. In this process, water is oxidized,
releasing electrons and forming oxygen gas as a byproduct. The
electrons are then used to reduce carbon dioxide to glucose, which
the plant uses for energy and growth.
Redox reactions also play a role in detoxification within cells. For
instance, the enzyme catalase converts toxic hydrogen peroxide
(H2O2), a byproduct of cellular metabolism, into water and oxygen.
This reaction involves a redox process where the hydrogen peroxide
is reduced to water, while oxygen is formed as the oxidation product.
To further understand the importance of redox reactions in biology,
consider the role of antioxidants. Antioxidants are molecules that
protect cells from damage caused by free radicals, which are unstable and highly reactive species that can cause oxidative stress.
Antioxidants work by undergoing redox reactions with free radicals,
neutralizing them and preventing them from damaging cellular
components such as DNA, proteins, and lipids.
In summary, redox reactions are chemical processes involving the
transfer of electrons between species, with one undergoing oxidation and the other undergoing reduction. These reactions are fundamental to many biological processes, including energy production,
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metabolism, and detoxification. Understanding redox reactions and
their importance in biology is crucial in understanding subsequent
concepts.
Figure 55: During the process of oxidizing and reducing a single
carbon, electrons are removed from the carbon atom as methane
is converted to carbon dioxide. This removal of electrons results
in the loss of energy. On the other hand, during the reduction of
carbon dioxide to methane, electrons are gained by the carbon
atom. This gain of electrons is usually accompanied by an increase
in potential energy and sometimes the addition of a proton (H+).
(Credit: openstax.org)
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6.8.2 Electron Carriers
NADH and FADH2 are electron carriers that play important roles
in the production of energy in cells. They are used to transfer electrons from one molecule to another during the process of cellular
respiration, which is the process by which cells convert the chemical
energy stored in glucose and other organic molecules into a form
that can be used to power the various functions of the cell.
Both NADH and FADH2 are used to transfer electrons from the
breakdown of glucose to the electron transport chain, which is a
series of protein complexes located in the mitochondria of cells. The
electron transport chain is responsible for generating most of the
energy used by cells in the form of ATP, which is the main energy
currency of cells.
NADH and FADH2 carry potential energy because they are able to
transfer their electrons to the electron transport chain, which generates ATP through a process called chemiosmosis. During chemiosmosis, the transfer of electrons through the electron transport chain
pumps protons across the inner membrane of the mitochondria,
creating a proton gradient. The energy stored in this proton gradient
is then used to synthesize ATP.
NAD+ and FAD are involved in various metabolic pathways, including
glycolysis, the citric acid cycle (also known as the Krebs cycle or TCA
cycle), and the electron transport chain. Here, we’ll discuss their
roles in these pathways:
Glycolysis: This is the first step in breaking down glucose to produce
energy. During glycolysis, glucose is converted into two molecules
of pyruvate. In this process, NAD+ is reduced to NADH, capturing
some of the energy stored in glucose. This NADH will later be used
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to produce ATP in the electron transport chain.
Citric Acid Cycle (TCA Cycle): After glycolysis, pyruvate is transported into the mitochondria, where it is converted into Acetyl-CoA.
Acetyl-CoA then enters the citric acid cycle, where it is further broken down to release energy. In this cycle, NAD+ is reduced to NADH
and FAD is reduced to FADH2. Both NADH and FADH2 serve as electron carriers that will transport the captured energy to the electron
transport chain.
Electron Transport Chain (ETC): The ETC is the final stage of cellular respiration, where the majority of ATP is produced. NADH and
FADH2 donate their electrons to the ETC, which is a series of protein
complexes embedded in the inner mitochondrial membrane. As
electrons move through the ETC, they release energy that is used
to pump protons across the membrane, creating a proton gradient.
This gradient is harnessed by ATP synthase to generate ATP. At the
end of the ETC, electrons are transferred to oxygen, which is reduced
to water.
In summary, electron carriers like NAD+ and FAD are crucial for
cellular respiration and metabolism. They participate in various
metabolic pathways, capturing and transferring energy stored in
chemical bonds to generate ATP. Understanding the roles of electron carriers in these processes is essential for students preparing
for the IMAT, as they form a fundamental aspect of cellular energy
production.
6.9
Glucose
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6.9.1 Why is Glucose Important in Metabolism?
Glucose is a simple sugar that plays a vital role in the energy metabolism
of living cells. It is produced by plants through the process of photosynthesis, in which light energy is converted into chemical energy
and stored in the bonds of glucose molecules.
When animals and other organisms consume glucose, either directly or by consuming plants that contain glucose, the glucose is
used as a source of energy. The energy stored in glucose is used
to power the chemical reactions that occur within cells, such as
the synthesis of proteins, the production of ATP (the main energy
currency of cells), and the transport of ions and molecules across
cell membranes.
The study of glucose and its role in metabolism is particularly important because it is the primary fuel source for most living organisms.
When we consume carbohydrates in our diet, such as bread or pasta,
they are broken down into glucose in our digestive system and then
transported to our cells through the bloodstream. Once inside the
cell, glucose is used as a substrate for cellular respiration and energy
production.
One of the reasons that glucose is such an effective energy storage
molecule is because of its chemical structure. It has a simple, sixcarbon structure that allows it to be easily broken down into smaller
molecules during the process of cellular respiration.
Cellular respiration is the process by which cells convert the chemical
energy stored in glucose and other organic molecules into a form
that can be used to power the various functions of the cell. During
cellular respiration, glucose is broken down in a series of reactions
that release energy in the form of ATP. This ATP can then be used
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by cells to power their various functions.
The main biochemical pathways involved in metabolism, such as
glycolysis, the Krebs cycle, the electron transport chain, and oxidative phosphorylation, are all centered around the metabolism of
glucose. These pathways work together to break down glucose into
smaller molecules, such as pyruvate, that can be further processed
to produce ATP. Understanding the role of glucose in these pathways is essential for understanding how the body generates energy
and maintains proper cellular function.
In addition to its role as an energy storage molecule, glucose also
plays important roles in other cellular processes, such as signaling
and the regulation of gene expression. For example, the concentration of glucose in the bloodstream is tightly regulated by hormones
such as insulin and glucagon, which help to maintain a stable supply
of energy for the body’s cells.
Additionally, disorders related to glucose metabolism, such as diabetes, are a significant health concern. Diabetes is a condition in
which the body is unable to properly regulate blood glucose levels, leading to high blood sugar levels that can damage organs
and tissues over time. Understanding the biochemical pathways
involved in glucose metabolism is crucial for developing treatments
for diabetes and other related conditions.
In conclusion, the study of glucose and its role in metabolism is
fundamental for understanding the functioning of living organisms,
as it provides energy and is involved in a wide range of cellular processes. It is also important for understanding and treating disorders
related to glucose metabolism, such as diabetes.
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6.9.2
GLUTs
For cells to use glucose, it must first enter the cell. This process
occurs through a transport protein called a glucose transport protein,
or GLUT.
There are several different types of GLUT proteins, which are classified based on their structure and function. The most common
type of GLUT protein is GLUT1, which is found in most cells and is
responsible for the basal uptake of glucose. Other types of GLUT proteins, such as GLUT2, GLUT4, and GLUT12, are specialized for specific
tissues or functions.
It is not necessary to memorize the specific types of GLUT proteins
and their locations in the body for the IMAT exam, but it is important
to understand that the transport of glucose across the cell membrane through a GLUT protein is an example of facilitated diffusion.
This type of passive transport does not require the expenditure of
energy and occurs due to a concentration gradient, with the movement of glucose happening from an area of higher concentration
to an area of lower concentration.
Once glucose enters the cell, it can be phosphorylated or modified
by the addition of a phosphate group. This process occurs through
a series of reactions called glycolysis, which takes place in the cytoplasm of the cell. The first step of glycolysis is the phosphorylation
of glucose to form glucose-6-phosphate. This step is catalyzed by
the enzyme hexokinase, which uses ATP as a source of energy.
Remember: glucose enters the cell through a transport protein
called a GLUT protein, which allows it to move through facilitated
diffusion, a type of passive transport. Once inside the cell, glucose can be phosphorylated through glycolysis, a series of reactions
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that begins with the phosphorylation of glucose to form glucose-6phosphate. This step is powered by ATP.
6.10 Glycolysis
Glycolysis is a crucial metabolic pathway that occurs in the cytosol
of cells in all living organisms. It is responsible for breaking down
glucose, a simple sugar, into two molecules of pyruvate. This process generates a small amount of adenosine triphosphate (ATP)
and nicotinamide adenine dinucleotide (NADH). As the first step in
cellular respiration, glycolysis is an anaerobic process, meaning it
does not require oxygen.
6.10.1 Steps in Glycolysis
Let’s break down the process into two stages: the energy-investment
stage and the energy-payoff stage. Each stage consists of several
steps, some of which are reversible, while others are irreversible.
In the energy-investment stage, the cell gets ready to make energy, ATP is used to activate glucose and convert it into a molecule
called glucose-6-phosphate. This molecule is then converted into
fructose-6-phosphate, which is then converted into two molecules
of a compound called glyceraldehyde-3-phosphate (G3P). In the
energy-payoff stage, the G3P molecules are converted into pyruvate,
producing ATP and NADH in the process. Let us look at these steps
in more detail:
Energy-Investment Stage (Steps 1-5):
1. Glucose phosphorylation (irreversible): An ATP molecule donates a phosphate group to glucose, forming glucose-6-phosphate
(G6P) and ADP. This step, catalyzed by the enzyme hexokinase,
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is irreversible.
2. Glucose-6-phosphate isomerase (reversible): G6P is converted
into fructose-6-phosphate (F6P) through a reversible isomerization reaction.
3. Phosphofructokinase (irreversible): Another ATP molecule donates a phosphate group to F6P, producing fructose-1,6-bisphosphate
(F1,6BP) and ADP. This step, catalyzed by phosphofructokinase,
is irreversible and a key regulatory point in glycolysis.
4. Aldolase (reversible): F1,6BP is split into two molecules: glyceraldehyde3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
5. Triose phosphate isomerase (reversible): DHAP is converted into
another G3P molecule, resulting in two G3P molecules in total.
Figure 56: The energy-investment stage of glycolysis. (Credit: openstax.org)
Energy-Payoff Stage (Steps 6-10):
6. Glyceraldehyde-3-phosphate dehydrogenase (reversible): Each
G3P molecule is oxidized and gains a phosphate group, producing 1,3-bisphosphoglycerate (1,3BPG). In this process, two NAD+
molecules are reduced to two NADH molecules.
7. Phosphoglycerate kinase (reversible): The phosphate group on
each 1,3BPG is transferred to ADP, generating two ATP molecules
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and two 3-phosphoglycerate (3PG) molecules.
8. Phosphoglycerate mutase (reversible): The phosphate group on
each 3PG molecule is shifted from the 3rd carbon to the 2nd
carbon, creating two 2-phosphoglycerate (2PG) molecules.
9. Enolase (reversible): Each 2PG molecule undergoes dehydration,
forming phosphoenolpyruvate (PEP).
10. Pyruvate kinase (irreversible): The phosphate group on each PEP
molecule is transferred to ADP, generating two ATP molecules
and two pyruvate molecules. This step, catalyzed by pyruvate
kinase, is irreversible.
Figure 57: The energy-payoff stage of glycolysis. (Credit: openstax.org)
Overall, glycolysis produces a net gain of two ATP molecules, two
pyruvates and two NADH molecules per glucose molecule. It is an
important metabolic pathway that allows cells to produce energy
and support their functions.
It is important to remember that the entire pathway of glycolysis and
cellular respiration always occurs twice for each glucose molecule.
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This means that for each glucose molecule broken down, the cell
goes through the energy yield process of glycolysis and cellular
respiration twice.
This is important because sometimes the IMAT exam writers may
ask about the yield of glucose or pyruvate from one or both cycles. It
is essential to pay attention to this detail and to understand whether
the yield being discussed is from one cycle or both cycles.
6.10.2
Reversible and Irreversible Steps in Glycolysis
The significance of reversible and irreversible steps in glycolysis lies
in the regulation of the pathway and the control of the rate of glucose metabolism. Reversible steps are those that can go in either
direction, depending on the conditions and the requirements of the
cell. Irreversible steps, on the other hand, can only proceed in one
direction, which makes them key regulatory points in the pathway.
The reversible steps in glycolysis are important because they allow
the cell to adjust the rate of glucose metabolism in response to
changes in energy demand or substrate availability. For example, if
the cell needs more ATP, the reversible steps can be pushed in the
direction of ATP synthesis, while if there is excess ATP, the reversible
steps can be pushed in the direction of glucose synthesis or other
pathways.
In contrast, the irreversible steps in glycolysis serve as checkpoints
to ensure that the pathway proceeds in a controlled and regulated
manner. These steps are typically catalyzed by enzymes that are
specific to the pathway and are regulated by feedback inhibition or
other mechanisms.
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For example, the enzyme hexokinase catalyzes the first step of glycolysis, in which glucose is phosphorylated to produce glucose-6phosphate. This step is irreversible and serves as a regulatory point
in the pathway, as it ensures that glucose is committed to the pathway and cannot be easily converted back to free glucose. Similarly,
the conversion of phosphoenolpyruvate to pyruvate by pyruvate
kinase is also an irreversible step that regulates the rate of glycolysis.
In summary, the significance of reversible and irreversible steps
in glycolysis lies in the regulation and control of the pathway. Reversible steps allow the cell to adjust the rate of glucose metabolism
in response to changes in energy demand or substrate availability, while irreversible steps serve as checkpoints to ensure that the
pathway proceeds in a controlled and regulated manner. Understanding the regulation of glycolysis is important for understanding
the metabolism of glucose and the functioning of living organisms.
6.10.3 Pyruvate's Fate
After glycolysis, pyruvate can be converted into a number of different molecules depending on the type of cell and the availability of
oxygen. Here are the main pathways that pyruvate can take:
Aerobic respiration: In cells that have access to oxygen, pyruvate
can be converted into Acetyl-CoA, which then enters the citric acid
cycle (also known as the Krebs cycle) to produce ATP. This process is
known as aerobic respiration and is the most efficient way for cells
to produce ATP.
Lactic acid fermentation: Sometimes cells do not have enough
oxygen available to support the process of cellular respiration. This
can happen during intense exercise, when the demand for energy
is high but the oxygen supply is limited. In these situations, cells
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must find another way to generate energy from glucose.
One way that cells can generate energy from glucose when oxygen
is limited is by converting pyruvate to lactic acid. This process, called
anaerobic respiration, allows cells to continue producing energy
even when oxygen is not available.
During anaerobic respiration, pyruvate is converted to lactic acid
through a series of reactions that release energy. This energy can
then be used by the cells to power their various functions.
While anaerobic respiration is not as efficient as cellular respiration
at producing energy, it allows cells to continue generating energy
when oxygen is limited. This is why it is important for pyruvate to
be able to convert to lactic acid in working cells when oxygen is not
available.
In addition to allowing cells to continue producing energy when
oxygen is limited, the conversion of pyruvate to lactic acid is also
important because it helps to regenerate NAD+, which is a critical
molecule needed for the continuation of glycolysis.
During glycolysis, NAD+ is used to accept electrons from the breakdown of glucose. This results in the production of NADH, which is a
molecule that carries the electrons that have been removed from
glucose.
However, in order for glycolysis to continue, NAD+ must be regenerated from NADH. This is where the conversion of pyruvate to lactic
acid comes in.
During the conversion of pyruvate to lactic acid, NADH is used to
accept electrons from pyruvate. This results in the production of
lactic acid and the regeneration of NAD+. This regenerated NAD+
can then be used to continue the process of glycolysis.
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Alcoholic fermentation: During alcoholic fermentation, pyruvate
is converted to ethanol and carbon dioxide. This process releases
energy that can be used by the cell to power its various functions.
While animal cells do rely on anaerobic respiration to generate energy when oxygen is limited, they do not use alcoholic fermentation
as a means of producing energy. Instead, muscle cells use another
type of anaerobic respiration called lactic acid fermentation, in which
pyruvate is converted to lactic acid.
Lactic acid fermentation allows muscle cells to continue producing
energy when oxygen is limited, but it does not produce ethanol as
a byproduct. Instead, lactic acid is produced, which can then be
converted back to pyruvate and used for energy production once
oxygen becomes available again.
Lactic acid can be converted back to pyruvate through a process
called lactic acid degradation, which occurs in the liver. During lactic
acid degradation, lactic acid is broken down into pyruvate, which can
then be used for energy production once oxygen becomes available
again.
Conversion to other molecules: Pyruvate can also be used to synthesize other molecules, such as amino acids, lipids, and nucleotides.
The fate of pyruvate after glycolysis depends on the type of cell and
the availability of oxygen. It can enter into aerobic respiration, lactic
acid fermentation, alcoholic fermentation, or be used to synthesize
other molecules.
6.10.4
The Link Reactions
The link reactions are a crucial step in cellular respiration, as they
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tion of ATP. The conversion of pyruvate to acetyl-CoA is a key step in
this process, as it prepares the acetyl group for entry into the citric
acid cycle.
The pyruvate dehydrogenase complex is a large multi-enzyme complex that catalyzes the conversion of pyruvate to acetyl-CoA. This
reaction involves several steps, including the removal of a carboxyl
group from pyruvate and the transfer of electrons to NAD+, which is
reduced to NADH. The resulting acetyl group is then attached to CoA
to form acetyl-CoA, which can be transported into the mitochondria
for further processing in the citric acid cycle.
The citric acid cycle is a complex series of enzymatic reactions that
occur in the mitochondria and produce ATP, NADH, and FADH2.
During the citric acid cycle, acetyl-CoA is oxidized to CO2, and the
energy released is used to generate ATP through oxidative phosphorylation. This cycle involves the oxidation of acetyl-CoA to CO2 and
the reduction of NAD+ to NADH and FAD to FADH2. The NADH and
FADH2 molecules generated during the citric acid cycle are then
used as electron carriers in the electron transport chain, which is
the final step in cellular respiration and results in the production of
ATP.
The link reactions are also important because they allow for the
integration of different metabolic pathways in the cell. Pyruvate,
which is produced during glycolysis, can be used to generate energy through the link reactions and citric acid cycle. Alternatively,
pyruvate can be converted to other molecules, such as lactate or
alanine, which can be used in other metabolic pathways in the cell.
Overall, the link reactions are a critical step in cellular respiration,
as they allow for the complete oxidation of glucose to CO2 and the
production of ATP. They also play an important role in integrating
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different metabolic pathways in the cell and are essential for the
proper functioning of cells and organisms.
6.11 The Krebs Cycle
The Krebs cycle, also known as the citric acid cycle or the tricarboxylic
acid (TCA) cycle, is a series of chemical reactions that occurs in the mitochondria of cells. It is an integral part of cellular metabolism, as it
provides the energy necessary for a wide range of cellular processes.
It is also closely linked to other metabolic pathways, including glycolysis, the process by which glucose is broken down into pyruvate,
and the electron transport chain, which is responsible for the final
stage of ATP production. Together, these metabolic pathways work
in concert to ensure that the cell has a constant supply of energy to
carry out its functions.
This complex series of chemical reactions ultimately generates several essential products, including ATP, NADH, and FADH2. ATP is
the primary energy source for the cell, and it is generated during
the Krebs cycle through a process called oxidative phosphorylation.
NADH and FADH2 are coenzymes that are involved in the production of ATP through oxidative phosphorylation.
The study of the Krebs cycle and its role in cellular metabolism is important for understanding not only how the body produces energy,
but also how it regulates energy production and use. Dysfunctions in
the Krebs cycle can lead to a variety of metabolic disorders, including
mitochondrial diseases, which can cause symptoms ranging from
muscle weakness to neurological problems. By gaining a deeper
understanding of the Krebs cycle and its regulation, researchers can
work to develop new treatments and therapies for these conditions.
The Krebs cycle begins with the conversion of a molecule called
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acetyl-CoA into a compound called citrate. Acetyl-CoA is produced
from the breakdown of glucose and other nutrients, and it serves as
the starting point for the Krebs cycle. Citrate is then converted into a
series of intermediates, each of which is converted into the following
intermediate through a series of enzyme-catalyzed reactions.
6.11.1
Essential Products of the Krebs Cycle
The Krebs cycle also generates several essential products, including:
ATP: The energy currency of the cell, ATP is produced during the
Krebs cycle through a process called oxidative phosphorylation.
NADH: NADH is a coenzyme that plays an important role in the
production of ATP through oxidative phosphorylation. It is produced
during the Krebs cycle when NAD+ is reduced to NADH, which can
then be used in the electron transport chain to generate ATP.
FADH2: FADH2 is another coenzyme involved in the production of
ATP through oxidative phosphorylation. Like NADH, it is produced
during the Krebs cycle when FAD is reduced to FADH2.
In addition to ATP, NADH, and FADH2, the Krebs cycle also generates
several other essential products. These include:
Carbon dioxide (CO2): Carbon dioxide is a byproduct of the Krebs
cycle and is released into the bloodstream. The CO2 is transported
to the lungs where it is exhaled out of the body.
Oxaloacetate: Oxaloacetate is a four-carbon molecule that is regenerated at the end of the Krebs cycle. It is involved in the continuation
of the cycle by reacting with acetyl-CoA to form citrate, which starts
the cycle again.
GTP: GTP (guanosine triphosphate) is a molecule that is similar to
ATP and is also used as a source of energy by cells. GTP is produced
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during the Krebs cycle and is used in various cellular processes, such
as protein synthesis.
The Krebs cycle is an essential component of cellular respiration
and is a key pathway for producing ATP, which is the main source
of energy for cells. Understanding the Krebs cycle and the various
products it generates is critical for understanding the processes by
which cells produce energy and maintain proper functioning.
The Krebs cycle plays a vital role in the production of energy for cells.
It is an essential part of cellular respiration, which is the process
by which cells produce energy from nutrients. Understanding the
Krebs cycle is important for understanding how cells produce energy
and how they function.
6.11.2 The Cycle
The Krebs cycle, also known as the citric acid cycle or tricarboxylic
acid cycle, is a crucial part of cellular respiration. It takes place in the
mitochondria, where it completes the oxidation of glucose and generates energy through the production of ATP, NADH, and FADH2.
These molecules are essential for the final stage of cellular respiration, the electron transport chain (ETC), and oxidative phosphorylation. Here is a step-by-step breakdown of the Krebs cycle:
1. Formation of citrate (reversible): Acetyl-CoA, produced from one
pyruvate molecule during the conversion of pyruvate to acetylCoA, combines with oxaloacetate to form citrate. This reaction
is catalyzed by the enzyme citrate synthase.
2. Isomerization of citrate (reversible): Citrate is isomerized to form
isocitrate by the enzyme aconitase. This step involves the reversible interconversion of citrate and isocitrate through the
intermediate cis-aconitate.
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3. Oxidation and decarboxylation of isocitrate (reversible): Isocitrate is oxidized and decarboxylated by the enzyme isocitrate
dehydrogenase, generating alpha-ketoglutarate, NADH (from
NAD+), and releasing a molecule of CO2.
4. Oxidation and decarboxylation of alpha-ketoglutarate (irreversible):
Alpha-ketoglutarate undergoes oxidation and decarboxylation
by the alpha-ketoglutarate dehydrogenase complex, forming
succinyl-CoA, NADH (from NAD+), and releasing another molecule
of CO2.
5. Conversion of succinyl-CoA to succinate (reversible): SuccinylCoA is converted into succinate by the enzyme succinyl-CoA
synthetase, producing one molecule of ATP (or GTP in some
organisms) through substrate-level phosphorylation.
6. Oxidation of succinate (reversible): Succinate is oxidized to form
fumarate by the enzyme succinate dehydrogenase. This step
generates one FADH2 molecule by reducing FAD to FADH2.
7. Hydration of fumarate (reversible): Fumarate is converted to
malate by the enzyme fumarase through the addition of a water
molecule.
8. Oxidation of malate (reversible): Malate is oxidized to form oxaloacetate by the enzyme malate dehydrogenase, generating
one NADH molecule (from NAD+).
After the completion of the Krebs cycle, the products include (per
pyruvate molecule):
• 3 NADH molecules
• 1 FADH2 molecule
• 1 ATP or GTP molecule
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Figure 58: Within the citric acid cycle, an acetyl group from acetyl
CoA merges with a four-carbon molecule called oxaloacetate, creating a six-carbon compound called citrate. A sequence of reactions
then takes place in which citrate gets oxidized, leading to the release
of two carbon dioxide molecules per acetyl group that enters the
cycle. Concurrently, three NAD+ molecules are transformed into
NADH, one FAD molecule becomes FADH2, and one molecule of
ATP or GTP (depending on the type of cell) is produced via substratelevel phosphorylation. Since the final product of the citric acid cycle
is also its starting material, the cycle proceeds continuously as long
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as there are sufficient reactants238
present. (Credit: openstax.org)
• 2 CO2 molecules (released)
Since one glucose molecule produces two pyruvate molecules, the
cycle occurs twice for each glucose molecule. Thus, the total products for one glucose molecule are:
• 6 NADH molecules
• 2 FADH2 molecules
• 2 ATP or GTP molecules
• 4 CO2 molecules (released)
The NADH and FADH2 molecules produced during the Krebs cycle,
as well as those generated during glycolysis and the conversion of
pyruvate to acetyl-CoA, are used in the electron transport chain (ETC)
and oxidative phosphorylation to produce additional ATP molecules.
The ETC and oxidative phosphorylation take place in the inner mitochondrial membrane, where a series
Let us then summarize what you have read for better consolidation.
During the Krebs cycle, the breakdown of one glucose molecule
through glycolysis produces two molecules of acetyl-CoA, which are
the starting point for the Krebs cycle. For one cycle (one pyruvate),
two molecules of carbon, three molecules of NADH, one molecule
of FADH2, and one molecule of ATP or GTP are produced. The cycle
occurs twice for each glucose molecule. In total, two Krebs cycles
can produce a maximum of six NADH molecules and two FADH2
molecules.
It’s important to note that the Krebs cycle is just one part of the
process of cellular respiration. In addition to the Krebs cycle, cellular
respiration also involves glycolysis, the electron transport chain, and
oxidative phosphorylation. Together, these processes allow cells to
produce energy from nutrients like glucose.
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NADH and FADH2 are important coenzymes that are involved in
the production of ATP through oxidative phosphorylation. They
are produced during the Krebs cycle and other parts of cellular
respiration, and they play a vital role in the generation of energy for
cells.
Key Takeaways
IMAT Tip: don’t let the exam writers confuse you, the electron
carries are not enzymes, but coenzymes
6.12
The Electron Transport Chain
The electron transport chain (ETC) is a critical component of cellular
metabolism that plays an essential role in producing ATP, the primary energy source for cells. The ETC works in concert with other
biochemical pathways, such as glycolysis and the Krebs cycle, to
generate energy for cellular processes.
Glycolysis breaks down glucose into pyruvate, which is then converted into acetyl-CoA and fed into the Krebs cycle. The Krebs cycle
generates NADH and FADH2, which provide electrons for the ETC.
The ETC then uses these electrons to pump protons across the mitochondrial membrane, creating a proton gradient that drives the
production of ATP through a process called oxidative phosphorylation.
The electron transport chain (ETC) occurs in the inner membrane
of the mitochondria. The ETC is a series of protein complexes and
electron carriers that are responsible for the production of ATP, the
energy currency of cells. The four protein complexes in the ETC
are vital for the transport of electrons and the establishment of the
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proton gradient. These four are: Complex I (NADH dehydrogenase),
Complex II (succinate dehydrogenase), Complex III (cytochrome c
reductase), and Complex IV (cytochrome c oxidase). Electron carriers, such as coenzyme Q and cytochrome c, transfer electrons
between the protein complexes. Each complex plays a unique role
in the transport of electrons from NADH and FADH2 to molecular
oxygen, the final electron acceptor. As electrons flow through the
ETC, energy is released, which is used to pump protons across the
mitochondrial membrane, ultimately leading to the generation of
ATP.
As the electrons move through the electron transport chain, they
give off energy. This energy is used to pump protons across a special
membrane in the mitochondria. This creates a difference in the
concentration of protons on either side of the membrane, which is
called a proton gradient.
The proton gradient is like a hill of water. If you let the water flow
down the hill, it will generate energy as it goes. The same is true for
the protons in the electron transport chain. When they flow down
the proton gradient, they generate energy that is used to produce
ATP, the energy currency of the cell.
In addition to producing ATP, the ETC also plays a critical role in regulating cellular metabolism. The rate of electron transport through
the ETC is tightly regulated by feedback mechanisms, ensuring that
ATP production is appropriately matched to the energy needs of the
cell. This regulation is essential for maintaining cellular homeostasis
and preventing cellular damage that can result from excessive ATP
production.
In summary, the electron transport chain is a vital component of
cellular metabolism that allows cells to produce ATP, the primary
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energy source for cellular processes. It works in concert with other
biochemical pathways, such as glycolysis and the Krebs cycle, to
generate energy from nutrients like glucose. The tight regulation
of the ETC is essential for maintaining cellular homeostasis and
preventing cellular damage from excessive ATP production.
6.12.1 The Components of The ETC
The electron transport chain (ETC) is like a relay race, with each
protein complex passing the baton (in this case, electrons) to the
next. The electron transport chain involves several specific proteins,
here are some examples:
NADH dehydrogenase: This protein accepts electrons from NADH,
a coenzyme that is produced during the Krebs cycle. NADH dehydrogenase is like the starting runner, accepting the electrons (the
baton) from the Krebs cycle (the previous runner) and passing them
on to the next complex.
Cytochrome c: This protein carries electrons from one part of the
electron transport chain to another. Cytochrome c is like the middle
runner, carrying the electrons (the baton) from one part of the ETC
(the track) to another. It’s like a baton carrier running around the
track to hand off the baton to the next runner.
Cytochrome c oxidase: This protein accepts electrons from cytochrome c and uses them to pump protons across the membrane.
Cytochrome c oxidase is like the final runner, receiving the electrons
(the baton) from cytochrome c and using them to pump protons
across the membrane. This is like the final runner pushing hard to
cross the finish line and win the race.
ATP synthase: This protein uses the energy generated by the flow of
protons down the proton gradient to produce ATP. ATP synthase is
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Figure 59: The electron transport chain consists of a sequence of
electron carriers found within the inner mitochondrial membrane.
Its primary function is to transfer electrons from NADH and FADH2
to molecular oxygen. As a result, protons are pumped from the
mitochondrial matrix into the intermembrane space, and oxygen
is converted into water through the process of reduction. (Credit:
openstax.org)
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like the coach waiting at the finish line, using the energy generated
by the flow of protons down the proton gradient to produce ATP.
On the IMAT exam, we may not see a specific question about the
components of the electron transport chain, but it is important to
understand the general concept of it. Remember that the movement of protons from the inside of the mitochondria to the space
between the inner and outer membranes creates something called
an ”electrochemical gradient.” This is because protons are positively
charged, which creates an electrical gradient, and also lowers the
pH due to the presence of protons (which are just hydrogen atoms
without electrons).
Together, these proteins work together to generate ATP, which is
used by cells to power their activities. The electron transport chain
is an essential part of cellular respiration and is involved in the production of the majority of ATP in the body.
6.13
Oxidative Phosphorylation
Oxidative phosphorylation is a process that occurs in the mitochondria of cells. It is an integral part of cellular respiration, the process
by which cells produce energy from nutrients.
Oxidative phosphorylation is like a machine that converts the energy
stored in the electrochemical gradient into energy that is available
to turn ADP+Pi back into ATP. It does this by using the kinetic energy
and the movement of protons through a unique structure called
ATP Synthase, while the protons flow back through it, the movement
is converted into enough energy to ”charge the battery (from our
previous analogy).
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6.13.1 ATP Synthase
ATP Synthase is like a hydroelectric dam. Just as a dam uses the
energy of falling water to generate electricity, ATP synthase uses the
energy of flowing protons to produce ATP, the energy currency of
the cell.
The energy for ATP synthase comes from the movement of protons
across a special membrane in the mitochondria. This movement of
protons creates a difference in the concentration of protons on either
side of the membrane, causing a ”proton gradient”. The proton
gradient is like a hill of water, and when the protons flow down the
hill, they generate energy.
ATP synthase captures this energy and uses it to produce ATP. It
does this by using a special mechanism that rotates like a turbine
as the protons flow through it. The rotation of the turbine drives the
synthesis of ATP from adenosine diphosphate (ADP) and inorganic
phosphate (Pi).
In summary, ATP synthase is an important part of cellular respiration
because it allows cells to produce energy from nutrients like glucose.
It’s like a machine that converts the energy stored in nutrients into
a form that cells can use to power their activities.
6.14 Summary of the Yields of the Different Biochemical Pathways
Here is a summary of the yields for glycolysis, Krebs cycle, electron transport chain, and oxidative phosphorylation per 1 glucose
molecule:
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Glycolysis:
• 2 pyruvate molecules
• 2 NADH molecules
• 4 ATP molecules (gross yield); 2 ATP molecules (net yield, as 2
ATP are used during the energy-investment stage)
Krebs Cycle (2 cycles per glucose molecule):
• 6 NADH molecules
• 2 FADH2 molecules
• 2 ATP or GTP molecules
• 4 CO2 molecules (released)
Electron Transport Chain and Oxidative Phosphorylation (estimations based on P/O ratios):
• 10 NADH molecules in total (2 from glycolysis, 2 from pyruvate decarboxylation, 6 from Krebs cycle) producing 25 ATP molecules
(2.5 ATP per NADH)
• 2 FADH2 molecules in total (from Krebs cycle) producing 3 ATP
molecules (1.5 ATP per FADH2)
• 28-32 ATP molecules in total (the exact number may vary depending on the efficiency of the process and shuttle systems for
NADH produced in glycolysis)
In summary, one glucose molecule yields approximately 36-38 ATP
molecules through glycolysis, the Krebs cycle, and the electron transport chain combined with oxidative phosphorylation. The exact
number can vary depending on specific cellular conditions and the
efficiency of the processes involved.
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7 Photosynthesis
Photosynthesis is the process by which green plants, algae, and
some bacteria convert light energy into chemical energy in the
form of glucose. This chemical energy is stored in the bonds of glucose molecules and is used by cells for various metabolic processes,
including cellular respiration.
As we discussed in the previous chapter, cellular respiration is the
process by which cells convert the chemical energy stored in glucose
into ATP, the primary energy currency of cells. This process occurs
in the mitochondria of cells and involves the breakdown of glucose
through a series of chemical reactions that release energy.
In this chapter, it is crucial to remember that both photosynthesis
and cellular respiration are essential processes for life on Earth, and
both occur in plants. Photosynthesis is the primary source of energy
for almost all living organisms, as it provides the energy needed
for the synthesis of organic compounds and the release of oxygen
into the atmosphere. Cellular respiration, on the other hand, is the
process by which cells use the energy stored in organic compounds
to power their various functions and activities.
While photosynthesis and cellular respiration are separate processes,
they are also interconnected. The glucose produced through photosynthesis is used as a source of energy during cellular respiration,
and the oxygen produced during photosynthesis is partially used as
a reactant during cellular respiration. Overall, the balance between
photosynthesis and cellular respiration is essential for maintaining
the energy balance in living organisms and the overall health of the
planet.
Light energy is used to drive the conversion of carbon dioxide and
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water into glucose and oxygen through a series of chemical reactions. These reactions can be divided into two main stages: the
light-dependent reactions and the light-independent reactions.
The light-dependent reactions occur in the thylakoid membranes
of the chloroplasts and involve the absorption of light energy by
pigment molecules, the transfer of this energy to ATP and NADPH,
and the release of oxygen as a byproduct.
Pigments are molecules that are able to absorb light, and they
do this because they contain electrons that can be excited by the
energy of light. When light is absorbed by a pigment, it causes the
electrons in the pigment to become excited and move to a higher
energy level.
The color of a pigment is determined by the wavelengths of light
that it absorbs. Different pigments absorb different wavelengths
of light, and this is what gives them their characteristic colors. For
example, chlorophyll, the pigment found in plants, absorbs light in
the blue and red wavelengths, which is why plants appear green
(the color that is not absorbed is reflected and appears to our eyes
as the color of the object).
Pigments are important for a variety of biological processes, including photosynthesis in plants, vision in animals, and the synthesis
of vitamins in some microorganisms. They are also used in many
everyday products, such as paints, dyes, and food coloring.
The light-independent reactions, also known as the Calvin cycle,
occur in the stroma of the chloroplasts and involve the conversion
of carbon dioxide into glucose using the ATP and NADPH produced
during the light-dependent reactions.
Overall, the process of photosynthesis can be summarized by the
following equation:
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6 CO2 + 6 H2 O + light → C6 H12 O6 + 6 O2
This equation shows that photosynthesis involves the conversion of
carbon dioxide and water into glucose and oxygen using light energy
as the driving force. The glucose produced during photosynthesis
is used by cells for various metabolic processes, including cellular
respiration, and oxygen is released into the atmosphere.
7.1 Light-Dependent Reactions
The light-dependent reactions of photosynthesis are a series of
chemical reactions that occur in the thylakoid membranes of the
chloroplasts of plant cells. These reactions involve the absorption
of light energy by pigment molecules, the transfer of this energy to
ATP and NADPH, and the release of oxygen as a byproduct.
The light-dependent reactions are initiated when light energy is absorbed by pigment molecules, such as chlorophyll, in the thylakoid
membranes. This energy is used to transfer electrons from water
molecules to NADP+, a process known as photolysis. The electrons
are then used to power the synthesis of ATP and NADPH, which are
important energy-carrying molecules used in the light-independent
reactions.
The light-dependent reactions are also known as the electron transport chain because they involve the transfer of electrons from one
molecule to another. This transfer of electrons is accompanied by
the release of energy, which is used to power the synthesis of ATP.
Here is a more detailed explanation of the light-dependent reactions:
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Figure 60: There are two distinct stages of photosynthesis: the lightdependent reactions and the Calvin cycle. The light-dependent
reactions occur in the thylakoid membrane and utilize light energy
to produce ATP and NADPH. The Calvin cycle, which takes place in
the stroma, uses the energy obtained from these compounds to
generate G3P from carbon dioxide. (Credit: openstax.org)
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1. Photosystem II (PSII) activation:
• Light energy is absorbed by chlorophyll and accessory pigments
within the light-harvesting complexes (LHCs) of PSII.
• The absorbed energy is transferred to the reaction center of PSII,
where it excites a pair of chlorophyll a molecules called P680.
• Excited P680 releases an electron, which is captured by the primary electron acceptor, a molecule called plastoquinone (PQ).
2. Photolysis of water:
• To replace the lost electron, PSII extracts electrons from water
molecules through a process called photolysis.
• Photolysis results in the production of oxygen gas (O2) as a
byproduct and the release of protons (H+) into the thylakoid
lumen.
3. Electron transport chain (ETC) and proton gradient:
• The electrons from PQ are transferred along the ETC, which
includes the cytochrome b6f complex and plastocyanin (PC).
• As the electrons move through the ETC, protons are pumped
from the stroma into the thylakoid lumen, creating a proton
gradient across the thylakoid membrane.
4. Photosystem I (PSI) activation:
• Meanwhile, light energy is also absorbed by the LHCs of PSI,
and the energy is transferred to the PSI reaction center, where
it excites a pair of chlorophyll a molecules called P700.
• Excited P700 releases an electron, which is captured by the
primary electron acceptor, a molecule called ferredoxin (Fd).
5. Formation of NADPH:
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• The electrons from Fd are transferred to NADP+ through the enzyme ferredoxin-NADP+ reductase (FNR), reducing it to NADPH.
• NADPH is then used as an energy source in the light-independent
reactions (Calvin cycle).
6. ATP synthesis (photophosphorylation):
• The proton gradient generated by the ETC and photolysis drives
the synthesis of ATP through a process called photophosphorylation.
• ATP synthase, an enzyme embedded in the thylakoid membrane, uses the energy from the proton gradient to produce ATP
from ADP and inorganic phosphate (Pi).
In summary, the light-dependent reactions of photosynthesis involve the absorption of light energy, the generation of a proton gradient through electron transport, and the synthesis of ATP and NADPH.
These energy-rich molecules are used in the light-independent reactions (Calvin cycle) to fix carbon dioxide and produce sugars. The
light-dependent reactions also produce oxygen gas as a byproduct, which is released into the atmosphere through the process of
photolysis.
7.2
Light Independent Reaction
The light-independent reactions of photosynthesis, also known as
the Calvin cycle or the dark reactions, are a series of chemical reactions that occur in the stroma of the chloroplasts of plant cells.
These reactions involve the conversion of carbon dioxide into glucose
using the ATP and NADPH produced during the light-dependent
reactions.
The light-independent reactions are initiated when carbon dioxide
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enters the stroma of the chloroplasts. The carbon dioxide is then
converted into a carbohydrate, such as glucose, through a series of
chemical reactions known as the Calvin cycle.
During the Calvin cycle, the carbon dioxide is combined with a 5carbon compound called ribulose bisphosphate (RuBP) to form a
6-carbon compound called glyceraldehyde 3-phosphate (G3P). This
reaction requires the energy provided by ATP and NADPH produced
during the light-dependent reactions.
The G3P produced during the Calvin cycle is then converted into
glucose or other organic compounds through a series of additional
chemical reactions. The glucose can be used by cells for various
metabolic processes, including cellular respiration, or it can be stored
in the form of starch for later use.
Here is a more detailed explanation of each phase:
1. Carbon fixation:
• Carbon dioxide (CO2) enters the stroma of the chloroplasts and is
fixed to a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP)
by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase
(RuBisCO).
• This reaction results in an unstable 6-carbon intermediate compound, which quickly splits into two molecules of a 3-carbon
compound called 3-phosphoglycerate (3-PGA).
2. Reduction:
• Each molecule of 3-PGA is phosphorylated by ATP, producing
two molecules of 1,3-bisphosphoglycerate (1,3-BPGA).
• Next, NADPH, produced during the light-dependent reactions,
donates high-energy electrons to each 1,3-BPGA molecule, reducing them to two molecules of glyceraldehyde 3-phosphate
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(G3P).
• One molecule of G3P is set aside for every three turns of the
Calvin cycle (corresponding to three fixed CO2 molecules) to be
used for glucose synthesis or other organic compounds.
3. Regeneration:
• The remaining G3P molecules are used to regenerate RuBP, the
initial 5-carbon sugar.
• This process involves a series of complex reactions, during which
G3P molecules are rearranged and phosphorylated using ATP,
ultimately producing RuBP.
• Regenerated RuBP allows the Calvin cycle to continue, fixing
more CO2 and producing additional G3P.
The Calvin cycle must complete three turns to produce one net G3P
molecule. This is because three CO2 molecules are fixed to generate
six G3P molecules, but only one G3P molecule is used for glucose
synthesis or other organic compounds, while the other five G3P
molecules are used to regenerate three RuBP molecules.
In summary, the light-independent reactions, or the Calvin cycle,
involve carbon fixation, reduction, and regeneration, ultimately converting CO2 into glucose or other organic compounds using the
energy from ATP and NADPH produced during the light-dependent
reactions. The Calvin cycle plays a crucial role in the synthesis of
glucose and other organic molecules necessary for plant growth
and development.
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7.3
The Interplay between Light-dependent and
Light-independent Reactions
Now that you are now aware of the different reactions that go
through each phase, it would be useful to know how those 2 reactions interplay with each other in the bigger picture. They are closely
interconnected, working together to convert light energy into chemical energy stored in glucose and other organic compounds. Here is
an explanation of how these two sets of reactions are related:
1. Energy transfer:
• The light-dependent reactions capture light energy and convert
it into chemical energy in the form of ATP and NADPH.
• The ATP and NADPH produced during the light-dependent reactions are then used as energy sources for the light-independent
reactions (Calvin cycle).
2. Electron flow:
• Electrons excited by light energy in the light-dependent reactions are used to reduce NADP+ to NADPH.
• In the Calvin cycle, NADPH donates these high-energy electrons
to convert 1,3-bisphosphoglycerate (1,3-BPGA) into glyceraldehyde 3-phosphate (G3P), playing a crucial role in the formation
of glucose and other organic compounds.
3. Proton gradient and ATP synthesis:
• The light-dependent reactions generate a proton gradient across
the thylakoid membrane by transferring electrons through the
electron transport chain and splitting water molecules.
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• This proton gradient drives ATP synthesis through ATP synthase
via a process called photophosphorylation.
• The ATP generated during the light-dependent reactions is utilized in the Calvin cycle for the phosphorylation of 3-phosphoglycerate
(3-PGA) and the regeneration of ribulose-1,5-bisphosphate (RuBP).
4. Cycling of reactants and products:
• The light-dependent reactions produce oxygen as a byproduct,
which is released into the atmosphere.
• The light-independent reactions (Calvin cycle) consume carbon
dioxide from the atmosphere and use it to synthesize glucose
and other organic compounds.
• The continuous cycling of reactants and products in both sets
of reactions maintains a balance between the consumption of
CO2 and the release of O2, playing a critical role in the global
carbon and oxygen cycles.
In summary, the light-dependent and light-independent reactions
of photosynthesis are closely connected through the transfer of
energy in the form of ATP and NADPH, the flow of electrons, and the
cycling of reactants and products. The light-dependent reactions
capture light energy and produce ATP, NADPH, and O2, while the
light-independent reactions (Calvin cycle) use the ATP and NADPH
to fix CO2 and synthesize glucose and other organic compounds,
ultimately storing the energy captured from sunlight in a form that
can be used by the plant and other organisms.
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8
The Cell Reproduction Cycle
Cell division is the process by which cells replicate and produce new
cells. It is an essential process for the growth and repair of tissues
and the continuation of life. Without cell division, organisms would
not be able to grow, repair damaged tissues, or reproduce.
Cell division occurs in two main types: mitosis and meiosis. Mitosis is the process by which a single cell divides into two identical
daughter cells, each with the same number of chromosomes as the
parent cell. Meiosis, on the other hand, is the process by which cells
divide into four non-identical daughter cells with half the number
of chromosomes as the parent cell.
During mitosis, the chromosomes in the parent cell are replicated
and then divided equally into the two daughter cells. This ensures
that each daughter cell has the same genetic information as the
parent cell. Mitosis is essential for the growth and repair of tissues
and the continuation of life.
Meiosis, is important for sexual reproduction. It occurs in the formation of eggs and sperm and produces four non-identical daughter
cells with half the number of chromosomes as the parent cell. This
ensures that when an egg and sperm combine during fertilization,
the resulting offspring will have the appropriate number of chromosomes. But before Mitosis and Meiosis, the cell must undergo
through the cell cycle which allows it to prepare to the M phase
(Meiosis or Mitosis)
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8.1 The Cell Cycle
The cell cycle is the series of events that occurs in a cell as it grows
and prepares to divide into daughter cells. It consists of two main
phases: interphase and cell division.
Interphase, which accounts for the majority of the cell cycle, is a
period of intense activity where the cell prepares for division. During
this time, the cell grows, duplicates its DNA, and synthesizes the
proteins and other cellular components necessary for cell division.
Interphase is divided into three main sub-phases:
G1 phase: The G1 phase is particularly significant because it is the
first phase of interphase, during which the cell grows and synthesizes the proteins and other cellular components needed for DNA
replication and cell division. The cell also checks for DNA damage
and any other abnormalities during this phase, ensuring that the
DNA is suitable for replication.
S phase: The S phase is critical because it is when DNA replication
occurs. DNA replication is an essential process, as each daughter cell
must receive a complete set of genetic material to function correctly.
Any errors in DNA replication can lead to mutations, which can have
severe consequences, including the development of cancer.
G2 phase: The G2 phase is important for checking that the cell has
replicated its DNA correctly and that there are no other errors in the
genetic material. If any abnormalities are detected during the G2
phase, the cell will not enter the final phase of the cell cycle, known
as cell division, until the errors have been corrected.
The second phase of the cell cycle is cell division, which is divided
into two main types: mitosis and meiosis.
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G0 phase: Some cells may enter a state called G0 phase, where
they are temporarily quiescent and do not actively divide. Cells in
G0 phase may still perform their normal functions, but they are
not actively preparing to divide. Examples of cells that may enter
G0 phase include differentiated cells, such as some nerve cells and
muscle cells, which do not divide after they have matured.
Mitosis: During mitosis, the cell’s replicated chromosomes are evenly
divided into two daughter cells, resulting in two identical daughter
cells with the same number of chromosomes as the parent cell.
Meiosis: During meiosis, the cell’s replicated chromosomes are
divided into four non-identical daughter cells with half the number
of chromosomes as the parent cell. This process is vital for sexual
reproduction, as it ensures that the resulting offspring will have the
appropriate number of chromosomes.
8.1.1 The S phase
The S phase, or the synthesis phase, is the second phase of the
interphase in the cell cycle. It is characterized by the replication of
the cell’s DNA, meaning a copy of each chromosome is made. This
is an essential step in preparation for cell division, as it ensures that
each daughter cell will have the same genetic information as the
parent cell.
During the S phase, the number of DNA molecules in the cell increases from n to 2n. This is because each chromosome is replicated,
resulting in two copies of each chromosome. An analogy to understand this process is to think of a book. Just as a book contains
information that can be copied onto a second book, the DNA in a cell
contains genetic information that can be copied onto the second
set of chromosomes.
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Figure 61: In multicellular organisms, the cell cycle comprises of
two main stages: interphase and the mitotic phase. Throughout
interphase, the cell grows and duplicates its nuclear DNA. Next, the
mitotic phase begins, during which the replicated chromosomes
are separated and allocated into two daughter nuclei. After the
process of mitosis is completed, the cytoplasm usually splits through
cytokinesis, forming two daughter cells that are genetically identical
to the parent cell. (Credit: openstax.org)
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As an example, let’s consider the fruit fly Drosophila melanogaster.
The fruit fly has 4 pairs of chromosomes, for a total of 8 chromosomes. During the S phase, each of these 8 chromosomes is replicated, resulting in a total of 16 chromosomes in the cell. These 16
chromosomes are then evenly divided into two daughter cells during the process of mitosis, resulting in two daughter cells with 8
chromosomes each.
During the S phase, or synthesis phase, of the cell cycle, the cell’s
DNA is replicated. This process involves the synthesis of a new complementary strand of DNA for each existing strand of DNA, resulting
in the doubling of the number of DNA molecules in the cell.
The significance of the S phase lies in its essential role in preserving the integrity of genetic information during cell division. By accurately replicating the cell’s DNA, the S phase ensures that each
daughter cell receives the same genetic information as the parent
cell. This process is crucial for maintaining the normal functioning
of cells, tissues, and organs, as well as ensuring the continuity of
genetic information across generations.
Errors during the S phase can lead to various problems and medically
relevant conditions, including:
• Mutations: Errors in DNA replication can introduce mutations,
which are changes in the DNA sequence. These mutations can
be harmless, beneficial, or detrimental depending on their nature and location in the genome. Some mutations can lead to
genetic disorders, increase the risk of developing diseases, or
cause cancer.
•
• Chromosomal abnormalities: Errors during the S phase can
result in abnormal chromosome numbers or structures. These
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abnormalities can lead to conditions such as Down syndrome,
Turner syndrome, or Klinefelter syndrome, which are caused by
an extra or missing chromosome.
•
• Replication stress: Incomplete or stalled replication can cause
replication stress, which can lead to DNA damage and genomic
instability. This instability can contribute to the development of
various diseases, including cancer.
•
• Cell cycle arrest and cell death: Errors during the S phase can
activate cell cycle checkpoints, which halt cell division to allow
time for repair. If the damage is too severe, the cell may undergo
programmed cell death, or apoptosis, to prevent the propagation of faulty genetic information.
•
• Cancer: Many types of cancer are associated with errors in DNA
replication during the S phase. These errors can lead to genomic instability, uncontrolled cell division, and the formation
of tumors.
In summary, the S phase is essential for the accurate replication of
genetic information, which is crucial for maintaining the normal
functioning of cells and the continuity of genetic information. Errors
during the S phase can have significant consequences, leading to
various medical conditions and diseases, including cancer.
8.1.2 DNA Replication (In Detail)
DNA replication is the process by which cells create a copy of their
genetic material, DNA, in preparation for cell division. It is a crucial
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process that ensures the accurate transmission of genetic information from one generation of cells to the next. In this section, we
will delve into the details of DNA replication, including the enzymes
involved, the direction of replication, and the role of Okazaki fragments.
Before we begin, it is important to understand the structure of
DNA. DNA, or deoxyribonucleic acid, is a double-stranded molecule
made up of two strands of nucleotides that are held together by
hydrogen bonds. Each nucleotide consists of a sugar molecule
called deoxyribose, a phosphate group, and one of four nitrogenous
bases: adenine (A), cytosine (C), guanine (G), or thymine (T). The
order of these bases, or the sequence of the DNA, is what carries
the genetic information.
Now, let’s talk about the process of DNA replication. DNA replication occurs in the S phase of the cell cycle, which is the period
between cell division. It starts with the unwinding of the double
helix structure of the DNA molecule by an enzyme called helicase.
This creates a replication fork, where the two strands of the DNA
molecule separate and expose the bases.
Next, an enzyme called primase adds short RNA primers to the
template strands of DNA. These primers serve as a starting point for
the synthesis of new strands of DNA.
Once the primers are in place, another enzyme called polymerase
starts synthesizing the new strands of DNA. There are several different types of polymerases, but the one primarily responsible for
DNA replication is called DNA polymerase III. DNA polymerase III
adds nucleotides to the template strands of DNA by matching the
correct base to the template strand using the base-pairing rule: A
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cation because one of the original strands is conserved and used as
a template for the synthesis of the new strand.
As DNA polymerase III moves along the template strand, it synthesizes the new strand in the 5’ to 3’ direction, meaning that it starts at
the 5’ end and moves towards the 3’ end. This is important because
it ensures that the new strands are synthesized in the correct order
and that the genetic information is accurately transmitted.
However, there is a problem with this process. DNA polymerase III
can only add nucleotides in the 5’ to 3’ direction, which means that
it cannot synthesize the new strand continuously. Instead, it has to
synthesize short stretches of DNA called Okazaki fragments. These
fragments are named after the scientist who discovered them, Reiji
Okazaki.
Once an Okazaki fragment is synthesized, an enzyme called ligase
seals the gap between the fragments by creating a covalent bond
between the sugar and phosphate groups. This creates a continuous
strand of DNA.
During DNA replication, the leading strand, which is the strand
being synthesized in the same direction as the replication fork, can
be synthesized continuously by DNA polymerase III. However, the
lagging strand, which is the strand being synthesized in the opposite
direction of the replication fork, is synthesized in a discontinuous
manner, with Okazaki fragments being synthesized and then sealed
together by ligase.
In summary, DNA replication is a complex process that involves
several enzymes, including helicase, primase, DNA polymerase III,
and ligase. It is a semi-conservative process that involves the synthesis of new strands of DNA using the template strands as a guide.
The direction of replication is from 5’ to 3’ and is important for the
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accurate transmission of genetic information. Okazaki fragments
play a crucial role in the synthesis of the lagging strand, which is
synthesized in a discontinuous manner.
It is worth noting that the process of DNA replication is not always
perfect and mistakes, or errors, can occur. These errors, called mutations, can alter the sequence of the DNA and potentially have
consequences for the organism. To ensure the accuracy of DNA
replication, cells have a number of mechanisms in place to detect
and repair errors.
One such mechanism is proofreading, which is carried out by DNA
polymerase III. DNA polymerase III has the ability to recognize and
remove mismatched bases before they are incorporated into the
new strand of DNA. Another mechanism is mismatch repair, which
involves the identification and correction of mismatched bases after
they have been incorporated into the new strand.
In addition to these mechanisms, cells also have a number of enzymes called exonucleases that can remove nucleotides from the
ends of DNA strands. These enzymes can be used to remove any
excess nucleotides that may have been added during replication.
Overall, DNA replication is a vital process that ensures the accurate
transmission of genetic information from one generation of cells to
the next. Understanding the details of this process, including the
enzymes involved and the role of Okazaki fragments, is important
for a number of fields, including molecular biology and medicine.
8.1.3
The Important Aspects of DNA Replication
DNA replication is a complex process that involves many different
components and enzymes working together to ensure the accurate
duplication of genetic information. Here are some of the most
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important aspects of DNA replication:
• Helicase: This enzyme is responsible for unwinding the doublestranded DNA helix to form a replication fork, which is the site
where replication occurs. Helicase does this by breaking the
hydrogen bonds that hold the two strands of DNA together. It
moves along the DNA strand in a 5’ to 3’ direction, separating the
strands and creating a single-stranded template for replication.
Helicase is essential for DNA replication to occur, as the doublestranded DNA helix must be unwound before the polymerases
can begin to synthesize new strands.
• Single-strand binding proteins: These proteins bind to and stabilize single-stranded DNA, preventing it from forming secondary
structures that would interfere with replication. They coat the
single-stranded template, preventing it from re-forming into
a double-stranded helix. Without these proteins, the singlestranded template would quickly become re-paired with its
complementary strand, halting the replication process.
• Primase: This enzyme synthesizes a short RNA primer at the
replication fork, providing a 3’-OH group to which DNA polymerase can add nucleotides. Primase is essential for DNA replication because DNA polymerase can only add nucleotides to an
existing 3’-OH group. Without the primer provided by primase,
DNA polymerase would not be able to start synthesizing the
new DNA strands.
• DNA polymerase: This enzyme is responsible for extending the
RNA primer by adding nucleotides to the 3’-OH group. There
are multiple DNA polymerases involved in DNA replication, each
with a different function. DNA polymerase III is the main polymerase responsible for elongating the new DNA strands, while
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DNA polymerase I is involved in removing the RNA primers and
replacing them with DNA. DNA polymerase also has a proofreading function, which allows it to correct errors in base pairing
that may occur during replication.
• DNA ligase: This enzyme joins the Okazaki fragments on the
lagging strand by catalyzing the formation of a phosphodiester
bond between adjacent nucleotides. DNA ligase is essential for
DNA replication because the lagging strand is synthesized in
short fragments that must be joined together to form a continuous strand.
• Topoisomerase: This enzyme relaxes the supercoiling ahead of
the replication fork, preventing the DNA strands from becoming
too tightly wound. Supercoiling occurs when the DNA helix
twists upon itself as it unwinds, and if left unchecked, it can
cause breaks in the DNA strand. Topoisomerase relieves this
tension by breaking the DNA strand, allowing it to unwind, and
then re-joining the strand.
• Proofreading: DNA polymerase proofreads newly synthesized
DNA, correcting errors in base pairing. This function ensures that
the new DNA strands are as accurate as possible, minimizing
the risk of mutations and errors that can lead to disease.
• Telomerase: This enzyme extends the ends of linear chromosomes by adding repetitive DNA sequences called telomeres.
Telomeres are essential for protecting the ends of chromosomes
from degradation and fusion with other chromosomes. Without
telomeres, the ends of chromosomes would gradually erode
with each cell division, leading to genomic instability and cell
death.
• Replication origin: This is the specific sequence of DNA where
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replication begins. Replication origin sequences are recognized
by proteins that initiate replication by recruiting helicase and
other replication enzymes to the site. The location and number
of replication origins varies between organisms and cell types,
depending on the size and complexity of their genomes.
Understanding these different aspects of DNA replication is essential for understanding how genetic information is transmitted from
one generation to the next, and how mutations and errors can occur
during replication.
8.2 Mitosis
Mitosis is a vital cellular process that results in a single cell dividing
into two genetically identical daughter cells, each possessing the
same number of chromosomes as the parent cell. This process is
crucial for the growth, repair, and maintenance of tissues, as well as
the perpetuation of life.
Mitosis can be subdivided into four primary stages, followed by
cytokinesis, the actual division of the cytoplasm:
1. Prophase: Prophase marks the beginning of mitosis. During
this stage, the chromatin in the cell’s nucleus condenses into
visible chromosomes, and the nucleolus disappears. Each chromosome consists of two identical sister chromatids joined at
the centromere. The centrosomes, which are responsible for
organizing the mitotic spindle, begin to migrate to opposite
poles of the cell. The mitotic spindle, composed of microtubules,
starts to form between the centrosomes. Additionally, the nuclear envelope starts to break down, allowing the spindle fibers
to access the chromosomes.
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2. Metaphase: In metaphase, the chromosomes align along the
cell’s equator, forming the metaphase plate. Each chromosome’s centromere becomes attached to the spindle fibers,
which extend from the centrosomes at the cell’s poles. This
connection ensures that each daughter cell will receive one
copy of each chromosome during cell division.
3. Anaphase: Anaphase is characterized by the separation of sister
chromatids. The centromere of each chromosome splits, and
the sister chromatids are pulled to opposite poles of the cell by
the mitotic spindle fibers. This movement ensures that each
daughter cell will have an identical set of chromosomes.
4. Telophase: During telophase, the cell starts to divide into two
daughter cells. A new nuclear envelope forms around each set of
chromosomes, and the nucleolus reappears within each nucleus.
The chromosomes begin to decondense back into chromatin,
and the mitotic spindle disassembles.
5. Cytokinesis: Cytokinesis is the final stage of cell division, during which the cytoplasm and cellular organelles are divided
between the two emerging daughter cells. In animal cells, a
contractile ring composed of actin and myosin filaments constricts at the cell’s equator, creating a cleavage furrow. This
furrow deepens until the cell is pinched into two separate cells.
In plant cells, a cell plate forms between the two daughter cells,
eventually developing into a new cell wall, which separates the
two cells.
In summary, mitosis is a critical process that ensures the accurate distribution of genetic material to two daughter cells. It consists of four
main stages—prophase, metaphase, anaphase, and telophase—followed by cytokinesis. This process plays a pivotal role in the growth,
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Figure 62: Metaphase. The microtubule spindle has finished positioning the chromosomes at the metaphase plate, getting ready
for the separation of sister chromatids during anaphase. (Credit:
openstax.org)
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repair, and continuation of life across various organisms.
8.3
Meiosis
Meiosis is a specialized cell division process that results in four nonidentical daughter cells, each with half the number of chromosomes
as the parent cell. This process is crucial for sexual reproduction,
as it ensures that offspring will have the appropriate number of
chromosomes. Meiosis consists of two main stages: meiosis I and
meiosis II, each with its respective substages.
Meiosis I: Meiosis I comprises the following substages:
1. Prophase I: During prophase I, chromosomes condense and
become visible. The nuclear envelope begins to break down,
and the spindle fibers start to form. Homologous chromosomes
(one from each parent) come together in a process called synapsis, forming a structure called a bivalent or tetrad. At this stage,
genetic material may be exchanged between non-sister chromatids in a process called crossing over, which generates genetic diversity in the resulting gametes.
2. Metaphase I: In metaphase I, the homologous chromosome
pairs align at the cell’s equator, forming two columns, unlike
the single column seen in mitosis. This arrangement is due to
the prior synapsis, where chromosomes paired up with their
homologous partners. Spindle fibers attach to the centromeres
of each homologous chromosome pair.
3. Anaphase I: During anaphase I, the spindle fibers pull the homologous chromosome pairs to opposite poles of the cell. Unlike mitosis, the sister chromatids remain attached at their centromeres and do not separate.
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4. Telophase I: In telophase I, the chromosomes reach the poles,
and the cell begins to divide, resulting in two daughter cells.
Each daughter cell has half the number of chromosomes as the
parent cell, but the chromosomes still consist of two chromatids.
5. Cytokinesis I: Cytokinesis I follows telophase I, resulting in the
separation of the two daughter cells.
Meiosis II: Meiosis II is similar to mitosis and includes the following
substages:
1. Prophase II: During prophase II, the nuclear envelope dissolves
again in the two daughter cells, and the spindle fibers begin to
form.
2. Metaphase II: In metaphase II, the chromosomes align at the
equator of each daughter cell, with spindle fibers attaching to
the centromeres.
3. Anaphase II: During anaphase II, the centromeres of each chromosome finally split, and the sister chromatids are pulled to
opposite poles of the cell by the spindle fibers.
4. Telophase II: In telophase II, the chromatids reach the poles,
and the nuclear envelope starts to reform around the separated
chromatids, which are now considered individual chromosomes.
5. Cytokinesis II: Cytokinesis II follows telophase II, dividing each
daughter cell from meiosis I into two new cells, resulting in a
total of four non-identical daughter cells, each with half the
number of chromosomes as the parent cell.
In summary, meiosis is a critical process for sexual reproduction,
generating non-identical daughter cells with half the number of
chromosomes as the parent cell. This process comprises two main
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ing over during prophase I contributes to genetic diversity among
the resulting gametes, which is essential for the evolution and adaptation of species.
8.3.1
Prophase I
Prophase 1 is the first stage of meiosis, a type of cell division that
produces gametes, or sex cells, such as sperm and eggs. In meiosis,
a single cell divides into four daughter cells, each of which contains
half the number of chromosomes as the parent cell. This reduction
in chromosome number is important for sexual reproduction, as it
allows for the exchange of genetic material between the parents
and the production of genetically diverse offspring.
During prophase 1, the chromosomes in the nucleus of the cell become visible and begin to condense. This is important because it
allows for the precise separation of the chromosomes during the
subsequent stages of meiosis. The chromosomes are made up of
DNA, which is tightly packed into a compact structure called chromatin. During prophase 1, the chromatin condenses and becomes
visible under the microscope as discrete threads called chromosomes.
As prophase 1 progresses, the nucleolus disappears and the nuclear
envelope breaks down. The centrosomes, which are responsible for
organizing the mitotic spindle, also begin to move to opposite poles
of the cell. The mitotic spindle is a structure that is composed of
microtubules and is responsible for separating the chromosomes
during cell division.
One of the key events that occurs during prophase 1 is crossing
over, also known as recombination. This is the exchange of genetic
material between homologous chromosomes, which are pairs of
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chromosomes that contain the same genes in the same order. Crossing over occurs during prophase 1 and allows for the production of
genetically diverse offspring by shuffling the genetic material of the
parents.
The exchange of genetic material between homologous chromosomes occurs at specific points called chiasmata, which are visible
as small, X-shaped structures under the microscope. Chiasmata
form when the homologous chromosomes pair up and exchange
genetic material. This process is facilitated by a protein structure
called the synaptonemal complex, which holds the chromosomes
in close proximity to each other.
During crossing over, the homologous chromosomes pair up and
the synaptonemal complex forms between them. The chromosomes then exchange genetic material at the chiasmata, creating
a new combination of genes on each chromosome. This process
can occur at multiple points along the length of the chromosomes,
resulting in the shuffling of genetic material.
After crossing over is complete, the homologous chromosomes
begin to separate and move towards opposite poles of the cell. This
is the beginning of metaphase 1, the next stage of meiosis.
In summary, prophase 1 is the first stage of meiosis and is characterized by the condensation of the chromosomes, the breakdown
of the nuclear envelope, and the movement of the centrosomes to
opposite poles of the cell. It is also the stage during which crossing
over, or recombination, occurs, allowing for the exchange of genetic
material between homologous chromosomes. The exchange of
genetic material occurs at chiasmata, which are facilitated by the
synaptonemal complex. Understanding the process of prophase 1 is
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tion and the production of genetically diverse offspring.
8.3.2
Prophase I Steps
Leptotene, zygotene, pachytene, diplotene, and diakinesis are stages
of prophase 1 in meiosis, a type of cell division that produces gametes, or sex cells, such as sperm and eggs. During meiosis, a single
cell divides into four daughter cells, each of which contains half the
number of chromosomes as the parent cell. This reduction in chromosome number is important for sexual reproduction, as it allows
for the exchange of genetic material between the parents and the
production of genetically diverse offspring.
Leptotene is the first stage of prophase 1 and is characterized by the
appearance of thin, elongated chromosomes under the microscope.
The chromosomes are made up of DNA, which is tightly packed into
a compact structure called chromatin. During leptotene, the chromatin begins to condense and becomes visible as discrete threads
called chromosomes.
Zygotene is the next stage of prophase 1 and is characterized by the
formation of the synaptonemal complex, a protein structure that
holds homologous chromosomes, or pairs of chromosomes that
contain the same genes in the same order, in close proximity to each
other. The synaptonemal complex is visible under the microscope
as a series of parallel lines between the homologous chromosomes.
Pachytene is the third stage of prophase 1 and is characterized by the
complete formation of the synaptonemal complex and the beginning of crossing over, also known as recombination. During crossing
over, the homologous chromosomes exchange genetic material at
specific points called chiasmata, which are visible as small, X-shaped
structures under the microscope. This process shuffles the genetic
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Figure 63: Crossover is a phenomenon that happens between nonidentical chromatids of homologous chromosomes. It leads to the
swapping of genetic material between these chromosomes. (Credit:
material of the parents and allows for the production of genetically
diverse offspring.
Diplotene is the fourth stage of prophase 1 and is characterized by
the separation of the homologous chromosomes. The synaptonemal complex begins to break down and the chromosomes start to
move towards opposite poles of the cell. This is the beginning of
metaphase 1, the next stage of meiosis.
Diakinesis is the final stage of prophase 1 and is characterized by
the complete separation of the homologous chromosomes and the
formation of the mitotic spindle. The mitotic spindle is a structure
that is composed of microtubules and is responsible for separating
the chromosomes during cell division.
In summary, leptotene, zygotene, pachytene, diplotene, and diakinesis are stages of prophase 1 in meiosis. They are characterized by the
condensation of the chromosomes, the formation and breakdown
of the synaptonemal complex, and the exchange of genetic material between homologous chromosomes through crossing over.
Understanding these stages is important for understanding the
mechanisms of sexual reproduction and the production of genetically diverse offspring.
8.4
The Main Differences Between Mitosis and
Meiosis
Mitosis and meiosis are both essential cell division processes, but
they serve different purposes and have distinct outcomes. Thus,
now that you are aware of the different processes that are involved
in each, it will be useful to consolidate all of this information by
appreciating the differences among those two. Here is a detailed
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comparison of their main differences to better understand their
respective significance:
Purpose:
• Mitosis is responsible for growth, repair, and maintenance of
tissues in an organism. It is also involved in asexual reproduction, where a single parent cell divides to produce genetically
identical offspring.
• Meiosis, on the other hand, is crucial for sexual reproduction, as it
produces gametes (sperm and egg cells) with half the number of
chromosomes as the parent cell. This ensures that the resulting
offspring will have the appropriate number of chromosomes
when gametes fuse during fertilization.
Number of divisions:
• Mitosis involves a single cell division, resulting in two genetically
identical daughter cells.
• Meiosis comprises two cell divisions (meiosis I and meiosis II),
resulting in four genetically unique daughter cells.
Chromosome number in daughter cells:
• In mitosis, the daughter cells have the same number of chromosomes as the parent cell (diploid).
• In meiosis, the daughter cells have half the number of chromosomes as the parent cell (haploid).
Genetic variation:
• Mitosis produces genetically identical daughter cells, with no
variation in genetic material.
• Meiosis generates genetic diversity through two processes: crossing over during prophase I, which involves the exchange of geEnterMedSchool.com
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netic material between homologous chromosomes, and independent assortment during metaphase I, where the homologous chromosomes align randomly at the cell’s equator.
Type of cells involved:
• Mitosis occurs in somatic (non-reproductive) cells.
• Meiosis takes place in germ cells, which are specialized cells that
give rise to gametes (sperm and egg cells).
Chromosome alignment:
• In mitosis, individual chromosomes line up along the cell’s equator during metaphase, forming a single column.
• In meiosis I, homologous chromosomes pair up and align along
the cell’s equator during metaphase I, forming two columns.
Sister chromatid separation:
• In mitosis, sister chromatids separate and are pulled to opposite
poles during anaphase.
• In meiosis, sister chromatids do not separate during anaphase
I; instead, homologous chromosomes are pulled apart. Sister
chromatids separate during anaphase II of meiosis II.
By understanding these key differences between mitosis and meiosis, students can better appreciate their respective roles in growth,
repair, and reproduction, as well as the generation of genetic diversity in sexually reproducing organisms.
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9
Mendel’s Experiments and Heredity
Gregor Mendel was an Austrian monk and scientist who is known
as the ”father of modern genetics.” He is famous for his experiments
on heredity, which he conducted with pea plants in the mid-1800s.
Mendel’s experiments were designed to investigate how traits, such
as flower color and seed shape, are inherited from one generation
to the next. He began by carefully selecting pea plants that had a
particular trait, such as round seeds, and crossbreeding them with
plants that had the opposite trait, such as wrinkled seeds.
Mendel then recorded the traits of the offspring and found that, in
many cases, the traits of the offspring were a combination of the
traits of the parent plants. For example, if he crossbred a plant with
round seeds with a plant with wrinkled seeds, the offspring would
have intermediate, or ”smooth,” seeds.
This led Mendel to conclude that traits are determined by ”factors,”
which we now know as genes. He also discovered that these factors
come in pairs, and that one factor from each pair is inherited from
each parent. This is known as the principle of segregation.
Mendel’s work laid the foundation for the modern science of genetics and has had a profound impact on our understanding of
heredity. Today, his principles are still widely accepted and have
helped scientists make important discoveries about the genetic
basis of inherited traits and diseases.
9.1 Phenotypes and Genotypes
Phenotypes and genotypes are fundamental concepts in genetics,
which help us understand the relationship between an organism’s
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physical traits and its underlying genetic information.
A phenotype encompasses the observable characteristics or traits
of an organism, including both external features (such as eye color,
hair color, and height) and internal aspects (such as blood type
and metabolic processes). Phenotypes result from the interaction
between an organism’s genotype and its environment, which can
influence the expression of certain traits.
A genotype, on the other hand, is the complete set of genetic information within an organism’s DNA, including all its genes and
alleles. An organism’s genotype serves as a blueprint for the development and function of its various traits. Genes are segments of
DNA that code for specific proteins or functional RNA molecules,
which contribute to the formation and regulation of an organism’s
traits.
Alleles are different versions of a gene that arise from mutations or
variations in the DNA sequence. Each individual inherits two alleles
for each gene, one from each parent. The combination of these
alleles determines the individual’s genotype for a particular trait.
The relationship between genotype and phenotype can be understood through the principles of dominant and recessive inheritance,
as illustrated by the example of eye color:
• Dominant and Recessive Alleles: Alleles can be dominant or
recessive, which affects their expression in the phenotype. Dominant alleles are expressed even when only one copy is present
(heterozygous), while recessive alleles are only expressed when
two copies are present (homozygous).
• Inheritance Patterns: The inheritance of alleles follows Mendel’s
laws of segregation and independent assortment. Segregation
states that each parent contributes one allele for each gene
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to their offspring, while independent assortment states that
the inheritance of one gene does not affect the inheritance of
another.
Using the eye color example, let’s assume the brown eye allele (B) is
dominant, and the blue eye allele (b) is recessive. There are three
possible genotypes for eye color:
• Homozygous dominant (BB): Brown eyes
• Heterozygous (Bb): Brown eyes, as the dominant brown allele is
expressed
• Homozygous recessive (bb): Blue eyes
Understanding phenotypes and genotypes is crucial for various applications, including predicting the risk of genetic diseases, guiding
selective breeding in agriculture, and contributing to personalized
medicine. By studying the relationship between phenotypes and
genotypes, researchers can identify the genetic basis of inherited
traits, understand how they are transmitted through generations,
and develop strategies to prevent or treat genetic disorders.
9.2
Sex-Linked Disorders
Sex-linked disorders are genetic conditions that arise due to mutations in genes located on the sex chromosomes (X and Y). Understanding how these disorders are inherited and analyzing related
questions on the IMAT requires a good grasp of the concepts of
sex-linked inheritance and pedigree analysis. In order to analyze
sex-linked related disorders, it’s essential to understand the basics
of sex chromosomes, inheritance patterns, and specific examples of
sex-linked disorders. We will start by discussing the foundational
concepts and then move on to examples and strategies for tackling
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such questions on an exam.
• Basics of sex chromosomes: Humans have 23 pairs of chromosomes, with one pair being the sex chromosomes, X and Y. Females have two X chromosomes (XX), while males have one X
and one Y chromosome (XY). Genes located on these sex chromosomes are responsible for sex-linked traits and disorders.
• Inheritance patterns: Sex-linked disorders can be either X-linked
or Y-linked, depending on the chromosome where the mutation occurs. X-linked disorders are more common and can be
either dominant or recessive. Y-linked disorders, on the other
hand, are rare and always dominant since males have only one
Y chromosome.
• X-linked disorders: These disorders are caused by mutations in
genes located on the X chromosome. Males are more likely to
be affected by X-linked disorders because they have only one X
chromosome, and any mutation on that chromosome will be
expressed.
• X-linked recessive: In this case, a female with two copies of the
mutant allele will be affected, while a female with one copy
will be a carrier. Males with the mutant allele will always be
affected. Examples include hemophilia and Duchenne muscular
dystrophy.
• X-linked dominant: In this case, a female with at least one copy of
the mutant allele will be affected, while males with the mutant
allele will also be affected. Examples include Rett syndrome and
fragile X syndrome.
• Y-linked disorders: These are rare genetic disorders caused by
mutations on the Y chromosome. Since males have only one
Y chromosome, Y-linked disorders are always dominant and
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exclusively affect males. Examples include Y-linked hearing loss
and some cases of male infertility.
Now that we have covered the basics, let’s discuss how to approach
sex-linked disorder questions on an exam:
1. Determine the type of disorder: Identify if the disorder is X-linked
or Y-linked, and if it is dominant or recessive. This information is
crucial for understanding the inheritance pattern and predicting
the likelihood of an individual being affected or being a carrier.
2. Analyze the given information: Carefully read the question and
understand the family history or pedigree provided. Note the
affected individuals and their relationships to one another.
3. Apply the inheritance pattern: Using the information about
the disorder and the given family history, apply the inheritance
pattern to predict the chances of an individual being affected,
being a carrier, or passing the disorder to their offspring.
Example: A question about an X-linked recessive disorder, like
color blindness:
Given: A woman with normal color vision (unaffected) has a father
who is colorblind. She marries a man with normal color vision. What
is the probability that their children will be colorblind?
Solution:
• Determine the type of disorder: Color blindness is an X-linked
recessive disorder.
• Analyze the given information: The woman’s father is colorblind,
meaning she must be a carrier (X’X) since she has normal color
vision. Her husband has normal color vision (XY). (Note: the ’ in
this case is to denote the affected chromosome)
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• Apply the inheritance pattern: Since the woman is a carrier, there
is a 50 chance she will pass on the affected X chromosome (X’)
to her children. If she has a son (XY), there’s a 50 percent chance
he will inherit the affected X chromosome and be colorblind
(X’Y). If she has a daughter (XX), there’s a 50 percent chance the
daughter will be a carrier like her mother (X’X).
In conclusion, understanding the basics of sex chromosomes, inheritance patterns, and specific sex-linked disorders is crucial for
analyzing questions related to these topics.
9.3
Mendelian Genetics
According to Mendelian genetics, there are three main modes of
inheritance: dominant, recessive, and co-dominant.
Dominant inheritance: In this mode of inheritance, a single copy
of the dominant allele is sufficient to express the trait. The dominant allele is usually represented by an uppercase letter, while the
recessive allele is denoted by a lowercase letter. In a heterozygous
individual (having two different alleles for a gene), the dominant
allele masks the expression of the recessive allele.
Example: Huntington’s disease is a dominant genetic disorder caused
by a single dominant allele (H). An individual with one copy of the
dominant allele (Hh) will develop the disease, while a person with
two recessive alleles (hh) will not.
Recessive inheritance: Recessive traits require two copies of the
recessive allele for the trait to be expressed. If an individual carries
only one recessive allele, they are considered a carrier and do not
exhibit the trait. Recessive disorders are often inherited from carrier
parents.
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Figure 64: If an individual carries a recessive X-linked disorder, their
male offspring have a 50 percent chance of being affected by the
disorder. Females, on the other hand, have a 50 percent chance of
inheriting the recessive gene and becoming a carrier of the disorder.
However, they will not be affected by the disorder itself. (Credit:
openstax.org)
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Example: Cystic fibrosis is a recessive genetic disorder caused by
two copies of the recessive allele (f). An individual with two copies
of the recessive allele (ff) will have the disease, while a person with
one dominant and one recessive allele (Ff) will be a carrier but not
affected.
Co-dominant inheritance: In co-dominant inheritance, both alleles
are expressed equally, and both contribute to the phenotype. This
results in a unique phenotype that is distinct from the homozygous
dominant or homozygous recessive state.
Example: Human blood types A, B, and O are determined by three
alleles: IA, IB, and i (O). The IA and IB alleles are co-dominant, while
the i (O) allele is recessive. An individual with one IA and one IB allele
will have blood type AB, as both alleles are expressed. An individual
with two i (O) alleles will have blood type O, while an individual with
two IA alleles will have blood type A.
Although there are more intricate aspects of genetics beyond the
three modes of inheritance, it is not necessary to study genetic
diseases or complex syndromes for the IMAT exam.
9.3.1 The Punnett Square
The Punnett square is a tool used in genetics to predict the possible
outcomes of a genetic cross or breeding experiment. It is named
after Reginald Punnett, who developed the concept in the early
1900s.
In summary, to use a Punnett square, you need to know the genotypes of the parent organisms. The genotypes are represented by
letters, with each letter representing a specific allele. For example, if
a gene has two alleles, ”A” and ”a,” ”A” could represent the dominant
allele and ”a” could represent the recessive allele. You then draw a
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grid with two rows and two columns. The genotypes of the parent
organisms are placed along the top and left side of the grid. The
genotypes of the offspring are then determined by combining the
alleles of the parent genotypes in each of the boxes in the grid.
Let’s explore the Punnett square in more depth and provide a stepby-step guide on how to create and interpret one.
1. Determine the genotypes of the parent organisms: Before creating a Punnett square, you need to know the genotypes of
both parents. For instance, let’s consider a cross between two
pea plants, where the trait of interest is the seed color (yellow or
green). One parent has a homozygous dominant genotype (AA)
for yellow seeds, while the other has a homozygous recessive
genotype (aa) for green seeds.
2. Create the grid: Draw a grid with two rows and two columns. This
creates four boxes within the square. If the organisms involved
are diploid, the Punnett square will have four boxes. However, if
they are haploid or if multiple genes are being considered, you
may need to create a larger grid.
3. Label the rows and columns: Write the alleles of the first parent’s
genotype along the top of the grid and the alleles of the second
parent’s genotype along the left side of the grid. In our example,
the grid would have the ”A” alleles along the top and the ”a”
alleles along the left side.
4. Fill in the boxes: Combine the alleles from the top row with
those from the left column in each box, resulting in the possible
genotypes for the offspring. In our example, all four boxes would
have the ”Aa” genotype since each box combines one ”A” allele
from the first parent with one ”a” allele from the second parent.
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ferent genotypes among the offspring. In our example, the
genotypic ratio would be 100
6. Determine the phenotypic ratio: Based on the offspring’s genotypes, predict their phenotypes. In our example, since the ”A”
allele is dominant for yellow seeds, all ”Aa” offspring will have
yellow seeds. The phenotypic ratio would be 100
7. Analyze the results: Interpret the results of the Punnett square
and draw conclusions about the inheritance patterns and outcomes of the cross. In this case, the cross between a homozygous dominant (AA) and homozygous recessive (aa) parent results in offspring that are all heterozygous (Aa) and have the
dominant phenotype (yellow seeds).
Understanding how to create and interpret Punnett squares is essential for predicting the genotypic and phenotypic outcomes of
genetic crosses. This skill is valuable for solving genetics problems
on exams like the IMAT, as well as for understanding inheritance
patterns in real-life scenarios.
9.3.2
Calculating the Phenotypic and Genotypic Ratios
When using a Punnett square, you can derive several ratios, such
as genotypic and phenotypic ratios, which are helpful for understanding the outcomes of a genetic cross. Here’s a step-by-step
explanation of how to calculate these ratios:
1. Complete the Punnett square: First, create a Punnett square as
previously described, filling in the possible offspring genotypes
based on the parents’ genotypes.
2. Count the different genotypes: After completing the Punnett
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square, identify the unique genotypes of the offspring and count
their occurrences. For example, consider a cross between two
heterozygous parents with the genotypes Aa (where ”A” is the
dominant allele and ”a” is the recessive allele).
In this case, the offspring genotypes are AA, Aa, and aa.
Calculate the genotypic ratio: To calculate the genotypic ratio, express the number of occurrences of each genotype as a ratio. Using
our example above, the genotypic ratio would be:
• AA: 1 (1/4 or 25
• Aa: 2 (2/4 or 50
• aa: 1 (1/4 or 25
The genotypic ratio is 1:2:1 (AA:Aa:aa).
1. Determine the phenotypes: Based on the offspring genotypes,
predict the phenotypes of the offspring. In our example, since
the ”A” allele is dominant, the offspring with the AA or Aa genotype will exhibit the dominant phenotype, while the offspring
with the aa genotype will exhibit the recessive phenotype.
2. Calculate the phenotypic ratio: To calculate the phenotypic ratio,
express the number of occurrences of each phenotype as a ratio.
Using our example, the phenotypic ratio would be:
• Dominant phenotype (AA or Aa): 3 (3/4 or 75
• Recessive phenotype (aa): 1 (1/4 or 25
The phenotypic ratio is 3:1 (dominant:recessive).
By calculating genotypic and phenotypic ratios using Punnett squares,
you can easily predict the outcomes of genetic crosses and understand the distribution of traits among offspring. This skill is essential
for solving genetics problems on exams and in real-life applications.
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9.3.3 Different Types of Genetic Crosses
Punnett squares can be used to analyze different types of genetic
crosses, including monohybrid crosses, dihybrid crosses, and more.
Here’s a detailed explanation of each type:
Monohybrid cross: A monohybrid cross involves a single trait controlled by one gene with two alleles. In this type of cross, you’re
examining the inheritance pattern of one characteristic, such as eye
color or flower color. The Punnett square for a monohybrid cross
is a 2x2 grid, as there are two possible alleles for each parent. The
genotypic and phenotypic ratios can then be calculated from the
resulting offspring genotypes.
Example: A cross between two heterozygous pea plants with purple
flowers (Pp) would be a monohybrid cross.
The genotypic ratio would be 1:2:1 (PP:Pp:pp), and the phenotypic
ratio would be 3:1 (purple:white).
Dihybrid cross: A dihybrid cross involves two traits, each controlled
by a different gene with two alleles. In this type of cross, you’re examining the inheritance patterns of two characteristics simultaneously,
such as seed shape and seed color. The Punnett square for a dihybrid cross is a 4x4 grid, as there are four possible combinations of
alleles for each parent. The resulting offspring genotypes can then
be used to calculate the genotypic and phenotypic ratios for both
traits.
Example: A cross between two heterozygous pea plants with round
yellow seeds (RrYy) would be a dihybrid cross.
The genotypic ratio for this cross would be 9:3:3:1 (RRYY:RrYy:Rryy:rryy),
and the phenotypic ratio would also be 9:3:3:1 (round yellow: round
green: wrinkled yellow: wrinkled green).
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Figure 65: An example of a dihybrid cross. (Credit: openstax.org)
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Other types of crosses: Punnett squares can also be used to analyze more complex crosses involving multiple genes, incomplete
dominance, co-dominance, or sex-linked traits. The size of the Punnett square will vary depending on the number of alleles and genes
involved. As the number of traits and alleles increases, the Punnett square becomes larger and more complex, making it more
challenging to calculate genotypic and phenotypic ratios.
In summary, Punnett squares are valuable tools for analyzing various types of genetic crosses, including monohybrid and dihybrid
crosses. By using these tools, you can predict the possible offspring
genotypes and phenotypes and better understand the inheritance
patterns of different traits.
9.4 Genetic Pedigrees
A genetic pedigree is a visual representation of the inherited characteristics of a particular family or group of individuals. It is commonly
used to track inherited traits and conditions, such as genetic diseases
or predispositions to certain health conditions. Genetic pedigrees
are often used in medical and scientific research to understand
the genetic basis of inherited traits and conditions, and to identify
potential genetic risk factors for certain health problems.
To read a genetic pedigree, you will need to understand a few basic
symbols and conventions. These symbols and conventions are used
to represent the relationships between individuals in the pedigree
and the inheritance of certain traits or conditions.
Here are some of the common symbols and conventions used in
genetic pedigrees:
A square shape is used to represent a male individual. A circle shape
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is used to represent a female individual. A horizontal line connecting
two individuals represents a marriage or partnership. A vertical line
connecting an individual to their offspring represents a parent-child
relationship. A filled-in symbol represents an individual who has
the trait or condition being studied. An open symbol represents an
individual who does not have the trait or condition being studied. To
read a genetic pedigree, start by identifying the individuals at the top
of the chart. These individuals are the founders of the pedigree, and
their traits and conditions are traced down through the generations.
Look for patterns of inheritance, such as if the trait or condition
is present in every generation or if it skips a generation. You can
also look for relationships between individuals, such as if the trait or
condition is more commonly found in one gender or if it is inherited
from a specific parent.
9.4.1 Tricks
There are a few general patterns that can help you determine the
mode of inheritance when reading a genetic pedigree. Here are a
few tips that may be useful:
Autosomal dominant inheritance: If a trait or condition is present in
every generation and appears to be inherited from one parent, it is
likely to be inherited in an autosomal dominant manner.
Autosomal recessive inheritance: If a trait or condition appears to
skip generations and is equal in amount in both females and males,
it is likely to be inherited in an autosomal recessive manner.
X-linked inheritance: If a trait or condition is more commonly found
in males and appears to be inherited from the mother, it is likely to
be inherited in an X-linked manner.
Mitochondrial inheritance: If a trait or condition is present in every
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generation and appears to be inherited exclusively from the mother,
it is likely to be inherited in a mitochondrial manner. Every individual
will be affected if the mother has a mitochondrial disease as the
mitochondria is inherited (For the IMAT) by the mother’s egg.
It is also important to note that some traits and conditions may
be inherited in a more complex manner, such as through multiple
genes or through a combination of genetic and environmental factors. In these cases, it may be more difficult to determine the mode
of inheritance based on the pattern of inheritance alone. Luckily, it
is out of the scope of the IMAT to know those.
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10
The Central Dogma
The genetic code is a vital component of molecular biology, as it
governs the relationship between the sequence of nucleotides in
DNA and the sequence of amino acids in proteins. The genetic
code consists of codons, which are groups of three nucleotides
that correspond to specific amino acids or stop signals. With four
possible nucleotides (A, T, C, and G), there are 64 possible codons in
the genetic code. Each codon is assigned to a specific amino acid,
with some amino acids being represented by multiple codons. In
addition, three codons function as stop signals that mark the end
of a protein-coding sequence.
The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to proteins within a cell. This
process can be broken down into two main steps: transcription and
translation.
1. Transcription: During transcription, an enzyme called RNA polymerase binds to a specific region on the DNA molecule called the
promoter, which marks the start of a gene. The RNA polymerase
unwinds and opens the double-stranded DNA molecule, exposing the nucleotides on each strand. One of the DNA strands,
known as the template strand, is used as a guide for RNA synthesis. The RNA polymerase reads the template strand in the 3’
to 5’ direction and synthesizes a complementary RNA molecule,
known as messenger RNA (mRNA), in the 5’ to 3’ direction. This
mRNA molecule carries the genetic information from the DNA
to the cytoplasm, where it will be translated into a protein.
2. Translation: Translation occurs in the cytoplasm on ribosomes,
which are complex molecular machines made up of ribosomal
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RNA (rRNA) and proteins. The ribosome binds to the mRNA
molecule and reads its codons. Transfer RNA (tRNA) molecules,
each carrying a specific amino acid, recognize the mRNA codons
through their complementary anticodons. The ribosome facilitates the formation of a peptide bond between adjacent amino
acids, elongating the growing polypeptide chain. This process
continues until a stop codon is encountered, signaling the end
of translation. The completed polypeptide chain is then released
and can fold into its functional three-dimensional structure or
undergo additional modifications.
The DNA remains in the nucleus to protect and preserve the cell’s
genetic information. By transcribing genes into mRNA molecules,
the cell can selectively utilize and regulate specific portions of its
genetic information without exposing the entire DNA molecule to
the cytoplasm. This separation between the DNA and the proteinsynthesizing machinery in the cytoplasm allows for precise regulation of gene expression and minimizes the risk of damage to the
DNA.
Understanding the genetic code and the central dogma of molecular biology is crucial for comprehending how genetic information
is transmitted and how it influences an organism’s traits. Knowledge of these processes is essential for various applications, such as
genetic engineering, gene therapy, and the development of novel
therapeutics for various diseases.
Think of the DNA in the nucleus as a cookbook that contains the
recipes for all the dishes that can be made in the kitchen (i.e., the
rest of the cell). The cookbook (DNA) is stored in a special cabinet
(the nucleus) that is separate from the kitchen (the rest of the cell).
In order to use the recipes in the cookbook (DNA), the cook (RNA
polymerase) must first transcribe the recipes into a form that can be
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used in the kitchen (mRNA). The cook (RNA polymerase) transcribes
the recipes by reading the cookbook (DNA) and writing down the
instructions (mRNA). The mRNA then leaves the cabinet (nucleus)
and carries the instructions to the kitchen (the rest of the cell), where
it is used to synthesize the proteins specified by the instructions.
So, the DNA doesn’t leave the nucleus because it is not in a form
that can be used directly in the rest of the cell. Instead, it must
be transcribed into mRNA, which can then be used to synthesize
proteins in the rest of the cell. This allows the genetic information
in the DNA to be used to make the proteins that are needed for the
functions of the cell.
10.1
Coding and Template Strands
The coding strand and template strand are two complementary
DNA strands that make up the double-helix structure of DNA. DNA
consists of two strands that run in opposite directions and are held
together by chemical bonds between the bases of each strand. The
coding strand carries the genetic information that is used to produce
a protein, while the template strand is the one that is copied to make
RNA.
Let’s simplify these concepts:
• Coding Strand: Imagine that the coding strand is like the correct sentence in a book, containing all the words (genetic information) in the right order to make sense. This strand has the
information that will be used to create proteins in the cell.
•
• Template Strand: The template strand is like a mirror image of
the coding strand. When the cell needs to make RNA, it uses this
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mirror-image strand as a guide to create a new RNA molecule
that is similar to the original coding strand.
Both DNA strands have a specific orientation or direction, which is
determined by their structure. The two ends of the DNA strand are
called the 3’ end and the 5’ end, named after the carbon atoms in the
sugar part of the DNA backbone. The 3’ end has an -OH (hydroxyl)
group, and the 5’ end has a -PO4 (phosphate) group. Scientists
always refer to the direction of DNA strands from the 5’ end to the 3’
end.
When the cell needs to make a protein, it first creates an RNA copy
of the relevant DNA segment. This process is called transcription.
The enzyme responsible for this process, RNA polymerase, reads
the template strand in the 3’ to 5’ direction and creates a new RNA
molecule in the 5’ to 3’ direction.
Since the coding and template strands are complementary, their
bases match up with each other: adenine (A) pairs with thymine (T),
and cytosine (C) pairs with guanine (G). When the RNA molecule is
made, it has the same sequence as the coding strand, except that
thymine (T) is replaced with uracil (U) in RNA.
To sum it up, the coding strand holds the genetic information for
making proteins, and the template strand is used to create a copy
of that information in the form of RNA. The 3’ and 5’ ends of the
DNA strands refer to the carbon atoms in the sugar-phosphate
backbone, and RNA is created by adding nucleotides to the growing
RNA molecule using the template strand as a guide.
Here is an example of a short double-stranded DNA molecule and
the corresponding RNA transcript that would be produced from it:
Double-stranded DNA:
5’ ATGCTAGCTAGCTAGC 3’
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3’ TACGATCGATCGATCG 5’
Template strand (used for RNA synthesis):
3’ TACGATCGATCGATCG 5’
RNA transcript:
5’ AUGCUAGCUAGCUAGC 3’
It is important to note that the coding strand and the RNA transcript
are identical, with the exception of the fact that RNA contains uracil
(U) instead of thymine (T). When studying for the IMAT exam, it can
be helpful to focus on the coding strand and the RNA transcript, as
they are identical and can save you time during the exam.
10.2
Replication
10.2.1 Eukaryotic Replication
DNA replication in eukaryotes is an essential process that ensures
the accurate duplication of genetic material before cell division.
Here is a detailed yet understandable explanation for students
preparing for the IMAT:
1. Replication origins: In eukaryotes, DNA replication starts at specific sequences called replication origins. Each chromosome
has multiple replication origins, which allows different regions
of the DNA to be replicated simultaneously, making the process
faster.
2. Pre-replication complex formation: At the replication origin, a
group of proteins called the pre-replication complex (pre-RC)
assembles. This complex consists of various proteins, including
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the origin recognition complex (ORC), Cdc6, Cdt1, and the MCM
helicase complex. The formation of the pre-RC is crucial for
initiating DNA replication and is strictly regulated to ensure
replication occurs only once per cell cycle.
3. DNA unwinding: The MCM helicase complex, part of the preRC, unwinds the DNA double helix, creating two single strands.
This unwinding exposes the DNA template for the replication
machinery to use.
4. Primer synthesis: DNA polymerases, the enzymes responsible
for adding new nucleotides to the growing DNA strand, cannot
start from scratch. They need a short sequence called a primer
to begin. The enzyme primase synthesizes these RNA primers
on the exposed single-stranded DNA templates.
5. Elongation: DNA polymerases attach to the primers and start
adding new nucleotides, complementary to the template strand.
In eukaryotes, the main DNA polymerases are DNA polymerase
delta and epsilon. The new DNA strands grow in the 5’ to 3’
direction.
6. Leading and lagging strands: Due to the antiparallel nature of
DNA, replication occurs continuously on the leading strand and
discontinuously on the lagging strand. The lagging strand is
synthesized in short fragments called Okazaki fragments, which
are later connected by an enzyme called DNA ligase.
7. Removal of primers and gap filling: Once the replication process
is almost complete, the RNA primers are removed, and the gaps
are filled with DNA. This task is performed by enzymes such as
RNase H, which removes the RNA primers, and DNA polymerase
delta, which fills in the gaps with the appropriate DNA sequence.
8. Termination: DNA replication in eukaryotes does not have a
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specific termination site. Instead, it ends when replication forks
from adjacent replication origins meet and the newly synthesized DNA strands are ligated together by DNA ligase.
DNA replication in eukaryotes is a highly regulated and accurate
process that ensures the faithful duplication of genetic material,
essential for growth, development, and reproduction.
10.2.2
Prokaryotic Replication
DNA replication in prokaryotes is a critical process that ensures the
precise duplication of genetic material before cell division. Here’s a
detailed yet comprehensible explanation for students preparing for
the IMAT:
1. Replication origin: In prokaryotes, DNA replication begins at a
single specific sequence called the origin of replication (oriC).
This is different from eukaryotes, which have multiple replication
origins on each chromosome.
2. DNA unwinding: At the origin, several proteins work together
to open up the DNA double helix, creating a replication bubble.
The enzyme DNA helicase unwinds the DNA, forming two singlestranded DNA templates.
3. Primer synthesis: Just like in eukaryotes, DNA polymerases in
prokaryotes cannot start synthesis without a primer. The enzyme primase synthesizes short RNA primers on the exposed
single-stranded DNA templates.
4. Elongation: DNA polymerase III, the primary DNA polymerase
in prokaryotes, attaches to the primers and starts adding new
nucleotides complementary to the template strand. The new
DNA strands grow in the 5’ to 3’ direction.
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5. Leading and lagging strands: Due to the antiparallel nature of
DNA, replication occurs continuously on the leading strand and
discontinuously on the lagging strand. The lagging strand is
synthesized in short fragments called Okazaki fragments, which
are later connected by an enzyme called DNA ligase.
6. Removal of primers and gap filling: Once replication is nearly
complete, the RNA primers are removed, and the gaps are filled
with DNA. DNA polymerase I replaces the RNA primers with the
appropriate DNA sequence, and DNA ligase seals the gaps.
7. Termination: In prokaryotes, DNA replication usually ends at a
specific termination site called the terminus region (ter). When
the replication forks reach this region, replication is halted, and
the two circular daughter chromosomes are separated by a
process called decatenation.
8. Topoisomerase and gyrase: During replication, the DNA becomes supercoiled ahead of the replication fork. Topoisomerase
and gyrase enzymes help in relieving these supercoils, allowing
replication to proceed smoothly.
DNA replication in prokaryotes is a highly regulated, efficient, and
accurate process that ensures the faithful duplication of genetic
material, crucial for the survival and reproduction of these simple,
single-celled organisms.
10.2.3 Differences between Eukaryotic and Prokaryotic Replication
Now that you are aware of the different steps that are involved in
eukaryotic and prokaryotic replication, it would be useful to conolidate this knowledge by emphasizing the differences among the
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two:
1. Genome organization: Prokaryotes have a single, circular DNA
molecule, while eukaryotes possess multiple, linear chromosomes. This difference in genome organization affects the initiation and termination of DNA replication.
2. Replication origins: Prokaryotes have a single origin of replication (oriC) on their circular DNA molecule. In contrast, eukaryotic
chromosomes have multiple replication origins, which allows for
the simultaneous replication of various segments of the large
eukaryotic genome.
3. Replication speed: Prokaryotic DNA replication is faster than eukaryotic replication. Prokaryotes have a single DNA polymerase
(DNA polymerase III) responsible for the majority of DNA synthesis, whereas eukaryotes have several DNA polymerases (such as
polymerases �, �, and �) with specific functions.
4. Replication enzymes: Both prokaryotes and eukaryotes share
some common enzymes, such as helicase, primase, and DNA
ligase. However, eukaryotes have more specialized enzymes and
additional proteins, like the sliding clamp PCNA (Proliferating
Cell Nuclear Antigen) and the replication protein A (RPA), which
help stabilize single-stranded DNA during replication.
5. Telomeres and telomerase: Due to the linear nature of eukaryotic chromosomes, eukaryotic cells face the end-replication
problem, in which the ends of chromosomes (telomeres) are
not completely replicated. To counteract this issue, eukaryotes
have evolved telomeres, repetitive DNA sequences at chromosome ends, and the enzyme telomerase, which elongates telomeres. Prokaryotes, with their circular DNA, do not face the endreplication problem and thus lack telomeres and telomerase.
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6. Compartmentalization: In eukaryotes, DNA replication occurs
inside the nucleus, while prokaryotes lack a nucleus, and replication occurs within the cytoplasm. This difference in compartmentalization can influence the regulation and coordination of
replication with other cellular processes.
7. Regulation: Eukaryotic DNA replication is tightly regulated and
coordinated with the cell cycle. Prokaryotic replication can occur
independently of the cell cycle, and a single prokaryotic cell may
have multiple rounds of replication happening simultaneously.
10.3 Eukaryotic and Prokaryotic Transcription
Prokaryotic transcription is the process of synthesizing RNA from a
DNA template in prokaryotic cells. Eukaryotic transcription is the
process of synthesizing RNA from a DNA template in eukaryotic
cells. While both processes involve the synthesis of RNA from a DNA
template, there are some key differences between prokaryotic and
eukaryotic transcription.
One of the main differences between prokaryotic and eukaryotic
transcription is the complexity of the cells and the structure of their
genomes. Prokaryotes are single-celled organisms that have a relatively simple cellular structure and a relatively small genome that
is usually contained in a single chromosome. Eukaryotes, on the
other hand, are more complex organisms that have a more complex
cellular structure and a larger genome that is usually contained in
multiple chromosomes.
Another difference between prokaryotic and eukaryotic transcription is the number and types of transcription factors involved in
the process. Prokaryotes have a smaller number of transcription
factors compared to eukaryotes, and these transcription factors are
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less diverse in terms of their functions. Eukaryotes have a much
larger number of transcription factors, which are more diverse in
terms of their functions and play a more complex role in regulating
transcription.
Prokaryotic transcription also differs from eukaryotic transcription in
the number and types of RNA polymerases involved in the process.
Prokaryotes have a single type of RNA polymerase that is responsible
for transcribing all of the genes in the genome, while eukaryotes
have multiple types of RNA polymerases that are responsible for
transcribing different classes of genes.
In addition to the differences mentioned above, prokaryotic and
eukaryotic transcription also have different mechanisms of initiation, elongation, and termination, as well as differences in RNA
processing.
Initiation:
• In prokaryotic transcription, the sigma factor associates with
RNA polymerase to form the holoenzyme, which recognizes and
binds to specific promoter sequences on the DNA. The sigma
factor then dissociates, and the core RNA polymerase proceeds
with transcription.
• In eukaryotic transcription, the initiation process is more complex and involves a series of transcription factors. The TATAbinding protein (TBP) and other general transcription factors
assemble at the TATA box (a consensus sequence found in many
eukaryotic promoters). The assembled complex, called the preinitiation complex, recruits RNA polymerase II, which then initiates transcription.
Elongation:
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• Prokaryotic transcription elongation is relatively simple, with
the RNA polymerase synthesizing the RNA transcript by adding
nucleotides to the 3’ end of the growing RNA molecule.
• Eukaryotic transcription elongation is more complicated, as it
involves the RNA polymerase II enzyme and a variety of elongation factors. These factors help to ensure the proper processivity
of the polymerase and the accuracy of the transcription process.
Termination:
• Prokaryotic transcription termination can occur through two
mechanisms: Rho-dependent termination, which involves the
Rho protein binding to the RNA transcript and dissociating the
RNA polymerase, and Rho-independent termination, which involves the formation of a hairpin loop structure in the RNA transcript that causes the RNA polymerase to dissociate from the
DNA template.
• Eukaryotic transcription termination is more complex and typically involves the cleavage and polyadenylation of the pre-mRNA
transcript, followed by the release of the RNA polymerase II enzyme.
RNA Processing:
• Prokaryotic mRNA transcripts are usually ready for translation
immediately after transcription, as there is no nucleus separating transcription and translation processes. Furthermore,
prokaryotic mRNA does not undergo extensive processing.
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cations are necessary for mRNA stability, nuclear export, and
efficient translation.
In conclusion, prokaryotic and eukaryotic transcription share the
basic principle of synthesizing RNA from a DNA template but differ
in several aspects, such as cellular complexity, genome organization,
the number and types of transcription factors and RNA polymerases,
and the mechanisms of initiation, elongation, termination, and RNA
processing. Understanding these differences is essential for comprehending the intricacies of gene expression and regulation in
different types of organisms.
10.3.1
The General Steps of Transcription
Transcription is the process of converting genetic information stored
in DNA into an RNA molecule. In this case, we will focus on the synthesis of messenger RNA (mRNA), which carries the information
needed for protein synthesis. Here are the general steps of transcription, presented in a comprehensive and straightforward manner
suitable for high school students preparing for the IMAT:
Initiation: a. The enzyme RNA polymerase binds to a specific region
on the DNA called the promoter. The promoter is a sequence of
nucleotides that signals the beginning of a gene and helps regulate
when and where transcription occurs. b. The DNA double helix
unwinds, separating the two strands and exposing the template
strand that will be used for RNA synthesis.
Elongation: a. RNA polymerase reads the template strand of DNA in
the 3’ to 5’ direction, synthesizing a complementary mRNA molecule
in the 5’ to 3’ direction. b. As RNA polymerase moves along the
template strand, it adds RNA nucleotides that are complementary
to the DNA nucleotides. Adenine (A) in DNA pairs with uracil (U) in
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RNA, and cytosine (C) pairs with guanine (G). c. The growing mRNA
molecule is composed of a series of codons, which are groups of
three nucleotides that correspond to specific amino acids or stop
signals in the genetic code.
Termination: a. Transcription ends when RNA polymerase encounters a specific DNA sequence called the terminator. This sequence
signals the end of the gene and the completion of the mRNA molecule.
b. RNA polymerase releases the newly synthesized mRNA molecule
and detaches from the DNA template. c. The DNA double helix
reforms, and the two strands rejoin.
Post-transcriptional modifications (in eukaryotes): a. Before the
mRNA molecule leaves the nucleus, it undergoes several modifications. These include the addition of a 5’ cap and a 3’ poly-A tail,
which protect the mRNA molecule from degradation and help with
stability, transportation, and translation. b. In eukaryotes, the mRNA
molecule may also undergo splicing, a process where non-coding
sequences called introns are removed, and the remaining coding
sequences called exons are joined together to form the final, mature
mRNA molecule.
After transcription and any necessary post-transcriptional modifications, the mature mRNA molecule is transported out of the nucleus
and into the cytoplasm, where it will be used as a template for protein synthesis during the process of translation.
10.4 Translation
10.4.1
Ribosomes
Ribosomes are the cellular structures responsible for synthesizing
proteins. They are found in all living cells and are composed of two
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subunits: a small subunit and a large subunit. The small subunit
is responsible for reading the mRNA molecule and selecting the
appropriate amino acid for each codon, while the large subunit is
responsible for linking the selected amino acids together to form a
protein chain.
The structure of ribosomes is complex and highly conserved across
all living organisms. Both the small and large subunits are made
up of proteins and ribosomal RNA (rRNA). In prokaryotes, the small
subunit is called 30S, and the large subunit is called 50S. In eukaryotes, the small subunit is called 40S, and the large subunit is called
60S. The proteins contribute to the overall shape and stability of
the ribosome, while the rRNA molecules play a crucial role in the
catalytic activity of the ribosome.
The ribosome has several distinct areas that are important for its
function. These include:
• The A (aminoacyl) site: This is the binding site for incoming
aminoacyl-tRNA molecules, which bring the appropriate amino
acid for the current codon in the mRNA molecule. The A site is
where the new amino acid is added to the growing polypeptide
chain.
• The P (peptidyl) site: This is the binding site for the peptidyltRNA molecule, which carries the growing protein chain. The P
site holds the tRNA molecule that is attached to the polypeptide
chain, and it is where the peptide bond formation between the
amino acids occurs.
• The E (exit) site: This is the exit site for completed tRNA molecules
that have delivered their amino acids to the growing protein
chain. After the amino acid has been transferred from the tRNA
to the polypeptide chain, the now empty tRNA molecule moves
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to the E site, from where it is released back into the cytoplasm
to be recharged with a new amino acid.
• The mRNA binding site: This is the site where the ribosome
binds to the mRNA molecule and reads the genetic code. The
small subunit of the ribosome is responsible for recognizing and
binding to specific sequences on the mRNA molecule, such as
the start codon (AUG) during the initiation phase of translation.
• The polypeptide exit tunnel: This is a narrow tunnel within the
large subunit through which the growing polypeptide chain
passes as it is synthesized. The tunnel keeps the newly synthesized protein protected until it is long enough to fold into
its functional three-dimensional structure after being released
from the ribosome.
During translation, the ribosome moves along the mRNA molecule,
reading the sequence of codons and synthesizing the protein chain
accordingly. It accurately selects the correct amino acids and catalyzes the formation of peptide bonds between them to create a
functional protein.
The structure and function of ribosomes are essential for the synthesis of proteins, which are important for the proper functioning
of cells and organisms. Dysfunctional ribosomes can lead to various
diseases and disorders, such as congenital dysmorphism, DiamondBlackfan anemia, and reticulate acropigmentation of Kitamura, but
don’t worry, they are out of the scope of the IMAT.
10.4.2 The Steps of Translation
Translation is the process of synthesizing a protein from an mRNA
molecule. It is the final step in the process of gene expression, which
begins with the transcription of DNA into mRNA. Translation occurs
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in the ribosomes, which are the cellular structures responsible for
synthesizing proteins.
The process of translation involves the following steps:
Initiation: a. The small subunit of the ribosome binds to the mRNA
molecule. b. The initiator tRNA, which carries the first amino acid
(methionine), binds to the start codon (AUG) on the mRNA molecule.
c. The large subunit of the ribosome joins the complex, forming
the complete ribosome. The ribosome now has three sites for tRNA
binding: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit)
site.
Elongation: a. A charged tRNA, carrying its corresponding amino
acid, binds to the A site of the ribosome. The tRNA’s anticodon must
match the mRNA codon present in the A site for binding to occur.
b. A peptide bond is formed between the amino acid carried by the
tRNA in the A site and the growing polypeptide chain attached to
the tRNA in the P site. c. The ribosome moves (translocates) one
codon down the mRNA molecule, shifting the tRNA from the A site
to the P site and the tRNA from the P site to the E site. The tRNA in
the E site is released, and the A site is now open for the next charged
tRNA to bind. d. This process repeats, with the ribosome moving
down the mRNA molecule, adding amino acids to the growing
protein chain until a stop codon is reached.
Termination: a. When the ribosome encounters a stop codon (UAA,
UAG, or UGA), a release factor protein binds to the A site instead
of a charged tRNA. b. The release factor promotes the cleavage of
the peptide chain from the tRNA in the P site. c. The ribosome
then dissociates into its small and large subunits, and the newly
synthesized protein is released into the cytoplasm for folding and
further processing.
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Each step in the translation process ensures the accurate and efficient synthesis of proteins, with various mechanisms in place to
maintain the fidelity of the genetic code. These mechanisms include
the highly conserved nature of the genetic code, the proofreading
ability of the ribosomes, and the error-correcting mechanisms of
the cell.
10.5
Answering Questions on Complementary
DNA strands
When answering questions related to the concept of complementary DNA strands, base pairing, and RNA transcription, it is crucial to
have a solid understanding of the basic principles. Here is a step-bystep guide to help you approach exam questions on these topics:
Review the key concepts: DNA is double-stranded and composed
of complementary strands.
• Base pairing rules: Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G) in DNA. In RNA, Thymine (T) is
replaced by Uracil (U).
• The coding strand contains the genetic information for protein
synthesis, while the template strand is used to create a complementary RNA copy.
Determine the proportion of bases:
• Remember Chargaff’s rules: The amount of adenine (A) equals
the amount of thymine (T), and the amount of cytosine (C) equals
the amount of guanine (G) in a DNA molecule.
• If the question provides the proportion or percentage of some
bases, you can calculate the proportion of the remaining bases
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Figure 66: The ribosome is a cellular structure responsible for converting mRNA into protein. It is composed of two subunits, a large
using Chargaff’s rules.
Identify the coding and template strands:
• The coding strand has the same sequence as the resulting
mRNA, with the exception that thymine (T) is replaced by uracil
(U) in RNA.
• The template strand is complementary to both the coding strand
and the resulting mRNA.
Calculate complementary sequences:
• If you are given a DNA sequence and asked to determine the
complementary sequence, use the base pairing rules: A pairs
with T, and C pairs with G.
• If you are given a template DNA strand and asked to determine
the resulting mRNA sequence, use the base pairing rules, but
remember to replace T with U in RNA: A pairs with U, and C pairs
with G.
Now, let’s apply this guide to some example questions:
Example 1: If 30 percent of the bases in a DNA molecule are adenine
(A), what is the percentage of the other bases?
• According to Chargaff’s rules, the percentage of thymine (T) will
also be 30 percent (since A pairs with T).
• The remaining percentage (100 - 30 - 30 = 40 percent) is split
equally between cytosine (C) and guanine (G), so each will be
20 percent.
Example 2: Given the template DNA strand 3’-TACGAT-5’, what is the
sequence of the coding strand and the resulting mRNA?
• The coding strand (5’-3’) is complementary to the template
strand: 5’-ATGCTA-3’.
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• The resulting mRNA has the same sequence as the coding
strand, with T replaced by U: 5’-AUGCUA-3’.
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11 Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms or biological systems to create or modify products or
processes for specific use. In recent years, it has become a trend for
medical school admission exams to include questions on biotechnology, particularly on the topic of genetic modifications. These
modifications have become increasingly common and are seen as
a potential future for medicine.
One key aspect of biotechnology is the use of recombinant DNA
technology, which involves the manipulation of DNA sequences to
create new genetic combinations or to introduce new traits into
organisms. This can be done through techniques such as gene
splicing or gene knockout, which involve the insertion or deletion
of specific genes in an organism’s genome.
Biotechnology also plays a significant role in the fields of medicine
and healthcare, with applications ranging from the development of
new drugs and therapies to the use of biotechnology for diagnostic
purposes. In addition, biotechnology has potential applications in areas such as energy production, food processing, and environmental
management.
Overall, biotechnology has the potential to revolutionize various
industries and has the potential to solve many of the world’s most
pressing problems.
11.1
Common Subjects
Some common biotechnology topics that may be studied as part of
the IMAT syllabus include:
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Recombinant DNA technology: This involves the manipulation of
DNA sequences to create new genetic combinations or to introduce
new traits into organisms.
Genetic engineering: This involves the introduction of new genes
into an organism’s genome through techniques such as gene splicing or gene knockout.
Genetic modification: This refers to the process of altering an organism’s genetic makeup through the introduction of new DNA.
Biomedical applications of biotechnology: This includes the development of new drugs and therapies, as well as the use of biotechnology
for diagnostic purposes.
Environmental applications of biotechnology: This includes the use
of microorganisms to clean up environmental pollutants and the
development of genetically modified crops and animals.
Ethics and regulation of biotechnology: This includes discussions
on the ethical implications of biotechnology and the regulations
that govern its use.
Industrial applications of biotechnology: This includes the use of
biotechnology in industries such as food processing and energy
production.
11.1.1 PCR
Polymerase chain reaction (PCR) is a widely-used laboratory technique for amplifying specific DNA sequences. It has a broad range of
applications in biotechnology, molecular biology, genetics, forensics,
and diagnostic testing.
The steps of PCR can be summarized as follows:
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• Denaturation: The DNA template is heated to a high temperature (typically around 95°C) to separate the two strands of the
double helix. This process ”unzips” the DNA, making the individual strands accessible for the primers to bind.
• Annealing: Short pieces of DNA called primers, which are complementary to specific regions of the template DNA, are added
to the mixture. The temperature is lowered (typically between
50°C and 65°C) to allow the primers to bind to their target sequences on the separated DNA strands. These primers serve as
starting points for the new DNA strands to be synthesized.
• Extension: An enzyme called DNA polymerase is added to the
mixture, along with free nucleotides (A, T, C, and G) that serve
as building blocks for the new DNA strands. The temperature is
raised (typically around 72°C) to facilitate the DNA polymerase
activity. The enzyme synthesizes new DNA strands using the
template DNA as a guide and the primers as starting points,
resulting in the production of two copies of the target DNA
sequence.
• Repeating: The process of denaturation, annealing, and extension is repeated multiple times (usually around 25-30 cycles),
leading to the exponential amplification of the target DNA sequence. With each cycle, the number of target DNA copies
doubles, resulting in a rapid increase in the quantity of the desired DNA fragment.
An analogy that may help you understand the PCR process is to
think of it as making copies of a book chapter. The book represents
the DNA template, and the chapter represents the specific DNA
sequence you want to amplify. To make copies of the chapter, you
first need to open the book (denaturation) to expose the pages.
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Then, you use bookmarks (primers) to mark the beginning and end
of the chapter (annealing). Next, you photocopy the pages of the
chapter (extension) using the bookmarks as a guide. If you repeat
this process multiple times, you will end up with many copies of the
desired chapter (amplification). This allows you to study, share, or
analyze the chapter in greater detail.
11.1.2 Gel Electrophoresis
Gel electrophoresis is a widely used laboratory technique in biotechnology and molecular biology for separating and analyzing molecules,
including DNA, RNA, and proteins, based on their size and charge.
The technique relies on the principles of molecular movement within
a gel matrix under an applied electric field.
At the core of gel electrophoresis are the following fundamental
principles:
• Molecular charge: Molecules such as DNA, RNA, and proteins
carry a net charge. In the case of DNA and RNA, the phosphate
backbone imparts a negative charge, while proteins possess
charges based on the composition of their amino acids. When
an electric field is applied, the charged molecules migrate toward the electrode with the opposite charge, moving through
the gel matrix.
•
• Gel matrix: The gel matrix, typically composed of agarose or
polyacrylamide, forms a three-dimensional lattice with pores of
varying sizes. This structure acts as a molecular sieve, allowing
smaller molecules to pass through more easily while impeding
the movement of larger molecules.
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•
• Size-based separation: As molecules move through the gel, they
experience resistance from the gel matrix. Smaller molecules
navigate the pores more easily and migrate more rapidly than
larger ones. Consequently, after a set period, molecules separate
based on their size, with smaller molecules traveling farther
through the gel.
•
• Buffer system: The buffer system used in gel electrophoresis
maintains a stable pH environment and provides ions that support the electric field. The buffer ensures consistent migration
rates and prevents degradation of the molecules during the
separation process.
•
• Visualization and analysis: After the electrophoresis, the separated molecules can be visualized using staining techniques
specific to the target molecules. The resulting pattern of bands
represents the separated molecules, which can be analyzed by
comparing their migration distances to known standards or
molecular weight markers.
In summary, gel electrophoresis is a powerful and versatile technique for separating and analyzing a variety of molecules based
on their size and charge. The fundamental principles of molecular
charge, gel matrix composition, size-based separation, buffer system, and visualization underlie the effectiveness of this method in
studying molecular structure, function, and differences between
samples.
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11.1.3
Genetic Transformation
The process of introducing a gene into a plasmid and then inserting
that plasmid into bacteria is known as genetic transformation. This
technique is widely used in biotechnology to produce proteins or
other molecules of interest in large quantities.
The detailed steps of genetic transformation are as follows:
1. Preparation of the plasmid: The first step involves preparing
the plasmid, which is a small, circular piece of DNA that can
replicate independently of the bacterial genome. The plasmid
is typically modified to contain a gene of interest, along with
regulatory elements that control its expression. The gene of
interest is inserted into the plasmid using techniques such as
restriction enzyme digestion and ligation or gene synthesis.
2. Preparation of the bacteria: The next step is to prepare the bacteria that will be used to host the plasmid. This typically involves
growing the bacteria in a culture medium, then washing and
concentrating the cells to obtain a pure population suitable for
transformation.
3. Transformation: The plasmid is introduced into the bacteria
using a process called transformation. This can be done using various methods, such as chemical transformation, where
bacteria are treated with calcium chloride to make them more
permeable, or electroporation, where a brief electrical pulse is
applied to create temporary pores in the bacterial cell membrane. During transformation, the plasmid enters the bacterial
cells and becomes incorporated into the bacterial genome or
remains as an independent, self-replicating entity.
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ture containing a selective agent, such as an antibiotic, that will
only allow cells carrying the plasmid to survive and grow. This
ensures that only cells that have successfully taken up the plasmid will be present in the final population. For example, if the
plasmid carries an antibiotic resistance gene, only transformed
bacteria will be able to grow in the presence of the antibiotic.
5. Expression: The gene of interest contained within the plasmid
will be expressed by the bacteria, leading to the production of
the protein or other molecule encoded by the gene. Factors
such as induction conditions, temperature, and growth media
can be optimized to maximize protein expression. The expressed
protein can then be harvested from the bacterial culture and
purified using various techniques, such as chromatography or
gel filtration.
In summary, genetic transformation is a powerful technique for
producing proteins or other molecules of interest in large quantities
using bacteria as hosts. The process involves the preparation of
a plasmid carrying the desired gene, transformation of bacteria
with the plasmid, selection of transformed cells, and expression and
purification of the protein or molecule of interest.
This process is commonly used to produce proteins such as insulin,
which is used to treat diabetes. It is also used to produce a wide
range of other proteins and molecules, including enzymes, hormones, and vaccines.
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12
Evolution
If you thought biology was just about memorizing the different parts
of a cell or learning about photosynthesis, think again. Evolution is
where the rubber really hits the road in the field of biology. It’s the
driving force behind the diversity of life on Earth, and it’s one of the
most fascinating subjects you’ll ever study.
But don’t let that intimidate you. Evolution may seem complicated
at first, but once you wrap your head around it, you’ll realize that
it’s actually quite simple (and also kind of hilarious). Imagine this:
millions of years ago, there was a group of apes swinging through
the trees. One day, one of these apes decided to try walking on two
legs instead of four. And voila! Humans were born.
Okay, it’s a bit more complicated than that, but you get the idea.
Evolution is all about the survival of the fittest and the continual
adaptation of organisms to their environment. It’s the reason why
there are so many different species on Earth, and it’s why we humans
are so darn smart (we had to be to survive).
So buckle up and get ready to dive into the world of evolution. Trust
us, it’s a wild ride that you won’t want to miss.
12.1
Mechanism
here are a few key ways that evolution occurs, and they’re all pretty
darn cool.
First up, we have natural selection. This is the process by which
certain traits become more or less common in a population over
time. Let’s say there’s a group of giraffes with long necks and a
group with short necks. If the long-necked giraffes are able to reach
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higher leaves on trees to eat, they’ll be more likely to survive and
reproduce. As a result, the trait of having a long neck will become
more common in the population over time. On the other hand, if
the short-necked giraffes struggle to find food and don’t reproduce
as much, the trait of having a short neck will become less common.
Next, we have genetic drift. This is when certain traits become more
or less common in a population by chance, rather than because of
any adaptive advantage. Imagine a small group of animals on a
deserted island. If one of the animals happens to have a certain trait,
that trait might become more common in the population just by
chance, even if it doesn’t give the animal any advantage.
Finally, we have mutation. This is when there’s a change in an organism’s DNA, which can lead to new traits. Mutations can be caused
by things like radiation or errors during DNA replication. Some mutations might be harmful, while others might be neutral or even
beneficial.
So there you have it! Those are the three main mechanisms behind
evolution. Natural selection, genetic drift, and mutation. And remember, these mechanisms are what drive the amazing diversity
of life on Earth, so embrace them and never stop learning!
12.1.1 Natural Selection
Natural selection is a fundamental principle in evolutionary biology
and acts as a driving force behind the adaptation and diversification
of species. This process is responsible for shaping the biological
characteristics of organisms over time, by favoring the survival and
reproduction of individuals with traits that are advantageous in a
given environment.
Here’s how it works: let’s say there’s a population of giraffes with
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both long necks and short necks. The long-necked giraffes are able
to reach higher leaves on trees to eat, while the short-necked giraffes
struggle to find food. As a result, the long-necked giraffes are more
likely to survive and reproduce, while the short-necked giraffes are
less likely to do so. Over time, the trait of having a long neck will
become more common in the population, while the trait of having
a short neck will become less common.
This process is known as ”survival of the fittest,” because the organisms with traits that are best suited to their environment are more
likely to survive and pass on those traits to their offspring. It’s important to note that ”fitness” in this context doesn’t necessarily mean
physical strength or agility. It simply refers to an organism’s ability
to survive and reproduce in its environment.
The process of natural selection can be broken down into four key
components:
• Variation: Within any population, there is a natural genetic variation among individuals. These variations can be in the form
of different physical or behavioral traits, such as size, color, or
foraging strategies.
• Inheritance: The genetic basis of these variations means that
traits can be passed down from parents to offspring through
their DNA. This allows for the continuity of certain traits within a
population across generations.
• Selection pressure: Environmental factors, such as availability of
resources, predation, or climate, can exert a selection pressure
on the population. This means that some individuals with certain traits will have a higher chance of survival and reproduction
than others.
• Adaptation: Over time, the frequency of advantageous traits
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within the population will increase, as individuals with these
traits are more likely to survive and reproduce. This leads to an
overall adaptation of the population to its environment.
To further illustrate the concept of natural selection, let’s consider another example: the peppered moth. Before the industrial revolution
in England, the peppered moth population consisted predominantly
of light-colored moths, which were well-camouflaged against the
light-colored trees they inhabited. However, as the industrial revolution progressed, soot from factories darkened the trees, making the
light-colored moths more visible to predators. Dark-colored moths,
which were initially rare, became better camouflaged and were less
likely to be eaten. As a result, the dark-colored moths had a higher
survival rate and produced more offspring, leading to an increase in
the frequency of the dark coloration within the population.
Natural selection can occur at different rates and can be influenced
by various factors, such as the size of the population, the degree
of competition for resources, and the rate at which new mutations
arise. It’s also important to remember that natural selection doesn’t
necessarily lead to the ”perfection” of a species. It simply leads to
the development of traits that are well-suited to the current environment. If the environment changes, the traits that were once
advantageous may no longer be so, and the population may need
to adapt in order to survive.
In summary, natural selection is a gradual process that operates over
many generations, constantly shaping the traits and characteristics
of organisms. It plays a crucial role in the evolution of species by
favoring the survival and reproduction of individuals with traits that
are advantageous in their environment. As environments change,
natural selection ensures that species continue to adapt in order to
maximize their chances of survival and reproduction.
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12.1.2
Genetic Drift
Genetic drift is a key evolutionary force that alters allele frequencies
in a population due to random events rather than selection pressures. It plays a particularly significant role in small populations,
where random fluctuations can have a more substantial impact on
gene distribution. As genetic drift continues, it can lead to changes
in allele frequencies over generations and potentially the fixation or
loss of specific alleles.
There are two main types of genetic drift, each with unique implications for population genetics:
1. Founder effect: This occurs when a small group of individuals
from a larger population becomes isolated and establishes a
new population. The gene pool of this new population is limited to the genes of the founding individuals. This limited gene
pool may not accurately represent the genetic diversity of the
larger population, causing certain alleles to be over-represented,
underrepresented, or even completely absent in the new population. For example, if a small group of animals becomes isolated
on an island, the genetic diversity of the island population may
be quite different from that of the mainland population.
2. Bottleneck effect: This occurs when a population experiences
a drastic reduction in size due to an event such as a natural
disaster, disease, or human intervention. The surviving individuals may not be a representative sample of the original population’s genetic diversity. As the population recovers in size,
the gene pool may be significantly altered, with some alleles
becoming more common or even fixed, while others may be lost.
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tion in the 19th century, leaving only about 20-30 individuals.
The population has since recovered to over 100,000 individuals, but genetic diversity is still severely reduced compared to
pre-exploitation levels.
In both cases, genetic drift can lead to a loss of genetic diversity
and an increased likelihood of inbreeding, which can further reduce genetic diversity and increase the risk of genetic disorders.
This is a particular concern in conservation efforts, as preserving
genetic diversity is critical to the long-term survival and adaptability
of species. By understanding and studying genetic drift, researchers
can better predict and mitigate the impacts of these processes on
populations and develop strategies for maintaining genetic diversity
in threatened species.
12.1.3
Mutation
Mutation is an essential mechanism of evolution, as it introduces
genetic variation into a population. Genetic variation is crucial for
evolution, as it provides the raw material for natural selection to
act upon, allowing populations to adapt to changing environments
and pressures. Mutations are random changes in an organism’s
genetic material (DNA or RNA) and can be caused by various factors
such as replication errors, exposure to radiation, or chemical agents.
They can result in the alteration, deletion, or addition of genetic
information.
There are different types of mutations:
Point mutations: These occur when a single base pair in the DNA
sequence is changed. Point mutations can be classified into three
categories:
• Silent mutations: These do not result in a change in the amino
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acid sequence of the protein. For example, if the codon GCU
(which codes for the amino acid Alanine) mutates to GCC, the
amino acid sequence remains unchanged, as both GCU and
GCC code for Alanine.
• Missense mutations: These result in a change in the amino acid
sequence of the protein, potentially altering its function. For
example, a mutation in the gene responsible for producing the
hemoglobin protein can lead to sickle cell anemia, a condition
where red blood cells become misshapen and less effective at
transporting oxygen.
• Nonsense mutations: These introduce a premature stop codon
in the protein-coding sequence, leading to the production of
a truncated, non-functional protein. For instance, a nonsense
mutation in the dystrophin gene can cause Duchenne muscular
dystrophy, a severe muscle wasting disorder.
Chromosomal mutations: Chromosomal mutations are genetic
changes that involve a large segment of DNA, such as an entire
chromosome or a large portion of it. These mutations can result in
changes in the structure or number of chromosomes, leading to
altered gene expression and potential genetic disorders.
Deletions occur when a segment of DNA is lost from a chromosome,
resulting in the loss of genetic information. This can lead to the
disruption of gene function and potentially cause genetic disorders.
Duplications occur when a segment of DNA is duplicated, resulting
in extra copies of genetic information. This can also lead to altered
gene expression and potential genetic disorders.
Inversions occur when a segment of DNA breaks off from a chromosome, flips around, and reattaches in the opposite orientation.
This can disrupt the normal functioning of genes in the inverted
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segment, potentially leading to genetic disorders. Translocations occur when a segment of DNA breaks off from one chromosome and
attaches to another chromosome. This can also disrupt the normal
functioning of genes, potentially leading to genetic disorders.
Down syndrome, also known as trisomy 21, is an example of a chromosomal mutation that occurs when there is an extra copy of chromosome 21. This leads to the overexpression of genes on chromosome 21, which can cause developmental delays, intellectual disability, and other physical and health problems. Other examples
of chromosomal mutations include Turner syndrome, Klinefelter
syndrome, and Cri-du-chat syndrome.
Mutations can be beneficial, neutral, or harmful, depending on their
effect on an organism’s fitness in a particular environment. Beneficial mutations increase an organism’s fitness and are more likely
to be passed on to future generations through natural selection.
Neutral mutations have no significant impact on an organism’s fitness, while harmful mutations decrease fitness and may be selected
against over time.
An analogy for understanding mutation in evolution is to think of a
population’s genetic makeup as a library of books. Each book represents a different gene, and each letter in the book represents a DNA
base pair. As mutations occur, they introduce ”typos” or changes
in the text. Some of these changes may be inconsequential (silent
mutations), while others may introduce new information (beneficial
mutations) or create errors that hinder the book’s understanding
(harmful mutations). Over time, these mutations accumulate, leading to genetic variation and, ultimately, evolution.
In summary, mutation is a crucial mechanism of evolution as it introduces genetic variation into populations. This variation allows
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organisms to adapt and evolve in response to changing environments and selective pressures, ensuring their long-term survival
and the continued diversification of life on Earth.
12.2 Formation of New Species
Species formation, or speciation, is the process by which new species
arise from existing populations due to genetic and phenotypic
changes over time. Speciation is a fundamental concept in biology and is driven by various mechanisms that lead to reproductive
isolation between populations. The main types of speciation are:
1. Allopatric speciation: This occurs when populations are geographically separated, preventing gene flow between them.
Over time, genetic and phenotypic differences accumulate, leading to the formation of distinct species. Geographic barriers,
such as mountains, rivers, or habitat fragmentation, can cause
allopatric speciation.
2. Sympatric speciation: This involves the formation of new species
within a shared geographic area. Sympatric speciation may result from factors such as ecological niche differentiation, sexual
selection, or genetic mutations that cause reproductive isolation
without the need for a physical barrier.
3. Parapatric speciation: This type of speciation occurs when adjacent populations experience different selective pressures and
gradually diverge, eventually becoming distinct species. While
there is limited gene flow between parapatric populations, reproductive isolation eventually develops due to the accumulation
of genetic differences and local adaptations.
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patric speciation occurs when a small, isolated population on
the periphery of a larger population diverges due to genetic
drift and selection pressures unique to its specific environment.
In summary, species formation is the process by which new species
arise through genetic and phenotypic changes that ultimately result
in reproductive isolation. The main types of speciation - allopatric,
sympatric, parapatric, and peripatric - are distinguished by the circumstances and mechanisms driving the divergence of populations
and the formation of distinct species.
12.2.1 Allopatric Speciation
Allopatric speciation is a fundamental process in evolutionary biology, responsible for the formation of new species when populations
of a species become geographically isolated from one another. This
isolation can result from various events, such as the emergence of a
physical barrier like a mountain range or a body of water, or longdistance dispersal to a new area. With the interruption of gene flow
between the separated populations, they are unable to interbreed
and gradually diverge due to different selective pressures, ultimately
leading to the development of new species.
Allopatric speciation is a widespread mechanism for the generation
of biodiversity, particularly in cases where a physical barrier divides a
population into isolated groups. The isolated populations may experience different environmental conditions and selective pressures,
leading to the evolution of unique traits, adaptations, and genetic
differences.
Several mechanisms can contribute to allopatric speciation, including:
1. Physical barriers: When a physical barrier arises and separates
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two populations of a species, they become isolated from one
another, with no opportunity for gene flow between them. Over
time, they evolve independently, accumulating differences that
may ultimately result in distinct species.
2. Dispersal: In some cases, individuals from a population may
disperse to a new, geographically distant area and establish a
new population. The physical distance between the original and
the new population can limit gene flow, leading to allopatric
speciation.
3. Vicariance: Vicariance occurs when a population is divided into
two or more isolated groups by the emergence of a physical
barrier, such as a river or a volcanic eruption. The isolated populations evolve separately under different selective pressures,
ultimately giving rise to new species.
4. Adaptive radiation: In some instances, allopatric speciation can
lead to an adaptive radiation, where multiple new species evolve
rapidly from a single ancestral species. This typically occurs
when the isolated populations occupy different ecological niches,
experiencing unique selective pressures that drive the evolution
of diverse adaptations.
Understanding allopatric speciation is crucial in fields such as biology, ecology, and conservation, as it helps reveal the mechanisms
of evolution and speciation. Moreover, it allows scientists to develop
strategies for preserving biodiversity by taking into account the
factors that contribute to the emergence of new species and the
maintenance of genetic diversity within populations.
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12.2.2
Sympatric Speciation
Sympatric speciation is an intriguing process in which new species
evolve while living in the same geographic area, without physical barriers separating the populations. This type of speciation can occur
when populations within a species become reproductively isolated
due to various factors such as behavioral, ecological, or genetic adaptations. Over time, these isolated populations accumulate enough
differences to be considered distinct species.
Though sympatric speciation is considered less common than allopatric speciation, it still plays a crucial role in understanding the
diverse mechanisms of evolution and speciation.
Several mechanisms can contribute to sympatric speciation:
• Polyploidy: Polyploidy is a genetic mutation that results in an
individual having extra sets of chromosomes. This phenomenon
is common in plants and can lead to the development of a new
species that is reproductively isolated from the parent population. For example, wheat (Triticum aestivum) is a hexaploid
species derived from the hybridization of three diploid ancestral
species. The resulting polyploid wheat is unable to breed with
its diploid ancestors, effectively creating a new species.
• Ecological divergence: When populations within a species occupy different habitats or ecological niches, they experience
different selective pressures that may drive their evolution in
distinct directions. For example, the apple maggot fly (Rhagoletis pomonella) originally fed on hawthorn fruit but later diversified to feed on apples. This led to divergent selection on
apple and hawthorn flies, resulting in different mating times
and preferences, ultimately reducing gene flow between the
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two populations and promoting sympatric speciation.
• Reproductive isolation: Behavioral or physiological adaptations
that prevent interbreeding between populations can also contribute to sympatric speciation. For example, in African cichlid
fish species from Lake Malawi, sexual selection for different male
color patterns has led to reproductive isolation between populations, as females preferentially mate with males displaying their
preferred color pattern.
• Disruptive selection: Sometimes, extreme phenotypes within
a population are more advantageous than intermediate ones,
leading to disruptive selection. This can result in the formation
of two distinct subpopulations with unique adaptations, which
may eventually become reproductively isolated. For instance,
the apple maggot fly mentioned earlier experienced disruptive
selection based on their host fruit preference, contributing to
the evolution of separate species.
Understanding sympatric speciation is essential in fields such as
biology, ecology, and conservation, as it sheds light on the complex mechanisms of evolution and speciation. This knowledge can
help scientists develop strategies for preserving biodiversity and
maintaining genetic diversity within populations.
12.2.3
Parapatric Speciation
Parapatric speciation is an intriguing form of speciation that occurs when populations within a species evolve into separate species
while living in adjacent geographic areas. In this process, populations experience different selective pressures due to environmental
gradients or ecological boundaries, leading to the development of
unique adaptations and, eventually, reproductive isolation.
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Parapatric speciation is considered an intermediate form of speciation between allopatric and sympatric speciation, as populations are
neither completely separated by physical barriers nor in the same
geographic area. This type of speciation highlights the importance
of understanding various evolutionary mechanisms and the diverse
ways in which new species can arise.
Several factors can contribute to parapatric speciation:
• Environmental gradients: When populations of a species inhabit
areas with different environmental conditions, they may experience distinct selective pressures that drive their evolution in
separate directions. For example, the grass Anthoxanthum odoratum grows along a gradient of soil composition, from soils rich
in copper to those with lower copper concentrations. Different
populations have evolved to tolerate varying copper levels, leading to genetic differentiation and reduced gene flow between
populations.
• Ecotones and hybrid zones: Ecotones are transitional areas between different ecosystems, while hybrid zones are regions
where genetically distinct populations interbreed. These areas
can create unique selective pressures that promote divergence
between adjacent populations. For instance, the European firebellied toad (Bombina bombina) and the yellow-bellied toad
(Bombina variegata) hybridize in a narrow zone where their
ranges overlap. The hybrids have reduced fitness, which maintains a sharp genetic boundary between the two species despite
their close proximity.
• Disruptive selection: Disruptive selection can also drive parapatric speciation, as populations with extreme phenotypes may
have a selective advantage over intermediate phenotypes. This
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process can result in the formation of distinct subpopulations
that eventually become reproductively isolated. In the case of
the grass Anthoxanthum odoratum mentioned earlier, disruptive selection based on copper tolerance has contributed to the
evolution of distinct populations.
• Secondary contact: Sometimes, previously allopatric populations may come into secondary contact after a period of geographic separation. In this scenario, if the populations have
accumulated sufficient genetic differences, they may maintain
their distinctiveness despite being in close proximity. This can
result in parapatric speciation, where the two species are now
adjacent but remain separate due to reduced gene flow and
reproductive isolation.
Understanding parapatric speciation is crucial in fields such as biology, ecology, and conservation, as it provides insights into the
complex mechanisms of evolution and speciation. This knowledge
can help scientists develop strategies for preserving biodiversity and
maintaining genetic diversity within and between populations.
12.2.4
Peripatric Speciation
Peripatric speciation is a form of speciation that occurs when a
small, peripheral population becomes isolated from the main population and evolves into a separate species. This process is similar
to allopatric speciation, but the focus is on the isolation of a small,
peripheral group rather than the separation of two large populations
by a physical barrier. The founder effect and genetic drift play significant roles in peripatric speciation, as the small, isolated population
experiences different selective pressures and evolutionary forces
compared to the main population.
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Peripatric speciation is a fascinating process that highlights the role
of peripheral populations in the evolution of new species and the
impact of genetic drift and founder effects on speciation.
Several factors contribute to peripatric speciation:
• Founder effect: When a small group of individuals becomes
isolated from the main population and establishes a new population, the gene pool of the new population is limited to the
genes of the founders. Any rare alleles present in the larger
population may be lost, and this can lead to the rapid fixation
of certain alleles in the new population and the loss of genetic
diversity. Over time, the isolated population may evolve unique
adaptations that lead to the development of a new species.
•
• Genetic drift: In small, isolated populations, genetic drift can
have a significant impact on the frequency of alleles, as random
changes in allele frequencies can occur more rapidly than in
larger populations. This can lead to an increased likelihood of
certain alleles becoming fixed due to chance, and it may result
in the evolution of unique traits and adaptations that contribute
to the formation of a new species.
• Different selective pressures: The peripheral population may
experience different environmental conditions and selective
pressures than the main population, leading to the evolution of
unique adaptations. These adaptations may eventually result
in reproductive isolation, preventing the peripheral population
from interbreeding with the main population and leading to
the formation of a new species.
• Geographic isolation: Although the primary focus in peripatric
speciation is on the isolation of a small, peripheral population,
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geographic barriers can still play a role in separating the peripheral population from the main population, further limiting gene
flow and promoting speciation.
An example of peripatric speciation is the formation of the Galápagos finches, which evolved from a common ancestor that colonized
the islands. The small, isolated populations that formed on different
islands experienced different selective pressures based on factors
such as food availability and environmental conditions. Over time,
these populations evolved distinct beak shapes and sizes, which
eventually led to the development of several unique species.
Understanding peripatric speciation is essential in fields such as
biology, ecology, and conservation, as it provides insights into the
mechanisms of evolution and speciation. This knowledge can help
scientists develop strategies for preserving biodiversity and maintaining genetic diversity within and between populations.
12.3 Population Evolution
Population evolution is the study of how populations of organisms
change over time. It is a subfield of evolutionary biology, which
focuses on how species evolve and change through time. Population evolution is concerned with understanding how changes in
the genetic makeup of a population can lead to changes in the
characteristics and traits of the organisms within that population.
There are several mechanisms that can drive population evolution,
including natural selection, genetic drift, and gene flow. Natural
selection is the process by which certain traits become more or
less common in a population over time due to their ability to help
organisms survive and reproduce. Genetic drift is a process that
occurs when the frequency of a gene variant, or allele, changes in a
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population due to chance rather than natural selection. Gene flow
is the movement of genes into or out of a population, which can
occur through processes such as migration or interbreeding.
Population evolution is an important area of study in fields such
as biology, conservation, and genetics, as it helps us understand
the mechanisms behind the evolution of species and the factors
that influence their evolution. It can also provide insight into the
processes that have shaped the diversity of life on Earth and inform
efforts to protect and conserve endangered species.
12.3.1
Gene Flow
Gene flow, also known as gene migration, is a critical mechanism
in population evolution, as it involves the exchange of genetic information between populations. It can occur when individuals or
their gametes (sperm or eggs) move from one population to another, leading to the mixing of genetic material. This transfer of
genetic variation can significantly impact the genetic makeup and
characteristics of a population, ultimately shaping its evolution.
There are several ways in which gene flow contributes to population
evolution:
Introduction of new genetic variation: Gene flow can introduce new
alleles or genetic combinations into a population, potentially increasing the overall genetic diversity. This introduction of novel genetic
material can provide populations with new traits that may enhance
their ability to adapt to changing environmental conditions. Greater
genetic diversity generally promotes a population’s adaptability, as
it offers more potential responses to selective pressures.
Counteracting the effects of genetic drift and inbreeding: Genetic
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cially in small populations. Gene flow can counteract these effects
by introducing new genetic material, which helps to maintain a
higher level of genetic diversity within the population. This is particularly important for the long-term survival and adaptability of a
population.
Homogenizing populations: High levels of gene flow between populations can cause the populations to become more genetically
similar. This genetic homogenization can be advantageous if it promotes the spread of beneficial traits across populations. However, it
can also be detrimental if it leads to the loss of locally adapted traits
that are important for the survival of individual populations under
specific environmental conditions.
Speciation and reproductive isolation: In some cases, gene flow can
be reduced or prevented due to physical, ecological, or behavioral
barriers. When gene flow is limited or nonexistent, populations
may diverge genetically, ultimately leading to speciation and the
formation of new species. In this sense, gene flow can play a role in
both promoting and inhibiting the formation of new species.
Adaptation to environmental changes: Gene flow can facilitate the
spread of advantageous alleles across populations, allowing them
to adapt more rapidly to changing environmental conditions. For
example, if a population encounters a new disease or parasite, gene
flow from other populations carrying resistance alleles can help the
affected population develop resistance more quickly.
An example of gene flow’s impact on population evolution can be
observed in the peppered moth (Biston betularia) in the United
Kingdom. The moth has two primary color forms: a light form and a
dark form. During the Industrial Revolution, pollution led to the darkening of tree trunks, and the dark form of the moth became more
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common due to its increased camouflage. As pollution decreased
and tree trunks lightened, the light form regained prevalence. This
shift in moth coloration was facilitated by gene flow, as moths migrated between different areas, spreading the advantageous color
forms.
Understanding gene flow is essential for fields such as biology, ecology, and conservation, as it provides insights into the mechanisms
of population evolution and the maintenance of genetic diversity.
This knowledge can help scientists develop strategies for preserving
biodiversity and maintaining genetic diversity within and between
populations.
12.4 The Hardy-Weinberg equilibrium
The Hardy-Weinberg equilibrium is a theoretical concept in population genetics that describes the frequency of different genotypes
(variants of a gene) in a population under specific conditions. It is
named after G. H. Hardy and Wilhelm Weinberg, who independently
published work on the concept in 1908 and 1909, respectively.
The Hardy-Weinberg equilibrium states that, in a population that is
not evolving (that is, not undergoing natural selection, migration,
mutation, or any other forces that could change the frequencies
of genotypes), the frequencies of different genotypes will remain
constant from generation to generation. This means that the proportion of individuals with a particular genotype in the population
will stay the same over time, unless some force acts to change it.
The Hardy-Weinberg equilibrium is an important concept in evolutionary biology because it provides a baseline against which we
can compare real populations. If we observe a population and see
that the frequencies of different genotypes are not what we would
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expect based on the Hardy-Weinberg equilibrium, we can conclude
that some force, such as natural selection or mutation, is acting on
the population and causing it to evolve.
The Hardy-Weinberg equilibrium can be described mathematically
using a set of equations that describe the frequencies of different
genotypes in the population. These equations allow us to calculate
the expected frequencies of genotypes based on the frequencies of
the alleles (different versions of a gene) in the population.
The Hardy-Weinberg equilibrium can be described mathematically
using the following equation:
p2 + 2pq + q 2 = 1
where p and q are the frequencies of the two alleles in the population. The frequency of the dominant allele (represented by p) is
squared, and the frequency of the recessive allele (represented by
q) is squared and added to the result. The sum of these two values
(p2 and q2 ) represents the frequency of individuals with the dominant and recessive genotypes, respectively. The term 2pq represents
the frequency of individuals with the heterozygous genotype (an
individual with one copy of each allele).
For example, consider a population in which the frequency of the
dominant allele (A) is 0.7 and the frequency of the recessive allele (a)
is 0.3. The frequency of individuals with the homozygous dominant
genotype (AA) would be p2 = 0.72 = 0.49, the frequency of individuals
with the homozygous recessive genotype (aa) would be q2 = 0.32 = 0.09,
and the frequency of individuals with the heterozygous genotype
(Aa) would be 2pq = 2 ∗ 0.7 ∗ 0.3 = 0.42. When we add these three frequencies together, we get 1, which is what we expect based on the
Hardy-Weinberg equilibrium.
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13
Viruses
Welcome to the exciting world of viruses! These tiny infectious
agents may be small, but they pack a big punch. With their ability
to infect and replicate within host cells, viruses have shaped the
course of history and continue to be a major force in biology today.
But don’t let their fearsome reputation deter you – studying viruses
can be both fun and rewarding. In this textbook, you’ll learn about
the amazing diversity of viruses and the ways in which they interact
with their hosts. You’ll also discover the many ways in which viruses
are used in medicine and biotechnology, from vaccines to gene
therapy.
So buckle up and get ready to dive into the fascinating world of
viruses. Who knows, you might even catch a virus of your own – the
intellectual kind, that is!
13.1 Main Characteristics of Viruses
Viruses are unique entities in the biological world, as they possess
distinct characteristics that set them apart from other organisms.
The following is a detailed discussion of the main characteristics of
viruses that make them distinct:
1. Infectious, acellular pathogens: Viruses are acellular, meaning
they are not made up of cells. Instead, they consist of genetic
material (DNA or RNA) surrounded by a protein coat called a
capsid. Some viruses also have an outer lipid envelope. Because
they lack cellular structures and cannot carry out the basic functions of life on their own, viruses are often considered to be on
the borderline between living and non-living entities.
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2. Obligate intracellular parasites with host and cell-type specificity: Viruses are obligate intracellular parasites, which means
they can only reproduce inside a host cell. They cannot replicate on their own and must hijack the host cell’s machinery to
reproduce. Viruses often exhibit host and cell-type specificity,
meaning they can only infect specific hosts or cell types within
those hosts. This specificity is due to the interactions between
viral surface proteins and host cell surface receptors, which determine whether a virus can enter a cell and initiate an infection.
3. DNA or RNA genome (never both): Viruses contain either DNA
or RNA as their genetic material, but never both. This is a unique
feature, as other cellular organisms, such as bacteria, archaea,
and eukaryotes, contain DNA as their genetic material. The type
of genetic material in a virus can be single-stranded or doublestranded, linear or circular, and segmented or non-segmented.
The genome size of viruses varies greatly, from just a few genes
to over a thousand.
4. Genome is surrounded by a protein capsid and, in some cases, a
phospholipid membrane studded with viral glycoproteins: The
viral genome is protected by a protein coat called a capsid, which
is made up of protein subunits called capsomeres. The capsid
provides structural stability and helps the virus attach to and
enter host cells. Some viruses, called enveloped viruses, have
an additional outer lipid envelope derived from the host cell
membrane. This envelope is studded with viral glycoproteins
that play a crucial role in recognizing and binding to host cell
receptors, facilitating viral entry into the host cell.
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tion because they lack many of the essential genes and enzymes required for replication, transcription, and translation.
They must, therefore, exploit the host cell’s genetic machinery
and resources to reproduce. This exploitation can cause damage
to the host cell, often leading to its death. The viral replication
process can also trigger immune responses in the host, which
may contribute to the symptoms and pathology of viral infections.
These unique characteristics of viruses make them distinct from
other living organisms and present challenges for the development
of antiviral therapies and vaccines. Understanding the specific features of viruses is crucial for the study of viral infections and the
development of effective strategies to combat them.
13.2 Structure
At their most basic level, viruses are simply collections of genetic
material (DNA or RNA) surrounded by a protein coat. But don’t let
their simplicity fool you – the intricacies of viral structure are what
allow them to be so effective at infecting host cells and replicating
themselves.
One important aspect of viral structure is the capsid, which is the
protein coat that surrounds the viral genetic material. The capsid
is made up of subunits called capsomeres, which are arranged in a
specific pattern to create the overall shape of the virus. The shape of
the capsid can be important for the virus’s ability to enter and infect
host cells.
In addition to the capsid, some viruses also have an outer envelope
made of lipids. This envelope is derived from the host cell’s membrane and helps the virus interact with host cells and evade the host
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immune system.
13.2.1 Virus Morphology
Viral morphology is the study of the shapes and structures of viruses.
Viruses are tiny infectious agents that can infect all living organisms,
including humans, animals, plants, and even bacteria. They are
composed of genetic material (either DNA or RNA) enclosed in a
protein coat called a capsid. Based on their capsid structure, viruses
can be classified into three main types: helical, icosahedral, and
complex. In this discussion, we will explore each of these types in
detail, focusing on their structures, examples, and significances.
Helical viruses: Helical viruses have a capsid that forms a helix or
spiral shape around the viral genetic material. The capsid proteins
are arranged in a cylindrical manner, creating a rod-like structure.
One of the most well-known examples of helical viruses is the tobacco mosaic virus (TMV), which infects plants. In humans, the
influenza virus and the rabies virus also exhibit helical morphology.
The helical structure provides stability and protection for the viral
genetic material and facilitates entry into host cells.
Icosahedral viruses: Icosahedral viruses have a capsid structure that
resembles a geometric shape called an icosahedron, which consists
of 20 equilateral triangular faces. This shape is highly symmetrical and efficient in terms of enclosing the maximum volume with
the minimum amount of capsid proteins. Many common human
viruses, such as the adenovirus (which causes respiratory infections),
the poliovirus, and the human papillomavirus (HPV), have icosahedral structures. The icosahedral shape offers stability and helps the
virus to withstand harsh environmental conditions.
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cate structure than helical or icosahedral viruses. They may possess
additional components, such as tails, fibers, or envelopes, which can
aid in attachment and entry into host cells. Bacteriophages, which
are viruses that infect bacteria, are a prime example of complex
viruses. They have a head (which can be icosahedral or helical) that
contains the genetic material, and a tail that helps in attaching to
the bacterial cell surface. The complex structure of these viruses
allows for specialized functions, such as injecting genetic material
into the host cell.
In conclusion, understanding viral morphology is essential for grasping how viruses function and interact with their hosts. Helical, icosahedral, and complex viruses exhibit distinct structures that play a
crucial role in their ability to infect host cells and cause disease. By
studying these structures, you will be able to gain valuable insight
into the fascinating world of viruses and their interactions with living
organisms.
13.2.2
List of Structures
• Nucleic Acid: The genetic material of a virus is contained within
its nucleic acid, which can be either DNA (deoxyribonucleic
acid) or RNA (ribonucleic acid). The nucleic acid contains the
instructions for viral replication, the synthesis of viral proteins,
and the assembly of new virus particles. The organization of viral
genomes can vary greatly, ranging from small circular molecules
to complex linear structures with multiple segments.
• Capsid: The capsid is a protein coat that surrounds and protects the viral nucleic acid. It is composed of subunits called
capsomeres, which are arranged in a specific pattern to create
the overall shape of the virus. The shape of the capsid can be
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Figure 67: Viral morphology can be classified into 3 main types: (a)
Helical, (b) Icosahedral, and (c) Complex (Credit: openstax.org)
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helical, icosahedral, or complex, and it can play a crucial role
in the virus’s ability to enter and infect host cells. The capsid
also serves as an antigenic determinant, stimulating the host’s
immune response.
• Envelope: Some viruses have an outer envelope made of lipids
that is derived from the host cell’s membrane. This envelope
provides additional protection and helps the virus interact with
host cells by facilitating viral attachment and entry. The envelope can also play a role in the virus’s ability to evade the host
immune system by presenting host-derived molecules on its
surface, making it harder for the immune system to recognize
the virus as foreign.
• Spike proteins and other surface glycoproteins: Some viruses,
such as the coronavirus that causes COVID-19, have spike proteins on their surface. These proteins help the virus bind to and
enter host cells by interacting with specific host cell receptors.
Other surface glycoproteins, such as hemagglutinin and neuraminidase in influenza viruses, also play critical roles in virus
attachment, entry, and release from host cells.
• Replication machinery: Most viruses encode a set of enzymes
and other proteins that are necessary for viral replication. These
may include polymerases (which replicate the viral nucleic acid),
proteases (which cleave viral proteins into functional pieces),
and other enzymes that help the virus replicate, modify host
cell machinery, and assemble new viruses. These viral proteins
can be attractive targets for antiviral drug development.
• Host range and tissue tropism: The host range of a virus refers
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ers are highly specialized and can only infect specific types of
cells or organisms. The host range is determined by the interaction between viral surface proteins and host cell receptors, as
well as the ability of the virus to exploit the host cell’s machinery for replication. Tissue tropism refers to the specific types of
tissues within a host organism that a virus can infect, which is
also determined by the presence of specific receptors and the
compatibility of the host cell machinery.
In summary, viruses are complex entities that possess a range of
structures and characteristics that enable them to infect host cells,
replicate, and evade host immune responses. Understanding these
components and their functions is critical for the study of viral infections and the development of effective strategies to combat them,
including antiviral drugs and vaccines.
13.2.3 HIV
Human Immunodeficiency Virus (HIV) is a retrovirus that causes Acquired Immunodeficiency Syndrome (AIDS), a condition that weakens the immune system and makes individuals more susceptible to
infections and diseases. HIV is a complex virus with several important components that contribute to its structure and function.
Nucleic Acid: HIV has a single strand of RNA as its genetic material.
The RNA contains the instructions for the synthesis of viral proteins
and enzymes, as well as for the replication of the virus.
Capsid: The capsid of HIV is a cone-shaped protein coat that surrounds the viral RNA. It is composed of subunits called capsomeres,
which are arranged in a specific pattern to create the overall shape
of the virus.
Envelope: HIV has an outer envelope made of lipids that is derived
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from the host cell’s membrane. The envelope is studded with proteins called gp120 and gp41, which help the virus bind to and enter
host cells.
Spike proteins: HIV has two types of spike proteins on its surface:
gp120 and gp41. These proteins help the virus bind to and enter
host cells, and they are also important for evading the host immune
system.
Replication machinery: HIV has a set of enzymes and other proteins
that are necessary for viral replication. These include reverse transcriptase, an enzyme that converts the viral RNA into DNA; integrase,
an enzyme that inserts the viral DNA into the host cell’s genome;
and protease, an enzyme that cleaves viral proteins into functional
pieces.
Host range: HIV can infect a wide range of host cells, including CD4+
T cells, macrophages, and dendritic cells.
13.3 Viral Life Cycles
13.3.1
Lytic and Lysogenic Cycles
The lytic cycle is the process by which a virus invades a host cell,
replicates itself, and then lyses (breaks open) the host cell to release
the new virus particles. This is a fast and efficient way for the virus to
spread, but it also causes damage to the host cell and can ultimately
lead to the death of the host.
On the other hand, the lysogenic cycle involves the integration of the
virus’s genetic material into the host cell’s DNA. This genetic material
remains dormant, or latent, until triggered by some external factor,
such as stress or a change in the host’s environment. When this
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trigger occurs, the virus’s genetic material is activated and begins
replicating, eventually leading to the lytic cycle and the release of
new virus particles.
The lytic and lysogenic cycles are two distinct modes of viral replication, each with its own unique characteristics and impacts on host
cells. In this discussion, we will delve into the details of each cycle
and emphasize their differences.
Lytic cycle:
• Infection: In the lytic cycle, a virus attaches itself to a host cell by
binding to specific receptors on the cell surface. Once attached,
the virus injects its genetic material (either DNA or RNA) into
the host cell.
• Replication: The viral genetic material hijacks the host cell’s machinery to replicate its own genome and synthesize viral proteins.
This process can be rapid, sometimes producing thousands of
new viral particles within hours.
• Assembly: The newly synthesized viral components, including
the genetic material and capsid proteins, are assembled into
complete viral particles within the host cell.
• Lysis: As the number of viral particles increases, the host cell
becomes overwhelmed and eventually ruptures, releasing the
new virus particles to infect neighboring cells. This lysis of the
host cell can cause damage to the host organism and lead to
symptoms of viral infection.
Lysogenic cycle:
• Infection and Integration: Similar to the lytic cycle, the lysogenic
cycle begins with a virus attaching itself to a host cell and injecting its genetic material. However, instead of immediately
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hijacking the host cell machinery, the viral genetic material integrates itself into the host cell’s DNA, becoming a prophage.
This integration allows the viral genome to remain dormant, or
latent, within the host cell.
• Dormancy: During the lysogenic cycle, the host cell continues
to function and replicate normally, with the integrated viral DNA
replicating along with the host’s own genetic material. The virus
remains dormant until some external factor, such as stress, UV
radiation, or a change in the host’s environment, triggers its
activation.
• Activation: When the viral genome is activated, it excises itself
from the host cell’s DNA and initiates the lytic cycle. This involves replicating its own genome, synthesizing viral proteins,
assembling new viral particles, and ultimately lysing the host
cell to release new virus particles.
The main differences between the lytic and lysogenic cycles lie in
their impacts on host cells and their replication strategies. In the lytic
cycle, the virus rapidly replicates and destroys the host cell, causing
direct harm and potentially leading to symptoms of infection. In the
lysogenic cycle, the virus maintains a more covert presence within
the host cell, remaining dormant until activated by external factors.
This dormancy allows the host cell to continue functioning normally
until the viral genome is activated, at which point the lytic cycle is
initiated, and new viral particles are produced.
Understanding the differences between these two modes of viral
replication is essential for the study of viral infections and the development of antiviral strategies, as the approach to treatment may
differ depending on whether a virus primarily undergoes a lytic or
lysogenic cycle.
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For the IMAT exam, it is not necessary to have a comprehensive
understanding of viruses. The exam is unlikely to cover viruses beyond HIV. Therefore, it is advisable to focus on memorizing only the
information that is directly relevant to the exam, such as HIV and
genetic transformation. Additionally, it is likely that the exam will
include questions about the structure and components of viruses.
13.3.2 Virulent vs. Temperate Viruses
Virulent and temperate viruses exhibit different life cycles when
infecting host cells. The primary distinction between the two lies in
their strategies for survival and replication.
Virulent viruses: These viruses follow a lytic life cycle, which involves
rapid replication and destruction of the host cell. Upon infecting the
host cell, the virulent virus hijacks the host’s cellular machinery to
replicate its genetic material and produce new viral particles. Once
the replication process is complete, the newly formed viral particles
burst out of the host cell, leading to its destruction or lysis. This
process releases a large number of viruses into the surrounding
environment, allowing them to infect more host cells. Examples of
virulent viruses include some bacteriophages (viruses that infect
bacteria) and many human pathogenic viruses, such as the influenza
virus.
Temperate viruses: These viruses follow a lysogenic life cycle, which
allows them to integrate their genetic material into the host’s genome
without immediately destroying the host cell. The viral genetic material, called a prophage in the case of bacteriophages, remains
dormant within the host’s DNA and is replicated along with the
host’s genetic material during cell division. This dormant state can
persist for an extended period until certain environmental factors
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Figure 68: A virulent phage only follows the lytic cycle, which involves
the replication of the phage and the lysis of the host cell. (Credit:
openstax.org)
or stressors trigger the activation of the viral genes. Once activated,
the virus switches to the lytic cycle, leading to the production of new
viral particles and the eventual lysis of the host cell. An example of a
temperate virus is the bacteriophage lambda.
In summary, the key difference between virulent and temperate
viruses is their life cycle strategy. Virulent viruses follow a lytic life
cycle, which involves rapid replication and destruction of the host
cell, while temperate viruses follow a lysogenic life cycle, integrating
their genetic material into the host’s genome and remaining dormant until specific conditions trigger their activation and transition
to the lytic cycle.
13.3.3 Transduction
Viral transduction is the process by which a virus transfers genetic
material from one bacterium to another. This process can have
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Figure 69: A bacteriophage that is temperate undergoes both the
lysogenic and lytic cycles. During the lysogenic cycle, the DNA of
the phage integrates with the host genome, forming a prophage
that can be passed down to future generations of cells. If the cell
encounters environmental stressors such as exposure to toxins or
starvation, the prophage may be removed and the phage can enter
the lytic cycle. (Credit: openstax.org)
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change of genetic information between different bacterial species,
leading to the acquisition of new traits, such as antibiotic resistance.
Transduction is carried out by bacteriophages, viruses that specifically infect bacteria. There are two main types of transduction: generalized transduction and specialized transduction. We will discuss
each type in detail, emphasizing their differences.
Generalized transduction:
• In generalized transduction, the bacteriophage accidentally
packages a segment of the host bacterium’s DNA into its capsid
instead of its own viral DNA during the lytic cycle. This can occur
when the host cell’s DNA is mistakenly recognized as viral DNA
by the bacteriophage’s replication machinery.
• When the bacteriophage infects a new host bacterium, it injects
the mistakenly packaged bacterial DNA into the new host cell.
The recipient bacterium can then incorporate this foreign DNA
into its own genome through a process called recombination.
As a result, the recipient bacterium acquires new genetic traits
from the donor bacterium.
• Generalized transduction is considered a random process, as any
portion of the donor bacterium’s DNA can be accidentally packaged into the bacteriophage and transferred to the recipient
bacterium.
Specialized transduction:
• Specialized transduction occurs when a bacteriophage that
has integrated its genetic material into the host bacterium’s
genome (as a prophage) during the lysogenic cycle, excises itself from the host DNA to initiate the lytic cycle. However, during
this excision process, the bacteriophage may accidentally excise
a small portion of the host bacterium’s DNA adjacent to the
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prophage, incorporating it into the viral genome.
• When the bacteriophage infects a new host bacterium, it injects
its viral DNA, along with the excised bacterial DNA, into the
recipient cell. The recipient bacterium can then incorporate
this foreign DNA into its own genome through recombination,
acquiring new genetic traits from the donor bacterium.
• Unlike generalized transduction, specialized transduction is not
random; it only transfers specific genes located near the prophage’s
integration site within the donor bacterium’s genome.
Figure 70: Specialized Transduction. It involves the excision of an
integrated phage that brings a segment of the DNA adjacent to
its insertion point. When the phage infects a new bacterium, its
DNA is integrated along with the genetic material obtained from
the previous host. (Credit: openstax.org)
Let us use analogies to make consolidate what you have read so far
regarding transduction:
Specialized transduction is like a courier who delivers a package to
a specific address. The courier knows exactly where the package
needs to go, and only delivers it to that address. In the same way,
specialized transduction involves a virus that can only infect specific
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cells and deliver its genetic material to a specific location in the
host’s DNA.
Generalized transduction, on the other hand, is like a postal worker
who delivers mail to every mailbox on a given route. The postal
worker doesn’t know which mailbox belongs to whom, so every
mailbox gets the same mail. In the same way, generalized transduction involves a virus that can infect a wide range of cells and insert
its genetic material into random locations in the host’s DNA.
In summary, viral transduction is a mechanism by which bacteriophages transfer genetic material between bacteria. Generalized
transduction involves the random packaging of bacterial DNA during the lytic cycle, while specialized transduction occurs during the
excision of a prophage from the host bacterium’s genome during
the lysogenic cycle. Both types of transduction can lead to the transfer of genetic traits between bacterial species, with implications for
bacterial evolution and the spread of antibiotic resistance.
13.4
Viruses in the Field of Medicine
Viruses have played a significant role in the field of medicine and
biotechnology, providing valuable tools for research, diagnostics,
and therapeutics. In this discussion, we will explore several ways
viruses are utilized in these fields, including vaccines, gene therapy,
oncolytic viruses, and viral vectors for research.
Vaccines: Vaccines are one of the most successful applications of
viruses in medicine. They are designed to stimulate the immune system to produce an immune response against a specific virus without
causing the disease itself. Traditional vaccines often use inactivated
or attenuated (weakened) viruses, while newer vaccines, such as the
COVID-19 mRNA vaccines, use viral components or genetic material
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to induce an immune response. Examples: The smallpox vaccine
(using the vaccinia virus) played a crucial role in eradicating smallpox, while vaccines against measles, mumps, and rubella (MMR
vaccine) have significantly reduced the incidence of these diseases.
Gene therapy: Gene therapy involves the use of viruses to deliver
genetic material into a patient’s cells to treat or prevent genetic
disorders. Viral vectors, such as adeno-associated viruses (AAV) or
lentiviruses, are engineered to carry therapeutic genes and efficiently deliver them to target cells without causing disease. Examples: Luxturna is an AAV-based gene therapy for a rare form of
inherited blindness. Zolgensma, another AAV-based therapy, treats
spinal muscular atrophy in young children.
Oncolytic viruses: Oncolytic viruses are viruses that selectively infect and destroy cancer cells while leaving healthy cells largely unharmed. These viruses can be naturally occurring or genetically
engineered to specifically target cancer cells. They work by infecting cancer cells, replicating within them, and ultimately causing
cell death, while also stimulating the immune system to attack cancer cells. Examples: Talimogene laherparepvec (T-VEC or Imlygic)
is an FDA-approved oncolytic virus for the treatment of advanced
melanoma. It is a modified herpes simplex virus that selectively
replicates in cancer cells and produces a protein that stimulates the
immune system.
Viral vectors for research and diagnostics: Viruses are also used
as tools for molecular biology research and diagnostics. Viral vectors, such as lentiviruses or adenoviruses, can be used to deliver
genes, small RNAs, or other molecules of interest into cells for functional studies or to create stable cell lines for research purposes.
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linked immunosorbent assay (ELISA), or nucleic acid sequencing to
confirm the presence of a viral infection.
Figure 71: Adenovirus vectors can be utilized in gene therapy to treat
certain genetic disorders that are caused by a faulty gene. (Credit:
openstax.org)
You need not know these examples in detail for your exam, but it is
useful to be aware of the interesting ways viruses have been utilized
extensively in medicine and biotechnology, providing valuable tools
for research, diagnostics, and therapeutics. From vaccines to gene
therapy and oncolytic viruses, these applications have transformed
our understanding of biology and have led to the development of
life-saving treatments for numerous diseases.
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13.5 Viral Transmission
Viral transmission is the process by which a virus spreads from one
individual or organism to another. It is a crucial aspect of viral infections and plays a significant role in the epidemiology and evolution
of viruses. Understanding the various modes of viral transmission
helps in the development of strategies to control and prevent the
spread of viral diseases. In this discussion, we will explore different
methods of viral transmission, focusing on the roles of vectors and
hosts in the process.
Direct transmission: Direct transmission occurs when a virus is
transferred directly from an infected individual to a susceptible
individual without any intermediary. Common modes of direct
transmission include physical contact (touching, kissing, or sexual
contact), respiratory droplets (coughing or sneezing), and vertical
transmission (from mother to offspring during pregnancy, childbirth,
or breastfeeding).
Indirect transmission: Indirect transmission involves the transfer
of a virus from an infected individual to a susceptible individual
through an intermediary, such as an inanimate object or a living
organism. The intermediary is often referred to as a ”vector” or ”host.”
Vectors: Vectors are living organisms that can transmit a virus from
an infected host to a susceptible host. They are typically arthropods,
such as mosquitoes, ticks, and flies. Vectors acquire the virus by
feeding on the blood or other body fluids of an infected host and
then transmit it to a susceptible host during a subsequent feeding
event. Vector-borne viral transmission can be divided into two categories: mechanical and biological. Mechanical transmission: In
mechanical transmission, the virus is passively carried on the external surface of the vector. When the vector comes into contact with
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a susceptible host, the virus is transferred to the new host. This type
of transmission does not involve the replication of the virus within
the vector. Biological transmission: In biological transmission, the
virus actively replicates within the vector before being transmitted
to a susceptible host. This often involves a period of ”extrinsic incubation,” during which the virus must replicate and disseminate
within the vector before becoming infectious.
Hosts: Hosts are organisms that are infected by a virus and can
serve as a source of viral transmission to other susceptible individuals. Hosts can be classified into various categories based on their
role in the transmission cycle: Reservoir hosts: Reservoir hosts are
animals that maintain the virus in nature and can serve as a source
of infection for vectors or other susceptible hosts. Reservoir hosts
typically do not show signs of illness when infected with the virus.
Amplifying hosts: Amplifying hosts are organisms that become
infected with a virus, replicate it to high levels, and contribute to
its transmission to other susceptible hosts or vectors. Amplifying
hosts often play a critical role in the spread of viral diseases during
outbreaks or epidemics. Dead-end hosts: Dead-end hosts are organisms that become infected with a virus but do not contribute
to its transmission. Infections in dead-end hosts typically result in a
dead-end for the virus, as they are unable to transmit the virus to
other susceptible hosts or vectors.
In summary, viral transmission is the process by which viruses spread
from one individual or organism to another. The modes of transmission can be direct or indirect, involving vectors and various types
of hosts. Understanding the mechanisms of viral transmission is
essential for the development of strategies to control and prevent
the spread of viral diseases.
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14 Bacteria and Archaea
Bacteria and Archaea are two of the three domains of life, the other
being Eukarya. Both Bacteria and Archaea are prokaryotes, meaning
they are single-celled organisms that do not have a defined nucleus
or other membrane-bound organelles. Despite their small size and
simple cell structure, bacteria and archaea are incredibly diverse
and play important roles in various ecosystems.
One way to classify bacteria is by their cell wall structure. Bacteria
with a thick peptidoglycan layer in their cell wall are referred to as
gram-positive, while those with a thin peptidoglycan layer and an
outer membrane are gram-negative. The gram stain, a laboratory
technique developed by Danish bacteriologist Hans Christian Gram,
is used to differentiate between these two types of bacteria.
Bacteria and archaea can be found in almost every environment
on Earth, from the deepest oceans to the highest peaks of the Himalayas. They can be free-living or exist in symbiotic relationships
with other organisms. Bacteria are essential for various processes in
the environment, including nitrogen fixation, decomposition, and
the production of antibiotics. Archaea are often found in extreme
environments, such as hot springs and salt mines, and are thought
to be some of the oldest organisms on Earth.
Despite their many beneficial functions, bacteria and archaea can
also cause diseases in humans, animals, and plants. Understanding
the structure, behavior, and characteristics of these tiny organisms
is crucial for improving public health and maintaining the balance
of ecosystems.
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14.1 Main Characteristics of Bacteria
Bacteria are unicellular microorganisms that display several distinct
characteristics, setting them apart from other forms of life such
as viruses, fungi, and eukaryotic cells. In this discussion, we will
cover the main features of bacteria, including their cell structure,
reproduction, metabolic diversity, and genetic organization.
Cell structure:
• Bacteria are prokaryotic cells, meaning they lack a nucleus and
membrane-bound organelles. Their genetic material is organized in a single circular chromosome found in the nucleoid
region of the cell.
• The bacterial cell wall is composed of peptidoglycan, a unique
polymer providing structural support and protection. Bacterial
cell walls can be classified as either gram-positive (thick peptidoglycan layer) or gram-negative (thin peptidoglycan layer with
an additional outer membrane).
• Some bacteria have additional structures such as flagella (for
motility), pili (for adhesion and DNA transfer), and capsules (for
protection and evasion of the host immune system).
Reproduction:
• Bacteria reproduce asexually through binary fission, a process
in which a single cell divides into two identical daughter cells.
This process allows for rapid population growth under favorable
conditions.
• Some bacteria can also transfer genetic material between cells
through processes such as conjugation, transformation, and
transduction, which contributes to their genetic diversity and
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adaptation.
Metabolic diversity:
• Bacteria exhibit a wide range of metabolic capabilities, enabling
them to inhabit diverse environments and utilize various energy
and carbon sources.
• Based on their energy source, bacteria can be classified as phototrophs (using light for energy) or chemotrophs (using chemical compounds for energy).
• Based on their carbon source, bacteria can be classified as autotrophs (using inorganic carbon, such as CO2) or heterotrophs
(using organic carbon, such as glucose).
• Bacteria can also be classified based on their oxygen requirements: aerobic (requiring oxygen), anaerobic (not requiring oxygen), or facultative anaerobic (capable of growing with or without oxygen).
Genetic organization:
• Bacteria have a single, circular chromosome that contains all
their essential genetic information. In addition, they can carry
small circular pieces of DNA called plasmids, which often contain genes that provide additional functions, such as antibiotic
resistance or specialized metabolic capabilities.
• Bacteria can also undergo horizontal gene transfer, which allows them to acquire new genetic traits from other bacteria.
This process plays a significant role in the rapid evolution and
adaptation of bacterial populations.
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tion through binary fission. These features, along with their ability
to rapidly adapt and evolve through processes like horizontal gene
transfer, contribute to the incredible diversity and ubiquity of bacteria in our world.
14.2 Gram Positive and Negative
14.2.1 Gram Staining
Gram staining is a laboratory technique used to distinguish between
different types of bacteria. It involves using a series of dyes and
washes to stain and visualize bacterial cells under a microscope.
When a bacteria is ”Gram-positive,” it means that it appears purple
when stained using the Gram staining technique. This is because
the thick layer of peptidoglycan in the bacterial cell wall traps the
purple dye and prevents it from being washed away during the
staining process.
In contrast, when a bacteria is ”Gram-negative,” it appears pink or red
when stained using the Gram staining technique. This is because
the thinner layer of peptidoglycan in the bacterial cell wall allows the
purple dye to be washed away during the staining process. Instead,
a counterstain with a pink or red dye is used to stain the bacteria.
The terms ”Gram-positive” and ”Gram-negative” were coined in
honor of their discoverer, Danish bacteriologist Hans Christian Gram,
who first developed the technique in 1884.
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14.2.2
Main Differences Between Gram Positive and
Gram Negative
Gram-positive and gram-negative bacteria differ in their cell wall
structure and composition, leading to distinct staining properties
during the gram stain procedure and varied responses to environmental stress and antimicrobial agents. In this discussion, we will
delve deeper into the main differences between gram-positive and
gram-negative bacteria, providing examples to emphasize their
unique characteristics.
Cell wall composition:
• Gram-positive bacteria: These bacteria have a thick peptidoglycan layer, typically accounting for 40-90 percent of the cell wall
dry weight. This layer is located outside the plasma membrane
and lacks an outer membrane. Examples of gram-positive bacteria include Staphylococcus aureus, Streptococcus pneumoniae,
and Bacillus subtilis.
• Gram-negative bacteria: They possess a thin peptidoglycan
layer, which constitutes only 5-10 percent of the cell wall dry
weight. This layer is sandwiched between the plasma membrane and an outer membrane rich in lipopolysaccharides. Examples of gram-negative bacteria include Escherichia coli, Pseudomonas aeruginosa, and Neisseria gonorrhoeae.
Outer membrane:
Gram-positive bacteria: They lack an outer membrane, and their
thick peptidoglycan layer is often associated with teichoic acids,
which are polymers of glycerol or ribitol linked by phosphate groups.
These teichoic acids contribute to the overall negative charge of the
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cell surface and can play a role in adhesion, virulence, and resistance
to host defenses. Gram-negative bacteria: The outer membrane is a
defining feature of gram-negative bacteria. It contains lipopolysaccharides (LPS), lipoproteins, and phospholipids. LPS contributes
to the structural integrity of the cell and can act as an endotoxin,
triggering immune responses in the host. The outer membrane
also contains porins, which are proteins that form channels for the
passage of small molecules.
Permeability and antibiotic resistance:
• Gram-positive bacteria: The absence of an outer membrane
in gram-positive bacteria makes them more permeable to certain antibiotics, such as penicillin and vancomycin, which target
peptidoglycan synthesis. However, they can still develop resistance through various mechanisms, such as the production of
beta-lactamases or alterations in penicillin-binding proteins.
• Gram-negative bacteria: The outer membrane of gram-negative
bacteria acts as a barrier to many antibiotics and other antimicrobial agents. The presence of efflux pumps and the limited permeability of the outer membrane contribute to their increased
resistance to antibiotics, such as penicillin, which cannot easily
penetrate the outer membrane.
In summary, gram-positive and gram-negative bacteria exhibit key
differences in their cell wall composition, the presence of an outer
membrane, and their permeability to antibiotics. These differences
are crucial in determining their staining properties, susceptibility to
antimicrobial agents, and the strategies employed by these bacteria to survive in various environments. Examples of gram-positive
bacteria include Staphylococcus aureus and Streptococcus pneumoniae, while Escherichia coli and Pseudomonas aeruginosa are
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Figure 72: Structural differences between gram positive and gram
negative bacteria. (Credit: openstax.org)
examples of gram-negative bacteria..
14.3
Prokaryotic Metabolism
Prokaryotic metabolism refers to the chemical reactions that take
place within prokaryotic cells, which are single-celled organisms
that do not have a defined nucleus or other membrane-bound organelles. Prokaryotes include bacteria and archaea. These organisms require specific nutrients to carry out their metabolic processes,
including macronutrients and micronutrients.
Macronutrients are nutrients required in large amounts for the
proper functioning of prokaryotic cells. They include carbon, nitrogen, oxygen, hydrogen, phosphorus, and sulfur. Carbon is the
primary building block for cellular structures, while nitrogen is essential for the synthesis of proteins and nucleic acids. Oxygen and
hydrogen are critical for the production of water and cellular respiration. Phosphorus is required for the formation of nucleic acids
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and ATP, while sulfur is a key component of some amino acids and
vitamins.
Micronutrients, also known as trace elements, are required in smaller
amounts but are still essential for the proper functioning of prokaryotic cells. These include metals like iron, copper, zinc, magnesium,
and manganese. Micronutrients often serve as cofactors for enzymes, helping to facilitate various metabolic reactions.
Prokaryotic cells obtain energy from the breakdown of organic compounds, such as sugars and amino acids, through a process called
cellular respiration. There are several types of cellular respiration
that prokaryotes can use to generate energy, including aerobic respiration, anaerobic respiration, and fermentation.
Aerobic respiration is the process by which prokaryotes obtain energy by using oxygen as the final electron acceptor in the electron
transport chain. During aerobic respiration, glucose is broken down
into pyruvate, and the energy released is used to produce ATP. Oxygen is required for aerobic respiration, and the end products of this
process are water and carbon dioxide.
Anaerobic respiration is the process by which prokaryotes obtain
energy in the absence of oxygen. This type of respiration can occur in two ways: by using an inorganic compound, such as sulfur
or nitrate, as the final electron acceptor, or by using fermentation.
Fermentation is the process by which prokaryotes produce ATP by
breaking down glucose or other organic compounds in the absence
of oxygen. The end products of fermentation are typically alcohol or
lactic acid.
Some bacteria, known as chemolithotrophs, can obtain energy by
oxidizing inorganic compounds like hydrogen sulfide, ammonia, or
ferrous iron, while others, called phototrophs, can harness energy
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from sunlight through photosynthesis. These alternative energy
sources allow bacteria to survive in a wide range of environments,
from deep-sea hydrothermal vents to the upper atmosphere.
Prokaryotic metabolism is similar to cellular respiration in eukaryotic cells, which are cells that have a defined nucleus and other
membrane-bound organelles. Both prokaryotic and eukaryotic cells
use cellular respiration to generate energy from the breakdown of
organic compounds. However, there are some differences in the
pathways and mechanisms of cellular respiration between prokaryotes and eukaryotes.
For example, prokaryotes have a simpler set of cellular machinery,
and their cellular respiration often occurs in the cell membrane or
the cytoplasm, whereas eukaryotes have specific organelles, such
as mitochondria, where cellular respiration takes place. Additionally,
prokaryotes can employ a wider range of metabolic strategies and
energy sources, as mentioned earlier, enabling them to inhabit
diverse ecological niches.
In summary, prokaryotic metabolism encompasses a variety of processes that enable bacteria and archaea to obtain energy and nutrients. These organisms require macronutrients and micronutrients
to fuel their metabolic reactions, and they can utilize a range of
energy sources through aerobic respiration, anaerobic respiration,
fermentation, chemolithotrophy, and photosynthesis. While there
are similarities between prokaryotic and eukaryotic metabolism,
the differences in cellular structures and metabolic flexibility allow
prokaryotes to thrive in a wide array of environments.
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14.3.1
Electron Transport Chain in Bacteria?
In bacteria, the electron transport chain (ETC) is a crucial component
of cellular respiration and energy production. Similar to eukaryotic
cells, the ETC in bacteria is composed of a series of protein complexes
and enzymes that facilitate the transfer of electrons from donors
to acceptors, ultimately resulting in ATP production, which is the
primary energy source for cells.
The ETC in bacteria is typically situated within the cytoplasmic membrane, containing various electron carriers such as cytochromes,
quinones, and iron-sulfur clusters. These carriers are capable of accepting and transferring electrons, eventually delivering them to
an electron acceptor like oxygen or nitrogen. The electron transfer
process within the ETC generates a proton gradient across the membrane, which subsequently powers ATP synthesis through chemiosmosis.
Bacterial ETCs exhibit a remarkable diversity based on the electron
donors and acceptors they utilize. Some bacteria employ photosynthesis, using light as the electron donor to generate energy. Others
rely on chemical compounds like sugars or organic acids for electron donation, while some can even exploit inorganic substances
like hydrogen gas or sulfur compounds. The electron acceptors also
vary among bacteria, with some utilizing oxygen, nitrogen, sulfur,
or metal ions, depending on their specific metabolic needs and
environmental conditions.
This adaptability of bacteria to employ different types of ETCs based
on their available electron donors and acceptors showcases their versatility and ability to thrive in a wide range of environments. Understanding these diverse metabolic pathways in bacteria contributes
to our knowledge of their ecological roles and potential applications
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in biotechnology, such as bioenergy production or bioremediation.
14.4
Bacterial Infections
Bacterial infections occur when harmful bacteria invade the body
and multiply, causing damage to the host’s tissues and triggering an
immune response. These infections can range from mild to severe,
depending on the type of bacteria, the infected area, and the host’s
immune system.
Bacteria can enter the body through various routes, such as open
wounds, the respiratory system, ingestion, or direct contact with
contaminated surfaces. Once inside the body, they can adhere to
and colonize host tissues, producing harmful substances called virulence factors. These factors include toxins, enzymes, and other
molecules that can damage host cells, disrupt normal body functions, and evade the immune system.
The immune system plays a crucial role in fighting off bacterial
infections. White blood cells called phagocytes are responsible for
engulfing and destroying bacteria, while other immune cells release
chemicals that help to contain and eliminate the infection. The
immune system can also produce specific proteins called antibodies,
which recognize and neutralize bacteria or their toxins.
The primary treatment for bacterial infections is the use of antibiotics. Antibiotics are drugs that target specific structures or processes in bacterial cells, such as cell wall synthesis or protein production, ultimately leading to the bacteria’s death or inhibition. It is
essential to choose the appropriate antibiotic for each infection, as
different antibiotics are effective against specific types of bacteria.
Physicians may rely on diagnostic tests, such as bacterial culture
and antibiotic susceptibility testing, to determine the most effective
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treatment.
It is crucial to follow the prescribed antibiotic treatment to ensure
the complete eradication of the infection and prevent the development of antibiotic resistance. Antibiotic resistance occurs when
bacteria evolve mechanisms to withstand the effects of antibiotics,
making the drugs less effective or ineffective. To minimize the risk
of antibiotic resistance, it is essential to use antibiotics only when
necessary and as prescribed by a healthcare professional.
Preventing bacterial infections is also important, and it can be achieved
through various measures, including proper hygiene, food handling,
and vaccination. Washing hands regularly, cleaning and disinfecting surfaces, and preparing and storing food safely can help reduce
the risk of bacterial infections. Vaccines are available for some bacterial infections, such as tetanus, diphtheria, and meningococcal
disease, providing protection by stimulating the immune system to
recognize and eliminate the bacteria.
In summary, bacterial infections are caused by harmful bacteria
invading the body and causing damage to host tissues. The immune system plays a vital role in defending against these infections.
Treatment typically involves the use of antibiotics, and prevention
strategies include proper hygiene, food handling, and vaccination.
14.5 Bacteria and Biotechnology
Bacteria play a significant role in the field of medicine and biotechnology. They are utilized for various purposes, including the production of antibiotics, vaccines, enzymes, and other essential molecules,
as well as for gene therapy and bioremediation. Here is a detailed
discussion on some of the ways bacteria are used in medicine and
biotechnology:
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Antibiotics production: Many antibiotics, such as penicillin, streptomycin, and tetracycline, are derived from bacteria, particularly from
the genera Streptomyces and Bacillus. These antibiotics are used to
treat bacterial infections in humans and animals. Bacteria-derived
antibiotics have been a cornerstone of modern medicine, enabling
the successful treatment of previously lethal infections.
Vaccine production: Bacteria are used in the production of certain
vaccines, either as whole-cell vaccines or by producing recombinant
proteins. For example, the BCG vaccine, used against tuberculosis,
is derived from an attenuated strain of Mycobacterium bovis. In
another example, genetically engineered Escherichia coli are used
to produce recombinant proteins, such as the Hepatitis B surface
antigen, which is then used to develop the Hepatitis B vaccine.
Enzyme production: Bacteria are widely employed in the production of enzymes for various industrial and medical applications. For
instance, Bacillus subtilis is used to produce proteases and amylases,
which find applications in detergents, food, and textile industries.
E. coli and other bacteria are also used for producing medically important enzymes, such as insulin and tissue plasminogen activator
(tPA), which is used to treat blood clots in patients with stroke or
myocardial infarction.
Probiotics: Certain strains of bacteria, such as Lactobacillus and
Bifidobacterium, are used as probiotics to promote gut health and
support the immune system. Probiotics are live microorganisms
that, when ingested in adequate amounts, confer health benefits
to the host. They can be found in various food products, including
yogurt, fermented milk, and dietary supplements.
Gene therapy: Bacteria can be genetically engineered to deliver
therapeutic genes into human cells, offering a potential treatment
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for various genetic disorders. For example, researchers are investigating the use of modified Listeria monocytogenes to deliver cancerfighting genes directly into tumor cells, potentially providing a targeted treatment for cancer.
Bioremediation: Bacteria can be employed in bioremediation, a process that uses living organisms to degrade or detoxify hazardous substances in the environment. For instance, Pseudomonas species can
break down toxic chemicals like polychlorinated biphenyls (PCBs)
or petroleum hydrocarbons in contaminated soils, helping to clean
up polluted sites.
In conclusion, bacteria play a vital role in medicine and biotechnology, providing valuable resources for drug development, disease
prevention, and environmental remediation. Their versatility and
adaptability make them an essential component of modern biotechnological applications.
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15
Human's Physiology - The Basics
Human physiology is the study of how the human body functions
and maintains homeostasis, or balance, in its various systems. Understanding human physiology is essential for anyone interested in
pursuing a career in the medical field, whether as a doctor, nurse,
or other healthcare professional. It is also an important subject for
the IMAT.
But human physiology is not just important for medical studies
and exams – it is also a fascinating and engaging topic that can
help us better understand the amazing and complex processes
that keep our bodies running smoothly. From the beating of our
hearts and the flow of our blood, to the workings of our brain and
the functioning of our immune system, human physiology is full of
interesting and important concepts that can help us appreciate the
wonders of the human body.
So whether you are studying for the IMAT or just have a general
interest in how the human body works, exploring the field of human
physiology can be both educational and enjoyable. It is a vital and
fascinating subject that is essential for anyone interested in health
and medicine.
15.1 Homeostasis
Homeostasis is the process by which an organism or a system maintains a stable internal environment in response to changes in the
external environment. It is a key concept in biology, and is essential
for the proper functioning of an organism.
To understand homeostasis, it would be useful to be aware of the
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key aspects that comprise it in simple terms, to help you have a solid
foundation for the rest of this book:
Homeostasis is the process by which organisms maintain a stable
internal environment despite changes in their external surroundings.
This complex mechanism relies on four key components that work
together to regulate various physiological systems:
1. Sensors: These are specialized cells or structures within the
body that act as detectors, sensing changes in the external and
internal environment. They monitor variables such as temperature, pH, blood sugar levels, and blood pressure. For example,
thermoreceptors in the skin can detect changes in external temperature, while chemoreceptors in the carotid artery can sense
changes in blood oxygen levels.
2. Control Center: The control center is responsible for processing
information received from sensors and determining the appropriate response to maintain homeostasis. The primary control
center in the body is the brain, specifically the hypothalamus
and medulla oblongata, which coordinate the body’s response
to changes detected by sensors. For instance, if the body’s core
temperature increases, the hypothalamus receives this information from the thermoreceptors and decides on a course of
action to lower the temperature.
3. Effectors: Effectors are organs, tissues, or cells that carry out
the control center’s directives to restore homeostasis. They can
be muscles, glands, or other structures that help regulate the
specific variable being monitored. In the case of temperature
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tion to these, blood vessels can also play a role in temperature
regulation by dilating to dissipate heat or constricting to conserve heat, depending on the body’s needs.
4. Feedback loop: The feedback loop is a crucial aspect of homeostasis, as it ensures that the system remains balanced and
responsive to changes. There are two types of feedback loops:
negative and positive feedback loops. Negative feedback loops
are more common and work to counteract changes and return the system to its set point. For example, when the body’s
temperature increases, the control center sends signals to the
effectors (sweat glands) to cool the body down. As the body
cools, the sensors detect this change and send feedback to the
control center, signaling that the desired temperature has been
reached, and the cooling process can stop. Positive feedback
loops, on the other hand, amplify changes and are typically involved in processes that have a clear endpoint, such as blood
clotting or childbirth.
Overall, homeostasis is achieved through a dynamic balance between various physiological systems, which work together to maintain a stable internal environment. This process involves the continuous interaction of sensors, control centers, effectors, and feedback
loops. By closely monitoring and adjusting the body’s internal conditions, homeostasis ensures that essential functions are performed
efficiently and that the body can respond effectively to changes in
its surroundings.
One example of homeostasis in the human body is the regulation of
body temperature. The body has a set point for temperature, and it
uses various mechanisms to maintain this set point within a narrow
range. For example, if the body’s temperature starts to rise above
the set point, the body will sweat to cool down through evaporation.
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Figure 73: Negative feedback is a physiological process that helps
the body maintain homeostasis by counteracting deviations from
a set point. One example of negative feedback in action is the
regulation of body temperature. (Credit: openstax.org)
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If the body’s temperature starts to fall below the set point, the body
will start to shiver to generate heat.
Another example of homeostasis in the human body is the regulation of blood sugar levels. The body has a set point for blood sugar,
and it uses various mechanisms to maintain this set point within a
narrow range. If blood sugar levels start to rise above the set point,
the body will release insulin to lower blood sugar levels. If blood
sugar levels start to fall below the set point, the body will release
glucagon to raise blood sugar levels.
15.2 Epithelial Tissue
Epithelial tissue is a type of tissue that covers the surface of the body
and lines various organs and cavities. It plays important roles in
protecting and separating different parts of the body, as well as in
the absorption, secretion, and transport of substances.
The two main overall types of epithelium are surface epithelium and
glandular epithelium.
Surface epithelium is the type of epithelial tissue that covers the
surface of organs and body cavities, serving as a protective barrier
and regulating the exchange of materials between the body and
the environment. Surface epithelium can be further classified into
squamous, cuboidal, and columnar epithelium, depending on the
shape of the cells that make up the tissue.
1. Squamous epithelium: This type of epithelial tissue is made up
of flat, scale-like cells that are arranged in a single layer. It is
found in areas that need to be stretched, such as the lining of
the blood vessels and the alveoli (air sacs) in the lungs. They are
also often in areas subject to high amounts of diffusion because
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substances can more easily diffuse through these thin cells.
2. Cuboidal epithelium: This type of epithelial tissue is made up of
cube-shaped cells that are arranged in a single layer. It is found
in areas that need to absorb or secrete substances, such as the
lining of the kidney tubules and the pancreas.
3. Columnar epithelium: This type of epithelial tissue is made up
of tall, column-like cells that are arranged in a single layer. It is
found in areas that need to absorb or secrete substances, such
as the lining of the intestine and the stomach.
Glandular epithelium is the type of epithelial tissue that forms glands,
which are specialized structures that secrete substances, such as
hormones or digestive juices, into the body. Glandular epithelium
can be further classified into exocrine and endocrine glands, depending on the type of secretion they produce. Exocrine glands
secrete substances such as sweat, saliva, and digestive enzymes into
ducts that lead to the surface or directly into the lumen of an organ.
Endocrine glands secrete hormones directly into the bloodstream.
Both surface epithelium and glandular epithelium have important
functions in maintaining the health and proper functioning of the
body, and they work together to regulate the exchange of materials
between the body and the environment.
These epithelial types can also be further classified based on their
structure and arrangement, which will also tell us their location and
function:
Simple epithelium: A single layer of cells that allows for high rates
of diffusion/absorption. Often found in alveoli (simple squamous:
gas exchange) or in the gastrointestinal tract (simple columnar:
absorption of nutrients).
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Stratified epithelium: Layered tissue where cells are stacked on
each other. Commonly found in areas subject to abrasion/friction
(for example stratified epithelium can be found lining the alimentary
tract).
Pseudostratified epithelium: This type of epithelial tissue is made
up of cells that are of different shapes and sizes, and they appear to
be arranged in multiple layers, even though they are all attached to
the basement membrane. It is found in areas that need to secrete
mucus, such as the lining of the respiratory tract.
Kerantinized tissue: Keratin layer provides further protection and
can be found in places in contact with the external environment,
such as the skin.
When naming these tissues, we put simple/stratified/pseudostratified and the type after, for example simple squamous epithelium.
In addition, there are also several specialized types of epithelial tissue,
such as transitional epithelium, which is found in areas that need to
stretch and contract, such as the bladder and the ureter.
15.3 Glandular Epithelium
Glandular epithelium is a specialized type of epithelial tissue responsible for the secretion of various substances, including hormones,
enzymes, mucus, and sweat. This tissue plays a crucial role in maintaining homeostasis and regulating various physiological processes
in the body. Glandular epithelial cells can be found in various glands
throughout the body, such as the endocrine glands, exocrine glands,
and mixed glands.
Structure: Glandular epithelium can be classified based on their
structure and mode of secretion. In terms of structure, glands can
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Figure 74: Surface epithelium and its different organizations. (Credit:
openstax.org)
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be categorized as unicellular or multicellular:
Unicellular glands: These glands consist of a single secretory cell,
such as goblet cells. Goblet cells are found in the lining of the respiratory and digestive tracts and secrete mucus to lubricate and
protect these surfaces.
Multicellular glands: These glands are composed of multiple cells
that work together to produce and secrete their products. Multicellular glands can be further classified based on the arrangement
of their secretory cells (tubular, acinar, or tubuloacinar) and the
branching pattern of their ducts (simple or compound).
Methods of Secretion: Glandular epithelium can be classified based
on their mode of secretion, which includes merocrine, apocrine, and
holocrine secretion:
Merocrine (eccrine) secretion: In this method, the secretory products are released through exocytosis, where vesicles containing the
substances fuse with the cell membrane and release their contents
into the extracellular space without causing damage to the cell. Examples of merocrine glands include salivary glands, which secrete
saliva, and pancreatic acinar cells, which produce digestive enzymes.
Apocrine secretion: In apocrine secretion, the secretory product
accumulates at the apical portion of the cell, which then pinches off
and is released into the extracellular space, resulting in the loss
of some cytoplasm but without complete cell destruction. The
mammary glands, which produce milk, are an example of apocrine
glands.
Holocrine secretion: Holocrine secretion involves the complete
disintegration of the secretory cell as it fills with the secretory product. The cell’s contents, including the secretory product and cellular
debris, are then released into the extracellular space. Sebaceous
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glands, which produce sebum (an oily substance that lubricates the
skin and hair), are an example of holocrine glands.
Differences in Structures: The primary differences between the
various types of glandular epithelium lie in their cellular organization and mode of secretion. Merocrine glands typically have a
well-defined system of secretory vesicles and maintain their cellular integrity during secretion. Apocrine glands exhibit a unique
apical constriction that allows for the release of the secretory product along with a small portion of the cell’s cytoplasm. In contrast,
holocrine glands are characterized by the complete disintegration
of the secretory cell during the release of their products.
In conclusion, glandular epithelium is a specialized type of epithelial tissue responsible for secreting various substances essential for
bodily functions. These tissues can be classified based on their structure (unicellular or multicellular) and mode of secretion (merocrine,
apocrine, or holocrine). Understanding the differences in the structure and function of glandular epithelium is crucial for studying
the role of these tissues in maintaining homeostasis and overall
physiological processes.
15.4 Connective Tissue
Connective tissue is a type of tissue that supports, binds, and connects different parts of the body. It is made up of cells and extracellular matrix, which is a gel-like substance that surrounds the cells
and provides structural support. The extracellular matrix (ECM) is
like a glue that holds cells and tissues together and helps to keep
their shape. It is made of a mixture of proteins and sugar molecules
that are found outside of cells. The ECM provides support for cells
and helps them communicate with each other. It’s like a scaffold
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Figure 75: (a) Merocrine secretion involves the release of secretory
products through the intact cell membrane. (b) Apocrine secretion
involves the shedding of the apical portion of the cell membrane,
along with the secretory products. (c) Holocrine secretion involves
the complete destruction of the cell, releasing the accumulated
secretory products, including the remnants of the cell. (Credit: openstax.org)
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that cells can attach to. In some cases, problems with the ECM can
lead to diseases. Think of the ECM as the framework of a building it holds everything in place and helps make sure everything works
properly.
Figure 76: Connective tissue. Fibroblasts are responsible for producing the fibrous tissue found in connective tissue proper, which
also consists of fixed cells including fibrocytes, adipocytes, and mesenchymal cells. (Credit: openstax.org)
Connective tissue is an essential component of the body’s structure,
providing support, protection, and connection between various tissues and organs. Depending on the arrangement of its structural
components, connective tissue can be classified into two main categories: loose connective tissue and dense connective tissue. Each
type has distinct characteristics and functions, which are outlined
in further detail below.
Loose Connective Tissue: Loose connective tissue is characterized
by a relatively sparse arrangement of cells and fibers within the
extracellular matrix (ECM), which is composed of ground substance,
a gel-like substance that provides cushioning and support. The
three main types of loose connective tissue are areolar, adipose, and
reticular tissues.
Areolar tissue: This is the most common type of loose connective
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tissue, found in various parts of the body, including beneath the
skin (subcutaneous layer), around blood vessels, nerves, and organs.
It consists of fibroblasts, macrophages, and mast cells, along with
collagen, elastin, and reticular fibers. Areolar tissue provides support, elasticity, and cushioning while allowing for movement and
flexibility.
Adipose tissue: This type of loose connective tissue is primarily
composed of adipocytes, which store fat as an energy reserve. Adipose tissue serves as insulation, padding, and an energy source for
the body. It is found in various locations, including under the skin,
around internal organs, and within the bone marrow.
Figure 77: Adipose Tissue. This type of connective tissue, which
is loosely organized, comprises predominantly fat cells and has a
minimal extracellular matrix. It functions as an energy reserve by
storing fat and offers insulation for the body. (Credit: openstax.org)
Reticular tissue: Reticular tissue is composed of reticular fibers
and reticular cells, which form a delicate network that supports the
framework for organs such as the spleen, lymph nodes, and bone
marrow. This tissue helps filter and remove foreign particles and
microorganisms from the body.
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Dense Connective Tissue: Dense connective tissue is characterized
by a densely packed arrangement of fibers, primarily collagen, within
the ECM. The two main types of dense connective tissue are dense
regular and dense irregular connective tissues.
Dense regular connective tissue: This type of tissue is primarily
composed of parallel collagen fibers that are densely packed, providing significant tensile strength in a single direction. Fibroblasts
are the primary cell type present in this tissue. Examples of dense
regular connective tissue include tendons, which connect muscles
to bones, and ligaments, which connect bones to other bones.
Dense irregular connective tissue: In contrast to dense regular
connective tissue, the collagen fibers in dense irregular connective
tissue are arranged in a random, interwoven pattern. This tissue
provides multidirectional tensile strength and resistance to stress,
making it ideal for locations that require support and protection
from various forces. Examples of dense irregular connective tissue
include the dermis layer of the skin, the fibrous capsules surrounding
organs, and the outer layer of blood vessels (tunica adventitia).
In summary, loose connective tissue is characterized by a loose
arrangement of cells and fibers within the ECM, providing support,
flexibility, and cushioning to various structures in the body. Dense
connective tissue, on the other hand, is characterized by its densely
packed fibers, providing strong support and resistance to forces in
specific directions. Understanding the differences between these
two types of connective tissue is crucial for studying their roles in
maintaining the body’s structural integrity and overall function.
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category of connective tissue with distinct functions and structures
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Figure 78: (a) Collagenous fibers are densely packed in parallel bundles in dense regular connective tissue. (b) Dense irregular connective tissue has a mesh-like network of interwoven collagenous
fibers.(Credit: openstax.org)
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gans. Several types of specialized connective tissue exist, including:
Cartilage: This specialized connective tissue consists of chondrocytes, cells that produce a matrix containing collagen and elastin
fibers. Cartilage is present in areas requiring support and cushioning, such as the nose and ears. There are other types of cartilage,
including hyaline cartilage, which serves as a precursor to certain
bones before ossification. Additional cartilage types include fibrocartilage, the strongest variety due to its high collagen fiber content,
found in intervertebral discs and ligaments; and elastic cartilage,
found in structures like the epiglottis.
Bone: Bone is a specialized connective tissue comprised of osteocytes, which create a mineral matrix called hydroxyapatite. Osteocytes can be further classified into osteoblasts, responsible for bone
formation and strengthening, and osteoclasts, which participate
in bone cleanup and remodeling. Two types of bone exist: compact bone, which is dense and robust, and spongy (or cancellous)
bone, housing red and yellow marrow for new blood cell production.
This connective tissue, found in the skeleton, offers support and
protection to the body.
Blood: Blood, another specialized connective tissue, is composed of
red and white blood cells, along with platelets, all suspended in a liquid called plasma. Found in the circulatory system, blood transports
oxygen, nutrients, hormones, and other substances throughout the
body.
It may be confusing to consider blood a tissue, as it is suspended
in fluid, which is less consistent than typical examples of tissues
and organs. However, despite its lower density compared to other
tissues, blood is still classified as a connective tissue on the IMAT
exam.
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In summary, connective tissue plays a crucial role in supporting,
binding, and connecting various body parts and is vital for the body’s
proper functioning.
15.5 Muscle Tissues
Muscle tissue is a type of tissue found in the human body that has
the ability to contract and produce movement. There are three main
types of muscle tissue: skeletal, cardiac, and smooth.
Skeletal muscle:
1. Origin: Skeletal muscles are attached to bones and are under
voluntary control, meaning that we can consciously control their
movement.
2. Structure: Skeletal muscles are composed of long, cylindrical
muscle fibers that are arranged in parallel. Skeletal muscle is
rich in blood vessels and is able to regenerate if damaged.
3. Function: Skeletal muscles are responsible for movement, such
as walking, running, and jumping.
Smooth muscle:
1. Origin: Smooth muscle is found in the walls of internal organs,
such as the digestive tract, and is not under conscious control.
2. Structure: Smooth muscle fibers are spindle-shaped and arranged in circular or longitudinal layers and is not as well vascularized as skeletal muscle. They also contain gap junctions in
between cells to allow for a more rapid depolarization of cells.
3. Function: Smooth muscle is responsible for involuntary movements, such as the peristaltic waves in the digestive tract.
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Cardiac muscle:
1. Origin: Cardiac muscle is found in the heart and is also not under
conscious control.
2. Structure: Cardiac muscle fibers are shorter, thicker, branched,
and connected to each other by intercalated disks (like a special
gap junction) which allow for a more coordinated heart beat.
3. Function: Cardiac muscle is responsible for the rhythmic contraction and relaxation of the heart, which pumps blood through
the body.
In summary, the three main types of muscles in the human body are
skeletal, smooth, and cardiac muscle, and they differ in terms of their
origin, structure, and function. Skeletal muscle is attached to bones
and is under voluntary control, smooth muscle is found in the walls
of internal organs and is responsible for involuntary movements,
and cardiac muscle is found in the heart and is responsible for the
rhythmic contraction and relaxation of the heart.
15.6 Nervous Tissues
Nervous tissue is a specialized tissue in the human body responsible for processing and transmitting information. It comprises two
primary cell types: neurons and glial cells.
Neurons, also known as nerve cells, are responsible for transmitting
information throughout the body. These cells consist of three main
parts: the cell body, which houses the cell’s nucleus and other organelles; the dendrites, which receive signals from other neurons;
and the axon, which conveys signals to other neurons or muscles
or glands. Neurons can be compared to a home’s electrical wiring
system, which carries electrical signals to power various devices.
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Figure 79: (a) Striated muscle fibers are a characteristic feature
of skeletal muscle cells, with nuclei located at the periphery. (b)
Smooth muscle cells, in contrast, lack visible striations and contain
a single nucleus. (c) Cardiac muscle cells also exhibit striations and
have one nucleus. These cellular characteristics can be observed in
micrographs. (Credit: openstax.org)
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Just as the electrical wiring connects to a central power source and
different outlets, switches, and appliances, neurons receive electrical signals from other neurons or sensory cells and transmit these
signals to other neurons or effector cells, like muscles.
Glial cells, or glia, are cells that provide support and protection for
the neurons. Various types of glial cells exist, such as astrocytes,
oligodendrocytes, and microglia. Astrocytes offer support and nourishment to neurons, while oligodendrocytes generate the myelin
sheath, a protective layer enveloping the axons of certain neurons.
Microglia maintain the nervous system’s health by removing damaged or dead cells.
Nervous tissue is distributed throughout the body, with the highest
concentration in the brain and spinal cord. It plays a crucial role in
the body’s functionality by transmitting information and orchestrating the actions of all other bodily systems. The nervous system can
be divided into the central nervous system (CNS), which includes
the brain and spinal cord, and the peripheral nervous system (PNS),
which comprises all the nerves extending from the CNS to the rest
of the body. The CNS processes information and controls various
bodily functions, while the PNS connects the CNS to sensory receptors, muscles, and glands, enabling communication between the
CNS and the rest of the body.
In summary, nervous tissue is essential for the proper functioning
of the human body, as it enables the transmission and processing
of information, allowing for coordination and control of all other
systems.
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Figure 80: Nervous tissue consists of neurons and neuroglia, which
are specialized cells that are designed to transmit and receive impulses. (Credit: openstax.org)
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Figure 81: The 4 types of tissue that can be found in the human
body. (Credit: openstax.org)
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16
The Digestive System
The digestive system is a group of organs that work together to
break down food, absorb nutrients, and eliminate waste. It begins
at the mouth and ends at the anus, and includes the esophagus,
stomach, small intestine, large intestine, and rectum.
The anatomy of the digestive system includes the various organs
and structures that make up the system, as well as the muscles and
other tissues that help to move food through the body.
The physiology of the digestive system involves the processes that
occur within the system to break down food, absorb nutrients, and
eliminate waste. This includes the secretion of enzymes and other
substances that help to digest food, as well as the movements of the
muscles and tissues that help to propel food through the system.
In this lesson, we will be taking a closer look at the anatomy and
physiology of the digestive system, including the functions of each
of the major organs and the role they play in the process of digestion.
By the end of this lesson, you should have a better understanding
of how the digestive system works and the important role it plays in
maintaining the health of the body.
16.0.1 The Oral Cavity
The oral cavity, or mouth, is the starting point for the digestive process. It is here that food is mechanically broken down into smaller
particles by the chewing action of the teeth, known as mastication.
All mammals have teeth that allow them to chew their food.
The chemical digestion of food also begins in the mouth. As food
is being chewed, saliva produced by the salivary glands mixes with
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the food. There are three major salivary glands in the human body:
the parotid, submandibular, and sublingual glands. Saliva contains
mucus, which helps to moisten food and regulate its pH, as well as
immunoglobulins and lysozymes with antibacterial properties that
can help prevent tooth decay. Saliva also contains the enzyme salivary amylase, which begins the process of breaking down starches
in food into a disaccharide called maltose. Another enzyme called
lipase, produced by cells in the tongue, can help to break down fat
components in food.
These combined actions result in the formation of a mass of food
called the bolus, which is then moved to the back of the mouth
by the tongue for swallowing. The pharynx, or throat, is a crucial
part of the digestive system as it serves as a passageway for both
the respiratory and digestive systems. It has an opening called the
glottis, which is covered by a cartilaginous flap called the epiglottis.
The epiglottis acts as a gatekeeper and closes the glottis to prevent
food from entering the trachea, or windpipe, during swallowing.
This helps to keep food out of the respiratory system and prevent
choking. Instead of entering the trachea, the bolus is directed into
the esophagus by the closure of the epiglottis. The esophagus is
a muscular tube that connects the pharynx to the stomach and
is responsible for pushing the bolus down into the stomach. The
muscles in the esophagus contract in a coordinated manner to
propel the food downward. This process is known as peristalsis. This
arrangement helps to keep food out of the respiratory system and
prevent choking.
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lowed. To move food from the mouth to the stomach, the esophagus relies on a series of wave-like movements called peristalsis.
These involuntary reflexes are initiated by the act of swallowing and
help to push the food down towards the stomach. Peristalsis is a
unidirectional movement and food cannot move backwards in the
esophagus.
At the junction between the esophagus and the stomach, there is a
ring-like muscle called the gastro-esophageal sphincter or the lower
esophageal sphincter (LES). When food is swallowed, this sphincter
opens to allow the bolus of food to enter the stomach. When there
is no swallowing action, the sphincter remains closed to prevent
the contents of the stomach from traveling back up the esophagus.
However, the LES is not considered a true sphincter, as it does not
maintain a constant and complete pressure, like a true sphincter.
A true sphincter, such as the one present in the iris of the eye, can
open and close rapidly and maintain a constant pressure to control
the flow of fluid or gas. In contrast, the LES is a dynamic muscle
that undergoes changes in pressure, depending on various factors
such as food intake, gastric pressure, and abdominal pressure. It
can relax to allow food and gastric juices to pass into the stomach
and then contract to prevent reflux. However, this process is not
always consistent, and certain factors such as obesity, pregnancy, or
eating a large meal can cause the LES to relax, allowing stomach
contents to reflux into the esophagus, causing heartburn or other
symptoms of gastroesophageal reflux disease (GERD). This can be
uncomfortable or painful and may require medical treatment.
16.0.3
The Stomach
The stomach is a muscular organ located in the upper abdomen that
plays a vital role in digestion. When we eat, the food is mixed with
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Figure 82: The transfer of food
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esophagus is regulated by the upper esophageal sphincter, while the
lower esophageal sphincter governs the movement of food from
the esophagus to the stomach. (Credit: openstax.org)
gastric juices produced by the stomach, which contain enzymes
that break down protein and help extract nutrients. The stomach is
able to expand to hold large amounts of food, making it useful for
animals that need to eat when food is available.
Inside the stomach, proteins are broken down by the enzyme pepsin,
which is produced by the chief cells in an inactive form called pepsinogen. Hydrochloric acid, secreted by the parietal cells, helps to activate the pepsinogen and create a highly acidic environment in
the stomach. This acidity helps to kill bacteria and other microorganisms present in the food, as well as facilitating the hydrolysis of
proteins. The stomach’s smooth muscles contract and relax every
20 minutes, mixing the stomach contents and creating a partially
digested mixture called chyme.
To protect the stomach lining from being digested by pepsin, the
enzyme is produced in an inactive form along with other mechanisms the stomach uses. The stomach protects itself from ulcers by
producing a thick layer of mucus to act as a barrier, releasing bicarbonate to neutralize the acid, producing hormone-like substances
called prostaglandins that encourage the production of mucus and
bicarbonate and decrease acid production, increasing blood flow
to the area for healing, and producing a substance called intrinsic
factor that helps absorb vitamin B12 and also protects the stomach.
Despite these protections, some people may still develop ulcers
from factors such as taking certain medicines (such as the use of
nonsteroidal anti-inflammatory drugs (NSAIDs), having a bacterial
infection caused by the bacterium Helicobacter pylori or by drinking
too much alcohol. Treatment for ulcers often involves reducing acid
production, eliminating the infection, or reducing the use of certain
medicines.
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16.0.4
The Small Intestine
The small intestine is a long, narrow tube located in the lower abdomen that plays a key role in the digestion and absorption of nutrients from food. It is made up of three sections: the duodenum,
the jejunum, and the ileum.
The duodenum is the first part of the small intestine and is about
25.4 cm long. It receives partially digested food, called chyme, from
the stomach and mixes it with bile and digestive enzymes produced
by the liver and pancreas, respectively. These enzymes help to break
down fats, proteins, and carbohydrates into smaller molecules that
can be absorbed by the body.
The jejunum is the middle section of the small intestine and is about
2.44 meters long. It is responsible for absorbing most of the nutrients
from the chyme. The walls of the jejunum are lined with tiny fingerlike projections called villi, which greatly increase the surface area
for absorption. Nutrients are absorbed through the walls of the villi
and enter the bloodstream, where they are transported to the liver
for further processing and storage.
The ileum is the last section of the small intestine and is about 3.66
meters long. It absorbs any remaining nutrients from the chyme
and also absorbs vitamin B12, which is important for the production
of red blood cells. The ileum empties into the large intestine, also
known as the colon.
The small intestine is highly efficient at absorbing nutrients, and it is
estimated that about 90% of the nutrients from the food we eat are
absorbed in the small intestine. It is also protected by several layers
of muscles that contract and relax to move the chyme through the
intestine and prevent the backflow of undigested material.
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Figure 83: The 3 regions of the small intestine can be appreciated in
this diagram, namely the duodenum, jejenum, and ileum. (Credit:
openstax.org)
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16.0.5 The Large Intestine
The large intestine, also known as the colon, is a long tube-like organ
located in the lower abdomen that plays a key role in the digestion
and elimination of waste products from the body. It is about 1.52
meters long and is made up of several parts: the ascending colon,
the transverse colon, the descending colon, the sigmoid colon, and
the rectum.
The ascending colon is the first part of the large intestine and begins
at the ileum, the end of the small intestine. It travels up the right
side of the abdomen and passes under the liver.
The transverse colon is the second part of the large intestine and
runs across the abdomen from right to left. It is connected to the
ascending colon on the right and the descending colon on the left.
The descending colon is the third part of the large intestine and is
located on the left side of the abdomen. It travels downward and is
connected to the transverse colon on the left and the sigmoid colon
on the right.
The sigmoid colon is the fourth part of the large intestine and is
shaped like the letter S. It is located in the lower left side of the
abdomen and is connected to the descending colon on the left and
the rectum on the right.
The rectum is the final part of the large intestine and is a short,
muscular tube that stores solid waste products until they are ready
to be eliminated through the anus. The rectum is located just above
the anus and is connected to the sigmoid colon on the left.
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terial that enters the large intestine from the small intestine, and
converts it into solid stool. The large intestine also eliminates waste
products from the body through the rectum and anus.
Figure 84: The Large Intestine (Credit: openstax.org)
16.1 Digestive Enzymes
Digestive enzymes are proteins that are produced by the body to
help break down the food we eat into smaller molecules that can
be absorbed and used by the body. They are produced by various
organs in the digestive system, including the mouth, stomach, pancreas, and small intestine.
In the mouth, the enzyme amylase is produced by the salivary glands
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and helps to break down carbohydrates into simpler sugars. Amylase begins to work on the food as soon as it enters the mouth, and
continues to work on it as it travels down the digestive tract.
In the stomach, the enzyme pepsin is produced by the chief cells
and helps to break down proteins into smaller peptides. Pepsin is
activated by hydrochloric acid, which is produced by the parietal
cells in the stomach.
The pancreas is a gland located in the upper abdomen that produces a variety of digestive enzymes, including lipases, which break
down fats into fatty acids and glycerol; proteases, which break down
proteins into peptides; and lactase, which breaks down lactose, a
sugar found in milk. These enzymes are released into the small
intestine, where they continue to break down the food and extract
nutrients.
The small intestine does produce its own set of enzymes, including
lactase, sucrase, and maltase, which help to break down sugars; and
chymotrypsin, which helps to break down proteins.
16.1.1
The Main Nerve involved in the release of Digestive Enzymes
For context, it would be useful to understand the main nerve involved in the secretion of the various digestive enzymes that allow
our bodies to gather the nutrients that pass through our digestive
tracts. Knowing this will also be useful in future medical knowledge.
The main nerve involved in the release of digestive enzymes is the
vagus nerve (CN X). The vagus nerve originates in the brain and
travels down through the abdomen, where it branches out to the
various organs of the digestive system.
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The vagus nerve controls the release of acetylcholine, a neurotransmitter that stimulates the secretion of gastric juices, including hydrochloric acid and digestive enzymes, in response to the presence
of food in the stomach. When food enters the stomach, the vagus
nerve signals the stomach to secrete gastric juices, which help to
break down the food into smaller particles that can be absorbed
and used by the body.
In addition to stimulating gastric secretions, the vagus nerve also
controls the peristaltic contractions of the stomach wall, which mix
the food and gastric juices together and grind up the food into
smaller particles. This helps to ensure thorough mixing and exposure of the food to the digestive enzymes, leading to complete
breakdown and absorption of nutrients.
Overall, the vagus nerve plays a crucial role in the regulation of
gastric secretions, helping to ensure that the digestive process is
coordinated and efficient.
16.1.2 Enzymes and other secretions of the Stomach
The stomach is an important organ in the digestive system that
plays a major role in the digestion and absorption of nutrients from
the food we eat. It produces several enzymes and hormones that
aid in this process, including pepsin, gastric lipase, hydrochloric acid,
intrinsic factor, mucin, and gastrin.
There are several gastric enzymes involved in the breakdown of
nutrients in the stomach:
Pepsin: Pepsin is the main protease enzyme in the stomach and is
responsible for breaking down proteins into smaller peptides and
amino acids. It is activated from the inactive form called pepsinogen
by the acidic environment in the stomach created by hydrochloric
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acid (HCl).
Gastric lipase: Gastric lipase is a lipase enzyme that is secreted by the
chief cells of the stomach and is responsible for breaking down fats
into fatty acids and glycerol. It is more effective at breaking down
fats at low pH levels, which makes it important in the stomach,
where the pH is between 1.5 and 2.5.
Gastric amylase: Gastric amylase is an enzyme that breaks down
carbohydrates, but its role in digestion is relatively limited compared
to other digestive enzymes, as most carbohydrate digestion occurs
in the small intestine.
It is also important to note of the different non-enzymatic secretions
of the stomach that aid in the overall process of digestion:
Hydrochloric acid, also produced by the parietal cells, helps to denature proteins, kill bacteria and viruses present in the food, and
activate pepsinogen into pepsin. Intrinsic factor, also produced by
the parietal cells, is important for the absorption of vitamin B12 in
the ileum.
Mucin is produced by the goblet cells in the lining of the stomach.
Its primary function is to protect the stomach from the corrosive
effects of stomach acid (hydrochloric acid, HCl). Mucin forms a gellike layer over the surface of the stomach lining, creating a barrier
between the acid and the delicate mucosal cells. This protects the
stomach from being damaged by the acid and helps maintain the
integrity of the mucosal barrier. Mucin also has a role in bacterial
protection. The gel-like layer created by mucin can trap and prevent
the growth of harmful bacteria that may be present in the stomach.
The acidic environment created by HCl also helps to kill bacteria and
prevent the overgrowth of harmful bacteria in the stomach.
Intrinsic factor is a glycoprotein produced by the parietal cells of
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the stomach. Its primary function is to facilitate the absorption of
vitamin B12 in the small intestine. Vitamin B12 is required for the
proper functioning of the nervous system and the formation of red
blood cells. Intrinsic factor binds to vitamin B12 in the stomach and
protects it from degradation, allowing it to be absorbed in the small
intestine.
Gastrin is a hormone produced by the G cells in the antral part of
the stomach. Its primary function is to stimulate the secretion of
hydrochloric acid (HCl) and pepsin in the stomach, and also to stimulate the contractions of the stomach wall. The presence of food in
the stomach stimulates the release of gastrin, which in turn stimulates the release of HCl and pepsin, creating an acidic environment
for digestive enzyme activation. Gastrin also stimulates the production of mucus in the stomach, which protects the stomach lining
from the corrosive effects of HCl.
16.1.3 Pancreatic Enzymes
The pancreas is a gland that has both endocrine and exocrine functions. Its endocrine function involves the production and release
of hormones such as insulin and glucagon, which help to regulate
glucose metabolism in the body. The pancreas also has an exocrine
function, which involves the production of digestive enzymes and
fluids that are released into the small intestine through the pancreatic duct. These enzymes and fluids help to break down proteins,
fats, and other nutrients in the small intestine to aid in digestion.
There are two types of cells in the pancreas that are responsible for its
exocrine function: ductal cells and acinar cells. Ductal cells produce
bicarbonate, which neutralizes the acidity of the stomach contents
as they enter the small intestine. Acinar cells produce a number
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of inactive enzymes, known as zymogens, which are activated in
the small intestine and play a key role in digestion. These enzymes
include trypsinogen, chymotrypsinogen, carboxypeptidase, elastases, pancreatic lipase, sterol esterase, phospholipase, nucleases,
and pancreatic amylase.
The pancreas’s exocrine function is regulated by a number of biofeedback mechanisms, including secretin and cholecystokinin (CCK),
which are hormones produced in the small intestine in response
to the presence of acidic or fatty/protein-rich chyme. These hormones stimulate the production and release of digestive enzymes
and fluids by the pancreas. In addition, the hormone gastrin, which
is produced by cells in the stomach, plays a role in regulating the
production and release of these enzymes and fluids.
Deficiencies in the production of these enzymes and fluids can
lead to exocrine pancreatic insufficiency, which can be treated with
pancreatic enzyme supplements.
16.1.4 Intestinal Enzymes
The duodenum, which is the first part of the small intestine, plays a
key role in the digestion process. It is here that a number of enzymes
and hormones are produced, which help to further break down the
chyme (a mixture of stomach contents and digestive juices) as it
passes through the intestine.
Given that the duodenum is one of the most integral sites of nutrient
absorption, it would be useful to familiarize yourself with the over
view of the regulation of the various secretions of digestive enzymes
and other substances via a complex network of feedback mechanisms. These mechanisms ensure that the right amount of digestive
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der to properly digest the food and absorb nutrients. Some of the
key feedback mechanisms involved in duodenal secretions include
the following:
Hormones play a major role in the regulation of duodenal secretions.
For example, the hormone secretin is released by ”S cells” in response
to the presence of acidic chyme in the duodenum. Secretin stimulates the pancreas to secrete bicarbonate, which neutralizes the
acidic chyme, creating a more alkaline environment for digestive enzymes to function optimally. Similarly, the hormone cholecystokinin
(CCK) is released by ”I cells” in response to the presence of fats and
amino acids in the duodenum. CCK stimulates the pancreas to secrete digestive enzymes that break down fats and proteins, and it
also stimulates the gallbladder to release bile, which helps emulsify
fats. CCK also slows down gastric motility and emptying, giving
more time for the pancreatic juices to neutralize the acidity of the
chyme.
Other hormones produced in the duodenum include gastric inhibitory peptide (GIP), motilin, and somatostatin. GIP slows down
gastric motility, while motilin increases gastrointestinal motility and
somatostatin inhibits a variety of secretory mechanisms.
In addition to these hormones, the small intestine also contains a
number of brush border enzymes that help to further break down
the chyme into absorbable particles. These include exopeptidases
and endopeptidases, which convert peptones and polypeptides
into amino acids, as well as maltase, lactase, sucrase, and other
disaccharidases, which convert sugars into simpler forms that can
be absorbed by the body.
Aside from hormones and enzymes, neural regulation also plays
an important role as mentioned earlier in this chapter. The enteric
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nervous system, which is sometimes referred to as the ”second
brain,” plays a crucial role in the regulation of duodenal secretions.
For example, the vagus nerve stimulates the release of digestive
enzymes and bicarbonate from the pancreas and bile from the
gallbladder. The enteric nervous system can also modulate the
secretion of digestive enzymes in response to changing conditions
in the duodenum.
Local feedback is also in effect in terms of regulating duodenal secretions which means that the local environment of the duodenum
can also have an effect on digestive enzyme secretion. For example,
the presence of partially digested food in the duodenum can stimulate the release of additional digestive enzymes, while the presence
of partially absorbed nutrients can signal the reduction or cessation
of enzyme secretion.
These feedback mechanisms work together to regulate the release
of digestive enzymes and other substances in the duodenum, ensuring that the right amount of digestive enzymes are released at
the right time to properly digest and absorb nutrients.
16.1.5 Bile - Not an Enzyme?
Bile is a yellow-green fluid produced by the liver and stored in the
gallbladder. Its main function is to help with the digestion and
absorption of fats in the small intestine.
Bile is produced by the liver cells called hepatocytes, which secrete
it into small channels called bile ducts. From there, it flows into the
larger common bile duct, which carries it to the gallbladder. The
gallbladder is a small, pear-shaped organ that stores and releases
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tion, including bile acids, bile pigments, cholesterol, and electrolytes.
Bile acids, which are the main component of bile, help to emulsify
fats, breaking them down into smaller droplets that can be more
easily digested and absorbed by the body. This process is called
emulsification.
Bile pigments, such as bilirubin, are produced when the liver breaks
down old red blood cells. They give bile its yellow-green color and
are eliminated from the body through the stool. For some context,
bilirubin is a byproduct of hemoglobin breakdown in red blood cells.
When red blood cells break down, the iron in hemoglobin is recycled,
and the rest of the molecule becomes bilirubin. The liver then takes
up this bilirubin and processes it into a form that can be excreted in
the bile
Cholesterol is a type of fat that is important for many body functions.
It is found in high levels in bile, and helps to form micelles, which
are small structures that allow fats to be absorbed by the body.
Bile is released into the small intestine in response to the presence
of fats in the chyme (a mixture of stomach contents and digestive
juices). The hormone cholecystokinin (CCK), which is produced
by cells in the small intestine, stimulates the contraction of the
gallbladder and the release of bile into the small intestine. Bile
then mixes with the chyme and helps to digest and absorb fats and
fat-soluble vitamins.
Bile is not an enzyme. Enzymes are proteins that catalyze chemical
reactions in the body and are involved in a wide range of metabolic
processes. Bile, on the other hand, is a fluid produced by the liver
and stored in the gallbladder that plays a role in the digestion and
absorption of fats. While bile is not an enzyme, it does work with
other enzymes such as lipases, which help to digest fats. These
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enzymes are produced by the pancreas and are released into the
small intestine along with bile to aid in the digestion of fats.
16.1.6
Accessory Organs
The accessory organs of the digestive system are organs that play a
supporting role in the digestive process but are not part of the main
digestive tract. They include the salivary glands, the pancreas, the
liver, and the gallbladder.
The salivary glands play an important role as an accessory organ of
the digestive system. They produce and secrete saliva, which has
several important functions in the digestive process. Saliva helps to
moisten food and make it easier to swallow, and it contains an enzyme called amylase, which starts the breakdown of carbohydrates
in the mouth. Additionally, saliva helps to lubricate food and protect
the mouth, throat, and esophagus from injury. The main function
of the salivary glands is to prepare food for swallowing and initiate
the digestive process by starting the breakdown of carbohydrates.
The liver is the largest gland in the body and is located in the upper
right side of the abdomen, just beneath the diaphragm. It has a
number of important functions, including the production of bile,
which helps with the digestion and absorption of fats in the small
intestine. The liver also plays a role in the metabolism of carbohydrates, proteins, and lipids, and is involved in the detoxification of
harmful substances in the body.
The gallbladder is a small, pear-shaped organ that is located just
beneath the liver. It stores and releases bile into the small intestine
as needed to aid in the digestion and absorption of fats.
The pancreas is a gland located behind the stomach that has both
endocrine and exocrine functions. Its endocrine function involves
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the production and release of hormones such as insulin and glucagon,
which help to regulate glucose metabolism in the body. The pancreas also has an exocrine function, which involves the production of
digestive enzymes and fluids that are released into the small intestine through the pancreatic duct. These enzymes and fluids help to
break down proteins, fats, and other nutrients in the small intestine
to aid in digestion.
The accessory organs of digestion play important roles in supporting
the process of digestion and absorption of nutrients in the body.
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17 The Nervous System
The nervous system is a complex network of cells and tissues that
enables us to communicate with the world around us, perceive
and interpret sensory information, and make decisions and actions
based on that information.
At the heart of the nervous system is the action potential, a tiny but
mighty electrical impulse that travels along the nerves, transmitting
information from one part of the body to another. When a nerve
receives a signal, it generates an action potential, which is like a tiny
spark of electricity that travels down the nerve and sends a message
to the next nerve or muscle.
The nerves themselves are like tiny wires that carry these electrical
signals to and from different parts of the body. They are made up of
cells called neurons, which have special structures that allow them
to receive and transmit signals. These structures include dendrites,
which receive signals from other neurons, and an axon, which sends
signals to other neurons or muscles.
So next time you feel your heart racing or your muscles moving, remember that it’s all thanks to the amazing communication network
of the nervous system and the tiny but mighty action potentials
that keep everything running smoothly!
17.1 Parts of a Neuron
A neuron, also known as a nerve cell, is the basic building block of
the nervous system. It’s responsible for transmitting information
throughout the body in the form of electrical and chemical signals.
There are several key parts to a neuron that work together to make
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this happen. Let’s break them down:
1. Cell Body (Soma): The cell body is the central part of the neuron.
It contains the nucleus, which holds the cell’s genetic material
(DNA). The cell body is responsible for maintaining the overall
health and function of the neuron. It also contains other important structures called organelles, which help the neuron carry
out various tasks, such as producing energy and synthesizing
proteins.
2. Dendrites: Dendrites are branch-like extensions that come out
of the cell body. Their job is to receive information from other
neurons in the form of chemical signals called neurotransmitters.
When neurotransmitters are released by neighboring neurons,
they bind to receptor sites on the dendrites, which can then
generate an electrical signal. This signal is sent to the cell body
and, if strong enough, will continue along the axon.
3. Axon: The axon is a long, thin extension that transmits electrical
signals from the cell body to other neurons or target cells. It can
vary in length, ranging from just a few micrometers to over a
meter long in some cases. Axons are covered in a fatty substance
called myelin, which acts like insulation to help speed up the
transmission of electrical signals.
4. Myelin Sheath: The myelin sheath is a protective layer made
of fatty tissue that wraps around the axon. It helps to insulate
the axon and increase the speed of electrical signals, making
communication between neurons more efficient. In between
segments of myelin, there are small gaps called Nodes of Ranvier, which play a crucial role in helping electrical signals jump
from one segment to the next, further speeding up the transmission process.
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5. Axon Terminals (Synaptic Terminals): At the end of the axon,
there are small branches called axon terminals. These terminals
connect with other neurons or target cells at specialized junctions called synapses. Within the axon terminals, there are small
sacs called synaptic vesicles that store neurotransmitters. When
an electrical signal reaches the end of the axon, it triggers the
release of neurotransmitters into the synapse.
6. Synapse: The synapse is the tiny gap between the axon terminal
of one neuron and the dendrites or cell body of another neuron
(or target cell). This is where the exchange of information takes
place, as neurotransmitters released from the axon terminal of
the first neuron bind to receptors on the second neuron, creating
a new electrical signal.
In summary, neurons are specialized cells with unique structures,
such as dendrites, axons, myelin sheaths, and axon terminals, which
work together to transmit information throughout the nervous system. They communicate with each other at synapses, using chemical signals called neurotransmitters to pass messages from one
neuron to another.
17.2
Neural Communication
Neurons are specialized cells that facilitate communication within
the nervous system. As the primary cells responsible for transmitting
information throughout the body, they engage in a complex process
called neurotransmission to communicate with each other and with
other cells.
Neurotransmission can be broken down into three main steps:
Reception: Neurons receive signals from other neurons or sensory
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Figure 85: Parts of a Neuron: Neurons are specialized cells that
transmit signals throughout the nervous system. The main parts
of a neuron include the dendrites, axon, myelin sheath, and axon
terminals. (Credit: KhanAcademy.org)
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receptors via specialized structures called dendrites. Signals can
be either chemical, such as neurotransmitters, or electrical, like the
action potentials generated by adjacent neurons.
Integration: Upon receiving a signal, the neuron processes and
integrates the incoming information, considering both the strength
of the signal and the activity of other neurons within its network. The
integration of signals occurs in the cell body, also known as the soma,
where the neuron evaluates whether the combined input surpasses
a certain threshold needed to generate an action potential.
Transmission: If the neuron determines that the signal should be
transmitted further, it generates an action potential—a rapid, temporary change in the cell membrane potential. The action potential
propagates along the neuron’s axon, eventually reaching the axon
terminal. Upon arrival, the action potential triggers the release of
neurotransmitter molecules stored in synaptic vesicles. These neurotransmitters diffuse across the synaptic cleft, the gap between
neurons, and bind to specific receptors on the postsynaptic neuron
(or a muscle cell). This binding initiates a new electrical or chemical
signal in the postsynaptic cell, effectively transmitting the original
signal.
Neurotransmission allows neurons to communicate with each other
and with various other cells in the body. This intricate process involves the reception of signals, the integration of incoming information, and the transmission of the signal across the synapse using
action potentials and neurotransmitter chemicals. Through neurotransmission, the nervous system can coordinate and regulate
numerous bodily functions, enabling it to respond to internal and
external stimuli.
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17.3
Signal Propagation
Signal propagation among neurons is a fascinating topic that helps
us understand how our brain processes information. At a high school
level, let’s break this down into simpler terms by discussing neurons,
synapses, neurotransmitters, and the synaptic cleft.
Neurons: Neurons are the basic building blocks of the nervous system, responsible for transmitting information throughout the body.
They consist of three main parts: the cell body (soma), dendrites,
and axon. The cell body contains the nucleus and other organelles
that maintain the neuron’s functions. Dendrites are branch-like
structures that receive information from other neurons, while the
axon is a long, slender projection that transmits information to other
neurons or cells.
Synapses: The point at which two neurons communicate is called a
synapse. There are three components to a synapse: the presynaptic
neuron, the synaptic cleft, and the postsynaptic neuron. The presynaptic neuron is the one sending the signal, while the postsynaptic
neuron receives the signal.
Synaptic Cleft: The synaptic cleft is a tiny gap between the presynaptic and postsynaptic neurons, where the signal must cross for
the communication to occur. This gap allows for the chemical transmission of information using neurotransmitters.
Neurotransmitters: These are chemical messengers that transmit
signals across the synaptic cleft from one neuron to another. When
an electrical signal called an action potential travels down the axon
of the presynaptic neuron, it reaches the synaptic terminal (the end
of the axon). This triggers the release of neurotransmitters stored
in vesicles within the terminal. The neurotransmitters then travel
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across the synaptic cleft and bind to receptors on the postsynaptic
neuron’s dendrites.
Receptor activation: Once neurotransmitters bind to their specific
receptors, they can either excite or inhibit the postsynaptic neuron.
Excitatory neurotransmitters increase the likelihood of the postsynaptic neuron generating its own action potential, while inhibitory
neurotransmitters decrease that likelihood. This balance between
excitation and inhibition helps to fine-tune the communication between neurons.
Termination of the signal: After the neurotransmitters have done
their job, they need to be cleared from the synaptic cleft to prevent
continuous activation of the postsynaptic neuron. There are two
main ways this can happen: reuptake and enzymatic breakdown. In
reuptake, neurotransmitters are taken back up into the presynaptic
neuron to be reused. In enzymatic breakdown, enzymes in the
synaptic cleft break down the neurotransmitters, rendering them
inactive.
Neural plasticity: One of the most fascinating aspects of neuronal
communication is its ability to change and adapt over time. Neural
plasticity refers to the brain’s ability to reorganize its connections and
strengthen or weaken synapses based on experience. This process
is essential for learning and memory formation.
In summary, signal propagation among neurons involves a complex yet elegant process of electrical and chemical communication
through the release of neurotransmitters at synapses and across
the synaptic cleft. This exchange of information allows our brains to
process information, learn, and adapt to our ever-changing environment.
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Figure 86: (Credit: The key components in signal propagation from
one neuron to another are important in ensuring action potentials
are effectively transmitted. KhanAcademy.org)
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17.4
Action Potential
An action potential is a sudden, brief change in electrical voltage
that occurs in certain types of cells, such as neurons, muscle cells,
and certain types of gland cells. It is a way that these cells use to
transmit information, either within the cell or to other cells.
The action potential begins when a neuron receives a stimulus, such
as a chemical signal from another neuron or a physical stimulus such
as touch or pressure. This stimulus causes the neuron to become
more permeable to certain ions, such as sodium ions. As a result,
the concentration of positive ions inside the cell increases, while
the concentration of negative ions decreases. These changes in ion
permeability cause a rapid, brief change in the membrane potential,
or voltage, across the cell membrane. The membrane potential is a
crucial aspect of cell biology and refers to the difference in electrical
charge between the interior and exterior of a cell membrane. It
is maintained by a delicate balance of ion concentrations and ion
channels present in the membrane. Normally, the inside of the cell
is negatively charged relative to the outside, due to the presence of
more negatively charged ions, such as chloride, and fewer positively
charged ions, such as sodium.
Once the membrane potential reaches a certain threshold, called
the action potential threshold, it triggers an all-or-nothing response.
The membrane potential becomes more positive, reaching a peak
called the action potential peak, and then rapidly returns to its resting state, called repolarization. This rapid change in the membrane
potential is called an action potential. The precise nature of the
changes in ion permeability and membrane potential during an
action potential are complex and involve multiple ion channels and
ion pumps. Nevertheless, understanding the basics of membrane
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potential and its role in action potentials is crucial to understanding
the electrical signaling that occurs in cells.
The action potential then travels down the length of the neuron,
from the dendrite to the axon terminal, transmitting the electrical signal from one part of the cell to another. When the action
potential reaches the axon terminal, it triggers the release of neurotransmitter chemicals, which pass the signal on to the next neuron
or muscle cell. Neurotransmitters are chemical messengers that
transmit signals between neurons in the nervous system. As mentioned, they are released from the axon terminal of a presynaptic
neuron and bind to specific receptors on the postsynaptic neuron,
causing a change in the membrane potential of the postsynaptic
neuron. This change in membrane potential can lead to the initiation or inhibition of an action potential, which transmits the signal
from one neuron to another. Examples of neurotransmitters include
acetylcholine, dopamine, serotonin, and glutamate. Different neurotransmitters have different effects on the postsynaptic neuron,
depending on the type of receptors they bind to and the resulting
changes in membrane potential. Some neurotransmitters excite
the postsynaptic neuron, causing it to fire an action potential, while
others inhibit it, preventing it from firing an action potential.
Thus, neurotransmitters play a key role in the functioning of the
nervous system, and imbalances or disruptions in neurotransmitter
levels can lead to various neurological and psychiatric conditions,
such as depression, anxiety, and Parkinson’s disease.
17.4.1 Steps
The action potential is a series of changes that occur in the membrane potential of a cell in response to a stimulus. The steps of an
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action potential are as follows:
Resting state: At rest, the cell membrane is more permeable to
potassium ions than to sodium ions through the balanced activity
of ion channels and pumps in the cell membrane.. This results in
a negative voltage across the membrane, called the resting membrane potential. Let us explain that further; at rest, the concentration
of Na+ ions is higher outside the cell than inside, while the concentration of K+ ions is higher inside the cell than outside. This difference
in ion concentration creates a resting membrane potential that is
negative on the inside and positive on the outside. To maintain
this resting state, the cell has special ion channels, such as potassium leak channels, that allow K+ ions to passively diffuse out of the
cell and maintain the negative membrane potential. At the same
time, the cell has active ion pumps, such as the sodium-potassium
pump, that work to maintain the ion concentration gradient and
prevent an excessive accumulation of Na+ ions inside the cell. The
resting state ion permeability of neurons is important because it
sets the stage for the initiation of an action potential. A change in
the membrane potential, such as a depolarization, can trigger the
opening of voltage-gated ion channels and cause a rapid change in
the ion permeability of the cell, leading to the initiation of an action
potential.
Stimulus: The cell receives a stimulus, such as a chemical signal
from another neuron or a physical stimulus like touch or pressure.
This stimulus causes the cell to become more permeable to sodium
ions.
Depolarization: As the cell becomes more permeable to sodium ions,
the concentration of positive ions inside the cell increases, while
the concentration of negative ions decreases (Recall that this is the
opposite of the resting state as explained above). This causes a rapid,
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brief change in the membrane potential, called depolarization, as
the positive ions move into the cell and the negative ions move out.
Action potential threshold: Once the membrane potential reaches a
certain threshold, called the action potential threshold, it triggers an
all-or-nothing response. For more context, it is called a ”threshold”
because it is the point at which a sufficient amount of depolarization has occurred to trigger the initiation of an action potential.
Which means that, when the membrane potential is depolarized
to the threshold, it is positive enough to activate voltage-gated ion
channels, particularly voltage-gated sodium (Na+) channels. The
activation of these channels leads to a rapid and brief influx of Na+
ions into the cell, further depolarizing the membrane potential and
leading to the initiation of an action potential.
Action potential peak: The membrane potential becomes more
positive, reaching a peak called the action potential peak.
Repolarization: After reaching the action potential peak, the membrane potential rapidly returns to its resting state, called repolarization. This is due to the increased permeability of the cell membrane
to potassium ions, which causes the positive ions to move out of
the cell. Correlate this with what you know so far regarding resting
state and depolarization; At this repolarization stage, the inside of
the cell is becoming more negative because the potassium ions are
leaking out. Thus, it makes sense that this stage will bring it closer
back to the resting state since we are already aware that this state
involves the inside of the cell being more negative than the outside
of the cell.
Refractory period: After an action potential, the cell enters a refractory period, during which it is less responsive to stimuli. This
prevents the cell from firing another action potential too quickly and
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allows the signal to be transmitted more efficiently. The refractory
period has two distinct phases: the absolute refractory period and
the relative refractory period. During the absolute refractory period,
the ion channels are completely inactivated, and it is impossible to
trigger another action potential no matter how strong the stimulus.
The relative refractory period follows the absolute refractory period,
during which it is possible to trigger another action potential, but
only with a stronger stimulus than would normally be required. An
analogy for the refractory period of action potentials is the recovery
time after physical exercise. Just as our muscles need time to recover after a workout, neurons need time to recover after firing an
action potential. During the recovery period, the ion channels are in
a state of inactivation, and the membrane potential is recovering
to its resting state. Just as it is impossible to immediately start another workout without allowing time for recovery, it is impossible
to trigger another action potential without allowing time for the
refractory period.
Resting state: Once the refractory period is over, the cell returns to
its resting state, ready to receive and transmit another signal.
17.4.2
The Sodium-Potassium Pump
The sodium-potassium pump, also known as the Na+/K+ pump,
is an enzyme that helps maintain the resting potential of the cell
membrane. It does this by actively transporting ions across the cell
membrane, against their concentration gradients.
The resting potential of a cell is the electrical charge across the cell
membrane when the cell is at rest and not transmitting an action
potential. It is maintained by the balance of ions on either side of
the membrane and the relative permeability of the membrane to
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Figure 87: When graphing the voltage measured across a cell membrane over time, the action potential commences with depolarization, proceeds to repolarization, which exceeds the resting potential
and enters hyperpolarization, and then the membrane ultimately
returns to its resting state. (Credit: openstax.org)
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different ions.
At rest, the concentration of positive sodium ions is higher outside
the cell than inside, while the concentration of positive potassium
ions is higher inside the cell than outside. The cell membrane is
more permeable to potassium ions than to sodium ions, which
means that potassium ions can pass through the membrane more
easily.
The sodium-potassium pump helps maintain this balance by actively pumping three sodium ions out of the cell for every two potassium ions it pumps in. This helps to maintain the concentration
gradient of the ions and the resting potential of the cell.
Imagine a banana floating in a vast sea ocean. The sea ocean represents the exterior of the cell, while the banana represents the
interior of the cell. The sea ocean is filled with salty water, representing the high concentration of sodium ions outside the cell. The
banana, on the other hand, is high in potassium, representing the
high concentration of potassium ions inside the cell.
17.5
Channels
Nervous tissue is a specialized tissue in the human body responsible for processing and transmitting information. During an action
potential, various types of ion channels contribute to the changes
in a cell’s membrane potential. These ion channels include:
Voltage-gated sodium channels: These channels are responsible
for the initial depolarization of the cell during an action potential.
They are activated when the membrane potential reaches a specific
threshold, known as the action potential threshold. Upon activation,
they enable sodium ions to enter the cell, causing the membrane
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potential to become more positive.
Voltage-gated potassium channels: These channels are responsible for repolarizing the cell after the action potential peak. They are
activated following the action potential peak, allowing potassium
ions to exit the cell and returning the membrane potential to its
resting state.
Voltage-gated calcium channels: These channels play a role in the
release of neurotransmitter chemicals from the axon terminal at
the end of an action potential. They are activated when the action
potential reaches the axon terminal, permitting calcium ions to
enter the cell and trigger the release of neurotransmitter chemicals.
Figure 88: The opening of voltage-gated channels is triggered by
changes in the transmembrane voltage. These channels are sensitive to charge and contain amino acids in their protein structure
that cause the pore to selectively open to certain ions. (Credit: openstax.org)
Leakage channels: These channels are always open, allowing ions
to flow in and out of the cell even when it is at rest. They help
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maintain the cell’s resting potential by enabling ions to cross the
membrane to balance the concentration gradient of the ions.
Ligand-gated ion channels: Also known as ionotropic receptors,
these channels are activated by the binding of a specific neurotransmitter or other signaling molecule (ligand) to a receptor site on the
channel. Upon activation, they allow specific ions to pass through
the membrane, leading to changes in the membrane potential and
ultimately propagating or inhibiting nerve signals.
Figure 89: The opening of a channel protein is triggered when a
particular molecule, called a ligand, binds to a specific site on the
channel’s outer surface. In the case of the neurotransmitter acetylcholine, the binding causes the pore to open, allowing only certain
ions to pass through, such as cations of sodium, calcium, and potassium. (Credit: openstax.org)
Mechanically-gated ion channels: These channels are activated
by mechanical forces, such as pressure or stretch, acting on the cell
membrane. They are commonly found in sensory neurons, where
they help transduce mechanical stimuli into electrical signals. For
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example, in auditory hair cells, mechanically-gated ion channels
open in response to the movement of hair-like structures called
stereocilia, ultimately leading to the generation of an electrical signal.
Figure 90: When a mechanical force or a change in temperature
occurs in the surrounding tissue, ion channels respond by opening
or closing. In the case of pressure or touch, the channels are physically opened, while thermoreceptors detect temperature changes
and activate the channels through a physical mechanism. (Credit:
openstax.org)
In summary, ion channels play critical roles in the generation and
propagation of action potentials, allowing for the transmission of
information throughout the nervous system. The various types of ion
channels, including voltage-gated, ligand-gated, and mechanicallygated channels, work together to facilitate the complex processes
of neural communication and signaling.
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17.6 The Central Nervous System
The central nervous system (CNS) is the part of the nervous system
that consists of the brain and the spinal cord. It is the command
center of the body and is responsible for receiving, processing, and
interpreting sensory information, as well as initiating and coordinating voluntary and involuntary responses to that information.
The brain is the most complex organ in the body and is responsible for a wide range of functions, including controlling movement,
sensation, cognition, and emotion. It is made up of several interconnected areas, each with a specific function. The cerebrum is the
largest part of the brain and is responsible for higher brain functions
such as thinking, decision-making, and voluntary movement. The
cerebellum is responsible for coordinating movement and balance.
The brainstem controls basic functions such as heart rate, blood
pressure, and breathing. The hippocampus is involved in learning
and memory.
The spinal cord is a long, tubular structure that extends from the
brain down through the center of the body. It is encased in the
vertebral column and is protected by the spinal column. The spinal
cord serves as a major pathway for communication between the
brain and the rest of the body. It receives sensory information from
the body and sends motor signals to the muscles and organs.
17.6.1 The Spinal Cord
The spinal cord, a vital component of the central nervous system,
runs through the vertebral column and extends from the brainstem
to the lumbar region. It is divided into different regions, or segments,
based on the level of the vertebral column. These segments include
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cervical, thoracic, lumbar, and sacral, each responsible for controlling
a specific area of the body. The spinal cord is made up of gray matter,
which contains the cell bodies of neurons, and white matter, which
contains axons that transmit signals between neurons.
The spinal cord plays a crucial role in several functions, including
sensory processing, motor control, and autonomic control:
Sensory processing: The spinal cord receives sensory information
from the body through the dorsal root ganglia, which contain the cell
bodies of sensory neurons. Sensory information includes pain, temperature, touch, and proprioception (the awareness of the body’s
position in space). Once the sensory information enters the spinal
cord, it is transmitted to the brain for further processing via ascending tracts in the white matter. These tracts include the spinothalamic tract (responsible for pain and temperature sensations), the
dorsal column-medial lemniscus pathway (responsible for touch
and proprioception), and the spinocerebellar tract (involved in proprioception and coordination).
Motor control: The spinal cord sends motor signals to the muscles
and organs, controlling voluntary and involuntary movement and
reflexes. Motor neurons in the spinal cord’s ventral horn receive signals from the brain through descending tracts in the white matter,
such as the corticospinal tract (responsible for voluntary movement)
and the reticulospinal tract (involved in posture and muscle tone).
These motor neurons then send signals to the muscles through
their axons, which exit the spinal cord through the ventral roots and
form peripheral nerves.
Reflexes: The spinal cord is also responsible for reflexes, which are
automatic responses to stimuli. Reflex arcs involve sensory neurons,
interneurons in the spinal cord, and motor neurons that control
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muscle movement. An example of a reflex is the knee-jerk response,
in which tapping the patellar tendon causes the quadriceps muscle
to contract and the lower leg to extend.
Autonomic control: The spinal cord controls involuntary functions
through the autonomic nervous system, which consists of the sympathetic and parasympathetic divisions. These divisions regulate
functions such as heart rate, blood pressure, digestion, and body
temperature. Preganglionic neurons of the autonomic nervous system are found in the spinal cord’s lateral horn and send signals to
postganglionic neurons in autonomic ganglia. The sympathetic
division is responsible for the ”fight or flight” response, while the
parasympathetic division is involved in ”rest and digest” processes.
In summary, the spinal cord is a crucial component of the central
nervous system, responsible for sensory processing, motor control,
reflexes, and autonomic control. It contains distinct regions that
correspond to specific areas of the body and is composed of gray
matter, which houses the cell bodies of neurons, and white matter,
which contains the axons that transmit signals between neurons.
The spinal cord is a major pathway for communication between the
brain and the rest of the body. It receives sensory information from
the body and sends motor signals to the muscles and organs, as well
as controlling involuntary functions through the autonomic nervous
system. It is divided into different segments based on the level of the
vertebral column and is made up of gray matter and white matter.
Think of the segments of the spinal cord like different rooms in
a house, each with its own unique function and purpose. Just as
different rooms in a house have different functions (e.g., a kitchen, a
living room, a bedroom), different segments of the spinal cord have
different functions (e.g., controlling the legs, the arms, the torso).
This organization allows for efficient and specific communication
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between the brain and the rest of the body.
Figure 91: A cross section of the spinal cord. (Credit: openstax.org)
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17.6.2 Gray and White Matter
Gray matter and white matter are different types of tissue that make
up the central nervous system (CNS), which consists of the brain
and the spinal cord. Gray matter is made up of the cell bodies
of neurons, while white matter is made up of axons that transmit
signals between neurons.
Gray matter is so named because it appears gray in color due to the
presence of neuron cell bodies and their associated blood vessels.
It is found in the outer layers of the brain and the inner layers of
the spinal cord. Gray matter is responsible for processing and integrating sensory information, as well as controlling movement and
reflexes. It contains a higher concentration of neurons and a lower
concentration of myelin, a fatty substance that insulates axons and
helps them transmit signals more efficiently.
White matter is so named because it appears white in color due to
the presence of myelin. It is found in the inner layers of the brain
and the outer layers of the spinal cord. White matter is responsible
for transmitting signals between different areas of the brain and
between the brain and the rest of the body. It contains a lower
concentration of neurons and a higher concentration of myelin.
Gray matter and white matter are different types of tissue that make
up the central nervous system. Gray matter is made up of the cell
bodies of neurons and is responsible for processing and integrating
sensory information and controlling movement and reflexes. White
matter is made up of axons and is responsible for transmitting signals between different areas of the brain and between the brain and
the rest of the body.
It is important to note the difference of the locations of the white and
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gray matter among the brain and the spinal cord. The location of
gray and white matter in the brain and spinal cord differs in that the
gray matter is located in the cortical region while the white matter
is on the inner region of the brain. In the spinal cord, gray matter is
located in the shape of a butterfly in the center of the cord, while the
white matter is located in the peripheral region. This arrangement
in both the brain and spinal cord serves specific functions to support
communication and coordination of the body’s functions.
17.6.3
The Brain
The brain is the most complex organ in the body and is responsible for a wide range of functions, including controlling movement,
sensation, cognition, and emotion. It is made up of several interconnected areas, each with a specific function.
Here are the main areas of the brain and a brief description of their
functions:
Cerebrum: The cerebrum is the largest part of the brain and is divided into two hemispheres, the left and the right. It is responsible
for higher brain functions such as thinking, decision-making, and
voluntary movement. The cerebrum is also responsible for processing sensory information and controlling the senses, such as sight,
hearing, and touch.
Cerebellum: The cerebellum is located under the cerebrum and
is responsible for coordinating movement and balance. It receives
input from the sensory systems and the cerebral cortex and sends
output to the muscles, allowing for precise and coordinated movement. The cerebellum can be thought of as the conductor of an
orchestra, coordinating and adjusting the body’s movements for
smooth and graceful performance.
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Brainstem: The brainstem is located between the cerebrum and
the spinal cord and is responsible for controlling basic functions
such as heart rate, blood pressure, and breathing. It also plays a role
in the control of sleep and wakefulness, as well as the regulation of
unconscious reflexes such as swallowing and sneezing.
Hippocampus: The hippocampus is located in the temporal lobe
of the cerebrum and is involved in learning and memory. It plays
a crucial role in the formation and consolidation of new memories
and in the retrieval of long-term memories.
In summary, the brain is made up of several interconnected areas,
each with a specific function. The cerebrum is responsible for higher
brain functions such as thinking and decision-making, the cerebellum is responsible for coordinating movement and balance, the
brainstem controls basic functions such as heart rate and blood
pressure, and the hippocampus is involved in learning and memory.
17.6.4 The Cerebrum
The cerebrum is the largest part of the brain and is responsible
for higher brain functions such as thinking, decision-making, and
voluntary movement. It is divided into two hemispheres, the left
and the right, which are connected by a structure called the corpus
callosum. The corpus callosum is a thick band of nerve fibers that
connects the left and right hemispheres of the cerebrum, allowing them to communicate and share information. This allows for
seamless coordination and integration of the functions controlled by
each hemisphere, leading to efficient processing of information and
overall improved brain function. Each hemisphere is further divided
into four lobes: the frontal lobe, the parietal lobe, the temporal lobe,
and the occipital lobe.
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Here is a brief overview of the functions of the different areas of the
cerebrum:
Frontal lobe: The frontal lobe is located at the front of the brain
and is responsible for a wide range of functions, including planning,
problem-solving, and decision-making. It is also involved in the
control of movement and the regulation of emotions and behavior.
Parietal lobe: The parietal lobe is located towards the back of the
brain and is responsible for processing sensory information, such as
touch, temperature, and pain. It is also involved in spatial awareness
and the integration of sensory information.
Temporal lobe: The temporal lobe is located on the sides of the
brain and is responsible for processing auditory information and
language. It is also involved in memory and emotion.
Occipital lobe: The occipital lobe is located at the back of the brain
and is responsible for processing visual information. It is involved in
the interpretation of visual stimuli and the perception of color, form,
and depth.
In summary, the cerebrum is the largest part of the brain and is
responsible for higher brain functions such as thinking, decisionmaking, and voluntary movement. It is divided into four lobes: the
frontal lobe, the parietal lobe, the temporal lobe, and the occipital
lobe, each of which has specific functions related to the processing
of sensory information and the control of movement, emotion, and
behavior. These lobes interact with each other to support complex
processes such as perception, thought, and action. The functions of
each lobe can be influenced by damage or injury, so it is important
to understand their role in the overall functioning of the brain.
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Figure 92: The cerebral cortex is composed of four lobes, and its
surface area is expanded through extensive folding, providing more
space for cerebral functions. (Credit: openstax.org)
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17.6.5 The Cerebellum
The cerebellum is a small, butterfly-shaped structure located under
the cerebrum at the back of the brain. It is responsible for coordinating movement and balance and is essential for the smooth and
precise execution of voluntary movements.
The cerebellum receives input from the sensory systems and the
cerebral cortex and sends output to the muscles, allowing for precise
and coordinated movement. It is also involved in the regulation
of posture and balance, as well as the control of reflexes and the
initiation of some reflexes.
In addition to its role in movement and balance, the cerebellum
is also involved in cognitive and emotional functions. It has been
shown to play a role in language processing, attention, and learning.
The cerebellum is divided into three lobes: the anterior lobe, the
posterior lobe, and the flocculonodular lobe. Each of these lobes
has a specific function:
The anterior lobe: This lobe is responsible for controlling voluntary movements, such as those involved in walking, speaking, and
reaching. It also plays a role in motor learning, meaning it helps to
improve motor skills through repetition and practice.
The posterior lobe: This lobe is responsible for coordinating the
body’s movements and maintaining balance. It receives information
from the inner ear, the eyes, and other sensory receptors, and then
makes adjustments to muscle activity to maintain balance and
stability.
The flocculonodular lobe: This lobe is involved in regulating the
vestibular system, which is responsible for maintaining balance and
orientation. It also plays a role in coordinating eye movements and
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gaze stabilization.
Overall, the cerebellum plays a crucial role in motor control and
coordination, helping to ensure smooth and precise movements.
17.6.6 The Brain Stem
The brainstem is a crucial, cone-shaped structure situated between
the cerebrum and the spinal cord. It plays a significant role in controlling fundamental functions like heart rate, blood pressure, and
respiration. Additionally, it is involved in regulating sleep and wakefulness, as well as managing unconscious reflexes like swallowing,
sneezing, and coughing.
The brainstem can be divided into three main parts: the midbrain,
the pons, and the medulla oblongata. Each of these sections has
specific functions and responsibilities:
Midbrain: The midbrain, also known as the mesencephalon, is responsible for controlling eye movement by coordinating the muscles that control eye motion. It is also involved in processing auditory
and visual information and helps initiate reflexes in response to auditory and visual stimuli. The midbrain contains the substantia nigra,
which is crucial for the production of dopamine, a neurotransmitter
involved in the regulation of movement and reward.
Pons: The pons, located between the midbrain and medulla oblongata, serves as a relay station for sensory and motor signals between
the cerebrum and the spinal cord. It plays a role in regulating sleep
and wakefulness, as well as controlling respiration. The pons also
houses the reticular formation, which is involved in maintaining
consciousness and modulating arousal levels.
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ply as the medulla, is the most inferior part of the brainstem and
connects to the spinal cord. It is responsible for controlling vital
functions such as heart rate, blood pressure, and respiration. The
medulla also contains various reflex centers, such as those responsible for swallowing, vomiting, coughing, and sneezing. Additionally, it
plays a role in the regulation of balance and coordination by relaying
sensory information from the inner ear to the cerebellum.
In summary, the brainstem is a critical structure that controls essential life-sustaining functions and serves as a relay center for sensory
and motor signals between the cerebrum and the spinal cord. Understanding the brainstem and its components is crucial for grasping the overall functioning of the nervous system and how it maintains our body’s homeostasis.
17.6.7 The Hippocampus
The hippocampus is a small, seahorse-shaped structure located in
the temporal lobe of the cerebrum. It is involved in learning and
memory and plays a crucial role in the formation and consolidation
of new memories and in the retrieval of long-term memories.
While it is out of the scope of the IMAT exam to learn the specific
subregions of the hippocampus, it is important to understand the
general idea that the hippocampus is made up of several areas
each with a specific function related to memory formation and
consolidation. Some examples of these subregions include the
cornu ammonis (CA) fields, which are involved in the storage of new
memories and the consolidation of those memories into long-term
storage, the dentate gyrus, which is involved in the formation of
new memories and the processing of spatial information, and the
subiculum, which is involved in the consolidation of memories and
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Figure 93: The brain stem is made of 3 parts, the midbrain, pons,
and medulla. (Credit: openstax.org)
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the integration of information from the hippocampus with other
brain areas.
17.6.8
The Thalamus
The thalamus is a structure located deep within the brain that plays
a vital role in the processing and transmission of sensory information.
It is a round, egg-shaped structure that is divided into two parts,
one on the left side of the brain and one on the right. The thalamus
is connected to the cerebral cortex (the outer layer of the brain that
is responsible for higher functions such as thinking and perception)
and to a number of other brain structures, such as the basal ganglia
and the cerebellum.
The thalamus receives input from the senses (such as sight, sound,
touch, and taste) and sends this information to the appropriate areas
of the cerebral cortex for further processing. It also plays a role in
the regulation of sleep, attention, and arousal.
The thalamus is divided into several different nuclei, or clusters of
nerve cells, each of which has a specific function. For example, the
lateral geniculate nucleus (LGN) receives visual information from
the retina and sends it to the visual cortex for processing, while
the medial geniculate nucleus (MGN) receives auditory information
from the ear and sends it to the auditory cortex for processing.
In addition to its role in sensory processing, the thalamus is also
involved in the control of voluntary movement. It receives input
from the basal ganglia (a group of structures involved in the control
of movement) and sends this information to the motor cortex, which
controls the muscles.
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17.6.9 The Hypothalamus
The hypothalamus is a small, almond-shaped region located deep
within the brain, just above the brainstem. It is a key part of the
brain’s control center and plays a vital role in maintaining homeostasis, or balance, in the body.
During development, the hypothalamus arises from the ectoderm,
which is the outermost layer of cells in the embryo. It is one of the
earliest brain structures to develop, and it is essential for the proper
development of the rest of the brain and nervous system.
The hypothalamus is responsible for a wide range of functions, including controlling the body’s thirst, hunger, and appetite; regulating body temperature; and controlling the body’s sleep-wake cycle.
It also plays a role in controlling mood and behavior, as well as in
regulating the endocrine system, which produces hormones.
The hypothalamus produces and releases several hormones of its
own, including:
Vasopressin: Also known as antidiuretic hormone (ADH), vasopressin
helps regulate water balance in the body by controlling the amount
of water reabsorbed by the kidneys.
Oxytocin: This hormone is involved in childbirth and lactation, and
it also plays a role in social bonding and sexual behavior.
Aside from ADH and oxytocin, one of its primary functions is to
produce and release several important hormones called releasing
hormones, which regulate the activity of the pituitary gland, also
known as the ”master gland” of the body. The releasing hormones
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lates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland, which in turn regulate
the activity of the gonads (ovaries or testes).
Thyroid-releasing hormone (TRH): This hormone stimulates the
release of thyroid-stimulating hormone (TSH) from the pituitary
gland, which regulates the activity of the thyroid gland.
Corticotropin-releasing hormone (CRH): This hormone stimulates
the release of adrenocorticotropic hormone (ACTH) from the pituitary gland, which regulates the activity of the adrenal gland.
Growth hormone-releasing hormone (GHRH): This hormone stimulates the release of growth hormone (GH) from the pituitary gland,
which regulates growth and metabolism.
The hypothalamus receives input from and sends signals to other
parts of the brain and the body through a network of nerve fibers.
It communicates with the pituitary gland, which is located just below it, to regulate the production and release of hormones by the
pituitary gland. The hypothalamus also receives input from sensory
organs and sends signals to various organs and systems in the body
to help maintain homeostasis.
17.6.10 The Pituitary Gland
The pituitary gland, also known as the ”master gland,” is a small
endocrine gland located at the base of the brain, just below the hypothalamus. It is divided into two main parts: the anterior pituitary
(or adenohypophysis) and the posterior pituitary (or neurohypophysis).
The anterior pituitary is the larger of the two parts and is responsible
for producing several hormones, including:
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Growth hormone (GH): This hormone stimulates cell growth, division, and reproduction, as well as regulating metabolism. This is
an integral hormone involved in the growth of children and adolescents.
Thyroid-stimulating hormone (TSH): This hormone regulates the
activity of the thyroid gland, which produces hormones that regulate
metabolism.
Adrenocorticotropic hormone (ACTH): This hormone regulates
the activity of the adrenal gland, which produces hormones that
regulate stress and inflammation.
Follicle-stimulating hormone (FSH): This hormone regulates the
activity of the gonads (ovaries or testes), promoting the growth and
maturation of follicles and sperm.
Luteinizing hormone (LH): This hormone regulates the activity of
the gonads, stimulating the release of estrogen and testosterone.
Prolactin (PRL): This hormone stimulates lactation in females after
childbirth.
The posterior pituitary, on the other hand, is smaller and is actually an extension of the hypothalamus. Do not be confused! The
posterior pituitary gland is a extension of the hypothalamus and
does not produce its own hormones. Instead, it stores and releases
two hormones produced by the hypothalamus, which include the
following:
Oxytocin: This hormone is involved in regulating social behavior
and emotional bonds, as well as the contraction of the uterus during
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absorption in the kidneys.
The hormones produced by the hypothalamus and the pituitary
gland work together to regulate various functions in the body, including growth and development, metabolism, and the body’s stress
response. The hypothalamus produces hormones that stimulate
or inhibit the production and release of hormones by the pituitary
gland, which in turn sends signals to other endocrine glands in the
body to produce and release their own hormones. This complex
system helps maintain homeostasis, or balance, in the body.
17.7
The Peripheral Nervous System
The peripheral nervous system (PNS) is the part of the nervous system that is outside the brain and spinal cord. It is responsible for
transmitting information between the central nervous system (CNS)
and the rest of the body. The PNS is divided into two main divisions:
the somatic nervous system and the autonomic nervous system.
The somatic nervous system is responsible for transmitting sensory
information from the body to the CNS and for controlling voluntary
movement. It consists of sensory neurons and motor neurons. Sensory neurons carry information from sensory receptors in the body
to the CNS, while motor neurons carry signals from the CNS to the
muscles and organs.
The autonomic nervous system is responsible for controlling the
body’s involuntary functions, such as heart rate, blood pressure, and
digestion. It is divided into two main branches: the sympathetic
nervous system and the parasympathetic nervous system.
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Figure 94: On either side of the midline, there are two ovoid structures known as thalami, which touch each other at the center. The
hypothalamus is situated below and in front of the thalamus, forming a sharp angle to which the pituitary gland is attached. (Credit:
openstax.org)
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ical activity or stress. It increases heart rate, blood pressure, and
respiration and diverts blood flow to the muscles and away from the
digestive system.
The parasympathetic nervous system is responsible for activating
the body’s ”rest and digest” response, which promotes relaxation
and digestion. It slows heart rate, lowers blood pressure, and increases blood flow to the digestive system.
Both the sympathetic and parasympathetic nervous systems work
together to maintain homeostasis, or balance, in the body. They
act in opposition to each other, with the sympathetic nervous system increasing activity and the parasympathetic nervous system
decreasing activity. This balance is important for maintaining the
body’s normal functions.
17.7.1
Somatic Nervous System
The somatic nervous system (SNS) is a part of the peripheral nervous system (PNS) that deals with voluntary actions and controls
the body’s skeletal muscles. It plays a crucial role in our everyday
activities and is responsible for activities such as walking, talking, or
picking up objects.
The peripheral nervous system is divided into two main parts: the
somatic nervous system and the autonomic nervous system (ANS).
While the somatic nervous system controls voluntary actions, the
autonomic nervous system is responsible for involuntary processes,
like heart rate and digestion.
The somatic nervous system has two main components: sensory
neurons and motor neurons.
Sensory neurons: These neurons collect information from the body’s
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sensory receptors, such as those in the skin, muscles, and joints. They
transmit information about touch, pressure, temperature, and pain
to the central nervous system (CNS), which consists of the brain
and spinal cord. For example, when you touch a hot surface, sensory neurons send a signal to your brain that the surface is hot and
potentially harmful.
Motor neurons: These neurons carry signals from the central nervous system to the skeletal muscles, causing them to contract and
move. For example, if you want to lift your arm, your brain sends a
signal through motor neurons that tell the muscles in your arm to
contract and move.
The somatic nervous system operates using a process called the
reflex arc. A reflex arc is a quick and automatic response to a stimulus
that bypasses the brain and involves the spinal cord. For example,
when you accidentally touch a hot object, the somatic nervous
system initiates a reflex action that causes you to quickly pull your
hand away. This reflex arc involves sensory neurons that detect the
heat, the spinal cord that processes the information, and motor
neurons that cause your muscles to contract and move your hand
away from the hot surface.
In summary, the somatic nervous system plays a vital role in controlling our voluntary movements and receiving sensory information
from the environment. It consists of sensory and motor neurons
that work together to help us perceive and interact with the world
around us.
17.7.2 Sympathetic Nervous System
The sympathetic nervous system is a branch of the autonomic nervous system (ANS), which is a part of the peripheral nervous system
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(PNS). The primary function of the sympathetic nervous system is
to prepare the body for a ”fight or flight” response during situations
that require immediate action or are perceived as threatening. It
does this by increasing the heart rate, dilating the pupils, and redistributing blood flow to the muscles, among other actions.
Location and structure: The sympathetic nervous system consists
of a series of nerve cells called ganglia that are located along either
side of the spinal cord. These ganglia are interconnected with the
spinal cord through a set of nerve fibers called preganglionic and
postganglionic neurons.
• Preganglionic neurons: These neurons originate in the spinal
cord, specifically in the intermediolateral column of the thoracic
and upper lumbar regions (T1 to L2). Preganglionic neurons
release the neurotransmitter acetylcholine (ACh) and have relatively short axons that synapse with the cells within the sympathetic ganglia.
• Postganglionic neurons: These neurons have their cell bodies
within the sympathetic ganglia and extend long axons to various
target organs and tissues throughout the body. Postganglionic
neurons release the neurotransmitter norepinephrine (NE) to
stimulate the target organs.
Function: The sympathetic nervous system is responsible for a range
of physiological changes in the body that prepare it for the ”fight or
flight” response. Some of its main functions include:
• Increasing the heart rate and strength of contraction, which
helps pump more blood to the muscles and other vital organs.
• Dilating the bronchioles in the lungs, allowing for increased
airflow and oxygen uptake.
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• Dilating the pupils, which enhances visual acuity and the ability
to see in low-light conditions.
• Constricting blood vessels in the skin and gastrointestinal system, which redirects blood flow to the muscles and other vital
organs.
• Stimulating the release of glucose from the liver and fatty acids
from adipose tissue, providing the body with additional energy
during times of stress.
• Inhibiting processes that are not immediately essential during
an emergency, such as digestion, immune response, and reproduction.
Significance: The sympathetic nervous system is crucial for our survival, as it enables us to respond quickly and effectively to potentially
dangerous situations. It allows us to mobilize resources, such as
energy and oxygen, to the parts of the body that need them most
in order to deal with a threat or challenge. Understanding the sympathetic nervous system is essential for understanding the overall
functioning of the autonomic nervous system and its role in maintaining the body’s homeostasis.
17.7.3 Parasympathetic Nervous System
The parasympathetic nervous system is a branch of the autonomic
nervous system (ANS), which is a part of the peripheral nervous
system (PNS). Its primary function is to promote ”rest and digest”
activities, which help the body conserve energy, maintain homeostasis, and support normal physiological functions during periods
of relaxation and recuperation.
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sists of neurons that originate from the brainstem and the sacral
region of the spinal cord. These neurons are organized into two
groups, known as the cranial and sacral components:
• Cranial component: This includes the preganglionic neurons
that emerge from the brainstem via specific cranial nerves (such
as the oculomotor, facial, glossopharyngeal, and vagus nerves).
These neurons synapse with postganglionic neurons located in
ganglia close to or within the target organs.
• Sacral component: This consists of preganglionic neurons that
originate in the sacral region of the spinal cord (S2-S4). These
neurons exit the spinal cord through the ventral roots and form
the pelvic nerves, which synapse with postganglionic neurons
in ganglia near the target organs.
Both preganglionic and postganglionic parasympathetic neurons
release the neurotransmitter acetylcholine (ACh).
Function: The parasympathetic nervous system is responsible for
a range of physiological processes that promote relaxation, energy
conservation, and the maintenance of normal bodily functions. Some
of its main functions include:
• Decreasing the heart rate and strength of contraction, which
conserves energy and helps maintain a stable, resting state.
• Constricting the bronchioles in the lungs, which reduces airflow
and oxygen demand during periods of rest.
• Constricting the pupils, which protects the retina from excessive light exposure and supports normal vision during nonthreatening situations.
• Stimulating the secretion of digestive enzymes and the movement of the gastrointestinal tract, which promotes the digestion
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and absorption of nutrients.
• Stimulating the release of insulin from the pancreas, which helps
regulate blood glucose levels.
• Promoting the activity of the immune system, which supports
overall health and well-being.
Significance: The parasympathetic nervous system plays a crucial
role in maintaining the body’s overall health and well-being by supporting essential physiological functions during periods of rest and
relaxation. It counterbalances the ”fight or flight” responses of the
sympathetic nervous system, helping to restore homeostasis and
conserve energy after periods of stress or activity. Understanding
the parasympathetic nervous system is essential for comprehending the overall functioning of the autonomic nervous system and its
role in maintaining the body’s homeostasis.
17.7.4 Ganglia Location
Ganglia are clusters of nerve cells that serve as relay centers for the
autonomic nervous system. They are located outside the brain and
spinal cord that are part of the peripheral nervous system. Ganglia
are important in transmitting and processing information between
the central nervous system (CNS) and the rest of the body. One important difference between the sympathetic and parasympathetic
systems is the location of their ganglia.
In the parasympathetic nervous system, ganglia are located near
or within the organs that they innervate, such as the heart, lungs,
and digestive system. The parasympathetic nervous system is responsible for activating the body’s ”rest and digest” response, which
promotes relaxation and digestion.
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In the sympathetic nervous system, ganglia are located near the
spinal cord and are connected to the CNS by long nerve fibers called
preganglionic fibers. The sympathetic nervous system is responsible
for activating the body’s ”fight or flight” response, which prepares
the body for physical activity or stress.
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18 Sensory Systems
Did you know that all animals with bilateral symmetry, like humans,
have a highly specialized sensory system that has evolved over time
to suit their specific needs and environments? These senses are
constantly on the lookout for stimuli, whether it be light, sound, or
chemicals in the air or water. In fact, some animals have senses
that are completely foreign to us, like the shark’s ability to detect
electric fields produced by other creatures. This unique sense, called
electroreception, helps the shark locate its prey and gives it a competitive edge in the underwater world. However, this special sense is
not something that most land animals possess. So next time you’re
marveling at the incredible abilities of animals, remember that their
senses play a big role in their success and survival!
18.1 General and Special Senses in Humans
The human body possesses two categories of senses: general senses
and special senses. General senses are present in all animals and
are essential for basic survival, including touch, temperature, pain,
and balance. On the other hand, special senses are more intricate
and found only in certain animals, including humans. These senses
comprise sight, hearing, taste, smell, and proprioception (the sense
of body position and movement).
Sensory perception involves three crucial steps:
Stimulus: A stimulus represents any change in the environment
that can be detected by the body’s senses. For instance, a sound
wave serves as a stimulus for hearing, while light waves act as stimuli
for vision.
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Sensory receptors: Sensory receptors are specialized cells designed
to detect specific stimuli. For example, the retina in the eye contains photoreceptors sensitive to light, while the cochlea in the ear
contains hair cells sensitive to sound vibrations.
Perception: Perception entails interpreting and organizing sensory information within the brain. This process involves recognizing
patterns, understanding the meaning behind stimuli, and making
sense of the information received.
In sensory perception, the concept of just-noticeable difference
(JND) refers to the minimum change in a stimulus required for an
individual to perceive a difference. For instance, if a person is presented with two cups of coffee with slightly different temperatures,
the JND for temperature would be the minimum temperature difference between the two cups that the individual could detect. The
JND varies for each sense and depends on the specific stimulus and
the individual’s sensitivity to that stimulus.
Overall, understanding the process of sensory perception helps us
grasp how the human body detects, interprets, and responds to
various stimuli in the environment. This knowledge is essential
for understanding how we experience and interact with the world
around us.
18.1.1
Mechanoreceptors
Mechanoreceptors are sensory receptors specialized in detecting
mechanical stimuli, including touch, pressure, and vibration. In
human skin, there are four primary types of mechanoreceptors that
serve different functions:
Merkel cells: Found in the epidermis (the outer layer of the skin),
Merkel cells detect light touch, such as a feather’s touch or a gentle
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breeze on the skin. They provide information about the texture,
shape, and pressure of objects we touch, contributing to our ability
to discern fine details.
Meissner’s corpuscles: Located in the dermis (the layer of skin
beneath the epidermis), Meissner’s corpuscles are responsible for
sensing fine touch, such as the touch needed for reading Braille. Like
Merkel cells, they offer information about the texture and pressure
of objects we touch, enabling us to perceive subtle differences.
Ruffini endings: Situated in the dermis, Ruffini endings detect skin
stretching. They play a crucial role in our sense of touch by detecting
changes in skin position and providing information about our limbs’
and joints’ positions. This awareness helps us maintain balance and
coordinate our movements.
Pacinian corpuscles: Located in the deeper layers of the skin, Pacinian
corpuscles detect vibrations and deep pressure. They help us sense
vibrations produced by footsteps or machinery and provide information about deep pressure, such as that experienced during a hug or
a firm handshake.
The distribution of somatosensory receptors between glabrous (smooth)
and hairy skin is as follows:
Glabrous skin, found on the fingertips and palms, contains a high
density of mechanoreceptors, including Merkel cells and Meissner’s
corpuscles. This concentration allows for fine touch sensitivity and
the ability to distinguish between different objects. Hairy skin, found
on the arms and legs, has a lower density of mechanoreceptors. It
primarily contains Ruffini endings and Pacinian corpuscles, which
are responsible for detecting stretch and deep pressure sensations.
The diverse types of mechanoreceptors work together to provide a
comprehensive sense of touch, allowing us to navigate and interact
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with the world around us.
18.1.2
Other Receptors
Chemoreceptors: These receptors respond to chemical stimuli and
are involved in the detection of various substances such as oxygen,
carbon dioxide, and pH levels. There are two main types of chemoreceptors: peripheral chemoreceptors and central chemoreceptors.
Peripheral chemoreceptors are found in the carotid and aortic bodies and are sensitive to changes in arterial oxygen, carbon dioxide,
and pH levels. Central chemoreceptors are located in the brainstem
and respond to changes in the pH of cerebrospinal fluid, which can
indicate changes in blood carbon dioxide levels. For example, when
blood carbon dioxide levels rise, central chemoreceptors signal the
respiratory system to increase the rate and depth of breathing to
eliminate excess carbon dioxide.
Thermoreceptors: These receptors are responsible for detecting
changes in temperature. There are two types of thermoreceptors:
warm receptors and cold receptors. Warm receptors are activated
by increases in temperature, while cold receptors are activated by
decreases in temperature. For example, thermoreceptors in the skin
send signals to the brain when we touch a hot surface, triggering
the sensation of pain and causing us to reflexively withdraw our
hand.
Nociceptors: These are specialized receptors that respond to painful
stimuli, such as extreme heat or cold, pressure, or chemical irritants.
They are found throughout the body and are responsible for detecting and signaling the presence of tissue damage or injury. For
example, when you accidentally touch a hot stove, nociceptors in
your skin send signals to your brain, triggering the sensation of pain
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and prompting you to remove your hand.
Baroreceptors: These are specialized stretch receptors that detect
changes in pressure within the body, such as changes in blood
pressure. Baroreceptors are found in various locations in the body,
including the walls of the heart, blood vessels, and lungs. When
blood pressure rises, baroreceptors in the walls of the blood vessels
detect the change and send signals to the brain, which then signals
the heart to slow down and the blood vessels to relax and widen,
resulting in a decrease in blood pressure.
Overall, each type of receptor plays an important role in allowing our
bodies to detect and respond to various stimuli in the environment,
helping to maintain homeostasis and keep us safe from potential
harm.
18.2 Taste and Smell
The sense of taste and the sense of smell are both important for
detecting and evaluating the characteristics of different substances,
including food and other chemicals in the environment. The primary
tastes detected by humans include sweet, sour, bitter, salty, and
umami, while humans can sense around 10,000 different odors
using around 350 different olfactory receptor subtypes. These senses
work together to create the perception of flavor.
The reception and transduction of odors and tastes involve the interaction of molecules with specialized receptors in the olfactory
epithelium and taste buds, respectively. These receptors send signals directly to the brain, allowing us to perceive and evaluate the
characteristics of different substances. The sense of taste is important for detecting the nutritional value of different foods, while the
sense of smell allows us to detect the presence of other animals and
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chemicals in the environment that can impact our survival.
18.2.1
Gustation (Taste)
The sense of taste, also known as gustation, is the ability to detect
and evaluate the flavors of different substances. Taste buds, which
are small clusters of sensory cells located on the tongue and other
parts of the oral cavity, are responsible for detecting tastes.
Each taste bud contains around 50 to 100 taste cells, which are specialized sensory neurons that are responsible for detecting different
tastes. These cells have hair-like projections called microvilli, which
are coated in receptors that bind to specific molecules. When a substance is tasted, molecules of the substance bind to these receptors,
causing a change in the electrical charge of the cell. This change in
electrical charge triggers an action potential, which is a signal that
travels along the taste cell’s axon to the brain.
The brain interprets these signals as different tastes, including sweet,
sour, salty, bitter, and umami. The perception of taste is not limited
to the tongue, however. The walls of the pharynx and the epiglottis
also contain taste buds, allowing us to detect tastes in the back of
the throat and at the base of the tongue.
In addition to the taste cells, taste buds also contain supporting cells
and basal cells. The supporting cells help to maintain the structure
of the taste bud and provide nutrients to the taste cells, while the
basal cells are stem cells that can divide and differentiate into new
taste cells. This process of cell division and differentiation helps to
maintain the sensitivity and function of the taste buds over time.
Overall, the anatomy and physiology of taste involve the interaction
of specific molecules with taste receptors on the tongue and other
parts of the oral cavity, and the interpretation of these signals by the
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brain. This allows us to detect and evaluate the flavors of different
substances, which helps us to make decisions about what to eat
and drink.
18.2.2
Olfaction (Smell)
The sense of smell, also known as olfaction, is the ability to detect
and evaluate odors. It is mediated by specialized sensory cells called
olfactory neurons, which are located in a small patch of tissue in the
upper part of the nasal cavity called the olfactory epithelium.
Each olfactory neuron has a single dendrite, which extends from
the cell body and is coated in hair-like cilia. These cilia are covered
in receptors that bind to specific molecules, known as odorants.
When an odorant is inhaled, it dissolves in the mucus covering the
olfactory epithelium and binds to the receptors on the cilia. This
causes a change in the electrical charge of the olfactory neuron,
which triggers an action potential, or a signal that travels along the
axon of the neuron to the brain.
The brain interprets these signals as different odors, and the perception of smell is closely linked to the emotion and memory centers
of the brain. This is why certain smells can trigger strong emotional
responses and memories.
In addition to the olfactory neurons, the olfactory epithelium also
contains supporting cells and basal cells. The supporting cells help
to maintain the structure of the olfactory epithelium and provide
nutrients to the olfactory neurons, while the basal cells are stem
cells that can divide and differentiate into new olfactory neurons.
This process of cell division and differentiation helps to maintain the
sensitivity and function of the olfactory neurons over time.
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18.3 Hearing and Vestibular Sensation
Hearing and vestibular sensation are two important senses that
allow us to perceive and interpret sound and movement. The sense
of hearing allows us to detect and evaluate sounds in our environment, including speech, music, and other noises. It is mediated
by specialized sensory cells called hair cells, which are located in
the inner ear. The sense of vestibular sensation allows us to detect
and evaluate movement, including acceleration, deceleration, and
changes in orientation. It is mediated by specialized sensory cells
called hair cells, which are also located in the inner ear.
The anatomy and physiology of these senses involve the interaction
of mechanical stimuli with specialized receptors in the inner ear,
and the interpretation of these stimuli by the brain. Together, these
senses help us to navigate and orient ourselves in the world around
us, and to communicate with others through speech and other
forms of communication. In this lesson, we will explore the anatomy
and physiology of hearing and vestibular sensation in more detail,
including the structure of the inner ear, the types of stimuli that
these senses can detect, and the ways in which the brain interprets
these stimuli.
18.3.1 The Anatomy
The hearing system, also known as the auditory system, is responsible for detecting sound waves and converting them into electrical
signals that the brain can interpret as sound. It consists of three
main parts: the outer ear, the middle ear, and the inner ear, each
with its unique structures and functions.
Outer Ear: The outer ear is comprised of the pinna (the visible part
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of the ear) and the ear canal. The pinna is a cartilaginous structure
that collects and funnels sound waves into the ear canal. The ear
canal, a tubular passage, amplifies the sound waves as they travel
toward the middle ear.
Middle Ear: The middle ear contains the eardrum (tympanic membrane), a thin membrane that vibrates in response to incoming
sound waves. Behind the eardrum are three tiny bones called the ossicles—the malleus, incus, and stapes—which amplify the eardrum’s
vibrations and transmit them to the inner ear. The Eustachian tube
connects the middle ear to the throat, equalizing air pressure on
both sides of the eardrum.
Inner Ear: The inner ear houses the cochlea, which is responsible for
converting sound waves into electrical signals. This spiral-shaped,
fluid-filled structure contains tiny hair cells along its length. As
sound waves travel through the cochlear fluid, they cause these hair
cells to bend, generating electrical signals that are transmitted to
the brain via the auditory nerve.
The vestibular system, also located within the inner ear, is responsible for detecting changes in head position and movement, helping
us maintain our balance. It consists of the vestibule, the semicircular
canals, and the vestibular nerve.
Vestibule: The vestibule is a small, fluid-filled chamber within the inner ear that houses the utricle and saccule. These structures contain
hair cells sensitive to changes in gravity and linear acceleration, providing information about the head’s position relative to the ground.
Semicircular Canals: The semicircular canals are three loop-shaped
tubes situated around the vestibule. Each canal is oriented at a
different angle and filled with fluid, and it contains hair cells sensitive
to rotational acceleration. When the head moves, the fluid in the
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canals shifts, stimulating these hair cells and sending signals about
head rotation to the brain via the vestibular nerve.
Together, the auditory and vestibular systems enable us to perceive
sound and maintain balance, allowing us to effectively navigate and
interact with our environment.
One way to think about the semicircular canals and the utricle/saccule is to imagine a boat on the water.
The utricle and saccule can be compared to a flat boat floating on
calm water. Just like the boat remains level on the water, the utricle
and saccule detect linear acceleration and help us maintain our
balance when we’re standing still or moving in a straight line.
On the other hand, the semicircular canals can be compared to a
boat in choppy water. Just like a boat bobbing up and down on the
waves, the semicircular canals detect rotational acceleration and
help us maintain our balance when we’re turning or spinning.
So, together, the utricle/saccule and semicircular canals work like a
boat on the water, detecting both linear and rotational movements
to help us maintain our balance and orientation in space.
The vestibular nerve carries signals from the vestibule and the semicircular canals to the brain, which uses these signals to help maintain
balance and coordinate movement.
18.3.2 The Physiology of Hearing
The physiology of hearing involves the conversion of sound waves
into electrical signals that the brain can interpret as sound. When
a sound wave enters the ear, it travels through the outer ear and
into the ear canal. The sound wave then causes the eardrum, a thin
membrane at the end of the ear canal, to vibrate.
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Figure 95: The outer ear comprises the auricle, ear canal, and tympanic membrane, while the middle ear contains the ossicles and
is linked to the pharynx by the Eustachian tube. The cochlea and
vestibule, which are responsible for hearing and balance, respectively, are located in the inner ear. (Credit: openstax.org)
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The vibrations of the eardrum are transmitted to the ossicles, three
small bones located in the middle ear. The ossicles are called the
malleus, incus, and stapes. They amplify the vibrations of the eardrum
and transmit them to the cochlea, a spiral-shaped structure in the
inner ear.
The cochlea is filled with fluid and lined with tiny hair cells. As the
vibrations of the ossicles travel through the fluid in the cochlea, they
cause the hair cells to bend. This movement generates electrical
signals, which are then transmitted to the brain via the auditory
nerve.
The brain interprets these electrical signals as sound, and it processes the information based on the pitch, volume, and location
of the sound. It also uses information from the vestibular system,
which helps us maintain our balance, to help us orient ourselves in
relation to the sound.
18.3.3
Inner Ear
The inner ear is a highly intricate and essential component of both
the auditory and vestibular systems. It comprises the cochlea, responsible for hearing, and the vestibule, responsible for balance
and spatial orientation. Each of these structures contains unique
features that enable them to perform their specific functions.
Cochlea: The cochlea is a snail-shaped, fluid-filled structure within
the inner ear. Inside the cochlea is the organ of Corti, which contains
the hair cells responsible for transforming sound waves into electrical signals that the brain can process. The organ of Corti is situated
on the basilar membrane, which runs the length of the cochlea and
vibrates in response to sound waves. This vibration causes the hair
cells to bend, generating electrical signals that are transmitted to
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the brain via the auditory nerve.
The cochlea is divided into three separate fluid-filled chambers: the
scala vestibuli, scala media, and scala tympani. The scala media
contains endolymph, which is crucial for maintaining the proper
ionic balance and pressure within the cochlear duct. This ensures
optimal functioning of the hair cells and accurate transmission of
sound information to the brain.
Vestibule: The vestibule is a small, fluid-filled chamber within the
inner ear that contains the otolithic organs: the utricle and saccule.
These structures contain hair cells embedded in a gelatinous mass,
with tiny calcium carbonate crystals called otoliths resting on top.
As the head moves, the otoliths shift due to changes in gravity and
linear acceleration, stimulating the hair cells and sending signals to
the brain about the head’s position relative to the ground.
Semicircular Canals: The vestibule also houses the semicircular
canals, three loop-shaped tubes oriented at different angles. Each
canal is filled with endolymph and contains an ampulla, a bulge in
the canal that houses hair cells. As the head rotates, the endolymph
in the canals moves, causing the hair cells in the ampulla to bend,
generating electrical signals that inform the brain about the head’s
rotational acceleration.
In terms of the fluid found in the inner ear, it is important to be aware
of their distinction. Endolymph is a crucial fluid in the inner ear,
specifically in the cochlea. It plays a critical role in the functioning
of the ear by maintaining the proper ionic balance and pressure
within the cochlear duct. This is essential for transmitting sound
information to the brain and for the proper functioning of hair cells
in the cochlea.
Perilymph, on the other hand, serves a protective role in the inner
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ear. It is found in the spaces surrounding the cochlea, including the
vestibular system and the semicircular canals. Composition wise,
it is similar to cerebrospinal fluid and its purpose is to cushion and
protect the delicate structures in the inner ear.
So, to summarize, endolymph is extremely important for hearing
and the proper functioning of the cochlea, while perilymph serves a
protective role for the inner ear structures by providing cushioning
and protection.
18.3.4 The Physiology of The Vestibular System
The physiology of the vestibular system is vital for maintaining balance, coordinating movement, and perceiving spatial orientation.
The vestibular system comprises the vestibule, semicircular canals,
and vestibular nerve, each of which plays a unique role in sensing
head position and movement.
Vestibule: The vestibule is a small, fluid-filled chamber in the inner ear that houses the otolithic organs—the utricle and the saccule. These structures contain hair cells topped with a gelatinous
layer embedded with tiny calcium carbonate crystals called otoliths.
When the head moves or changes orientation relative to gravity, the
otoliths shift, causing the hair cells to bend. This bending generates electrical signals that correspond to linear acceleration and the
head’s position concerning gravity.
Semicircular Canals: Located around the vestibule, the semicircular canals are three fluid-filled tubes arranged at right angles to
each other. Each canal corresponds to a different plane of motion:
horizontal, anterior-vertical, and posterior-vertical. At the base of
each canal is an enlarged region called the ampulla, which contains
hair cells covered by a gelatinous structure called the cupula. As the
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head rotates, the fluid inside the canals (endolymph) lags behind
due to inertia, causing the cupula and hair cells to bend. This bending generates electrical signals proportional to the head’s rotational
acceleration.
Vestibular Nerve: The electrical signals generated by the hair cells in
the vestibule and semicircular canals are transmitted to the brain via
the vestibular nerve, a branch of the vestibulocochlear nerve (cranial
nerve VIII). These signals reach the vestibular nuclei in the brainstem
and the cerebellum, where they are processed and integrated with
information from the visual and proprioceptive systems.
Integration and Perception: The brain processes and integrates
the information from the vestibular, visual, and proprioceptive systems to generate a comprehensive perception of spatial orientation,
balance, and coordinated movement. This integration allows us to
maintain our balance while walking, running, or performing other
complex movements, and it helps us orient ourselves in our surroundings.
In summary, the vestibular system is crucial for detecting changes in
head position, movement, and acceleration. It works in conjunction
with the visual and proprioceptive systems to maintain balance
and coordinate movement, providing us with a sense of spatial
orientation. The vestibule, semicircular canals, and vestibular nerve
all contribute to this complex sensory system, ensuring we can
navigate and interact with our environment effectively.
18.4 Vision
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18.4.1 Structure of Vision
Vision is the process by which the brain interprets the images that
the eye sees. It is a complex process that involves several different
parts of the eye and the brain.
The eye is the organ that is responsible for capturing light and converting it into electrical signals that the brain can interpret. The eye
has several different parts that work together to allow us to see:
Cornea: The cornea is the outermost layer of the eye, providing protection to the internal structures. It is composed of five layers, each
with a specific function. These layers, from the outermost to the
innermost, are the epithelium, Bowman’s layer, stroma, Descemet’s
membrane, and the endothelium. The epithelium is the outermost
layer, providing protection and contributing to the cornea’s transparency. The stroma makes up the bulk of the cornea and is responsible for the cornea’s strength and elasticity.
Aqueous humor: After passing through the cornea, light enters the
anterior chamber of the eye, which is filled with a clear fluid called
the aqueous humor. This fluid provides nutrients to the cornea and
lens and helps maintain the eye’s intraocular pressure.
Iris and pupil: The iris contains two sets of muscles: the sphincter
pupillae and the dilator pupillae. The sphincter pupillae constricts
the pupil in response to bright light, while the dilator pupillae dilates
the pupil in response to low light. These muscles are innervated
by the parasympathetic and sympathetic nervous systems, respectively, as mentioned earlier.
Lens: The lens is held in place by the suspensory ligaments, which
are connected to the ciliary muscles. When these muscles contract
or relax, they alter the shape of the lens, allowing it to focus light from
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objects at various distances. This process is called accommodation.
Vitreous humor: After passing through the lens, light travels through
the vitreous humor, a clear, gel-like substance that fills the posterior
chamber of the eye. The vitreous humor provides structural support
to the eye and helps maintain its shape.
Retina: The retina contains millions of photoreceptor cells called
rods and cones. Rods are responsible for vision in low-light conditions and are more sensitive to light, while cones are responsible
for color vision and work best in bright light. There are three types
of cones, each sensitive to a different range of wavelengths, corresponding to the colors blue, green, and red.
Phototransduction: When light hits the photoreceptor cells, it initiates a process called phototransduction. In this process, the lightsensitive molecule, called rhodopsin in rods and photopsin in cones,
undergoes a chemical change that triggers a cascade of events
leading to the generation of electrical signals.
Signal processing: The electrical signals generated by photoreceptor cells are transmitted to other retinal cells, such as bipolar cells
and horizontal cells, which further process the information. This processing involves spatial and temporal filtering, which helps sharpen
the image and enhance its contrast.
Ganglion cells and optic nerve: The processed signals from bipolar
cells are transmitted to retinal ganglion cells, which in turn send the
signals via their axons to form the optic nerve. The optic nerve carries
these electrical signals to the brain’s lateral geniculate nucleus (LGN)
in the thalamus.
Visual cortex: From the LGN, the signals are sent to the primary
visual cortex (V1) in the occipital lobe of the brain. Here, the information is further processed and integrated with information from the
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other eye, forming a cohesive representation of the visual scene. The
information is then relayed to higher-order visual areas for further
processing, ultimately leading to our perception of the world around
us.
The process of vision begins when light rays enter the eye and are
focused by the cornea and lens onto the retina. The retina contains
cells called rods and cones that are sensitive to light. When these
cells are stimulated by light, they send electrical signals through
the optic nerve to the brain. The brain then interprets these signals
and creates an image of what we are seeing.
18.4.2 Rods and Cons
The rods and cones are two types of photoreceptor cells in the retina
of the eye. These cells are responsible for converting light into electrical signals that can be transmitted to the brain and interpreted
as visual images.
Rods are sensitive to low levels of light and are responsible for detecting light and dark, as well as for detecting movement. There
are about 120 million rods in the human eye, and they are mostly
concentrated in the periphery of the retina. Rods are not sensitive
to color and do not work well in bright light.
Cones are responsible for color vision and for seeing fine detail. There
are about 6-7 million cones in the human eye, and they are mostly
concentrated in the center of the retina, in an area called the fovea.
Cones require higher levels of light to function, so they are not as
sensitive as rods in low light conditions.
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pairments, such as night blindness or color blindness.
18.4.3
Vision Physiology
Phototransduction is the process by which light is transformed into
an electrical signal that can be processed by the brain. This process
occurs in the photoreceptor cells of the eye, specifically in the rods
and cones.
The first step in phototransduction is the absorption of light by
photopigments, which are the light-sensitive molecules in the rods
and cones. The photopigments in rods are called rhodopsin, while
those in cones are called cone opsins. Each cone cell contains a
different type of cone opsin, determining the color of light to which
the cell is most sensitive.
When light is absorbed by the photopigment, it triggers a chemical
reaction in which the photopigment molecule is broken down into
smaller molecules. In rods, this process is called bleaching and involves the separation of the protein opsin from a molecule called
retinal. This separation causes a conformational change in the opsin
molecule, leading to the opening of ion channels in the cell membrane. The ion channels in rods are selective for negatively charged
particles called chloride ions, and when these ion channels open,
chloride ions flow into the cell, generating an electrical signal.
In cones, the breakdown of the photopigment also involves the
separation of opsin from retinal, but the specific process and types
of ion channels involved can vary depending on the specific type of
cone opsin present in the cell.
The influx of ions into the cell generates an electrical signal that is
transmitted to the brain through the optic nerve, allowing us to see
and interpret light. This process of phototransduction is rapid and
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efficient, allowing us to respond quickly to changes in light levels
and perceive the world around us with high visual acuity.
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19
The Endocrine System
The endocrine system is a complex network of glands, hormones,
and receptors that regulate various bodily functions, including growth
and development, metabolism, and reproductive processes. It plays
a vital role in maintaining homeostasis in the body, which is the
balance of internal conditions necessary for optimal functioning.
Some of the major endocrine glands in the body include the pituitary gland, thyroid gland, and adrenal gland. The pituitary gland,
often referred to as the ”master gland,” is located at the base of
the brain and produces hormones that regulate many important
functions, including growth and development, metabolism, and
reproductive processes. The thyroid gland, located in the neck, produces hormones that regulate the body’s metabolism, including
the rate at which the body converts food into energy. The adrenal
gland, located just above the kidney, produces hormones that help
regulate the body’s stress response and help to maintain blood
pressure.
Other important endocrine organs in the body include the pancreas,
which produces hormones such as insulin and glucagon that regulate glucose metabolism, and the ovaries and testes, which produce
the hormones estrogen, progesterone, and testosterone, which play
key roles in sexual development and reproductive function.
Before we tackle the specific parts of the endocrine system, let us
discuss a few generalities first to further understand the complex
systems involved in hormone production and release:
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Figure 96: Endocrine glands are located throughout the body.
(Credit: openstax.org)
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19.1 Regulation of Hormone Production
Hormonal regulation refers to the way in which hormones are produced, released, and controlled in the body. The two main types of
hormonal regulation are negative feedback and positive feedback.
Negative feedback is a common mechanism of hormonal regulation in which the body works to maintain a stable, balanced state.
In negative feedback, the hormone or neurotransmitter being regulated acts as a messenger, sending a signal to the body to change a
particular function. For example, when the body’s blood sugar levels
become too high, the hormone insulin is released from the pancreas.
Insulin acts as a signal to cells throughout the body, telling them to
take in sugar from the bloodstream and use it for energy. This helps
to lower blood sugar levels back to normal.
Positive feedback, on the other hand, occurs when the hormone
or neurotransmitter being regulated amplifies or intensifies a particular function. Positive feedback is less common than negative
feedback, but it can be important in certain situations, such as during childbirth or lactation.
One example of positive feedback is the process of milk production
in lactating women. When a baby nurses, it stimulates the release
of the hormone oxytocin from the pituitary gland. Oxytocin acts
on the milk-producing cells in the breast, causing them to release
milk. This milk feeds the baby, which in turn stimulates the release
of more oxytocin, creating a positive feedback loop. This process
helps to ensure that the baby is well-fed and nourished.
Another example of positive feedback is the process of childbirth.
During labor, the cervix begins to dilate, which stimulates the release
of oxytocin. Oxytocin causes contractions of the uterus, which help
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to push the baby through the birth canal. These contractions also
stimulate the release of more oxytocin, creating a positive feedback
loop that helps to bring about the successful delivery of the baby.
19.2 Types of Hormones
Hormones are chemical messengers produced by glands in the
body that help to regulate various bodily functions. There are several different types of hormones, and they can have a wide range
of effects on the body depending on the specific hormones and
receptors involved.
Protein hormones: Protein hormones are made up of amino acids
and are typically larger and more complex than other types of hormones. Examples of protein hormones include insulin, which is produced by the pancreas and helps to regulate glucose metabolism,
and thyroid hormones, which are produced by the thyroid gland
and help to regulate metabolism. Protein hormones bind to specific receptors on the surface of cells, where they activate signaling
pathways inside the cells to bring about their effects.
Steroid hormones: Steroid hormones are made from cholesterol
and are lipid-soluble, meaning they can easily pass through cell
membranes. Examples of steroid hormones include testosterone,
which is produced by the testes in men and helps to regulate sexual
development and function, and estrogen, which is produced by
the ovaries in women and helps to regulate reproductive function.
Steroid hormones bind to specific receptors inside cells, where they
enter the cell nucleus and bind to DNA to bring about their effects.
Thyroid hormones: Thyroid hormones are produced by the thyroid
gland and help to regulate metabolism in the body. There are two
main thyroid hormones: thyroxine (T4) and triiodothyronine (T3).
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Thyroid hormones bind to specific receptors on the surface of cells,
where they activate signaling pathways inside the cells to bring
about their effects.
Amino acid-derived hormones: Amino acid-derived hormones are
made from amino acids and are typically smaller and simpler in
structure than protein hormones. Examples of amino acid-derived
hormones include epinephrine (adrenaline), which is produced by
the adrenal gland and helps to regulate the body’s stress response,
and melatonin, which is produced by the pineal gland and helps
to regulate sleep-wake cycles. Amino acid-derived hormones bind
to specific receptors on the surface of cells, where they activate
signaling pathways inside the cells to bring about their effects.
In Summary: Hormones are chemical messengers produced by
glands in the body that help to regulate various bodily functions.
They bind to specific receptors on target cells in order to bring about
their effects. Hormones can be classified into different types based
on their chemical structure, including protein hormones, steroid
hormones, thyroid hormones, and amino acid-derived hormones.
The location and structure of the receptors for a specific hormone
determines how it will bind to the target cell and bring about its
effects.
In terms of receptor binding, it would be useful to be reminded again
of the difference between protein and steroid hormones. Protein
hormones, such as insulin and thyroid hormones, bind to specific
receptors on the surface of cells and activate signaling pathways
inside the cells to bring about their effects. Steroid hormones, such
as testosterone and estrogen, are lipid-soluble and can diffuse across
cell membranes. They bind to specific receptors inside cells and
enter the cell nucleus, where they bind to DNA and bring about
their effects by modulating gene expression.
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Amino acid-derived hormones, such as epinephrine and melatonin,
are not lipid-soluble and cannot enter cells. They bind to specific
receptors on the surface of cells and activate signaling pathways
that bring about their effects. Thyroid hormones, which are also
amino acid-derived, can bind to receptors either on the surface of
cells or inside cells, depending on their chemical structure.
The binding of hormones to their receptors can alter cellular activity
and result in an increase or decrease in normal body processes. In
some cases, the number of receptors that respond to a hormone can
change over time, resulting in increased or decreased sensitivity to
the hormone. This process, called up-regulation or down-regulation,
can affect the sensitivity of cells to hormones and influence their
overall effects on the body.
19.3
Endocrine Glands
The endocrine system is a complex network of glands and hormones
that regulate the body’s functions. Endocrine glands are organs
that produce and secrete hormones, which are chemical messengers that travel through the bloodstream to target cells and organs.
Hormones play a crucial role in regulating various bodily processes,
including growth and development, metabolism, and reproductive
function.
There are several endocrine glands in the body, including the pituitary gland, thyroid gland, parathyroid gland, adrenal gland, and
pancreas. The pituitary gland, also known as the ”master gland,”
is located at the base of the brain and produces hormones that
regulate the function of other endocrine glands. The thyroid gland,
located in the neck, produces hormones that regulate metabolism.
The parathyroid glands, also located in the neck, produce hormones
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that regulate calcium levels in the body. The adrenal glands, located
on top of the kidneys, produce hormones that help the body respond to stress and regulate blood pressure. The pancreas, located
in the abdomen, produces hormones such as insulin and glucagon,
which regulate blood sugar levels.
In addition to these major endocrine glands, there are also several
other endocrine glands and tissues in the body, including the gonads (testes and ovaries), the thymus, the pineal gland, and the
hypothalamus. Each of these glands and tissues plays a specific role
in the endocrine system and helps to keep the body’s functions in
balance.
19.4
Hypothalamic-Pituitary Axis
The hypothalamic-pituitary axis (HPA) is a complex network of interactions between the hypothalamus, a small region of the brain, and
the pituitary gland, a small gland located at the base of the brain.
The HPA is responsible for maintaining homeostasis, in the body
through the production and regulation of hormones.
The hypothalamus is a part of the brain that serves as a control
center for the HPA. It receives input from various parts of the body
and sends signals to the pituitary gland to produce and release
hormones. The hypothalamus also produces its own hormones,
called releasing hormones and inhibiting hormones, which regulate
the production and release of hormones from the pituitary gland.
The pituitary gland, also known as the ”master gland,” is divided
into two parts: the anterior pituitary and the posterior pituitary. The
anterior pituitary produces hormones that regulate the function of
other endocrine glands, including the thyroid gland, adrenal gland,
and gonads (testes and ovaries). Some of the hormones produced
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by the anterior pituitary include thyroid-stimulating hormone (TSH),
adrenocorticotropic hormone (ACTH), and luteinizing hormone (LH).
The posterior pituitary stores and releases hypothalamic hormones
that regulate water balance in the body, including vasopressin (also
known as antidiuretic hormone or ADH) and oxytocin.
The HPA plays a crucial role in maintaining homeostasis in the body.
For example, when the body is under stress, the hypothalamus
sends a signal to the pituitary gland to produce and release ACTH,
which stimulates the adrenal glands to produce the stress hormone
cortisol. Cortisol helps the body respond to stress by increasing
blood sugar levels and blood pressure, among other effects.
Figure 97: Located inferior and anterior to the thalamus, the hypothalamus region is connected to the pituitary gland by the infundibulum, which acts as a stalk. The pituitary gland comprises
an anterior and posterior lobe that secrete different hormones in
response to signals from the hypothalamus. (Credit: openstax.org)
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19.5 Thyroid Gland
The thyroid gland is a butterfly-shaped endocrine gland located in
the neck, just below the Adam’s apple, which is a prominence in
the front of the neck that is formed by the laryngeal prominence
of the thyroid cartilage. It is an important part of the endocrine
system, producing hormones that regulate metabolism, growth,
and development.
Thyroid anatomy: The thyroid gland is composed of two lobes connected by a thin band of tissue called the isthmus. The gland is
comprised of numerous follicles, which are small, spherical structures lined by follicular cells (also called thyrocytes). The center of
each follicle is filled with a gel-like substance called colloid, which
stores thyroid hormones before they are released into the bloodstream.
Thyroid hormone synthesis: Thyroid hormones are synthesized
within the follicular cells and the colloid. Iodide, obtained from dietary sources, is actively transported into the follicular cells by a protein called the sodium-iodide symporter (NIS). Within the follicular
cells, iodide is oxidized to form iodine, which then binds to the amino
acid tyrosine on thyroglobulin, a large protein molecule within the
colloid. The coupling of iodinated tyrosines on thyroglobulin forms
the precursor molecules for T4 (thyroxine) and T3 (triiodothyronine).
When thyroid hormones are needed, thyroglobulin is taken back
into the follicular cells and proteolytically cleaved, releasing T4 and
T3 into the bloodstream.
Thyroid hormone transport: T4 and T3 are primarily transported
in the bloodstream bound to proteins, such as thyroxine-binding
globulin (TBG), transthyretin, and albumin. Only a small fraction of T4
and T3 circulates unbound or ”free,” which is the biologically active
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form. Once the hormones reach target tissues, T4 is converted into
T3 by deiodinase enzymes, as T3 is more potent and has a greater
affinity for thyroid hormone receptors.
Thyroid hormone actions: T3 and T4 influence various physiological processes by binding to nuclear thyroid hormone receptors,
which act as transcription factors. These hormones regulate cellular
metabolism, protein synthesis, and mitochondrial function, as well
as playing crucial roles in growth, development, and thermogenesis.
Hypothalamic-pituitary-thyroid (HPT) axis: The secretion of thyroid hormones is regulated by the HPT axis. The hypothalamus
produces thyrotropin-releasing hormone (TRH), which stimulates
the anterior pituitary gland to release thyroid-stimulating hormone
(TSH). TSH then acts on the thyroid gland, promoting the synthesis
and release of T4 and T3. Negative feedback mechanisms involving
T4 and T3 inhibit TRH and TSH production, maintaining hormonal
homeostasis.
Calcitonin: In addition to T4 and T3, the thyroid gland also produces calcitonin, which is synthesized and secreted by parafollicular
cells (C-cells) within the thyroid tissue. Calcitonin helps regulate
calcium homeostasis by inhibiting osteoclast activity, promoting
bone formation, and increasing renal calcium excretion.
Thyroid disorders: Thyroid dysfunction can lead to various health
issues. Hypothyroidism results from insufficient production of T4
and T3, causing symptoms such as fatigue, weight gain, and cold intolerance. Hyperthyroidism, on the other hand, occurs when the thyroid gland produces excessive T4 and T3, leading to symptoms like
weight loss, rapid heartbeat, and tremors. Autoimmune diseases,
such as Hashimoto’s thyroiditis and Graves’ disease, are common
causes of hypothyroidism and hyperthyroidism, respectively.
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19.6
Parathyroid Glands
The parathyroid glands are four small endocrine glands located in
the neck, near the thyroid gland. These glands produce a hormone
called parathyroid hormone (PTH), which helps to regulate calcium
levels in the body.
Calcium is an essential mineral that is important for many functions
in the body, including building strong bones, muscle function, and
nerve function. The parathyroid glands help to maintain the proper
levels of calcium in the body by releasing PTH when the levels of
calcium in the blood are too low. PTH works by increasing the absorption of calcium from the intestines, increasing the release of
calcium from the bones, and increasing the reabsorption of calcium
by the kidneys.
In addition to regulating calcium levels, PTH also helps to regulate
the levels of phosphate in the body. Phosphate is another essential
mineral that is important for many functions in the body, including
the production of energy and the synthesis of proteins and other
important molecules. PTH works by decreasing the absorption
of phosphate from the intestines and increasing the excretion of
phosphate by the kidneys.
The overall effect of parathyroid hormone (PTH) on the body is to
maintain normal calcium levels and the proper balance between
calcium and phosphate. PTH increases blood calcium levels by
stimulating the osteoclasts to break down bone tissue and release
calcium, the kidneys to reabsorb more calcium from the urine, and
the intestinal tract to absorb more calcium from the diet. PTH decreases blood phosphate levels by stimulating the renal excretion
of phosphate. These actions help to ensure that calcium and phosphate levels are in the proper balance, which is important for various
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physiological processes such as bone metabolism, nerve and muscle
function, and blood coagulation.
Problems with the parathyroid glands can lead to various health
issues. Hyperparathyroidism, or an overactive parathyroid gland,
occurs when the parathyroid gland is producing too much PTH. This
can lead to high levels of calcium in the blood, which can cause
symptoms such as kidney stones, bone pain, and muscle weakness.
Hypoparathyroidism, or an underactive parathyroid gland, occurs
when the parathyroid gland is not producing enough PTH. This can
lead to low levels of calcium in the blood, which can cause symptoms
such as muscle spasms and numbness and tingling in the fingers
and toes.
19.7 Calcium Level Regulation
Calcium regulation in the body is essential for maintaining healthy
bones, as well as proper functioning of muscles, nerves, and various
organs and tissues. Parathyroid hormone (PTH), produced by the
parathyroid glands, plays a central role in regulating calcium levels
in the body. These glands, located behind the thyroid gland in the
neck, are small endocrine glands that continuously monitor and
respond to fluctuations in blood calcium levels.
PTH acts on three main target organs - the bones, intestines, and
kidneys - to regulate calcium levels in the blood. When blood calcium levels drop too low, the parathyroid glands sense this change
and respond by increasing the secretion of PTH, which then triggers
several actions to raise calcium levels:
Bone resorption: PTH stimulates the activation of osteoclasts, specialized cells responsible for breaking down bone tissue and releasing stored calcium into the bloodstream. This process, known as
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bone resorption, is essential for maintaining calcium homeostasis.
Kidney reabsorption: PTH increases the reabsorption of calcium
in the kidneys, specifically in the distal convoluted tubules and the
ascending loop of Henle. By reducing the amount of calcium lost in
urine, the kidneys help preserve calcium levels in the bloodstream.
Intestinal absorption: Although PTH does not act directly on the
intestines, it stimulates the production of calcitriol (active vitamin D)
in the kidneys. Calcitriol, in turn, enhances the intestinal absorption
of calcium, further contributing to increased calcium levels in the
bloodstream.
Phosphate regulation: PTH reduces the reabsorption of phosphate
in the kidneys, leading to increased phosphate excretion in urine.
This is important because excessive phosphate can bind to calcium,
forming insoluble complexes that can deposit in tissues and impair
calcium’s biological functions.
When blood calcium levels are too high, the parathyroid glands decrease PTH secretion, allowing calcium levels to return to normal.
This decrease in PTH secretion results in reduced bone resorption,
decreased kidney reabsorption of calcium, and lower intestinal calcium absorption.
Proper calcium regulation is crucial for maintaining healthy bones
and preventing conditions such as osteoporosis, a disease characterized by decreased bone mass and increased fracture risk. Dysregulation of PTH and calcium homeostasis can lead to abnormal
calcium levels and associated health problems, including hypercalcemia (high calcium levels), hypocalcemia (low calcium levels), and
parathyroid disorders.
In addition to PTH, other hormones, such as calcitonin and calcitriol,
are also involved in calcium regulation. Calcitonin, produced by
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the thyroid gland, acts to lower blood calcium levels by inhibiting
bone resorption and increasing calcium excretion in the kidneys.
Calcitriol, the active form of vitamin D, aids in calcium absorption
from the intestines and helps maintain normal calcium levels in the
body.
In summary, calcium regulation is a complex process involving the
actions of parathyroid hormone on bones, kidneys, and intestines,
as well as the interplay of other hormones like calcitonin and calcitriol. Maintaining calcium homeostasis is vital for healthy bones
and proper functioning of muscles, nerves, and various organs and
tissues. Dysregulation of this process can lead to a variety of health
problems, highlighting the importance of understanding and maintaining calcium balance in the body.
19.8 The Adrenal Glands
The adrenal glands are a pair of small, triangular-shaped glands that
sit atop the kidneys in the abdominal cavity. They play a vital role in
the endocrine system by producing hormones that help regulate
various bodily functions, including the body’s response to stress,
metabolism, and the immune system.
The adrenal glands are made up of two layers: the outer layer, known
as the adrenal cortex, and the inner layer, known as the adrenal
medulla.
Adrenal cortex structure: The adrenal cortex is divided into three
distinct zones, each responsible for producing specific hormones.
These zones, from the outermost to the innermost, are the zona
glomerulosa, zona fasciculata, and zona reticularis.
Zona glomerulosa: This outermost layer of the adrenal cortex priEnterMedSchool.com
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marily synthesizes and releases mineralocorticoids, the most important of which is aldosterone. Aldosterone secretion is regulated by
the renin-angiotensin-aldosterone system (RAAS), which responds
to low blood pressure, low blood volume, or low sodium levels. In response to these stimuli, aldosterone increases sodium reabsorption
and potassium excretion by the kidneys, leading to increased water
retention and blood volume, ultimately raising blood pressure.
Zona fasciculata: This middle layer of the adrenal cortex produces
glucocorticoids, predominantly cortisol. Cortisol secretion is regulated by the hypothalamic-pituitary-adrenal (HPA) axis. In response
to stress or low blood glucose levels, the hypothalamus releases
corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary gland to produce adrenocorticotropic hormone (ACTH).
ACTH then acts on the zona fasciculata, promoting cortisol synthesis and release. Cortisol has various functions, including increasing
blood glucose levels, suppressing the immune system, and assisting
in the metabolism of fats, proteins, and carbohydrates.
Zona reticularis: The innermost layer of the adrenal cortex produces androgens, including dehydroepiandrosterone (DHEA) and
androstenedione. These hormones serve as precursors for the synthesis of testosterone and estrogen in other tissues. The production
of adrenal androgens is stimulated by ACTH from the anterior pituitary gland.
Adrenal medulla structure: The adrenal medulla is a specialized
neuroendocrine tissue that consists mainly of chromaffin cells. These
cells are responsible for synthesizing and releasing catecholamines,
such as adrenaline and noradrenaline.
Catecholamine synthesis and release: The synthesis of catecholamines
is regulated by the sympathetic nervous system. In response to
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stress or danger, the hypothalamus activates sympathetic preganglionic neurons, which release acetylcholine. Acetylcholine then
binds to nicotinic receptors on chromaffin cells, stimulating the synthesis and release of adrenaline and noradrenaline into the bloodstream. These hormones increase heart rate, blood pressure, and
blood flow to muscles, while also mobilizing glucose and fatty acids
for energy production.
Negative feedback regulation: The hormonal secretion by the
adrenal glands is tightly regulated by negative feedback loops. For
example, high cortisol levels in the bloodstream suppress the release
of CRH and ACTH, leading to decreased cortisol production. Similarly,
high aldosterone levels inhibit the release of renin, reducing the
activation of the RAAS.
In summary, the adrenal glands are essential endocrine organs
that produce hormones crucial for maintaining homeostasis and
responding to stress. The adrenal cortex, divided into three zones,
produces corticosteroids and androgens, while the adrenal medulla
synthesizes and releases catecholamines. The secretion of these
hormones is regulated by complex feedback mechanisms involving
the hypothalamus, pituitary gland, and other endocrine and neural
systems.
19.9 Pineal Gland
The pineal gland, also known as the pineal body or the epiphysis, is a
small, pinecone-shaped gland located in the brain. It is located in the
center of the brain, near the thalamus and the hypothalamus. The
pineal gland is responsible for producing the hormone melatonin,
which plays a role in the regulation of sleep and wakefulness.
Melatonin is produced in response to changes in light exposure
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Figure 98: The two adrenal glands are located on top of the kidneys
and are made up of an outer cortex and inner medulla, both of
which are enclosed by a connective tissue capsule. The cortex can
be further divided into different zones, each of which produces
distinct hormones. (Credit: openstax.org)
and helps regulate the body’s sleep-wake cycle, also known as the
circadian rhythm. During the day, the pineal gland is inactive and
does not produce melatonin. As the day turns to night and the
body is exposed to less light, the pineal gland begins to produce
melatonin, which helps the body prepare for sleep. Melatonin levels
in the body remain elevated throughout the night and gradually
decrease as the day begins, helping the body to wake up and feel
alert.
In addition to its role in sleep regulation, melatonin has also been
shown to have a number of other potential health benefits. These
include acting as an antioxidant, helping to regulate immune function, and possibly even playing a role in the prevention of certain
types of cancer.
19.10
The Gonads
Let’s explore the synthesis, regulation, and functions of gonadal hormones in greater depth, as well as their role in sexual development
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and other physiological processes.
Gonadal hormone synthesis: Gonadal hormone production is regulated by the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which
stimulates the anterior pituitary gland to produce and secrete two
gonadotropins: follicle-stimulating hormone (FSH) and luteinizing
hormone (LH). FSH and LH then act on the gonads to regulate the
production of sex hormones.
Estrogen synthesis and regulation: In females, estrogen is primarily synthesized in the granulosa cells of the ovarian follicles. FSH
stimulates the growth and maturation of ovarian follicles and promotes estrogen production. Additionally, estrogen can also be produced in smaller amounts in other tissues, such as adipose tissue
and the adrenal glands. Estrogen production is regulated through
a negative feedback loop involving the hypothalamus and pituitary
gland.
Progesterone synthesis and regulation: Progesterone is synthesized in the corpus luteum, a temporary endocrine structure formed
in the ovary after ovulation. LH stimulates the formation of the corpus luteum and the production of progesterone. Similar to estrogen,
progesterone is also involved in a negative feedback loop with the
hypothalamus and pituitary gland.
Testosterone synthesis and regulation: In males, testosterone is
primarily produced in the Leydig cells of the testes. LH stimulates
the production of testosterone, while FSH supports the maturation
of sperm cells. Testosterone levels are regulated through a negative
feedback loop involving the hypothalamus and pituitary gland.
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tion. Estrogen is responsible for the growth and maturation of the
reproductive system, the development of secondary sexual characteristics, and the regulation of the menstrual cycle. Progesterone is
essential for preparing the uterine lining for pregnancy, maintaining pregnancy, and the development of the mammary glands for
lactation.
Hormonal functions in males: Testosterone is crucial for male sexual development and reproductive function. It promotes the growth
and maturation of the reproductive system, the development of
secondary sexual characteristics, sperm production, and the maintenance of libido. Testosterone also has anabolic effects on muscle
and bone development.
Additional functions of gonadal hormones: Beyond their roles in
sexual development and function, gonadal hormones also influence
various other physiological processes. For example, estrogen has
been shown to have protective effects on the cardiovascular system and to promote bone health. Testosterone also plays a role in
the maintenance of bone density and muscle mass, as well as the
regulation of mood and energy levels.
In summary, gonadal hormones, including estrogen, progesterone,
and testosterone, are produced by the gonads and regulated by the
HPG axis. These hormones are essential for sexual development,
reproductive function, and the maintenance of secondary sexual
characteristics. Furthermore, they are involved in a variety of other
physiological processes, such as bone health, cardiovascular function, and mood regulation.
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19.11
Secondary Endocrine Function
The heart, kidneys, intestines, thymus, gonads, and adipose tissue
are all organs that have secondary endocrine functions in addition
to their primary non-endocrine functions.
The heart contains specialized cardiac muscle cells in the walls of
the atria that produce the hormone atrial natriuretic peptide (ANP).
ANP is released in response to increased blood volume and acts on
the kidneys to reduce the reabsorption of sodium, causing sodium
and water to be excreted in the urine. ANP also reduces the amount
of renin and aldosterone released by the kidneys and adrenal cortex,
further preventing the retention of water. This leads to a reduction
in blood volume and blood pressure, and a decrease in the concentration of sodium in the blood. You can think of ANP secretion as
a pressure relief valve in the heart, activated when blood pressure
becomes too high.
The gastrointestinal tract produces several hormones that aid in
digestion. These hormones, such as gastrin, secretin, and cholecystokinin, are secreted in the presence of food and act on other
organs such as the pancreas, gallbladder, and liver to stimulate the
release of gastric juices and help break down and digest food in the
gastrointestinal tract.
The kidneys also have endocrine functions, including the production
of renin in response to decreased blood volume or pressure, and the
production of calcitriol, which helps with the absorption of calcium
and phosphate ions. The kidneys also produce erythropoietin (EPO),
a protein hormone that triggers the production of red blood cells
in the bone marrow. EPO is released in response to low oxygen
levels and can improve endurance by increasing oxygen delivery
to muscle cells. However, artificially high levels of EPO can cause
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severe health risks due to the increased viscosity of blood which can
lead to complications such as hypertension, increased chances of
forming blood clots, and organ damage due to impaired blood flow.
The thymus is a gland located behind the sternum that is most
active in infants and becomes smaller in size as we age. It produces
hormones called thymosins that contribute to the development of
the immune response.
Adipose tissue, which is a type of connective tissue found throughout the body, produces the hormone leptin in response to food
intake. Leptin increases the activity of neurons that suppress appetite and decreases the activity of neurons that stimulate appetite,
leading to feelings of satiety after eating and reducing the desire to
eat more. Leptin is also involved in reproduction, as it is necessary
for the synthesis of GnRH and gonadotropins. Low levels of adipose
tissue in thin females can cause delays in puberty, but an increase
in adipose tissue can improve fertility by increasing the production
of leptin. Ghrelin, on the other hand, is produced mainly by the
stomach and small intestine and its main function is to stimulate
hunger and increase food intake. Ghrelin signals to the brain that
the body needs more energy and that it is time to eat.
19.12 Pancreatic Hormones
The pancreas is a gland located in the abdomen that plays an important role in regulating glucose metabolism in the body. It produces
several hormones that help to maintain a balance of blood sugar
levels, a process known as glucose homeostasis.
There are two main types of cells in the pancreas that produce
hormones: alpha cells and beta cells. Alpha cells, which make up
about 20 percent of the cells in the pancreas, produce the hormone
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glucagon. Beta cells, which make up about 80 percent of the cells
in the pancreas, produce the hormone insulin.
Glucagon is a hormone that helps to raise blood sugar levels. It
is released by the pancreas in response to low blood sugar levels,
such as those seen between meals or during periods of fasting.
Glucagon signals the liver to convert stored glycogen into glucose,
which is then released into the bloodstream. This process, called
glycogenolysis, increases the amount of glucose in the blood, helping to maintain normal blood sugar levels.
In addition to glycogenolysis, glucagon also stimulates the liver to
produce glucose from non-carbohydrate sources, such as amino
acids and fatty acids, through a process known as gluconeogenesis.
By increasing glucose production and release into the bloodstream,
glucagon helps to regulate blood sugar levels and provide energy
to the body when needed.
Insulin is a hormone that helps to lower blood sugar levels by facilitating the uptake of glucose from the bloodstream into cells, where
it can be used for energy. Insulin is released by the pancreas in
response to high blood sugar levels, such as those seen after a meal.
It signals to cells to take up glucose from the bloodstream, which
helps to lower blood sugar levels.
19.12.1 Insulin and Glucagon
Let’s dive deeper into the production, regulation, and roles of insulin
and glucagon in glucose metabolism, as well as their impact on
the metabolism of other nutrients and how other hormones also
contribute to glucose homeostasis.
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ized cells called beta cells, which are found in clusters called islets
of Langerhans within the pancreas. When blood glucose levels rise,
such as after consuming a carbohydrate-rich meal, the increased
concentration of glucose in the bloodstream triggers the release
of insulin from the beta cells. Insulin release is also influenced by
other factors, such as the presence of certain amino acids and hormones, like incretins, that are released by the gastrointestinal tract
in response to food intake.
Glucagon production and regulation: Glucagon is produced by
alpha cells, another type of specialized cell found in the islets of
Langerhans within the pancreas. Glucagon secretion is stimulated
when blood glucose levels are low, such as during fasting or in response to intense physical activity. It is also influenced by factors
like the presence of certain amino acids, as well as hormones like
adrenaline and cortisol, which can also affect blood sugar levels.
Insulin’s role in glucose metabolism: Insulin facilitates glucose
uptake into cells, primarily muscle and adipose tissue, by promoting
the translocation of glucose transporter proteins (GLUT4) to the cell
membrane. This allows cells to take up glucose from the bloodstream and use it for energy or store it as glycogen. Insulin also
suppresses glucose production in the liver by inhibiting gluconeogenesis (the process of making glucose from non-carbohydrate
sources) and glycogenolysis (the breakdown of glycogen to release
glucose).
Glucagon’s role in glucose metabolism: Glucagon stimulates glucose release from the liver by promoting glycogenolysis and gluconeogenesis. This helps to maintain blood glucose levels when
they are low, ensuring that cells have a constant supply of energy.
Glucagon also promotes the breakdown of stored fats in adipose
tissue (lipolysis) and the release of fatty acids into the bloodstream,
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which can be used as an alternative energy source by cells.
Insulin and glucagon’s impact on nutrient metabolism: Insulin
and glucagon have opposing effects on the metabolism of fats and
amino acids. Insulin promotes the storage of fats in adipose tissue
and the uptake of amino acids into cells for protein synthesis. In
contrast, glucagon stimulates the breakdown of stored fats and the
release of amino acids from muscle tissue to be used as an energy
source or for gluconeogenesis.
Other hormones involved in glucose homeostasis: Various hormones, such as adrenaline (epinephrine), cortisol, and growth hormone, can also influence blood glucose levels. For example, adrenaline
and cortisol are released in response to stress, and both hormones
stimulate glycogenolysis and gluconeogenesis, which raises blood
glucose levels. Growth hormone can impact glucose homeostasis
by affecting insulin sensitivity and glucose uptake in cells.
In summary, insulin and glucagon are two critical hormones produced by the pancreas that regulate glucose metabolism in the
body. Insulin lowers blood glucose levels by promoting glucose
uptake and storage, while glucagon raises blood glucose levels by
stimulating glucose release from the liver and other sources. The
balance between insulin and glucagon secretion is essential for
maintaining glucose homeostasis and proper metabolic function.
Disruptions in this balance can lead to conditions like diabetes. Additionally, other hormones, such as adrenaline, cortisol, and growth
hormone, also contribute to the body’s overall glucose homeostasis.
19.12.2
Insulin Mechanism of Action
Insulin is a hormone produced by the beta cells of the pancreas that
plays a critical role in regulating glucose metabolism in the body.
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Its main function is to help lower blood sugar levels by facilitating
the uptake of glucose from the bloodstream into cells, where it can
be used for energy.
Here is a detailed explanation of the mechanism of insulin:
Production and release of insulin: When blood sugar levels are high,
such as after a meal, the pancreas releases insulin into the bloodstream. The release of insulin is triggered by an increase in blood
sugar levels and is also influenced by other hormones and factors,
such as glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP). High blood sugar levels are detected by specialized cells
in the pancreas known as beta cells. Beta cells are located in the
islets of Langerhans, which are clusters of cells scattered throughout the pancreas. As mentioned, in addition to glucose-stimulated
insulin secretion, beta cells also respond to other signals such as
incretin hormones (GLP-1 and GIP), which are produced by the gut
in response to food intake, and neural signals from the autonomic
nervous system. These signals help to coordinate insulin secretion
with changes in glucose levels and other metabolic demands.
Insulin signaling: Once released into the bloodstream, insulin travels
to cells throughout the body and binds to specific receptors on the
surface of cells. This binding activates a signaling pathway inside
the cells, which ultimately results in the uptake of glucose from the
bloodstream into cells.
Uptake of glucose: Insulin signaling leads to the activation of a
glucose transporters. GLUT (glucose transporter) is a family of transmembrane proteins that mediate the transport of glucose across
cell membranes. These transporters play a critical role in regulating
the uptake of glucose into cells and the release of glucose from
cells. Different types of GLUTs have distinct functions, with GLUT1
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responsible for basal glucose uptake, GLUT2 found in liver, pancreas
and small intestine, and GLUT4 being the main insulin-responsive
glucose transporter found in muscle and fat tissues. These transporters play a key role in regulating blood glucose levels and glucose
homeostasis. Defects in GLUT function can lead to conditions such
as diabetes.
Conversion of glucose to glycogen: Once inside the cell, glucose is
converted into glycogen, in a process called glycogenesis, which is
a complex carbohydrate that can be stored in the liver and muscles
for later use. The conversion of glucose to glycogen is facilitated by
the enzyme glycogen synthase. When insulin levels are high, it stimulates the activation of glycogen synthase, the enzyme responsible
for catalyzing the formation of glycogen from glucose. This results
in an increase in glycogen synthesis. Conversely, when insulin levels
are low, glycogen synthase is inhibited, leading to decreased glycogen synthesis and the release of glucose from glycogen stores into
the bloodstream.
Lowering of blood sugar levels: The uptake of glucose from the
bloodstream into cells, along with its conversion to glycogen, helps
to lower blood sugar levels in the body. This process is essential for
maintaining a balance of blood sugar levels, a process known as
glucose homeostasis.
The mechanism of insulin involves a complex interplay of hormones,
signaling pathways, and enzymes that work together to regulate glucose metabolism in the body. Understanding this mechanism can
help to inform healthy dietary and lifestyle choices, as well as provide
insight into potential complications related to glucose metabolism,
such as diabetes.
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19.12.3 Glucagon Mechanism of Action
Glucagon is a hormone produced by the alpha cells of the pancreas
that plays a critical role in regulating glucose metabolism in the body.
Its main function is to help raise blood sugar levels by stimulating
the release of glucose from the liver into the bloodstream.
Here is a detailed explanation of the mechanism of glucagon:
Production and release of glucagon: When blood sugar levels are
low, such as between meals or during periods of fasting, the pancreas releases glucagon into the bloodstream. The release of glucagon
is triggered by low blood sugar levels and is also influenced by other
hormones and factors, such as glucose-dependent insulinotropic
peptide (GIP) and gastric inhibitory peptide (GIP).
Glucagon signaling: Once released into the bloodstream, glucagon
travels to the liver and binds to specific receptors on the surface of
liver cells. This binding activates a signaling pathway inside the liver
cells, which ultimately results in the release of glucose from the liver
into the bloodstream.
Conversion of glycogen to glucose: The main source of glucose for
the body is glycogen, which is a complex carbohydrate stored in
the liver and muscles. When activated by glucagon signaling, the
enzyme glycogen phosphorylase breaks down glycogen into glucose, which is then released into the bloodstream. In contrast, when
insulin levels are high and blood sugar levels are elevated, insulin
stimulates the inactivation of glycogen phosphorylase, inhibiting
the breakdown of glycogen.
Raising of blood sugar levels: The release of glucose from the liver
into the bloodstream helps to raise blood sugar levels in the body.
This process is essential for maintaining a balance of blood sugar
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levels, a process known as glucose homeostasis.
The mechanism of glucagon involves a complex interplay of hormones, signaling pathways, and enzymes that work together to
regulate glucose metabolism in the body. Understanding this mechanism can help to inform healthy dietary and lifestyle choices, as well
as provide insight into potential complications related to glucose
metabolism, such as diabetes.
19.13 Osmoregulation
Osmoregulation is a vital physiological process that maintains the
body’s water and electrolyte balance, ensuring optimal functioning
of cells and tissues. This complex process involves the interplay of
various organs, including the hypothalamus, pituitary gland, kidneys,
and the cardiovascular system, as well as numerous hormones, such
as antidiuretic hormone (ADH), aldosterone, and atrial natriuretic
peptide (ANP).
Hypothalamus and osmoreceptors: The hypothalamus contains
specialized cells called osmoreceptors that can detect changes in
the osmolarity (the concentration of solutes in a solution) of the
blood. When blood osmolarity increases (due to dehydration, for
example), osmoreceptors in the hypothalamus are stimulated and
trigger the release of ADH.
Posterior pituitary gland and ADH storage: ADH, also known as
vasopressin, is produced in the hypothalamus but stored and released by the posterior pituitary gland. In response to increased
blood osmolarity, the posterior pituitary gland releases ADH into the
bloodstream.
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ically the collecting ducts of the nephrons, where it increases the
permeability of the ducts to water. This allows more water to be
reabsorbed from the urine back into the bloodstream, resulting in
more concentrated urine and increased water conservation. The
reabsorption of water helps to restore blood osmolarity to normal
levels.
Renin-angiotensin-aldosterone system (RAAS) and sodium balance: The RAAS is another hormonal system involved in osmoregulation, primarily regulating sodium and water balance. In response
to low blood volume or low blood pressure, the kidneys release
an enzyme called renin. Renin then converts angiotensinogen, a
protein produced by the liver, into angiotensin I, which is subsequently converted to angiotensin II by the action of an enzyme
called angiotensin-converting enzyme (ACE). Angiotensin II is a
potent vasoconstrictor, which increases blood pressure, and also
stimulates the release of aldosterone from the adrenal cortex. Aldosterone increases sodium reabsorption in the kidneys, which in
turn promotes water reabsorption, helping to restore blood volume
and blood pressure.
Atrial natriuretic peptide (ANP) and regulation of blood volume:
ANP is a hormone produced by the heart, specifically the atria, in
response to increased blood volume or increased blood pressure.
ANP promotes vasodilation, decreases the reabsorption of sodium
in the kidneys, and inhibits the release of aldosterone, all of which
contribute to the excretion of excess water and electrolytes, helping
to restore blood volume and blood pressure to normal levels.
In summary, osmoregulation is a complex process that involves the
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ing the amount of water excreted in the urine. In addition, the
renin-angiotensin-aldosterone system (RAAS) and atrial natriuretic
peptide (ANP) also contribute to the regulation of water and electrolyte balance, ensuring optimal cellular function and overall body
homeostasis.
19.13.1
Renin-Angiotensin-Aldosterone System
The renin-angiotensin-aldosterone system (RAAS) is an intricate hormonal cascade that maintains blood pressure and fluid balance in
the body. It is a crucial regulatory system that comprises several
components, including renin, angiotensinogen, angiotensin I, angiotensin II, angiotensin-converting enzyme (ACE), aldosterone, and
various receptors.
Renin release: Specialized cells in the kidneys called juxtaglomerular cells monitor blood pressure and blood volume. When blood
pressure or volume is low, these cells release renin into the bloodstream. Several factors can stimulate renin release, including reduced renal blood flow, decreased sodium concentration in the
distal tubules, or increased sympathetic nervous system activation.
Angiotensinogen conversion: Angiotensinogen is a protein produced by the liver and constantly released into the bloodstream.
When renin is present, it cleaves angiotensinogen to produce angiotensin I.
Angiotensin I conversion: Angiotensin I is a relatively inactive hormone that is converted to angiotensin II by angiotensin-converting
enzyme (ACE). ACE is an enzyme located mainly in the endothelial
cells of blood vessels, particularly in the lungs.
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sure. It also stimulates the release of aldosterone from the adrenal
glands, promotes sodium reabsorption in the proximal tubules of
the kidneys, and increases thirst and the release of antidiuretic hormone (ADH) to encourage water intake and retention. Additionally,
angiotensin II enhances the sympathetic nervous system’s activity,
further contributing to increased blood pressure.
Aldosterone release and function: Aldosterone is a steroid hormone produced by the adrenal cortex. It acts on the distal tubules
and collecting ducts of the kidneys, stimulating sodium reabsorption and potassium secretion. The reabsorption of sodium leads
to increased water retention, which in turn helps restore blood volume and blood pressure. Aldosterone’s overall effect is to maintain
proper electrolyte balance while adjusting blood pressure.
Feedback mechanisms: The RAAS is regulated by negative feedback loops. As blood pressure and volume are restored, the stimuli
for renin release are diminished, leading to reduced production of
angiotensin II and aldosterone. This helps maintain blood pressure
and electrolyte balance within the appropriate physiological range.
RAAS and disease: Dysregulation of the RAAS can contribute to
various health conditions, including hypertension, heart failure, and
chronic kidney disease. Drugs that target the RAAS, such as ACE
inhibitors, angiotensin receptor blockers (ARBs), and aldosterone
antagonists, are widely used to treat these conditions.
In summary, the renin-angiotensin-aldosterone system is a vital
hormonal system that plays a central role in regulating blood pressure and electrolyte balance. Its complex cascade of components,
including renin, angiotensinogen, angiotensin I, angiotensin II, ACE,
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ditions, and pharmacological interventions targeting the RAAS are
common therapeutic strategies.
19.14 Reproductive System
The reproductive system is regulated by a complex interplay of
hormones produced by the hypothalamus, pituitary gland, and
gonads (testes in males and ovaries in females). The primary hormones involved in the regulation of the reproductive system are
gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH),
and follicle-stimulating hormone (FSH).
Gonadotropin-releasing hormone (GnRH) is produced and released
by the hypothalamus. It acts on the pituitary gland to stimulate
the production and release of LH and FSH. LH and FSH are gonadotropins produced by the pituitary gland that play important
roles in the development and function of the gonads.
In males, LH stimulates the Leydig cells in the testes to produce
testosterone, the primary male sex hormone. Testosterone has several important functions in the male body, including the regulation
of sperm production and the maintenance of male secondary sexual characteristics such as muscle mass and bone density. FSH, on
the other hand, stimulates the Sertoli cells in the testes to promote
spermatogenesis, the process of sperm production. FSH helps to
support the growth and maturation of sperm-producing cells called
spermatogonia, and also stimulates the production of a variety of
factors that are important for the process of sperm maturation.
In females, Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) play important roles in regulating the menstrual cycle
and the production of eggs in the ovaries. They act on two different
types of cells in the ovaries: the theca cells and the granulosa cells.
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The theca cells are located outside the follicles in the ovary and are
responsible for producing androgens, such as testosterone. LH acts
on the theca cells and stimulates them to convert androgens into
estrogens. The granulosa cells are located within the follicles in the
ovary and are responsible for producing estrogens. FSH acts on the
granulosa cells and stimulates them to increase their production
of estrogens. In this way, LH and FSH work together to regulate
the menstrual cycle and promote the growth and development of
follicles in the ovary.
19.15 Hormonal Regulation of Stress
The hormonal regulation of stress is a multifaceted process that involves the intricate interplay of several hormones, neurotransmitters,
and physiological systems in the body. This coordinated response
aims to maintain homeostasis and ensure the body is equipped to
effectively cope with stressors.
The stress response can be divided into two main components: the
hypothalamic-pituitary-adrenal (HPA) axis and the sympatheticadrenal-medullary (SAM) system.
The HPA axis is activated when the hypothalamus in the brain detects stress, leading to the release of corticotropin-releasing hormone (CRH). CRH then stimulates the anterior pituitary gland to
secrete adrenocorticotropic hormone (ACTH) into the bloodstream.
ACTH, in turn, triggers the adrenal cortex (the outer layer of the
adrenal glands) to produce and release cortisol. Cortisol, as the primary stress hormone, exerts a wide range of effects on the body
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tion. Furthermore, cortisol acts as a feedback inhibitor, regulating
the HPA axis by inhibiting the release of CRH and ACTH when cortisol levels are sufficiently elevated.
The SAM system is activated concurrently with the HPA axis during
the stress response. It involves the release of the catecholamines
adrenaline (epinephrine) and noradrenaline (norepinephrine) from
the adrenal medulla (the inner layer of the adrenal glands) and
sympathetic nerve terminals. Adrenaline and noradrenaline act as
both hormones and neurotransmitters, influencing various target
tissues to rapidly prepare the body for the ”fight or flight” response.
This response involves increased heart rate and blood pressure, dilation of the bronchioles, redistribution of blood flow to vital organs
and muscles, and the release of glucose and fatty acids for energy
production.
Another neurotransmitter involved in the stress response is serotonin, which is primarily found in the central nervous system and
the gastrointestinal tract. Serotonin plays a crucial role in regulating
mood, anxiety, appetite, and sleep. Imbalances in serotonin levels
have been linked to mood disorders such as depression and anxiety,
which are often associated with chronic stress.
The hormonal regulation of stress is an intricate and dynamic process that requires the delicate balance of various hormones, neurotransmitters, and physiological systems. The stress response aims to
ensure that the body can effectively adapt to and manage stressors,
maintain homeostasis, and recover once the stressor has subsided.
However, prolonged or excessive activation of the stress response
can have negative consequences on physical and mental health,
highlighting the importance of effective stress management strategies.
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While the stress response is a natural and necessary part of life,
chronic or excessive stress can have negative effects on the body.
It can lead to a variety of physical and mental health problems,
including anxiety, depression, and cardiovascular disease.
To help manage stress, it is important to adopt healthy coping mechanisms, such as regular exercise, good sleep hygiene, and stressreducing activities like meditation or yoga. It may also be helpful to
seek support from friends, family, or a mental health professional.
By taking care of our bodies and minds, we can better regulate the
hormonal response to stress and maintain good overall health and
well-being.
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20 The Musculoskeletal System
The musculoskeletal system is an essential part of the body that
allows us to move, support our weight, and maintain proper posture
and balance. It is made up of bones, muscles, tendons, ligaments,
and other connective tissues that work together to allow us to move
and function in our daily lives. In this lesson, we will delve into
the physiological processes that underlie the functioning of the
musculoskeletal system. We will explore how muscles contract and
produce movement, how bones grow and repair themselves, and
how the various components of the musculoskeletal system work
together to support and move the body. We will also discuss the
role of hormones and other signaling molecules in the regulation
of musculoskeletal function. By the end of this lesson, you should
have a strong understanding of the physiological processes that
underlie the functioning of the musculoskeletal system and how
they contribute to the overall health and well-being of the body.
Let’s get started!
20.1
Skeletal Muscles
Skeletal muscle, also known as striated muscle, is a type of muscle
tissue responsible for voluntary movement in the human body. It is
composed of long, multinucleated cells called muscle fibers, which
are organized into bundles called fascicles. Skeletal muscles are
attached to bones by tendons, and their contraction generates force
that enables various body movements, such as walking, running,
and lifting objects. This section will discuss the structure, function,
and physiology of skeletal muscle in detail.
Structure of Skeletal Muscle
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• Muscle Fiber: Skeletal muscle fibers are elongated, cylindrical
cells with multiple nuclei located at the periphery. Each muscle fiber is surrounded by a plasma membrane called the sarcolemma, which encloses the sarcoplasm, the cytoplasm of the
muscle cell. Within the sarcoplasm, muscle fibers contain numerous myofibrils, which are the contractile units responsible
for generating force.
• Myofibrils: Myofibrils are composed of repeating units called
sarcomeres, which are the smallest functional units of skeletal muscle. Sarcomeres contain two types of protein filaments:
thin (actin) and thick (myosin) filaments. These filaments are arranged in an overlapping pattern, creating a striated appearance
when viewed under a microscope. The regular arrangement of
sarcomeres gives skeletal muscle its characteristic appearance
and enables the generation of force during contraction.
• Sarcomeres: Sarcomeres are delimited by two Z-lines, which
anchor the thin filaments. Within each sarcomere, the thin
filaments extend from the Z-lines towards the center, while the
thick filaments are located in the middle. The overlapping region
of thin and thick filaments is called the A-band, while the lighter
region containing only thin filaments is the I-band. The H-zone
is the central region of the A-band, where only thick filaments
are present. The arrangement of filaments within the sarcomere
is crucial for muscle contraction.
Function of Skeletal Muscle
• Movement: Skeletal muscles work in pairs called agonist-antagonist
pairs to generate coordinated movement. When one muscle
contracts (agonist), the opposing muscle (antagonist) relaxes,
allowing for smooth and controlled movement of the joint. This
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coordinated action is essential for maintaining balance, stability,
and posture.
• Heat Production: Skeletal muscles also play a role in maintaining body temperature. During muscle contraction, energy is
released in the form of heat, which helps maintain the body’s
core temperature within a narrow range.
• Protection: Skeletal muscles protect internal organs by providing a layer of padding and support. They also contribute to
the maintenance of body posture and the stabilization of joints
during movement.
20.1.1
Skeletal Muscle Fiber
The skeletal muscle fibers are the individual cells that make up the
skeletal muscles. They are long and cylindrical in shape and are
composed of many smaller units called myofibrils. Each myofibril
is made up of even smaller units called myofilaments, which are
made up of proteins called actin and myosin.
The arrangement of these myofilaments is what gives skeletal muscle fibers their characteristic striped appearance. The myofilaments
are organized into repeating units called sarcomeres, which are the
functional units of contraction in a muscle fiber.
During muscle contraction, the actin and myosin filaments slide
past each other, which shortens the sarcomeres and causes the
muscle to contract. This process is known as the sliding filament
theory of muscle contraction.
Skeletal muscle fibers are also classified based on their size and
strength. There are two main types of skeletal muscle fibers: fasttwitch and slow-twitch. Fast-twitch fibers are responsible for quick,
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powerful movements and are used for activities like sprinting and
weightlifting. They are able to generate a lot of force, but tire quickly.
Slow-twitch fibers are responsible for more endurance-based activities and are used for activities like long-distance running and
cycling. They are able to sustain contractions for longer periods of
time, but are not as powerful as fast-twitch fibers.
Skeletal muscle cells are composed of multiple muscle fibers that
are surrounded by a plasma membrane called the sarcolemma.
These fibers are made up of smaller units called myofibrils, which
have a striped appearance due to the repeating bands of actin and
myosin proteins. These bands create darker areas called A bands
and lighter areas called I bands. The I bands contain a vertical line
called a Z disc, and the space between two consecutive Z discs is
called a sarcomere. The sarcomere is the functional unit of skeletal
muscle, and as these units contract, the myofibrils and muscle cells
shorten.
Skeletal muscle fibers are made up of small structures called myofibrils, which are made up of even smaller structures called myofilaments. The myofilaments are made up of thick filaments (made
of myosin protein) and thin filaments (made of actin, tropomyosin,
and troponin proteins). The thick filaments are found only in the
A bands, while the thin filaments attach to the Z disc and extend
along the I band and into the A band. The area where the thick
and thin filaments overlap has a dense appearance, creating the
H zone in the center of the A band. The M line holds the thick filaments together in the H zone. The myofibrils are connected by
intermediate filaments called desmin, which attach to the Z disc.
When the muscle is relaxed, the binding sites on actin for myosin
attachment are blocked by tropomyosin. When calcium ions bind
to troponin, the binding sites are exposed and muscle contraction
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occurs through the interaction of actin and myosin.
Figure 99: The sarcolemma, a plasma membrane enclosing sarcoplasm, the cytoplasm of muscle cells, encases each skeletal muscle fiber. The striated appearance of a muscle fiber is attributed to
the presence of many fibrils within the cell. (Credit: openstax.org)
The sliding filament model is a theory that explains how skeletal
muscles contract and produce force. According to this model, muscle contraction occurs when thin filaments made of the protein
actin slide over thicker filaments made of the protein myosin.
The sliding filament model of muscle contraction is a well-orchestrated
process that explains how muscles contract and generate force. This
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process can be broken down into a series of sequential steps that
involve the interaction between the thin (actin) and thick (myosin)
filaments within muscle fibers.
1. Resting state: In a resting muscle fiber, the actin and myosin
filaments are partially overlapped. The binding sites on actin for
myosin attachment are obstructed by tropomyosin, a regulatory
protein that wraps around the actin filament. This prevents
the myosin heads from binding to actin and initiating muscle
contraction.
2. Nervous system signal: When a nerve signal reaches the muscle,
it triggers the release of calcium ions from the sarcoplasmic reticulum, an intracellular storage site for calcium, into the muscle
cell’s cytoplasm.
3. Calcium binding: The released calcium ions bind to troponin, another regulatory protein that is associated with tropomyosin on
the thin filaments. This binding event induces a conformational
change in troponin, which subsequently causes tropomyosin to
shift and expose the actin binding sites for myosin.
4. Cross-bridge formation: With the binding sites now exposed, the
myosin heads extend and attach to the actin filaments, forming
cross-bridges between the thick and thin filaments.
5. Power stroke: The myosin heads pivot, pulling the actin filaments towards the center of the sarcomere, the basic functional
unit of a muscle fiber. This sliding of the thin filaments over
the thick filaments shortens the sarcomere, resulting in muscle
contraction.
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ening of the muscle fiber as a whole. Concurrently, the H zone, a
region in the A band containing only thick filaments, diminishes
in size.
7. ATP binding and cross-bridge detachment: Following the power
stroke, adenosine triphosphate (ATP) molecules bind to the
myosin heads. The binding of ATP causes the myosin heads
to detach from the actin filaments, breaking the cross-bridges.
8. Myosin head repositioning: The ATP molecules are then hydrolyzed, providing energy for the myosin heads to return to
their original extended position, ready for another cycle of crossbridge formation and power stroke.
9. Muscle relaxation: When the nerve signal ceases, calcium ions
are actively pumped back into the sarcoplasmic reticulum. The
decrease in calcium concentration causes troponin to revert
to its original conformation, prompting tropomyosin to once
again obstruct the actin binding sites. This prevents further
cross-bridge formation, leading to muscle relaxation and the
return of the sarcomere to its resting state.
In summary, the sliding filament model describes a sequential and
comprehensive process that involves the intricate interaction between actin and myosin filaments in response to nerve signals. This
interaction, regulated by calcium ions and regulatory proteins, results in the shortening and subsequent relaxation of muscle fibers,
allowing for muscle contraction and force generation.
20.2 ATP and Muscle Contraction
Muscle contraction occurs when myosin heads bind to actin and pull
the actin inward. This process requires energy, which is provided
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Figure 100: The sliding filament model. During contraction of a
sarcomere, the Z lines come closer, causing the I band to reduce in
size. In contrast, the A band retains its width. When the sarcomere is
fully contracted, the thin and thick filaments overlap entirely. (Credit:
openstax.org)
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Figure 101: Contraction is initiated when a cross-bridge is formed
between actin and myosin heads. The muscle fiber contains Ca++
ions that bind to troponin, which allows the cross-bridge to form.
As long as Ca++ ions are present and ATP is available, the muscle
fiber will continue to contract by shortening. (Credit: openstax.org)
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by ATP. Myosin binds to actin at a specific site on the actin protein,
while the myosin molecule also has a binding site for ATP.
1. ATP binds to myosin, and the enzyme ATPase hydrolyzes the
ATP molecule into ADP and an inorganic phosphate molecule
(Pi).
2. The energy released from ATP hydrolysis causes the myosin
head to change angle and become ”cocked”.
3. If the actin binding sites are uncovered, a cross-bridge forms
between the actin and myosin molecules.
4. The Pi molecule is released, allowing the stored energy in the
myosin head to be expended through a conformational change.
5. The myosin head moves towards the M line, pulling the actin
along with it, causing the sarcomere to shorten and the muscle
to contract.
6. After the contraction, ADP is released and the cross-bridge remains in place with actin and myosin still bound together.
7. ATP can then bind to myosin, allowing the cross-bridge cycle to
begin again and for further muscle contraction to occur.
20.2.1
Regulatory Proteins
In a resting muscle, actin and myosin are separated by regulatory
proteins that block the binding sites on myosin. Tropomyosin, in
particular, covers the myosin binding sites on actin, preventing crossbridge formation and muscle contraction without nervous input.
Troponin helps to position tropomyosin on the actin molecule and
also binds calcium ions.
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Figure 102: (a) As calcium binds to troponin, the active site on actin
is exposed.(b) The myosin head is attracted to actin and forms the
cross-bridge by binding to the actin at its actin-binding site.(c) The
phosphate generated in the previous contraction cycle is released
during the power stroke, resulting in the myosin head pivoting toward the center of the sarcomere. The attached ADP and phosphate
group are then released.(d) A new molecule of ATP attaches to the
myosin head, leading to the detachment of the cross-bridge.(e) The
myosin head hydrolyzes ATP to ADP and phosphate, returning the
myosin to the cocked position. (Credit: openstax.org)
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formation, revealing the myosin binding site on actin and allowing
cross-bridge formation. This process requires the presence of calcium, which is maintained at low concentrations in the sarcoplasm.
When calcium ions are present, they bind to troponin, causing it to
undergo conformational changes that allow tropomyosin to move
away from the myosin binding sites on actin. This exposes the binding sites, enabling cross-bridge formation and triggering muscle
contraction. The cross-bridge cycling continues until calcium ions
and ATP are no longer available, at which point tropomyosin covers
the binding sites on actin once again.
20.3 Excitation–Contraction Coupling
Excitation-contraction coupling refers to the process by which electrical signaling in a muscle leads to contraction. In skeletal muscles,
this process involves the end plate, a specialized region at the neuromuscular junction where the muscle fiber and nerve fiber meet.
When an action potential reaches the end plate, it triggers the release of neurotransmitter molecules called acetylcholine (ACh) from
the presynaptic terminal of the nerve fiber. These ACh molecules
bind to receptors on the postsynaptic membrane of the muscle fiber,
causing ion channels to open and allowing the influx of positive ions,
such as sodium and calcium.
The influx of positive ions causes a depolarization of the muscle
fiber membrane, which can propagate along the length of the fiber
and trigger an action potential. When an action potential reaches
the sarcoplasmic reticulum (a specialized endoplasmic reticulum in
muscle cells), it causes the release of stored calcium ions into the
sarcoplasm.
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cess of cross-bridge formation between actin and myosin, leading
to muscle contraction. When the action potential ends and the
calcium ions are removed from the sarcoplasm, the cross-bridges
between actin and myosin are broken, causing the muscle to relax.
Excitation-contraction coupling is essential for the proper function
of skeletal muscles, allowing them to contract and produce movement in response to nervous input.
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21
The Respiratory System
The respiratory system is a vital system in the human body responsible for the exchange of gases between the body and the environment. It includes the organs and structures involved in breathing,
such as the nasal cavity, pharynx, larynx, trachea, bronchi, and lungs.
The main function of the respiratory system is to bring oxygen into
the body and remove carbon dioxide, a waste product produced
during cellular respiration. Oxygen is essential for the body’s cells to
produce energy, while carbon dioxide is toxic and must be removed.
The respiratory system works in conjunction with the circulatory
system to deliver oxygen to the body’s cells and remove carbon
dioxide. It also plays a role in regulating the acid-base balance of
the body and maintaining proper pH levels.
Overall, the respiratory system plays a crucial role in the overall health
and well-being of the body. Dysfunction or disease of the respiratory
system can have serious consequences and can disrupt the body’s
ability to maintain homeostasis.
21.1 The Basic Principles of Gas Exchange
The basic principles of gas exchange refer to the exchange of gases,
such as oxygen and carbon dioxide, between an organism and its environment. This is important for the functioning of cells and tissues,
as oxygen is necessary for aerobic respiration and the production
of energy, while carbon dioxide is a byproduct of this process that
must be removed.
The respiratory system plays a vital role in gas exchange by bringing oxygen into the body and removing carbon dioxide. It can be
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Figure 103: The major respiratory structures that can be found in
our bodies. (Credit: openstax.org)
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thought of as a ”highway” connecting the body’s cells with the environment. Oxygen is like a delivery truck bringing supplies to the
cells, while carbon dioxide is like a garbage truck removing waste.
The exchange of gases takes place in the lungs, specifically in tiny
air sacs called alveoli. The thin walls of the alveoli allow oxygen
from the air we breathe to diffuse into the bloodstream, where it is
transported to the cells. At the same time, carbon dioxide diffuses
from the bloodstream into the alveoli and is expelled from the body
when we exhale.
One key concept in understanding gas exchange is partial pressure.
Partial pressure refers to the pressure exerted by a particular gas
in a mixture of gases. For example, in air, which is made up of a
mixture of gases including oxygen, nitrogen, and carbon dioxide,
the partial pressure of oxygen is the pressure that oxygen would
exert if it were the only gas present in the mixture.
The partial pressure of a gas can be affected by several factors, including the concentration of the gas, the temperature of the environment, and the volume of the container. In the case of gas
exchange in living organisms, the partial pressure of oxygen and
carbon dioxide in the air or water surrounding the organism, as well
as the partial pressure of these gases within the organism’s body,
play a role in the exchange of these gases.
In general, gases tend to move from areas of higher partial pressure
to areas of lower partial pressure. This means that, in the case of
gas exchange in living organisms, oxygen from the air or water will
tend to move into the organism’s body, while carbon dioxide will
tend to move out of the organism’s body and into the surrounding
environment.
The process of gas exchange in living organisms can take place
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Figure 104: Partial pressure refers to the pressure that a gas exerts
individually. When there is a mixture of gases, the total pressure is
equal to the sum of the partial pressures of each gas present in the
mixture. (Credit: openstax.org)
through a variety of mechanisms, including diffusion through cell
membranes, specialized respiratory organs, and circulation of fluids
through the body. These mechanisms allow organisms to obtain
the oxygen they need for cellular respiration and to remove excess
carbon dioxide, ensuring the proper functioning of their cells and
tissues.
21.2 Lung Volumes and Capacities
Lung volumes and capacities refer to the amount of air that can
be inhaled or exhaled by the lungs during different phases of respiration. Understanding these concepts can help us understand
how the respiratory system functions and how it can be affected by
various conditions.
There are several different lung volumes and capacities that can be
measured, including:
Tidal volume: This is the amount of air inhaled or exhaled during
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normal, quiet breathing. It is usually around 500 ml.
Inspiratory reserve volume: This is the amount of air that can be
inhaled beyond the tidal volume, or the extra air that can be inhaled
during deep or forced breathing. It is usually around 3000 ml.
Expiratory reserve volume: This is the amount of air that can be
exhaled beyond the tidal volume, or the extra air that can be exhaled
during forced breathing. It is usually around 1100 ml.
Residual volume is the amount of air that stays in the lungs after
a person exhales as much as they can. It is important for keeping
the lungs elastic and avoiding collapse of the alveoli. Although it is
typically around 1200 ml, residual volume is not something that can
be directly measured.
Vital capacity: This is the total amount of air that can be exhaled
after a maximal inhalation. It is equal to the sum of the tidal volume,
inspiratory reserve volume, and expiratory reserve volume, and is
usually around 4800 ml.
FRC, or functional residual capacity, refers to the amount of air that
remains in the lungs at the end of a normal, passive exhalation. It is
the volume of air present in the lungs after a person has breathed
out normally and reached a steady state, where there is no further
airflow in or out of the lungs. The FRC is important in respiratory
physiology because it determines the amount of air available for
gas exchange in the lungs. It is a measure of the lung’s reserve
capacity and is used to evaluate various respiratory conditions, such
as chronic obstructive pulmonary disease (COPD) and asthma, as
well as to monitor the effects of treatments for these conditions. In
healthy individuals, the FRC is typically around 2-3 liters, but it can
be affected by factors such as age, body position, and lung disease.
Total lung capacity: This is the total volume of air that the lungs
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can hold, and is equal to the vital capacity plus the residual volume.
It is usually around 6000 ml.
Figure 105: Respiratory volumes and capacities. (Credit: openstax.org)
Measuring these lung volumes and capacities can help us understand how the respiratory system is functioning and identify any
potential problems or abnormalities. For example, a decrease in vital
capacity may be a sign of impaired lung function, while an increase
in residual volume may indicate the presence of airway obstruction
or other respiratory issues.
21.3 Human Breathing Mechanism
Breathing, also known as pulmonary ventilation, is the process of
inhaling and exhaling air in and out of the lungs. This process facilitates the exchange of oxygen and carbon dioxide between the body
and the environment. The respiratory system, which includes the
lungs, bronchial tubes, and alveoli, controls this process, and it can
be influenced by various factors such as physical activity, emotional
state, and the body’s need for oxygen.
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Mechanics of Breathing Inhalation: The process of breathing starts
with the contraction of the diaphragm, a dome-shaped muscle at
the bottom of the lungs. When the diaphragm contracts, it moves
downward, expanding the chest cavity. Simultaneously, the external
intercostal muscles between the ribs contract, lifting the ribcage
and further increasing the volume of the chest cavity. This decrease
in pressure inside the chest cavity creates a negative pressure, or
subatmospheric pressure, which draws air into the lungs through
the nose or mouth. Exhalation: During exhalation, the diaphragm
and external intercostal muscles relax, allowing the chest cavity to
decrease in volume. This increase in pressure within the chest cavity
forces air out of the lungs through the nose or mouth. Exhalation is
typically a passive process; however, during forceful exhalation, such
as during exercise or coughing, the internal intercostal muscles and
abdominal muscles contract to expel air more forcefully.
Pathway of Air and Gas Exchange Air pathway: Inside the lungs,
the air passes through a network of tubes called bronchial tubes,
which branch out into smaller tubes called bronchioles. The bronchioles further divide and eventually terminate in tiny air sacs called
alveoli. Alveoli: Alveoli are the primary sites of gas exchange in the
lungs. They are surrounded by a network of blood vessels called
capillaries, which facilitate the exchange of oxygen and carbon dioxide. The walls of the alveoli and capillaries are very thin, allowing
for efficient diffusion of gases. Gas exchange: Oxygen from the
air in the alveoli diffuses across the alveolar and capillary walls and
binds to hemoglobin molecules in red blood cells. This oxygen-rich
blood is then transported to the heart and circulated throughout
the body. At the same time, carbon dioxide, a waste product of
cellular respiration, diffuses from the blood in the capillaries into the
alveoli, where it is exhaled.
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Figure 106: Normal inhalation and exhalation mechanisms. (Credit:
KhanAcademy.org)
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Regulation of Breathing Factors influencing breathing rate: The
rate of breathing can be influenced by various factors, such as physical activity, emotional state, and the body’s need for oxygen. For
example, during exercise, the body’s demand for oxygen increases,
leading to faster and deeper breathing. The medulla oblongata in
the brainstem contains the respiratory control center, which monitors blood levels of oxygen, carbon dioxide, and pH, and adjusts
the breathing rate accordingly. Chemoreceptors: Chemoreceptors
in the carotid and aortic bodies, as well as in the medulla oblongata, detect changes in blood oxygen, carbon dioxide, and pH levels.
When these levels deviate from the normal range, chemoreceptors
send signals to the respiratory control center, which in turn alters
the rate and depth of breathing to restore homeostasis.
In summary, breathing is a vital process that maintains the body’s
oxygen levels and removes excess carbon dioxide. It involves the coordinated action of respiratory muscles, particularly the diaphragm
and intercostal muscles, and the intricate structure of the lungs,
bronchial tubes, and alveoli. Understanding how breathing works is
essential for maintaining good respiratory health
21.4 Transport of Gases in Blood
The transport of gases in the human body is essential for maintaining proper physiological function. Oxygen is transported from the
lungs to the cells, while carbon dioxide is removed from the cells
and exhaled. This process is carried out by the circulatory system,
specifically the heart, blood vessels, and blood.
Gas Transport in Blood Oxygen Transport: The role of blood is
crucial in the transport of oxygen. When oxygen enters the lungs,
it diffuses across the alveoli into the capillaries, where it binds to
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hemoglobin in red blood cells (RBCs). Hemoglobin is a protein that
can carry up to four oxygen molecules, forming oxyhemoglobin.
Oxygen-rich blood is then pumped from the lungs to the heart,
which distributes it to the rest of the body through the arterial system. When the oxygenated blood reaches the cells, the oxygen
molecules are released from hemoglobin and diffuse into the cells
to be used in cellular respiration to produce energy. Carbon Dioxide
Transport: Meanwhile, carbon dioxide (CO2), a byproduct of cellular
respiration, is produced by the cells and must be removed. Three
primary mechanisms transport CO2 in the blood:
• Dissolved CO2: Approximately 7 percent of CO2 is transported
in the blood as dissolved CO2, which diffuses from the cells into
the blood plasma and is carried back to the lungs.
• Carbaminohemoglobin: About 23 percent of CO2 is transported
in the blood by binding to hemoglobin, forming carbaminohemoglobin. This process occurs primarily with deoxygenated
hemoglobin in venous blood.
• Bicarbonate Ions: The majority of CO2 (about 70 percent) is transported in the blood in the form of bicarbonate ions (HCO3-). This
process begins when CO2 diffuses from the cells into the RBCs,
where it reacts with water (H2O) in the presence of an enzyme
called carbonic anhydrase. This reaction forms carbonic acid
(H2CO3), which then dissociates into bicarbonate ions (HCO3-)
and hydrogen ions (H+). Bicarbonate ions are then transported
out of the RBCs and into the plasma in exchange for chloride
ions (Cl-), a process known as the chloride shift.
The concentration of carbon dioxide in the blood is regulated by the
respiratory system and the circulatory system. The respiratory system controls the concentration of carbon dioxide in the alveoli, while
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the circulatory system transports the carbon dioxide throughout
the body.
However, these percentages can vary depending on the body’s
needs and the conditions in which the blood is transported. For
example, during exercise, the body’s demand for oxygen increases,
leading to an increase in the partial pressure of carbon dioxide in the
alveoli. This can shift the balance of carbon dioxide transport in the
blood towards more carbon dioxide being bound to hemoglobin
and less being transported as bicarbonate ions.
It’s important to note that these percentages are approximate and
can vary somewhat depending on the specific circumstances.
Figure 107: Carbon dioxide is conveyed through three distinct means:
(a) within red blood cells; (b) following the creation of carbonic acid
(H2CO3), which is then dissolved in plasma; and (c) in plasma. (Credit:
openstax.org)
Respiratory and Circulatory Systems Coordination
The respiratory and circulatory systems work together to regulate
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the concentration of oxygen and carbon dioxide in the blood. The
respiratory system manages the concentration of gases in the alveoli,
while the circulatory system carries the gases throughout the body.
Oxygen and Carbon Dioxide Exchange: At the level of the alveoli, the
partial pressure of oxygen (PO2) is higher than in the blood, which
drives the diffusion of oxygen into the capillaries. Conversely, the
partial pressure of carbon dioxide (PCO2) is higher in the blood than
in the alveoli, facilitating the diffusion of CO2 out of the capillaries
and into the alveoli, where it is exhaled.
Chemoreceptors and Gas Regulation: Chemoreceptors located in
the carotid and aortic bodies and in the brainstem detect changes
in blood oxygen, carbon dioxide, and pH levels. When these levels
deviate from the normal range, chemoreceptors send signals to the
respiratory control center in the medulla oblongata, which in turn
alters the rate and depth of breathing to restore homeostasis.
In summary, the transport of gases in the human body is crucial for
maintaining proper physiological function. Blood plays a critical role
in this process, carrying oxygen to the cells and removing carbon
dioxide from the cells. The respiratory and circulatory systems work
together to ensure efficient gas exchange and transport
21.4.1
Oxygen Transport
Oxygen is carried in the blood through a protein called hemoglobin,
which is found in red blood cells. Hemoglobin has a high affinity for
oxygen and can bind to it reversibly, meaning it can easily release
and pick up oxygen as needed.
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hemoglobin. The oxyhemoglobin is then carried by the red blood
cells to the body’s cells, where it releases the oxygen and replenishes
the cells’ supply.
The amount of oxygen that the blood can carry is determined by the
concentration of hemoglobin in the blood. The more hemoglobin
there is, the more oxygen the blood can carry. Factors that can
affect the concentration of hemoglobin in the blood include anemia,
dehydration, and high altitude.
21.4.2 The Oxygen Dissociation Curve
The oxygen dissociation curve is a graphical representation of the relationship between the partial pressure of oxygen (PO2) and the percentage of oxygen saturation in hemoglobin (Hb). It is an important
concept in understanding how oxygen is carried and released by the
blood, and it reflects the cooperative binding of oxygen molecules
to hemoglobin.
Shape of the Oxygen Dissociation Curve
The oxygen dissociation curve typically has a sigmoid (S-shaped)
curve, with the steepest part in the middle. This shape represents
the cooperative binding of oxygen to hemoglobin, which means
that as one oxygen molecule binds to a subunit of hemoglobin,
the binding of subsequent oxygen molecules becomes easier. Consequently, hemoglobin’s affinity for oxygen increases as the PO2
increases, allowing it to efficiently pick up oxygen in the lungs and
release it to the body’s cells.
Factors Affecting the Oxygen Dissociation Curve
The oxygen dissociation curve can be influenced by various factors
that cause shifts to the right or left, affecting the release of oxygen
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Figure 108: The oxygen dissociation curve. This graph shows the
relationship between the partial pressure of oxygen and hemoglobin
saturation. (Credit: openstax.org)
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from hemoglobin to the body’s cells.
• Bohr Effect: The Bohr effect refers to the shift in the oxygen
dissociation curve to the right in response to an increase in
the concentration of carbon dioxide (CO2) or a decrease in pH
(a measure of acidity). This rightward shift makes it easier for
hemoglobin to release oxygen to the cells, allowing for more
efficient oxygen delivery to the body’s tissues, especially during periods of increased metabolic demand, such as exercise.
The Bohr effect occurs because CO2 and H+ ions can bind to
hemoglobin, reducing its affinity for oxygen and promoting oxygen release.
• Temperature: An increase in temperature can also cause the
curve to shift to the right, reflecting a decreased affinity of hemoglobin
for oxygen. This is because higher temperatures can promote
the release of oxygen from hemoglobin, facilitating oxygen delivery to tissues with increased metabolic activity and heat production, such as during exercise or fever.
• 2,3-Diphosphoglycerate (2,3-DPG): 2,3-DPG is a byproduct of
glycolysis and can bind to hemoglobin, reducing its affinity for
oxygen. An increased concentration of 2,3-DPG can cause the
curve to shift to the right, promoting oxygen release to the cells.
This effect is particularly important in situations where there is
a higher demand for oxygen, such as during anemia or at high
altitudes.
• Fetal Hemoglobin (HbF): Fetal hemoglobin has a higher affinity
for oxygen than adult hemoglobin (HbA), which allows the fetus
to extract oxygen from the maternal circulation. Consequently,
the oxygen dissociation curve for HbF is shifted to the left compared to HbA, enabling more efficient oxygen transfer across
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the placenta.
In summary, the oxygen dissociation curve is a graphical representation of the relationship between the partial pressure of oxygen
and the percentage of oxygen saturation in hemoglobin. It reflects
the cooperative binding of oxygen molecules to hemoglobin and
can shift to the right or left based on various factors, such as CO2
concentration, pH, temperature, and 2,3-DPG concentration. Understanding the oxygen dissociation curve is essential for understanding how oxygen is carried and released by the blood and how the
body adapts to various physiological conditions.
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22 The Circulatory System
Welcome to the fascinating world of the circulatory system! This
amazing system is responsible for pumping oxygen, nutrients, and
hormones throughout the body, as well as removing waste and
carbon dioxide. It is made up of the heart, blood vessels, and blood,
and it plays a crucial role in maintaining the health and wellbeing of
every organ and tissue in the body. In this chapter, we’ll take a closer
look at the components of the circulatory system, how it works, and
the important functions it serves. So let’s dive in and learn more
about this incredible system that keeps us alive and well!
22.1
The Circulatory System
The circulatory system is a complex network of organs and blood
vessels that is responsible for delivering oxygen, nutrients, and other
essential substances to all of the cells in the body. At the center of
the circulatory system is the heart, a powerful muscular organ that
pumps blood through the body via a network of arteries and veins.
The heart is divided into four chambers: the left and right atria, and
the left and right ventricles. The atria are the upper chambers of
the heart, while the ventricles are the lower chambers. Blood flows
into the atria from the veins and is then pumped into the ventricles,
from which it is sent out to the rest of the body.
The blood vessels that carry blood throughout the body include
arteries, veins, and capillaries. Arteries are blood vessels that carry
oxygen-rich blood away from the heart to the rest of the body. They
have thick walls and are able to withstand the high pressure of
the blood being pumped through them. Veins, on the other hand,
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carry oxygen-depleted blood back to the heart. They have thinner
walls and are responsible for returning the blood to the heart to be
oxygenated.
Capillaries are the smallest blood vessels in the body and are responsible for exchanging oxygen and nutrients with the cells. They have
very thin walls, allowing for the exchange of gases and substances
between the blood and the surrounding cells.
Important to remember, not all arteries contain oxygen-rich blood.
Arteries are blood vessels that carry oxygen-rich blood away from the
heart to the rest of the body. However, there are some exceptions
to this general rule.
The pulmonary artery is an exception to this rule. It carries oxygendepleted blood from the right ventricle of the heart to the lungs,
where it is oxygenated before being returned to the left atrium of
the heart via the pulmonary vein.
Another exception is the coronary artery. The coronary artery carries
oxygen-rich blood to the heart muscle itself. It branches off from
the aorta, the main artery that carries oxygen-rich blood from the
left ventricle of the heart to the rest of the body.
In general, however, arteries do carry oxygen-rich blood away from
the heart to the rest of the body
22.2 Blood and its Components
The blood is a vital fluid that circulates throughout the body, performing various essential functions. It transports oxygen, nutrients,
hormones, and other substances to the cells and tissues, and removes waste products and carbon dioxide. Blood is made up of
several different components, each with its own unique function.
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Red Blood Cells (Erythrocytes): Red blood cells are one of the main
components of blood, and they play a crucial role in transporting oxygen from the lungs to the body’s cells and tissues. These biconcaveshaped cells contain a protein called hemoglobin, which binds to
oxygen and facilitates its transport. Red blood cells are continuously
produced in the bone marrow and have a lifespan of about 120 days.
They make up about 40-45% of the total volume of blood.
White Blood Cells (Leukocytes): White blood cells are essential components of the blood, playing a critical role in the body’s immune
system by helping to defend against infection and disease. There
are several different types of white blood cells, each with specific
functions:
Neutrophils: These cells are the most abundant type of white blood
cells and are the first to arrive at the site of infection. They primarily
target and engulf bacteria, playing a significant role in the body’s
defense against bacterial infections. Lymphocytes: Lymphocytes
are involved in the adaptive immune response and are further divided into B cells, T cells, and natural killer (NK) cells. B cells produce
antibodies, while T cells have various roles in immune regulation, including the direct killing of infected cells. NK cells are involved in the
defense against viral infections and cancer cells. Monocytes: Monocytes differentiate into macrophages and dendritic cells, which are
involved in phagocytosis and antigen presentation, respectively.
White blood cells make up about 1% of the total volume of blood.
Platelets (Thrombocytes): Platelets are small, disc-shaped cell fragments that play a vital role in blood clotting. When a blood vessel is
damaged and bleeding occurs, platelets are activated and aggregate at the site of injury to form a clot. This process helps to prevent
excessive blood loss and initiates tissue repair. Platelets make up
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about 1-3% of the total volume of blood.
Plasma: Plasma is the liquid portion of the blood, making up about
55-60% of the total volume. It is a clear, straw-colored fluid that
contains a variety of proteins, minerals, hormones, enzymes, and
other substances that support the body’s functions. Some essential
components of plasma include:
Albumin: This protein helps to maintain the blood’s osmotic pressure and transport various substances, such as hormones and fatty
acids. Globulins: These proteins play a role in the immune system
and transport various substances, such as hormones and lipids. Fibrinogen: This protein is involved in blood clotting and is converted
to fibrin during the clotting process. Electrolytes: These ions, such
as sodium, potassium, and chloride, help to maintain the body’s
fluid balance and pH.
In addition to these main components, the blood also contains a
number of other substances, including nutrients, electrolytes, and
waste products. Together, these components work together to
maintain the health and function of the body and keep it running
smoothly. Understanding the composition and function of blood is
essential for comprehending various physiological processes and
the body’s response to different conditions.
22.3 The Heart
The heart is a powerful muscular organ that is responsible for pumping blood throughout the body. It is divided into four chambers: the
left and right atria, and the left and right ventricles.
The right atrium is the upper chamber on the right side of the heart.
It receives oxygen-depleted blood from the body via the superior
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Figure 109: Blood consists of cellular and non-cellular components.
The cellular elements are primarily composed of erythrocytes, while
leukocytes and platelets are present in relatively smaller numbers.
Plasma is the non-cellular component in which these formed elements are suspended. When blood is subjected to centrifugation,
the lighter component, plasma, floats to the top, separated from
the heavier component, erythrocytes, by a buffy coat of leukocytes
and platelets. The percentage of erythrocytes in the total blood
volume is called hematocrit, which can be used to assess the blood’s
oxygen-carrying capacity. Hematocrit levels can be low or high, and
such variations can have clinical implications. (Credit: openstax.org)
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and inferior vena cava and pumps it into the right ventricle.
The right ventricle is the lower chamber on the right side of the
heart. It pumps the oxygen-depleted blood from the right atrium
into the lungs, where it is oxygenated before being returned to the
left atrium via the pulmonary vein.
The left atrium is the upper chamber on the left side of the heart.
It receives oxygenated blood from the lungs and pumps it into the
left ventricle.
The left ventricle is the lower chamber on the left side of the heart.
It pumps the oxygenated blood from the left atrium out to the rest
of the body via the aorta, the main artery that carries oxygen-rich
blood from the heart to the rest of the body.
So, in summary, the order of blood flow through the heart is as
follows:
Oxygen-depleted blood from the body enters the right atrium via
the superior and inferior vena cava.
The right atrium pumps the oxygen-depleted blood into the right
ventricle.
The right ventricle pumps the oxygen-depleted blood into the lungs,
where it is oxygenated.
The oxygenated blood returns to the left atrium via the pulmonary
vein.
The left atrium pumps the oxygenated blood into the left ventricle.
The left ventricle pumps the oxygenated blood out to the rest of the
body via the aorta.
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Figure 110: Blood flows from the right atrium to the right ventricle,
where it is then pumped into the pulmonary circuit. The blood in the
pulmonary artery branches is oxygen-poor but relatively high in carbon dioxide. In the pulmonary capillaries, gas exchange takes place,
with oxygen moving into the blood and carbon dioxide moving out.
The oxygen-rich and carbon dioxide-poor blood then returns to the
left atrium, where it enters the left ventricle. The left ventricle then
pumps the oxygen-rich blood into the systemic circuit. In the systemic capillaries, there is an exchange of oxygen and nutrients from
the capillaries and carbon dioxide and waste products into them.
Finally, the blood returns to the right atrium and the cycle begins
anew. (Credit: openstax.org)
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22.3.1
Heart Valves
The heart is a muscular organ that functions as a pump, circulating
blood throughout the body. It is composed of four chambers and
four valves that regulate the flow of blood, ensuring that it moves
efficiently in the right direction. The valves are crucial for the proper
functioning of the heart and preventing blood from flowing backward.
Atrioventricular Valves (AV valves)
Tricuspid Valve: The tricuspid valve is located between the right
atrium and the right ventricle. It has three flaps, or cusps, and prevents blood from flowing backward from the right ventricle into the
right atrium when the ventricle contracts. During diastole, when
the right atrium contracts, the tricuspid valve opens, allowing blood
to flow from the right atrium into the right ventricle.
Mitral Valve (Bicuspid Valve): The mitral valve is situated between
the left atrium and the left ventricle. It has two cusps and helps
prevent blood from flowing backward from the left ventricle into
the left atrium when the ventricle contracts. During diastole, when
the left atrium contracts, the mitral valve opens, allowing blood to
flow from the left atrium into the left ventricle.
Semilunar Valves
Pulmonary Valve: The pulmonary valve is located between the right
ventricle and the pulmonary artery, which transports blood to the
lungs for oxygenation. The pulmonary valve has three cusps and
prevents blood from flowing backward from the pulmonary artery
into the right ventricle when the ventricle relaxes. During systole,
when the right ventricle contracts, the pulmonary valve opens, allowing blood to flow from the right ventricle into the pulmonary
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artery.
Aortic Valve: The aortic valve is located between the left ventricle
and the aorta, the main artery that carries oxygen-rich blood from
the heart to the rest of the body. It has three cusps and prevents
blood from flowing backward from the aorta into the left ventricle
when the ventricle relaxes. During systole, when the left ventricle
contracts, the aortic valve opens, allowing blood to flow from the
left ventricle into the aorta.
Valve Function and the Cardiac Cycle
The opening and closing of the heart valves are synchronized with
the cardiac cycle, which consists of two main phases: systole (contraction) and diastole (relaxation).
During systole:
The atrioventricular valves (tricuspid and mitral) close, preventing
blood from flowing back into the atria. The semilunar valves (pulmonary and aortic) open, allowing blood to flow into the pulmonary
artery and aorta.
During diastole:
The atrioventricular valves (tricuspid and mitral) open, allowing
blood to flow from the atria into the ventricles. The semilunar valves
(pulmonary and aortic) close, preventing blood from flowing back
into the ventricles.
The opening and closing of the heart valves are facilitated by changes
in blood pressure within the chambers of the heart, which helps regulate the flow of blood and maintain proper circulation throughout
the body. Understanding the structure and function of the heart
valves is essential for comprehending the cardiovascular system
and the importance of maintaining good heart health.
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Figure 111: A figure showing the heart valves with the major vessels
removed. (Credit: openstax.org)
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22.3.2 The Coronary Arteries
The coronary artery is a blood vessel that carries oxygen-rich blood
to the heart muscle itself. It branches off from the aorta and wraps
around the heart, supplying blood to the heart muscle. The coronary
artery is important for maintaining the health and function of the
heart muscle, and it is essential for the proper functioning of the
heart.
During the systolic phase of the cardiac cycle, the heart muscle
contracts, which causes the chambers of the heart to pump blood
out to the rest of the body. This contraction compresses the coronary
arteries, which can temporarily limit or even cut off blood flow to
the heart muscle itself.
However, this compression of the coronary arteries is only temporary and is actually an important aspect of the cardiac cycle. When
the heart muscle relaxes during diastole, the coronary arteries expand again and fill with oxygen-rich blood. This provides the heart
muscle with the necessary oxygen and nutrients to fuel its ongoing
contractions and keep the heart functioning properly.
So while the coronary arteries do not receive blood flow during
systole, this temporary interruption is essential for the overall functioning of the heart and allows for efficient blood flow to both the
heart and the rest of the body.
22.4 The Cardiac Cycle
The cardiac cycle is the series of events that occurs during each
heartbeat. It begins with the contraction of the heart’s upper chambers, the atria, and ends with the relaxation of the lower chambers,
the ventricles. The cardiac cycle is regulated by a complex system
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of electrical signals and chemical signaling pathways.
The cardiac cycle consists of two main phases: systole and diastole.
During systole, the heart muscle contracts and pumps blood out
to the rest of the body, which temporarily compresses the coronary
arteries and limits blood flow to the heart muscle. During diastole,
the heart muscle relaxes and the chambers fill with blood, which
allows the coronary arteries to expand and supply the heart muscle
with oxygen and nutrients. Overall, the difference between systole
and diastole is that systole is a contraction phase that pumps blood
out of the heart, while diastole is a relaxation phase that allows
blood to flow into the heart and supply the heart muscle with the
necessary oxygen and nutrients.
Here are the specific events that happen during systole and diastole
in the cardiac cycle:
During Systole: The ventricles of the heart contract and push blood
out of the heart and into the circulatory system. The atria of the
heart are in atrial diastole (relaxed) and fill with blood from the veins.
The atrioventricular (AV) valves, the tricuspid valve on the right side
and the mitral valve on the left side, close to prevent backflow of
blood into the atria. The semilunar valves, the pulmonary valve on
the right side and the aortic valve on the left side, open to allow
blood to be ejected from the ventricles and into the arteries.
During Diastole: The ventricles of the heart are in ventricular diastole
(relaxed) and fill with blood from the atria. The atria of the heart
contract and push blood into the ventricles (atrial systole). The AV
valves open to allow blood to flow from the atria into the ventricles.
The semilunar valves close to prevent blood from flowing back into
the ventricles. The coronary arteries, which supply oxygen and nutrients to the heart muscle, fill with blood during this phase as the
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heart muscle relaxes.
Therefore, keep in mind that ”systole” and ”diastole” are general
heart terms that comprise of various events happen in a specific sequence. While both the atria and ventricles undergo systole during
the cardiac cycle, systole in the atria and ventricles does not occur
at the same time. The atria contract during atrial systole, while the
ventricles contract during ventricular systole. This sequence of contraction and relaxation ensures that blood is efficiently pumped
through the heart and into the circulation.
Innervation of the heart is the process by which the heart is regulated by the nervous system. The heart has its own intrinsic electrical
system, called the cardiac conduction system, which coordinates the
contraction of the heart muscle. However, the cardiac conduction
system can be influenced by the sympathetic and parasympathetic
nervous systems.
The sympathetic nervous system increases heart rate and contractility, while the parasympathetic nervous system decreases heart rate
and contractility. This balance of sympathetic and parasympathetic
activity helps to maintain a normal heart rate and blood pressure.
The sympathetic innervation of the heart is primarily via the sympathetic trunk, which carries sympathetic fibers from the spinal
cord to the heart. The parasympathetic innervation of the heart is
via the vagus nerve, which carries parasympathetic fibers from the
brainstem to the heart.
22.5 Arteries, Veins, and Capillaries
Arteries, veins, and capillaries are the three types of blood vessels
that make up the circulatory system. Each type of blood vessel has a
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Figure 112: Overview of the Cardiac Cycle. (Credit: openstax.org)
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specific structure and function, and they work together to transport
blood, oxygen, and nutrients throughout the body.
Arteries are blood vessels that carry oxygenated blood away from
the heart. They have a thick muscular wall, which allows them to
withstand the high pressure of blood being pumped by the heart.
The inner layer of the arterial wall, called the endothelium, is smooth
and helps to prevent blood clotting.
Veins are blood vessels that carry deoxygenated blood back to the
heart. They have a thinner wall compared to arteries and are more
distensible, which allows them to accommodate the large volume
of blood returning to the heart. The walls of veins also have valves,
which prevent the backflow of blood and help to maintain the flow
of blood in a single direction.
As mentioned earlier, keep in mind the only exception to this, wherein
pulmonary arteries and veins are unique in that they carry oxygenpoor blood and oxygen-rich blood, respectively, which is opposite to
what occurs in the rest of the body. The pulmonary arteries carry
deoxygenated blood from the right side of the heart to the lungs,
where it is oxygenated through respiration. The oxygen-rich blood
then flows back to the left side of the heart through the pulmonary
veins and is pumped out to the rest of the body to deliver oxygen to
the tissues.
Capillaries are the smallest type of blood vessels, and they are responsible for exchanging nutrients, oxygen, and waste products
between the blood and the body’s tissues. They have a very thin
wall, only one cell thick, which allows for the exchange of substances
through diffusion.
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22.6 Blood Flow and Blood Pressure Regulation
Blood flow is the movement of blood through the circulatory system.
It is driven by the pumping action of the heart, which pushes blood
through the arteries, veins, and capillaries of the body. The blood
carries oxygen and nutrients to the body’s tissues and removes
waste products, such as carbon dioxide and lactic acid.
Blood pressure is the force of blood against the walls of the blood
vessels. It is measured in millimeters of mercury (mmHg) and is
expressed as two numbers: systolic blood pressure (the highest
pressure when the heart is contracting) and diastolic blood pressure
(the lowest pressure when the heart is relaxed). A normal blood
pressure reading is typically around 120/80 mmHg.
Blood flow and blood pressure are regulated by a complex system of
mechanisms that ensure that the body’s tissues receive an adequate
supply of oxygen and nutrients while also maintaining a normal
blood pressure.
One important factor in blood flow and blood pressure regulation is
the resistance of the blood vessels. The diameter of the blood vessels
can be adjusted through the process of vasodilation (widening) and
vasoconstriction (narrowing). Vasodilation increases blood flow and
reduces resistance, while vasoconstriction does the opposite.
The autonomic nervous system, which is responsible for the body’s
automatic functions, plays a key role in regulating blood flow and
blood pressure. The sympathetic nervous system increases heart
rate and contractility, and also causes vasoconstriction, which leads
to an increase in blood pressure. The parasympathetic nervous
system, on the other hand, decreases heart rate and contractility,
and causes vasodilation, which leads to a decrease in blood pressure.
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Other factors that can affect blood flow and blood pressure include
the volume of blood in the circulatory system, the viscosity (thickness) of the blood, and the elasticity of the blood vessels.
Problems with blood flow and blood pressure regulation can lead
to serious health issues such as hypertension (high blood pressure),
hypotension (low blood pressure), and circulatory problems. Understanding the mechanisms that regulate blood flow and blood
pressure is important for the diagnosis and treatment of these conditions.
22.6.1 Cardiac Physiology
Cardiac physiology focuses on the functioning of the heart and its
role in the circulatory system. Several key parameters are used to
describe and evaluate the heart’s performance, such as stroke volume, cardiac output, heart rate, preload, afterload, and contractility.
Understanding these concepts is crucial for understanding how
the heart works to pump blood throughout the body and maintain
proper circulation.
• Stroke Volume (SV): Stroke volume is the amount of blood pumped
by the left ventricle during each heartbeat. It is typically measured in milliliters (mL) and varies depending on factors such as
heart rate, preload, afterload, and contractility. Stroke volume is
calculated as the difference between the end-diastolic volume
(EDV) and the end-systolic volume (ESV):
• Stroke Volume (SV) = End-Diastolic Volume (EDV) - End-Systolic
Volume (ESV)
• Cardiac Output (CO): Cardiac output is the volume of blood
pumped by the heart per minute. It is an essential indicator of
the heart’s efficiency and is determined by the product of stroke
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volume and heart rate:
• Cardiac Output (CO) = Stroke Volume (SV) × Heart Rate (HR)
• Heart Rate (HR): Heart rate is the number of heartbeats per
minute. A normal resting heart rate for adults ranges from 60 to
100 beats per minute. Heart rate can be influenced by factors
such as age, fitness, emotional state, and the body’s need for
oxygen.
• Preload: Preload refers to the stretching of the ventricular walls
at the end of diastole, just before the heart contracts. It is directly
related to the volume of blood in the ventricles (end-diastolic
volume) and reflects the degree of ventricular filling. According
to the Frank-Starling law, an increase in preload results in an
increase in stroke volume, up to a certain point. This is because
the increased stretch of the ventricular walls allows for a more
forceful contraction.
• Afterload: Afterload is the resistance that the ventricles must
overcome to pump blood into the aorta and pulmonary artery.
It is influenced by factors such as blood pressure, vascular resistance, and ventricular wall tension. Increased afterload can
result in decreased stroke volume because the ventricles have
to work harder to overcome the resistance.
• Contractility: Contractility refers to the inherent ability of the
heart muscle to contract independently of external factors such
as preload and afterload. It is influenced by factors such as calcium levels, the availability of oxygen, and the responsiveness
of the muscle fibers to stimulation. An increase in contractility leads to an increase in stroke volume, while a decrease in
contractility leads to a decrease in stroke volume.
In summary, cardiac physiology encompasses the functioning of
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the heart, with key parameters such as stroke volume, cardiac output, heart rate, preload, afterload, and contractility playing crucial
roles in determining the heart’s performance. Understanding these
concepts is essential for evaluating the cardiovascular system and
maintaining good heart health.
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23 Osmotic Regulation and Excretion
The human body requires a daily intake of eight to ten glasses of
water to maintain proper hydration. This water is eliminated from
the body through urination, defecation, sweating, and respiration.
The fluids that surround the organs and tissues of the body need to
be kept at a consistent temperature, pH, and solute concentration to
maintain homeostasis. These solutes, which are mostly mineral salts
and sugars, are regulated through the process of osmotic regulation.
Osmotic homeostasis must be maintained despite external factors
such as temperature, diet, and weather.
The brain, which is 80% water, has a particularly important role in
osmotic balance. The balance of water and spinal fluid is crucial for
proper brain function, and deviations from this balance can cause
serious medical conditions. Researchers have recently discovered
that the flow of water between brain cells is controlled by water
channels called aquaporin channels, which are regulated by the
neurotransmitter gamma-aminobutyric acid.
23.1 The Lymphatic System
The lymphatic system is a crucial component of the body’s immune
system, which helps to defend against infection and disease. This
complex network consists of lymphatic vessels, lymph nodes, and
various organs, including the spleen, thymus, and tonsils. The lymphatic system works closely with the circulatory system to maintain
the body’s fluid balance and remove waste products from the tissues.
• Lymph vessels: The lymphatic vessels are a system of small,
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thin-walled tubes that transport lymph fluid throughout the
body. These vessels are similar to veins in their structure, with
one-way valves that prevent the backflow of lymph. Lymphatic
vessels collect excess fluid and waste products from the interstitial spaces between cells and transport them to the lymph
nodes for filtration.
• Lymph nodes: Lymph nodes are small, bean-shaped structures
located along the lymphatic vessels. They serve as filtering
stations for the lymph fluid, trapping and removing bacteria,
viruses, and other foreign substances before the fluid is returned
to the bloodstream. Lymph nodes also contain immune cells,
such as lymphocytes and macrophages, which help to recognize
and destroy pathogens.
• Spleen: The spleen is a large, flat organ located in the upper
left part of the abdomen. It plays several important roles in
the lymphatic and circulatory systems. The spleen filters blood,
removing old or damaged red blood cells and recycling their
components. It also serves as a reservoir for platelets and white
blood cells, and plays a vital role in the body’s immune response
by producing lymphocytes and antibodies.
• Thymus: The thymus is a gland located in the upper chest, just
below the sternum. It plays a critical role in the development
and maturation of T-lymphocytes, a type of white blood cell that
is essential for cell-mediated immunity. The thymus is most
active during childhood and adolescence, gradually shrinking
and becoming less functional with age.
• Tonsils and adenoids: These lymphatic tissues are located in the
throat and nasal cavity, respectively. They serve as the first line
of defense against inhaled or ingested pathogens, producing
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lymphocytes and antibodies to help neutralize infections.
• The lymphatic system relies on muscle movement and the rhythmic contraction of blood vessels to circulate lymph fluid, as it
lacks a central pump like the heart. As the muscles contract,
they squeeze the lymph vessels, propelling the fluid through
the system and towards the lymph nodes for filtration.
In addition to its immune functions, the lymphatic system also plays
a crucial role in maintaining fluid balance within the body. It removes excess fluid and waste products from tissues, preventing
swelling and ensuring the proper functioning of cells and organs.
In summary, the lymphatic system is an essential part of the body’s
immune defense and plays a critical role in maintaining fluid balance. It consists of a complex network of vessels, nodes, and organs,
including the spleen, thymus, and tonsils, which work together to
protect the body from infection and disease.
23.2
Osmotic Balance
Osmoregulation is the process by which the body maintains the
balance of water and electrolytes (mineral salts) in the body fluids.
Osmotic balance is important for many body functions, including
maintaining the proper concentration of substances in the blood
and tissues, regulating blood pressure, and supporting the function
of cells and organs.
Osmolarity is a measure of the concentration of dissolved particles in
a solution. It is typically measured in milliosmoles per liter (mOsm/L).
Osmolarity is important in the body because it helps to determine
the movement of water and electrolytes between cells and the
extracellular fluid.
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Figure 113: Overview of the Anatomy of the Lymphatic System (Credit:
openstax.org)
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Osmoregulators are animals that actively regulate their body’s osmotic balance. They do this by adjusting the concentration of electrolytes in their body fluids to match their environment. This allows
them to survive in a wide range of environments, even if the water
and electrolyte concentrations vary.
Osmoconformers, on the other hand, do not actively regulate their
body’s osmotic balance. Instead, they allow their body fluids to
match the osmotic concentration of their environment. This is a less
efficient way to maintain osmotic balance, but it requires less energy
and is found in animals that live in relatively stable environments.
While humans are not typically classified as strict osmoregulators
or osmoconformers, our bodies do have mechanisms to maintain
osmotic balance and respond to changes in the environment. For
example, the kidneys play a crucial role in regulating the body’s
water and electrolyte balance by excreting excess water and electrolytes or retaining them as needed. The body also has mechanisms
to regulate the secretion of hormones that control water balance,
such as antidiuretic hormone (ADH) and aldosterone, in response
to changes in blood pressure and electrolyte levels
23.2.1
Solutions
When two solutions or a solution and a cell have different concentrations of solutes, water will tend to move from the area of higher
water concentration to the area of lower water concentration in a
process called osmosis. The direction of this water movement is
influenced by the relative concentrations of solutes in each solution, and the resulting solution can be categorized as hypertonic,
hypotonic, or isotonic.
Hypertonic solutions have a higher concentration of solutes than
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the other solution or cell. This means that there is a higher osmotic
pressure in the hypertonic solution, and water will tend to move out
of the cell or the other solution and into the hypertonic solution,
causing the cell or other solution to shrink.
Hypotonic solutions have a lower concentration of solutes than the
other solution or cell. This means that there is a lower osmotic
pressure in the hypotonic solution, and water will tend to move into
the cell or the other solution from the hypotonic solution, causing
the cell or other solution to swell or burst.
Isotonic solutions have the same concentration of solutes as the
other solution or cell. This means that the osmotic pressure is the
same, and there will be no net movement of water between the
two solutions or the cell and the solution. The cells or solutions will
remain the same size and shape.
To differentiate between hypertonic, hypotonic, and isotonic solutions, you can use a technique called osmosis. This involves placing
a semipermeable membrane, such as a dialysis bag, between the
solution and a reference solution or a cell. The semipermeable
membrane allows small molecules such as water and electrolytes
to pass through, but it blocks the passage of larger molecules such
as sugars.
If the solution inside the dialysis bag becomes more concentrated
(has less water) than the reference solution or cell, it is hypertonic.
If the solution inside the dialysis bag becomes less concentrated
(has more water) than the reference solution or cell, it is hypotonic.
If there is no change in the concentration of the solution inside the
dialysis bag, it is isotonic.
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23.3
The Structure and Function of the Kidneys
The kidneys are vital organs situated in the abdomen of mammals
that perform several essential functions, including maintaining the
body’s homeostasis, regulating water and electrolyte balance, filtering waste products, and producing hormones. These bean-shaped
organs are key players in the body’s osmoregulatory system, ensuring optimal fluid balance and overall health.
Each kidney is composed of millions of functional units called nephrons,
which work together to filter blood and produce urine. Nephrons
contain two main components: the glomerulus, a network of tiny
blood vessels called capillaries, and the tubule, a tube-like structure
that processes the filtered fluid.
The glomerulus serves as the first stage in blood filtration. In a
process known as glomerular filtration, blood is forced through a
semipermeable membrane, allowing small molecules such as water, electrolytes, glucose, and amino acids to pass into the tubule.
Larger molecules like proteins and red blood cells are unable to pass
through the membrane and remain in the bloodstream.
Following filtration in the glomerulus, the tubule processes the filtered fluid to form urine through two primary processes: tubular
reabsorption and tubular secretion.
Tubular Reabsorption: As the filtrate moves along the tubule, essential substances like glucose, amino acids, and certain electrolytes
are reabsorbed back into the bloodstream. This process occurs
through various mechanisms, including passive diffusion, facilitated
diffusion, and active transport. Tubular reabsorption helps maintain the body’s electrolyte balance and prevents the loss of valuable
nutrients.
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Tubular Secretion: During tubular secretion, waste products and
excess substances are removed from the blood and transported
into the tubule. These substances include excess electrolytes, toxins,
and certain medications. Tubular secretion aids in maintaining the
body’s acid-base balance and ensures the elimination of harmful
substances from the body.
As the filtrate continues through the tubule, it eventually reaches
the collecting duct, where the final adjustments to the fluid composition are made. The collecting ducts from numerous nephrons join
together and drain the processed fluid into the renal pelvis, which
subsequently empties into the ureter. The ureter then transports
the urine to the bladder for temporary storage before it is expelled
from the body through the urethra.
In summary, the kidneys play a critical role in maintaining the body’s
homeostasis and overall health by regulating water and electrolyte
balance, filtering waste products, and producing hormones. The
structure and function of the nephrons within the kidneys are essential to these processes, with the glomerulus filtering blood and the
tubule processing the filtered fluid through tubular reabsorption
and tubular secretion to form urine.
23.4
Anatomy
The kidneys are a pair of bean-shaped organs that play a crucial role
in maintaining the body’s homeostasis. They are composed of three
main layers and three internal regions, which work together to filter
blood, excrete waste products, and regulate fluid and electrolyte
balance.
Renal Fascia: The outer renal fascia is a layer of dense connective
tissue that surrounds the kidneys and provides structural support. It
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also helps to hold the kidneys in place within the abdominal cavity.
Perirenal Fat Capsule: This layer is composed of adipose tissue and
is located directly beneath the renal fascia. The perirenal fat capsule serves as a cushion, protecting the kidneys from mechanical
damage and anchoring them in place.
Renal Capsule: The innermost layer, the renal capsule, is a tough,
fibrous membrane that tightly envelopes the kidney, providing additional protection and maintaining its shape.
The three internal regions of the kidney include:
Renal Cortex: The outermost region, the renal cortex, is characterized by its granular appearance due to the presence of numerous
nephrons. These nephrons are organized into lobes, with each lobe
consisting of a renal pyramid and an adjoining cortical region.
Renal Medulla: The renal medulla lies beneath the cortex and is
composed of multiple tissue masses called renal pyramids. These
pyramids have a conical shape, with the base facing the cortex and
the apex, or renal papilla, pointing towards the renal pelvis. The
renal columns are spaces filled with connective tissue and blood
vessels that separate the renal pyramids.
Renal Pelvis: Located at the hilum, the renal pelvis is an expanded,
funnel-shaped cavity that collects urine produced by the nephrons.
The renal pelvis branches into two or three major calyces, which
further divide into minor calyces that surround the renal papillae
of the pyramids. The minor calyces collect urine and transport it to
the major calyces, which then drains into the renal pelvis.
The kidneys are supplied with blood through an intricate network of
blood vessels. The renal arteries branch from the abdominal aorta
and deliver oxygen-rich blood to the kidneys. Within the kidneys,
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these arteries further subdivide into segmental, interlobar, arcuate, and cortical radiate arteries. Blood is then filtered through the
nephrons and returned to the venous system via a series of veins
that mirror the arterial structure.
Each of the million nephrons within a kidney consists of a glomerulus and a renal tubule. The glomerulus is a capillary network surrounded by Bowman’s capsule, where blood filtration occurs. The
filtered fluid, or filtrate, then travels through the renal tubule, which
is divided into the proximal convoluted tubule, the loop of Henle, and
the distal convoluted tubule. As the filtrate moves through these
segments, various substances are reabsorbed into the bloodstream
or secreted into the tubule for excretion.
The urine produced by the nephrons eventually drains into the minor
and major calyces, then into the renal pelvis, and finally flows into the
ureters. The ureters transport the urine to the urinary bladder, where
it is stored until it is ready to be expelled from the body through the
urethra during the process of micturition.
The kidneys play a vital role in maintaining the body’s balance of
fluids, electrolytes, and hormones. They filter waste products from
the blood, regulate blood pressure, and produce hormones that
help regulate the body’s metabolism. Dysfunction of the kidneys
can lead to a variety of health problems, including kidney disease
and kidney failure.
23.5
The Nephron
The nephron is the functional unit of the kidney, responsible for filtering the blood and producing urine. Each kidney contains around
one million nephrons, and each nephron is made up of several parts
with specific functions.
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Figure 114: Internal Anatomy of the Kidney (Credit: openstax.org)
The glomerulus is a network of tiny blood vessels located at the
beginning of the nephron. Blood flows into the glomerulus and is
filtered by a thin layer of cells called the glomerular endothelium.
The resulting filtrate is collected in a capsule called the Bowman’s
capsule.
The Bowman’s capsule surrounds the glomerulus and is made up
of two layers of cells: the parietal layer, which lines the inside of
the capsule, and the visceral layer, which surrounds the glomerulus.
The filtrate flows from the Bowman’s capsule into the renal tubules,
which are tubes that make up the majority of the nephron.
The renal tubules are divided into several segments, each with a
specific function in the process of producing urine. The proximal
tubule is the first segment of the renal tubule and is responsible
for reabsorbing most of the useful substances, such as glucose and
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amino acids, from the filtrate back into the bloodstream. The loop
of Henle is a longer, looping section of the renal tubule that helps
to concentrate the filtrate by allowing water and electrolytes to be
reabsorbed back into the bloodstream. The distal tubule is the final
segment of the renal tubule and is responsible for regulating the
concentration of electrolytes in the blood by adjusting the amount
of water and electrolytes in the filtrate.
The final step in the process of producing urine is the collection
of the remaining filtrate in a structure called the collecting duct.
The collecting duct empties into the renal pelvis, which is a central
channel in the kidney that collects urine and transports it to the
ureters, tubes that carry urine away from the kidney to the urinary
bladder.
23.5.1 The Proximal Tubule
The proximal tubule is the first segment of the renal tubule in the
nephron, the functional unit of the kidney responsible for filtering
the blood and producing urine. Located in the renal cortex, the
outer region of the kidney, the proximal tubule is responsible for
reabsorbing most of the useful substances, such as glucose and
amino acids, from the filtrate back into the bloodstream.
The filtrate that enters the proximal tubule contains a variety of
substances, including glucose, amino acids, electrolytes, and waste
products. The proximal tubular cells, which line the tubule, have
many microvilli, which are tiny finger-like projections that increase
the surface area of the cell and allow for efficient absorption of
substances from the filtrate. The proximal tubular cells also have
transport proteins, which are specialized proteins that help transport
specific substances across the cell membrane.
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Figure 115: This figure shows the nephron and the corresponding
blood supply. (Credit: openstax.org)
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Approximately 65-75% of the filtered glucose is reabsorbed in the
proximal tubule, along with about 75-85% of the filtered amino acids.
Most of the electrolytes, such as sodium, potassium, and calcium,
are also reabsorbed in the proximal tubule, with the exception of a
small amount of hydrogen ions, which are secreted into the tubule
to help regulate the pH of the blood. Waste products, such as urea
and creatinine, are not reabsorbed and remain in the filtrate as it
passes through the tubule.
For the IMAT exam, it is important to know that in a healthy human,
Glucose will be 100% reabsorbed back to the blood stream.
The amount of glucose that is reabsorbed in the nephron (the filtering units of the kidney) depends on several factors, including the
concentration of glucose in the filtrate, the presence of hormones
such as insulin and glucagon, and the activity of transporters in the
nephron that mediate glucose reabsorption.
Under normal conditions, the vast majority of the glucose that is
filtered by the kidney is reabsorbed in the proximal tubule, which is
the first part of the nephron. This is because the concentration of glucose in the filtrate is typically much higher than the concentration
of glucose in the plasma, which creates a concentration gradient
that drives the movement of glucose from the filtrate into the blood.
The reabsorption of glucose in the nephron is mediated by specialized transporters called SGLT2 (sodium-glucose cotransporter 2) and
GLUT2 (glucose transporter 2), which are located in the brush border
of the proximal tubule cells. These transporters move glucose from
the filtrate into the cells of the nephron, where it is then released
into the bloodstream.
The rate of glucose reabsorption in the nephron can be regulated
by several hormones, including insulin, which promotes glucose
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uptake, and glucagon, which promotes the breakdown of glycogen
and the release of glucose into the bloodstream.
Like glucose, the reabsorption of amino acids in the nephron is
mediated by specialized transporters located in the brush border of
the proximal tubule cells.
The reabsorption of amino acids in the nephron is important for
maintaining the balance of amino acids in the body and for conserving valuable nutrients. The reabsorption of amino acids is also
regulated by several hormones, including insulin, which promotes
amino acid uptake, and glucagon, which promotes the breakdown
of amino acids and the release of amino acids into the bloodstream.
Under normal conditions, the vast majority of the amino acids that
are filtered by the kidney are reabsorbed in the proximal tubule.
However, the specific amino acids that are reabsorbed and the exact
amounts that are reabsorbed can vary depending on the individual’s
nutritional status, the presence of certain hormones, and other
factors.
The reabsorption of electrolytes in the nephron (the filtering units of
the kidney) is an important process that helps to maintain the balance of electrolytes in the body and to regulate the volume and composition of the body’s fluids. Electrolytes are ions that can conduct
electricity when dissolved in water, and they include substances
such as sodium, potassium, calcium, and chloride.
The reabsorption of electrolytes in the nephron occurs primarily
in the proximal tubule, which is the first part of the nephron. The
reabsorption of electrolytes is mediated by specialized transporters
located in the brush border of the proximal tubule cells. The specific
electrolytes that are reabsorbed and the exact amounts that are
reabsorbed can vary depending on the individual’s needs and the
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presence of certain hormones. For example, the hormone aldosterone promotes the reabsorption of sodium and the secretion of
potassium in the distal tubule, which helps to maintain the proper
balance of these electrolytes in the body.
23.5.2 The Loop of Henle
The loop of Henle is a U-shaped structure located in the medulla
of the kidney, descending into the inner region (the renal medulla)
from the PCT, then ascending back to the cortex. It plays a key role
in the regulation of water and electrolyte balance in the body.
The loop of Henle consists of a descending limb and an ascending
limb. The descending limb is permeable to water, which allows water
to passively diffuse out of the nephron and into the surrounding
interstitial fluid. However, the descending limb is impermeable to
electrolytes such as sodium and chloride ions.
In contrast, the ascending limb is impermeable to water but is permeable to electrolytes, including sodium and chloride ions. These
electrolytes are actively transported out of the ascending limb and
into the interstitial fluid, which helps to create a concentration gradient that drives the reabsorption of water from the descending limb
in the next part of the nephron
The main function of the loop of Henle is to regulate the concentration of the interstitial fluid in the medulla of the kidney. This
is accomplished through a process called countercurrent multiplication, which involves the movement of water and electrolytes
through the loop of Henle.
As the filtrate (the fluid that has been filtered by the kidney) flows
through the descending limb of the loop of Henle, it comes into contact with the concentrated interstitial fluid in the medulla. Because
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the descending limb is permeable to water, but not to electrolytes,
water is able to move from the filtrate into the interstitial fluid, while
the electrolytes remain in the filtrate. This process allows the concentration of electrolytes in the filtrate to increase, while the volume
of the filtrate decreases.
As the filtrate continues to flow up the ascending limb of the loop
of Henle, it comes into contact with the more dilute interstitial fluid
in the cortex of the kidney. Because the ascending limb is permeable to electrolytes, they are able to move from the filtrate into the
interstitial fluid. This helps to further dilute the interstitial fluid in
the cortex. This process also helps to maintain the concentration
gradient that was established by the movement of water out of the
descending limb, which is necessary for the reabsorption of water
later in the nephron.
The macula densa, which is located between the descending and
ascending limbs, plays a role in regulating the flow of filtrate through
the nephron. It is a specialized region of the nephron that is sensitive
to changes in the concentration and volume of the filtrate. When
the concentration or volume of the filtrate increases, the macula
densa increases the flow of filtrate through the nephron, and when
the concentration or volume of the filtrate decreases, it decreases
the flow of filtrate.
In summary, the loop of Henle is a U-shaped structure in the nephron
that plays a key role in the regulation of water and electrolyte balance in the body. It consists of a descending limb and an ascending
limb, which are separated by the macula densa. The main function
of the loop of Henle is to regulate the concentration of the interstitial
fluid in the medulla of the kidney through a process called countercurrent multiplication. The macula densa plays a role in regulating
the flow of filtrate through the nephron.
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23.5.3
Distal Convoluted Tubule
The distal convoluted tubule (DCT) is a section of the nephron (the
filtering units of the kidney) that plays a key role in the regulation
of electrolyte balance in the body. It is located in the cortex of the
kidney, which is the outer layer of the kidney tissue.
The DCT is a small tube that is characterized by its convoluted
(twisted) shape, which increases its surface area and allows for more
efficient reabsorption of electrolytes. The DCT is lined with specialized cells called distal tubule cells, which are responsible for the
reabsorption of electrolytes from the filtrate (the fluid that has been
filtered by the kidney).
The main function of the DCT is to reabsorb electrolytes, such as
sodium, potassium, and calcium, from the filtrate and to secrete
excess electrolytes, such as hydrogen ions, into the filtrate. The reabsorption and secretion of electrolytes in the DCT is regulated by
several hormones, including aldosterone, which promotes the reabsorption of sodium and the secretion of potassium, and parathyroid
hormone, which promotes the reabsorption of calcium.
In addition to its role in electrolyte balance, the DCT also plays a role
in the regulation of blood pressure. The reabsorption of sodium in
the DCT helps to maintain the proper volume of extracellular fluid
in the body, which in turn helps to regulate blood pressure.
The DCT can be divided into several different regions, including the
macula densa, which is a specialized region of the nephron that is
sensitive to changes in the concentration and volume of the filtrate,
and the collecting duct, which is a tube that collects the filtrate from
the nephron and transports it to the renal pelvis, where it is then
excreted from the body.
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23.5.4 The Collecting Duct and ADH
The collecting duct is a tube that collects the filtrate (the fluid that
has been filtered by the nephron, the filtering units of the kidney)
from the nephron and transports it to the renal pelvis, where it is
then excreted from the body. It is located in the cortex of the kidney,
which is the outer layer of the kidney tissue.
The collecting duct is lined with specialized cells called collecting
duct cells, which are responsible for the reabsorption and secretion of electrolytes and water. The reabsorption and secretion of
electrolytes and water in the collecting duct is regulated by several
hormones, including antidiuretic hormone (ADH), which promotes
the reabsorption of water, and aldosterone, which promotes the
reabsorption of sodium and the secretion of potassium.
The main function of the collecting duct is to regulate the concentration and volume of the urine that is excreted from the body. By
reabsorbing water and electrolytes from the filtrate, the collecting
duct helps to conserve valuable nutrients and to maintain the proper
balance of electrolytes in the body.
The collecting duct can be subdivided into two main parts: the cortical collecting duct (CCD) and the medullary collecting duct (MCD).
The CCD is the initial part of the collecting duct and is located in
the renal cortex. It receives fluid from the distal convoluted tubule
(DCT) of several nephrons. The primary function of the CCD is to
reabsorb sodium ions and water from the fluid and to secrete potassium ions into it. The MCD, on the other hand, is the part of the
collecting duct that extends from the cortex to the medulla of the
kidney. It receives fluid from the CCD and further concentrates the
urine through water reabsorption and urea recycling. The MCD is
critical in regulating water balance and electrolyte concentration in
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the body.
Antidiuretic hormone (ADH), also known as vasopressin, is a hormone that is produced by the hypothalamus and released by the
posterior pituitary gland. It plays a key role in the regulation of water
balance in the body by promoting the reabsorption of water from
the filtrate in the collecting duct. When the body is dehydrated,
ADH levels increase, which promotes the reabsorption of water and
helps to conserve body fluids. When the body is properly hydrated,
ADH levels decrease, which promotes the excretion of excess water
in the urine.
In summary, the collecting duct is a tube that collects the filtrate
from the nephron and transports it to the renal pelvis, where it is
then excreted from the body. It is lined with specialized cells called
collecting duct cells, which are responsible for the reabsorption and
secretion of electrolytes and water. The collecting duct plays a key
role in the regulation of the concentration and volume of the urine
that is excreted from the body, and is regulated by hormones such
as ADH and aldosterone.
23.6 The Urea Cycle
The urea cycle, also known as the ornithine cycle or the KrebsHenseleit cycle, is a series of chemical reactions that occurs in the
liver and is responsible for the production and excretion of urea, a
waste product of protein metabolism. The main purpose of the urea
cycle in the body is to remove toxic ammonia, which is produced
by the breakdown of proteins, from the bloodstream and convert it
into less toxic urea, which can be excreted in urine.
The urea cycle begins with the conversion of amino acids, which are
the building blocks of proteins, into ammonia. Ammonia is a toxic
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compound that is produced as a byproduct of protein metabolism
and must be removed from the body..
The first step in the urea cycle is the conversion of ammonia into
another compound called carbamoyl phosphate by the enzyme
carbamoyl phosphate synthase. Carbamoyl phosphate is then combined with ornithine, by the enzyme ornithine transcarbamylase
(OTC), to form citrulline, which is the second intermediate in the
urea cycle.
The third intermediate in the urea cycle is argininosuccinate, which is
produced from citrulline by the enzyme argininosuccinate synthase.
Argininosuccinate is then broken down into arginine and fumarate
by the enzyme argininosuccinate lyase.
The final step in the urea cycle is the conversion of arginine into urea
by the enzyme arginase. Urea is then excreted from the body in the
urine, where it helps to remove excess nitrogen from the body.
The urea cycle plays a vital role in the body’s metabolism by helping
to remove excess nitrogen, a waste product of protein metabolism,
from the body. It is important for maintaining the proper balance of
nitrogen in the body and for preventing the accumulation of toxic
compounds such as ammonia.
In summary, the urea cycle is a series of chemical reactions that
occurs in the liver and is responsible for the production and excretion
of urea, a waste product of protein metabolism. The urea cycle
begins with the conversion of amino acids into ammonia and ends
with the conversion of arginine into urea. The urea cycle plays a vital
role in the body’s metabolism by helping to remove excess nitrogen
from the body and by maintaining the proper balance of nitrogen
in the body.
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23.6.1
Hormonal Osmoregulation
Osmoregulation is the process by which the body maintains the
proper balance of water and electrolytes (such as sodium, potassium,
and chloride) in the body. This is important because the body’s cells
need a specific balance of these substances to function properly.
Hormones play a key role in osmoregulation, as they help to regulate
the movement of water and electrolytes in and out of cells.
The RAAS system helps to regulate blood pressure by increasing the
production of a hormone called angiotensin, which constricts blood
vessels, raises blood pressure, and promotes the retention of salt and
water in the kidneys. This system is activated when blood pressure
is low, and it helps to maintain adequate blood flow to vital organs
in the body. The process starts with the release of an enzyme called
renin by the kidneys. Renin then cleaves a protein produced by the
liver to form angiotensin I, which is converted to angiotensin II in
the lungs. Angiotensin II has several effects that help to raise blood
pressure, including vasoconstriction and stimulation of aldosterone
production. The RAAS system is a complex mechanism that plays a
critical role in maintaining blood pressure and fluid balance in the
body.
Aldosterone as mentioned, is a hormone that is involved in the
RAAS and is produced by the adrenal glands. It helps to regulate
electrolyte balance in the body by increasing the reabsorption of
sodium and the excretion of potassium in the kidneys. This can help
to raise blood pressure and maintain proper electrolyte balance.
Atrial natriuretic peptide (ANP) is a hormone that is produced by the
heart and helps to regulate blood pressure and electrolyte balance.
It is released in response to high blood pressure or high blood volume, and helps to lower blood pressure by increasing the excretion
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of sodium and water in the urine.
It has already been mentioned in the section that details the function of the kidneys, but to reiterate, ADH is also an important aspect
of hormonal osmoregulation. ADH is produced by the hypothalamus and released by the posterior pituitary gland in response to an
increase in blood osmolarity or a decrease in blood volume. ADH
acts on the kidneys to promote the reabsorption of water and reduce urine output, thereby helping to maintain water balance in
the body.
Overall, these hormones play important roles in the body’s osmoregulation, helping to maintain proper electrolyte balance and blood
pressure. Dysregulation of these hormones can lead to imbalances
in electrolyte levels and blood pressure, which can have serious
health consequences.
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24 The Immune System
The immune system is a complex network of cells, tissues, and organs that work together to protect the body against infection and
disease. It is divided into two main branches: the innate immune
system and the adaptive immune system. The innate immune
system is the body’s first line of defense and includes physical barriers such as skin and mucous membranes, as well as cells such
as macrophages and neutrophils. The adaptive immune system is
slower to respond, but is more targeted and specific in its defense
against foreign substances. It is made up of white blood cells called
B cells and T cells, which produce antibodies to help identify and
destroy pathogens. In this chapter, we will focus on the most important aspects of the immune system that have been tested on past
IMAT exams, such as B and T cells, antibody production, and the
primary and secondary immune response. However, it is important
to note that the immune system is a vast and complex subject that
is often tested in greater detail on other admission exams.
24.1 The Innate Immune Response
The innate immune response is the body’s first line of defense
against infections and diseases. It is a non-specific response, meaning that it is not targeted to a specific pathogen, but rather provides
a general defense against a wide range of foreign substances. There
are several different components of the innate immune response,
including physical and chemical immune barriers, immediate and
induced immune responses, and natural killer cells.
Physical and chemical immune barriers are the body’s first line
of defense against pathogens. Physical barriers include skin and
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mucous membranes, which provide a physical barrier that prevents
pathogens from entering the body. Chemical barriers include sweat,
tears, and mucus, which contain enzymes and other substances
that can kill or inhibit the growth of pathogens.
The immediate immune response includes the activation of cells
such as macrophages and neutrophils, which can engulf and destroy
pathogens. These cells are activated as soon as a pathogen enters
the body, and they work to kill and remove the pathogen as quickly
as possible.
The induced immune response involves the activation of more specialized cells, such as dendritic cells, which are responsible for activating the adaptive immune response. Dendritic cells present
pieces of the pathogen to T cells, which recognize the pathogen
and trigger a more targeted and specific response.
Natural killer cells are a type of white blood cell that plays a key
role in the immune response. They are able to recognize and kill
abnormal cells, such as cancer cells and infected cells, without the
need for specific recognition.
Major histocompatibility class I (MHC-I) molecules are proteins found
on the surface of all cells in the body. They are used by the immune
system to identify the body’s own cells and distinguish them from
foreign cells.
The complement system is a group of proteins that work together
to destroy extracellular pathogens. When activated, the proteins in
the complement system can form a pore in the cell membrane of
the pathogen, causing the pathogen to leak and ultimately leading
to its death.
In summary, the innate immune response provides a rapid and
non-specific defense against a wide range of pathogens. It includes
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physical and chemical barriers, immediate and induced immune responses, natural killer cells, major histocompatibility class I molecules,
and the complement system, all of which work together to protect
the body against infection and disease.
24.1.1 Cytokines
Cytokines are small proteins that are released by immune cells and
other cells in response to various stimuli, such as infections or inflammation. They play a critical role in the immune response by
regulating the activity of immune cells and coordinating the immune response.
There are many different types of cytokines, including interleukins,
interferons, and tumor necrosis factors, each with specific functions.
Interleukins are proteins that are involved in the communication
between immune cells, and they play a role in activating and regulating immune cells. Interferons are proteins that are produced
by virus-infected cells and help to protect neighboring cells from
infection. Tumor necrosis factors are proteins that help to kill cancer
cells and are involved in the immune response to infections.
The release of cytokines can have both positive and negative effects
on the body. On the one hand, cytokines play a vital role in activating and coordinating the immune response, which is necessary to
protect the body against infections and diseases. On the other hand,
the excessive release of certain cytokines can lead to inflammation,
which can be harmful if not properly controlled.
For example, cytokines such as interleukin-1 and tumor necrosis
factor-alpha can cause inflammation, which can be beneficial in
the short term by helping to kill pathogens and remove damaged
tissue. However, if the inflammation becomes chronic, it can cause
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tissue damage and lead to a variety of health problems, such as
autoimmune diseases, allergies, and cancer.
24.1.2
Phagocytosis and Inflammation
Phagocytosis is a process by which immune cells called phagocytes
ingest and destroy foreign substances, such as bacteria and other
pathogens. Phagocytes are specialized cells that are able to recognize and engulf foreign substances, and they play a crucial role
in the immune response. There are two main types of phagocytes:
neutrophils and macrophages.
Neutrophils are the most common type of phagocyte, and they are
responsible for protecting the body against infections. They are able
to recognize and engulf bacteria and other pathogens, and they use
a variety of chemical and physical methods to destroy them.
Macrophages are another type of phagocyte, and they are found
in various tissues throughout the body. They are larger and longerlived than neutrophils, and they play a key role in the immune response to infections and other foreign substances. Macrophages
are able to recognize and engulf pathogens, and they also release
chemical signals that help to activate other immune cells.
Inflammation is a process that occurs in response to tissue damage or infection. It is characterized by swelling, redness, heat, and
pain, and it is a key component of the immune response. Inflammation is triggered by the release of chemical signals called cytokines,
which are produced by immune cells such as macrophages and
neutrophils.
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vation of immune cells, and it helps to remove damaged tissue
and pathogens from the body. Chronic inflammation, on the other
hand, is a long-term response that can last for weeks, months, or
even years. It is often associated with autoimmune diseases and
other chronic conditions, and it can lead to tissue damage and other
health problems if not properly controlled.
24.1.3
Natural Killer Cells
Natural killer (NK) cells are a type of white blood cell that plays a key
role in the immune response. They are part of the innate immune
system, which is the body’s first line of defense against infections
and other foreign substances. NK cells are able to recognize and kill
abnormal cells, such as cancer cells and infected cells, without the
need for specific recognition.
NK cells are able to recognize and kill target cells through a process
called cytotoxic activity or cytotoxic function, which involves the
release of cytotoxic granules containing perforin and granzymes that
induce apoptosis in the target cell. In addition to their direct killing
ability, NK cells also release chemical signals called cytokines, which
help to activate other immune cells and coordinate the immune
response. NK cell activation can be triggered by a variety of stimuli,
including infections, inflammation, and stress.
NK cells also play a role in the regulation of the immune response.
They are able to inhibit the activation of other immune cells, such as
T cells and B cells, which helps to prevent overactive or inappropriate
immune responses.
NK cells are important for maintaining overall health and wellness,
as they help to protect the body against infections, cancer, and
other diseases. They are also involved in the immune response to
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transplanted organs and tissue, and they play a role in preventing
the rejection of transplanted organs.
Major histocompatibility class I (MHC I) molecules are proteins that
are found on the surface of all cells in the body. They are used by
the immune system to identify the body’s own cells and distinguish
them from foreign cells. MHC I molecules are present on all nucleated cells, which are cells that contain a nucleus, and they play a
key role in the immune response.
NK cells are able to recognize MHC I molecules on the surface of
cells, and they use this information to determine whether a cell is
normal or abnormal. If an NK cell recognizes that a cell is expressing
MHC I molecules, it assumes that the cell is normal and leaves it
alone. If an NK cell does not recognize MHC I molecules, it assumes
that the cell is abnormal and kills it.
24.1.4
The Complement System
The complement system is a group of proteins that work together
to destroy extracellular pathogens, such as bacteria and viruses. It
is part of the innate immune system, which is the body’s first line
of defense against infections and other foreign substances. The
complement system is activated in response to the presence of a
pathogen, and it helps to kill the pathogen and remove it from the
body.
There are three main pathways by which the complement system
can be activated: the classical pathway, the lectin pathway, and the
alternative pathway. The classical pathway is activated by antibodies that are specific for the pathogen, and it involves the sequential
activation of several different proteins. The lectin pathway is activated by proteins called lectins, which are found in certain plant and
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animal products, and it involves the activation of a single protein
called mannan-binding lectin. The alternative pathway is activated
in the absence of antibodies, and it involves the activation of a single
protein called factor B.
Once activated, the complement system proteins work together to
attack the pathogen. They can form a pore in the cell membrane of
the pathogen, causing the pathogen to leak and ultimately leading
to its death through the formation of a membrane attack complex.
In addition to the membrane attack complex, they can also opsonize
or coat the pathogen, making it more visible to other immune cells
and making it easier to remove. The complement system also plays
a role in inflammation, as it can release chemical signals called
anaphylatoxins, which trigger the release of histamine and other
chemicals that cause swelling, redness, and heat.
In summary, the complement system is a group of proteins that
work together to destroy extracellular pathogens and remove them
from the body. It can be activated through three different pathways, and it plays a role in both killing pathogens and triggering
inflammation.
It is not necessary to memorize the complement system in detail
for the IMAT exam, but it is a topic that you may encounter again
during your medical studies. Don’t worry about memorizing all the
details for now, and focus on other areas of the exam. Good luck!
24.2
The Adaptive Immune Response
The adaptive immune response is the body’s second line of defense
against infections and other foreign substances. It is a specific and
targeted response, meaning that it is tailored to a specific pathogen
or foreign substance. The adaptive immune response is slower to
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Figure 116: The complement cascade and function. (Credit: openstax.org)
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develop than the innate immune response, but it provides a more
long-lasting and effective defense against infections and diseases.
There are two main types of adaptive immunity: cell-mediated immunity and humoral immunity.
Cell-mediated immunity is a type of immunity that involves the
activation of immune cells called T cells. T cells are responsible for
attacking infected cells directly, and they are also involved in the
regulation of other immune cells. There are two main types of T cells:
CD4+ T cells and CD8+ T cells. CD4+ T cells are also known as helper
T cells, and they are responsible for activating other immune cells,
including B cells and macrophages. CD8+ T cells are also known as
cytotoxic T cells, and they are responsible for killing infected cells
directly.
Humoral immunity is a type of immunity that involves the production of antibodies by immune cells called B cells. Antibodies are
proteins that are specific for a particular pathogen or foreign substance, and they help to identify and neutralize the pathogen. B
cells are activated by T cells, and they produce antibodies that are
specific for the pathogen. The antibodies can bind to the surface of
the pathogen, marking it for destruction by other immune cells.
The adaptive immune response is an important part of the body’s
defense against infections and diseases, and it provides a more longlasting and effective protection than the innate immune response.
It involves the activation of T cells and B cells, which work together
to identify and destroy pathogens and foreign substances.
In summary, the adaptive immune response is the body’s second
line of defense against infections and other foreign substances. It
is a specific and targeted response that is tailored to a particular
pathogen or foreign substance, and it provides a more long-lasting
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and effective defense than the innate immune response. The adaptive immune response involves the activation of T cells and B cells,
which work together to identify and destroy pathogens and foreign
substances.
24.2.1 B and T cells
B cells and T cells are two types of white blood cells that play a key
role in the adaptive immune response. They are responsible for
identifying and attacking foreign substances, such as bacteria and
viruses, and they work together to provide a more effective immune
response.
B cells, also known as B lymphocytes, are responsible for producing antibodies, which are proteins that are specific for a particular pathogen or foreign substance. When a B cell encounters a
pathogen, it is activated and begins to produce antibodies that are
specific for that pathogen. The antibodies can bind to the surface
of the pathogen, marking it for destruction by other immune cells.
T cells, also known as T lymphocytes, are responsible for attacking
infected cells directly. There are two main types of T cells: CD4+ T
cells and CD8+ T cells. CD4+ T cells, also known as helper T cells,
are involved in activating other immune cells, including B cells and
macrophages. They are activated by dendritic cells, which present
pieces of the pathogen to the T cells. Once activated, the CD4+ T cells
release chemical signals called cytokines, which help to coordinate
the immune response. CD8+ T cells, also known as cytotoxic T cells,
are responsible for killing infected cells.
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24.2.2
Antibodies
Antibodies, also known as immunoglobulins, are proteins that are
produced by the immune system in response to the presence of
foreign substances, such as bacteria, viruses, and toxins. Antibodies are specific for a particular foreign substance, and they help to
identify and neutralize the substance.
An antigen is a foreign substance that stimulates the production
of antibodies. Antigens can be proteins, carbohydrates, or lipids,
and they are typically found on the surface of pathogens or other
foreign substances. Antigens are recognized by the immune system
as being foreign, and they trigger the production of antibodies to
attack and neutralize the substance.
An epitope is a specific region on an antigen that is recognized by
an antibody. Epitopes are typically small, typically comprising only a
few amino acids, and they are located on the surface of the antigen.
Each antibody is specific for a particular epitope, and it is able to
bind to the epitope and neutralize the antigen.
Here are the five main types of antibodies, also known as immunoglobulins, that are produced by the immune system:
IgG: Immunoglobulin G (IgG) is the most abundant immunoglobulin
in the blood, accounting for approximately 3/4 of all antibodies. It
provides long-term protection against infections by recognizing
and binding to specific pathogens, marking them for destruction
by other cells in the immune system. IgG is also able to cross the
placenta, providing passive immunity to the fetus. Additionally, IgG
can activate the complement system, which helps to enhance the
immune response against pathogens. IgG can also bind to and
neutralize toxins and viruses, preventing them from causing harm
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to the body.
IgM: Immunoglobulin M (IgM) is the first antibody produced in response to an infection. It is primarily found in the blood and exists
as a pentamer, with five units linked together by a J chain. IgM
is efficient at activating the complement system, which helps to
enhance the immune response against pathogens. Additionally,
IgM has multiple binding sites, allowing it to bind more effectively
to certain pathogens. IgM levels in the blood are often used as a
marker of recent or ongoing infection.
IgA: Immunoglobulin A (IgA) is found in high levels in secretions
such as tears, saliva, and breast milk. It helps to protect mucosal
surfaces from infections by binding to and neutralizing pathogens.
IgA can exist as a monomer, which is the most common form, or as
a dimer, which is the primary form found in secretions. Additionally,
IgA can activate the innate immune response, helping to recruit
other immune cells to the site of infection.
IgE: Immunoglobulin E (IgE) is involved in allergic reactions and
defense against parasites. IgE binds to mast cells and basophils,
triggering the release of histamine and other chemicals that cause
inflammation and other symptoms of an allergic reaction. IgE levels
are typically low in the blood, but they can become elevated in
response to certain infections and diseases. In addition to its role
in allergic reactions, IgE also helps to defend against parasites by
binding to and neutralizing them.
IgD: Immunoglobulin D (IgD) is primarily found on the surface of B
cells, where it functions as a receptor for antigen. The exact function
of IgD is not yet fully understood, but it is thought to play a role in B
cell activation and differentiation. IgD is the least abundant of the
immunoglobulins, with levels in the blood typically less than 1
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In summary, there are five main types of antibodies that are produced by the immune system, and each has a specific role in the
immune response. IgM is the first type of antibody to be produced
in response to an infection or other foreign substance, and it has a
high affinity for antigens. IgG is the most common type of antibody
in the body, and it is produced in large quantities in response to an
infection or other foreign substance. IgA is found in mucosal tissues
and plays a key role in protecting these tissues from infections. IgE
is involved in the immune response to allergies, and it is able to
bind to allergens and trigger the release of histamine and other
chemicals that cause allergic symptoms. IgD is a relatively rare type
of antibody, and its function is not fully understood.
24.2.3
Antigen-Presenting Cells (APCs)
Antigen-presenting cells (APCs) are immune cells that are responsible for presenting pieces of pathogens or other foreign substances
to T cells. APCs are a key component of the immune system, as
they help to activate T cells and coordinate the immune response.
There are several different types of APCs, including dendritic cells,
macrophages, and B cells.
Dendritic cells are a type of APC that is found in various tissues
throughout the body. They are responsible for capturing and processing antigens, and they present pieces of the antigens to T cells.
Dendritic cells are able to recognize a wide range of antigens, and
they are able to present the antigens to T cells through a process
called antigen presentation.
Macrophages are another type of APC that is found in various tissues throughout the body. They are responsible for phagocytosis,
which is the process of ingesting and destroying foreign substances.
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Macrophages are able to recognize and engulf pathogens, and they
are able to present pieces of the pathogens to T cells through antigen presentation.
B cells are a type of APC that is responsible for producing antibodies.
When a B cell encounters a pathogen, it is activated and begins
to produce antibodies that are specific for that pathogen. The antibodies can bind to the surface of the pathogen, marking it for
destruction by other immune cells.
Remember, there are several different types of phagocytes in the immune system, including neutrophils, monocytes, and macrophages.
Neutrophils are a type of white blood cell that is responsible for
defending against infections. They are the most abundant type of
phagocyte in the body, and they are able to recognize and engulf
bacteria and other pathogens. Neutrophils are short-lived cells, and
they are rapidly replaced by the bone marrow when they are no
longer needed.
Monocytes are a type of white blood cell that is responsible for defending against infections. They are larger than neutrophils and
have a longer lifespan, and they are able to recognize and engulf
bacteria and other pathogens. Monocytes are able to differentiate
into macrophages, which are a type of phagocyte that is found in
various tissues throughout the body.
Macrophages are a type of phagocyte that is found in various tissues
throughout the body. They are responsible for phagocytosis, and
they are able to recognize and engulf bacteria and other pathogens.
Macrophages are also involved in the activation of other immune
cells, such as T cells and B cells, and they are able to present pieces
of pathogens to these cells through a process called antigen presentation.
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24.2.4
Immunological Memory
Immunological memory is the ability of the immune system to
remember a previous encounter with a pathogen or other foreign
substance and respond more quickly and effectively to a subsequent
encounter. It is an important aspect of the immune response, as it
allows the body to provide long-lasting protection against infections
and diseases.
There are two main types of immunological memory: innate immune memory and adaptive immune memory.
Innate immune memory is the ability of the innate immune system
to remember a previous encounter with a pathogen or other foreign
substance and respond more quickly and effectively to a subsequent
encounter. It is a short-lived form of memory, and it is characterized
by the activation of certain immune cells, such as natural killer cells
and macrophages, in response to a pathogen.
Adaptive immune memory is the ability of the adaptive immune
system to remember a previous encounter with a pathogen or other
foreign substance and respond more quickly and effectively to a
subsequent encounter. It is a long-lived form of memory, and it is
characterized by the activation of immune cells called memory T
cells and memory B cells in response to a pathogen. Memory T cells
and memory B cells are able to recognize and attack a pathogen
more quickly and effectively than naive T cells and B cells, which
have not encountered the pathogen before.
Immunological memory is an important aspect of the immune
response, as it allows the body to provide long-lasting protection
against infections and diseases. It involves the activation of memory
T cells and memory B cells, which are able to recognize and attack a
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pathogen more quickly and effectively than naive T cells and B cells.
Memory B cells are a type of immune cell that is involved in the
adaptive immune response. They are produced in response to a
previous encounter with a pathogen or other foreign substance, and
they are able to recognize and attack the pathogen more quickly
and effectively than naive B cells, which have not encountered the
pathogen before.
Memory B cells are produced as a result of the activation of B cells
during an immune response. When a B cell encounters a pathogen,
it is activated and begins to produce antibodies that are specific
for that pathogen. The antibodies can bind to the surface of the
pathogen, marking it for destruction by other immune cells. Some
of the activated B cells become memory B cells, which are able to
survive for long periods of time in the body.
Memory B cells are important for providing long-lasting protection
against infections and diseases. When a person is re-exposed to a
pathogen, the memory B cells are able to recognize and attack the
pathogen more quickly and effectively than naive B cells. This allows
the body to mount a more rapid and effective immune response,
and it helps to prevent or control the infection.
Memory T cells are a type of immune cell that is involved in the
adaptive immune response. They are produced in response to a
previous encounter with a pathogen or other foreign substance, and
they are able to recognize and attack the pathogen more quickly
and effectively than naive T cells, which have not encountered the
pathogen before.
There are two main types of memory T cells: CD4+ memory T cells
and CD8+ memory T cells. CD4+ memory T cells, also known as
helper T cells, are responsible for activating other immune cells,
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including B cells and macrophages. CD8+ memory T cells, also
known as cytotoxic T cells, are responsible for killing infected cells
directly.
Memory T cells are produced as a result of the activation of T cells
during an immune response. When a T cell encounters a pathogen,
it is activated and begins to attack the pathogen. Some of the
activated T cells become memory T cells, which are able to survive
for long periods of time in the body.
Memory T cells are important for providing long-lasting protection
against infections and diseases. When a person is re-exposed to a
pathogen, the memory T cells are able to recognize and attack the
pathogen more quickly and effectively than naive T cells. This allows
the body to mount a more rapid and effective immune response,
and it helps to prevent or control the infection.
24.3
Primary and Secondary Immune Response
There are two types of immune responses that involve the production of antibodies: primary and secondary immune responses.
The primary immune response is the initial response of the immune
system to an invading pathogen. When a pathogen enters the
body for the first time, it is recognized by the immune system as
foreign and the body’s immune cells mount an attack to eliminate
the invader. The primary immune response takes several days to
develop as the immune cells need time to recognize the pathogen
and produce antibodies. The antibodies produced during the primary immune response are generally of low affinity and may not
be sufficient to completely eliminate the pathogen. However, the
immune cells remember how to recognize and attack the pathogen,
so the next time the same pathogen enters the body, the immune
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system can mount a more rapid and effective response.
The secondary immune response is the immune response that occurs when the immune system encounters a pathogen that it has
encountered before. This response is faster and more effective than
the primary immune response because the immune cells have already been primed to recognize and attack the pathogen. During
the secondary immune response, memory B cells are activated
and produce antibodies of higher affinity and in greater quantities
than during the primary immune response. The memory T cells are
also activated, which helps to eliminate infected cells more quickly.
The secondary immune response is responsible for the long-term
immunity that is generated after a primary immune response.
24.3.1
Lymph Nodes
The lymph nodes are small, bean-shaped structures that are found
throughout the body. They are part of the immune system, and their
main function is to filter out harmful substances, such as bacteria
and viruses, from the lymph fluid that passes through them. Lymph
nodes are connected to each other by a network of lymphatic vessels,
which transport lymph fluid from the body tissues to the lymph
nodes.
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25 Human Reproduction
Reproduction is essential for the continuation of a species. In the
animal kingdom, there are various methods of reproduction. Asexual reproduction creates genetically identical offspring, while sexual
reproduction involves the fusion of genetic material from two individuals to produce offspring that are genetically diverse from their
parents. During sexual reproduction, the male gamete (sperm) may
be inserted into the female’s body for internal fertilization, or the
sperm and eggs may be released into the environment for external
fertilization. An example of the latter is the seahorse, where following a mating dance, the female lays eggs in the male seahorse’s
abdominal brood pouch, which are then fertilized. The eggs hatch
and the offspring develop in the pouch for several weeks.
Some animals can also reproduce asexually through a process called
parthenogenesis. This process is common in invertebrates but rare
in vertebrates. Initially, scientists believed that vertebrate parthenogenesis occurred only in captive animals such as birds, snakes, and
sharks. However, it was recently discovered that approximately one
in five smalltooth sawfish in Florida are the result of parthenogenesis. This endangered species of fish may be using this form of
reproduction as an adaptation to avoid extinction.
25.1 Human Reproductive Anatomy and Gametogenesis
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25.1.1
Male Reproductive Anatomy
The male reproductive system is a complex structure responsible for
producing and transporting sperm, as well as releasing hormones
that regulate the development of male characteristics. The system
includes various organs and glands, which work together to ensure
proper function and fertility.
Testes: The testes, or testicles, are a pair of oval-shaped organs located within the scrotum, a sac of skin that hangs outside the body.
This external location allows for temperature regulation, which is
essential for optimal sperm production. Each testis is composed of
numerous seminiferous tubules, where sperm is produced through
a process called spermatogenesis. Additionally, the testes contain
Leydig cells, which produce the hormone testosterone. Testosterone
is crucial for the development of male secondary sexual characteristics, such as increased muscle mass, facial and body hair, and a
deepened voice.
Epididymis: The epididymis is a tightly coiled tube located behind
each testicle. It serves as a storage and maturation site for sperm.
The sperm remains in the epididymis for several weeks, during which
they gain motility and become capable of fertilization.
Vas Deferens: The vas deferens, also known as the ductus deferens,
is a long, thin tube that connects the epididymis to the ejaculatory ducts within the prostate gland. During ejaculation, sperm
travels through the vas deferens and mixes with seminal fluid from
accessory glands to form semen.
Accessory Glands: The male reproductive system includes three
accessory glands that contribute to semen production: the seminal vesicles, the prostate gland, and the bulbourethral glands. The
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seminal vesicles are a pair of glands located behind the bladder
that produce a thick, alkaline fluid rich in fructose, which provides
energy for the sperm. The prostate gland surrounds the urethra just
below the bladder and produces a slightly acidic fluid that helps
to nourish and protect the sperm. The bulbourethral glands, also
known as Cowper’s glands, are located near the base of the penis
and produce a clear, viscous fluid that lubricates and neutralizes any
acidic urine residue within the urethra.
Urethra: The urethra is a dual-purpose tube that transports both
urine and semen out of the body through the penis. The urethra
is divided into three sections: the prostatic urethra, which passes
through the prostate gland; the membranous urethra, which extends through the pelvic floor muscles; and the penile (or spongy)
urethra, which travels the length of the penis.
Penis: The penis is the external male organ used for both sexual
intercourse and urination. It is composed of three main parts: the
shaft, the head (or glans), and the foreskin (which may or may not be
present depending on whether circumcision has been performed).
The shaft is the long, cylindrical part of the penis, while the head
is the rounded, sensitive tip. The head is covered by a layer of skin
called the foreskin in some males, which can be removed in a procedure called circumcision. The penis contains three columns of
erectile tissue: two corpora cavernosa on the dorsal side and one corpus spongiosum that surrounds the urethra. During sexual arousal,
these tissues fill with blood, causing the penis to become erect and
enabling penetration during intercourse.
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Figure 117: The testicular reproductive system comprises several
structures, including the testes, epididymides, penis, and various
ducts and glands involved in semen production and transport.
Sperm leave the scrotum via the ductus deferens, which is bundled in the spermatic cord. The seminal vesicles and prostate gland
contribute fluids to the sperm, resulting in the formation of semen.
(Credit: openstax.org)
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25.1.2 Female Reproductive Anatomy
The female reproductive system is a complex and highly coordinated system responsible for producing and nurturing offspring,
as well as releasing hormones that regulate the menstrual cycle
and female secondary sexual characteristics. The system includes
several organs and structures, each with specific functions to ensure
proper function and fertility.
Ovaries: The ovaries are a pair of small, almond-shaped organs located on either side of the uterus within the pelvic cavity. The primary function of the ovaries is to produce ova (eggs) and release the
hormones estrogen and progesterone. These hormones regulate
the menstrual cycle and the development of female secondary sexual characteristics, such as breast development and the growth of
body hair. The ovaries contain numerous follicles, each containing
an immature egg (oocyte). During each menstrual cycle, typically
one egg matures and is released in a process called ovulation.
Fallopian Tubes: The fallopian tubes are narrow, muscular tubes
that connect the ovaries to the uterus. Each fallopian tube is about
10-12 centimeters long and features finger-like projections called
fimbriae near the ovarian end. When an egg is released from the
ovary during ovulation, the fimbriae help to guide the egg into the
fallopian tube. Fertilization by sperm typically occurs within the
fallopian tube, after which the fertilized egg travels towards the
uterus for implantation.
Uterus: The uterus is a hollow, pear-shaped organ located in the
pelvis between the bladder and rectum. The primary function of the
uterus is to support fetal development during pregnancy. The uterus
consists of a thick, muscular outer layer called the myometrium,
which is responsible for the contractions that occur during labor.
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The inner lining of the uterus, known as the endometrium, thickens and becomes enriched with blood vessels in preparation for
implantation of a fertilized egg. If implantation does not occur, the
endometrium is shed during menstruation.
Cervix: The cervix is the lower, narrow end of the uterus that connects the uterine cavity to the vaginal canal. It is responsible for
secreting mucus that changes in consistency throughout the menstrual cycle to help facilitate or inhibit the passage of sperm. During
childbirth, the cervix dilates to allow the passage of the baby through
the birth canal.
Vagina: The vagina is a muscular, elastic tube that extends from the
cervix to the vulva, the external female genitalia. The vagina has
several functions, including serving as the site of sexual intercourse,
providing a passageway for menstrual blood to exit the body, and
acting as the birth canal during childbirth. The vaginal walls produce
a natural lubrication during sexual arousal to facilitate intercourse.
External Genitalia (Vulva): The vulva is the collective term for the external female genitalia, which includes the mons pubis, labia majora,
labia minora, clitoris, and the vaginal and urethral openings. The
vulva is involved in sexual arousal, and the clitoris, a highly sensitive
erectile structure, plays a significant role in female sexual pleasure.
These organs and structures work together to support the complex
processes of ovulation, fertilization, implantation, and gestation,
ensuring the continuation of human life.
25.2 Spermatogenesis
Spermatogenesis is the process by which the male body produces
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Figure 118: The Female Reproductive System. (Credit: openstax.org)
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ductive system, and it involves the production, maturation, and
transport of sperm cells. Spermatogenesis is essential for fertility
and the ability to reproduce.
The process of spermatogenesis begins in the testes, which are
located in the scrotum. Within the testes are small, tube-like structures called seminiferous tubules. These tubules are where spermatogenesis occurs.
Spermatogenesis is the process of sperm cell development in the
testes, which takes place within the seminiferous tubules. It begins
with a type of germ cell called spermatogonia, which are diploid
(having two sets of chromosomes) and located in the seminiferous
tubules of the testes. Spermatogonia are stem cells that have the
ability to divide and differentiate into sperm cells. As the spermatogonia divide, they undergo a series of changes that result in the
production of mature sperm cells.
Through a series of mitotic divisions, spermatogonia give rise to
primary spermatocytes, which are also diploid. These primary spermatocytes undergo meiosis I, which results in the formation of two
haploid (having one set of chromosomes) secondary spermatocytes.
Each secondary spermatocyte then undergoes meiosis II, resulting
in the formation of two more haploid cells: spermatids. Spermatids
are immature sperm cells that are not yet capable of fertilization,
and they undergo a process of maturation called spermiogenesis.
During spermiogenesis, the spermatids develop the characteristic
tail and head of a mature sperm cell, which enables them to swim
and fertilize an egg. Once this process is complete, the sperm cells
are fully mature and ready to be released from the testes.
Once the spermatids have completed spermiogenesis, they are
released into the lumen of the seminiferous tubules, where they are
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transported through the epididymis and vas deferens to the urethra,
the tube that carries urine and sperm out of the body.
Figure 119: (a) Spermatogonial stem cells undergo mitosis, which
results in two identical, diploid daughter cells (spermatogonia to
primary spermatocyte). Meiosis involves two rounds of cell division,
where the primary spermatocyte transforms into the secondary
spermatocyte, followed by the transformation of the secondary spermatocyte into the spermatid. This process results in the formation
of four haploid daughter cells (spermatids). (b) The electron micrograph displays a cross-section of a seminiferous tubule from a rat,
and the light-shaded area in the center represents the lumen. Primary spermatocytes are located near the basement membrane, and
the early spermatids are moving closer towards the lumen. (Credit:
openstax.org)
Spermatogenesis is a continuous process, and it occurs throughout
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a man’s life. It typically takes about 74 days for a spermatogonium
to mature into a fully mature sperm cell.
25.2.1
Hormonal Regulation of Spermatogenesis
The hormonal regulation of spermatogenesis, or the process by
which the male body produces sperm, is a complex process that
involves a number of hormones and signaling pathways. These
hormones and signaling pathways work together to control the
production, maturation, and transport of sperm cells within the
male reproductive system.
One of the key hormones involved in the hormonal regulation of
spermatogenesis is testosterone, a male sex hormone produced by
the testes. Testosterone plays a crucial role in the development of
male secondary sexual characteristics, such as facial and body hair,
a deep voice, and an increased muscle mass. It also plays a key role
in the regulation of spermatogenesis.
Testosterone is produced by cells called Leydig cells, which are located within the testes. The production of testosterone is regulated
by luteinizing hormone (LH), a hormone produced by the pituitary
gland. LH stimulates the Leydig cells to produce testosterone, which
then acts on the Sertoli cells within the seminiferous tubules, where
spermatogenesis occurs. The Sertoli cells provide the necessary
support and nourishment for the developing sperm cells.
In addition to testosterone, there are other hormones and signaling
pathways that are involved in the hormonal regulation of spermatogenesis. One of these hormones is follicle-stimulating hormone
(FSH), which is produced by the pituitary gland. FSH stimulates
the Sertoli cells, which are located in the seminiferous tubules, and
promotes their growth and function.
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In response to FSH, Sertoli cells produce a protein called inhibin,
which helps to regulate the production of testosterone. Inhibin
inhibits the secretion of follicle-stimulating hormone by a negative
feedback mechanism. This regulatory mechanism helps to maintain
the proper balance of hormones in the testes, which is important
for the proper functioning of the male reproductive system.
There are also a number of other factors that can affect the hormonal
regulation of spermatogenesis. One of these is stress, which can
disrupt the normal hormonal balance and affect fertility. Other
factors that can affect the hormonal regulation of spermatogenesis
include age, nutrition, and certain medical conditions.
25.3
Oogenesis
Oogenesis is the process by which the female body produces eggs,
or ova. It is a complex process that occurs within the female reproductive system and is essential for fertility and the ability to
reproduce.
Oogenesis begins in the fetus, before a baby is born. During fetal
development, the female fetus has a small number of primordial
follicles, which are clusters of cells that contain an immature egg
cell. These primordial follicles are formed from a small group of cells
called oogonia, which are present in the developing fetus.
At birth, the female fetus has a finite number of primordial follicles,
and this number decreases over time. The number of primordial
follicles a woman has at birth is determined by her genetic makeup
and is not influenced by environmental factors.
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genesis. During folliculogenesis, the primordial follicle undergoes a
series of changes that result in the production of a mature egg cell.
The first step in folliculogenesis is the growth and development of
the primordial follicle. As the follicle grows, it forms a fluid-filled sac
called a primary follicle. The primary follicle contains an immature
egg cell, or oocyte, surrounded by a layer of cells called granulosa
cells.
As the primary follicle continues to grow, it becomes a secondary
follicle. The secondary follicle contains a larger number of granulosa
cells and a thicker layer of theca cells, which are cells that produce
hormones. The secondary follicle also contains a fluid-filled cavity
called the antrum.
The final stage of folliculogenesis is the development of the tertiary,
or mature, follicle. The tertiary follicle contains a fully mature egg
cell, or oocyte, surrounded by granulosa cells and theca cells. The
tertiary follicle is also known as the Graafian follicle, and it is ready
to be released from the ovary during ovulation.
Ovulation is the process by which the mature egg is released from
the ovary. It typically occurs once a month, and it is regulated by
hormones produced by the hypothalamus and the pituitary gland.
During ovulation, the tertiary follicle ruptures, releasing the mature
egg into the fallopian tube.
If fertilization occurs, the fertilized egg will travel down the fallopian
tube and into the uterus, where it will implant itself in the uterine
lining and begin to grow. If fertilization does not occur, the egg will
be expelled from the body during menstruation.
Oogenesis is a continuous process that occurs throughout a woman’s
reproductive life. It typically takes about 90 days for a primordial
follicle to mature into a fully mature egg cell. As a woman ages,
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the number of primordial follicles decreases, and the rate of folliculogenesis slows. Eventually, the woman will run out of primordial
follicles, and she will no longer be able to reproduce. This is known
as menopause.
In summary, oogenesis is the process by which the female body
produces eggs. It begins in the fetus and continues throughout a
woman’s reproductive life. Oogenesis involves the maturation and
release of eggs from the ovaries, and it is essential for fertility and
the ability to reproduce. As a woman ages, the number of eggs
decreases, and eventually, she will run out of eggs and will no longer
be able to reproduce.
25.3.1 Hormonal Regulation of Oogensis
The hormonal regulation of oogenesis, or the process by which the
female body produces eggs, is a complex process that involves a
number of hormones and inhibitors. These hormones and inhibitors
work together to control the maturation and release of eggs from
the ovaries.
To further consolidate the cells involved in the hormonal regulation
of human reproduction, it would be noteworthy to be reminded of
the similarities in males and females. Both Leydig cells and Theca
cells are types of endocrine cells that are involved in the production
of sex hormones. Leydig cells are located in the testes and produce
testosterone, while Theca cells are located in the ovaries and produce
androgens.
Similarly, Sertoli cells and Granulosa cells are both types of supportive cells that are involved in the development of reproductive cells.
Sertoli cells are located in the testes and provide support for developing sperm cells, while Granulosa cells are located in the ovaries
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Figure 120: During oogenesis, the process of cell division results
in the production of a single haploid ovum and one to three polar
bodies, which eventually degenerate. For the ovum to be fertilized,
it must first be penetrated by a sperm cell. (Credit: openstax.org)
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and provide support for developing egg cells.
Overall, these cells play important roles in the hormonal regulation
and support of reproductive cells in both males and females.
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
are two important hormones involved in the hormonal regulation
of oogenesis, the process of egg cell development in females. FSH
is produced by the pituitary gland and stimulates the growth and
development of follicles within the ovaries, which contain the immature eggs. As the follicles grow and mature, they produce estrogen,
a hormone that helps to regulate the menstrual cycle. LH, also
produced by the pituitary gland, plays a key role in ovulation, the
process by which the mature egg is released from the ovary. Together, LH and FSH act on the Theca and Granulosa cells of the
ovaries to promote the production and maturation of eggs, which is
crucial for proper reproductive function in females.
LH acts primarily on the Theca cells, promoting their production
of androgens (such as testosterone). These androgens are then
converted into estrogens by the Granulosa cells.
FSH acts primarily on the Granulosa cells, promoting their growth
and function. In response to FSH, the Granulosa cells produce estrogens, which are important for the development of the reproductive
tract and the maturation of eggs.
Overall, the actions of LH and FSH on the Theca and Granulosa cells
are important for the proper functioning of the female reproductive
system, including the production and maturation of eggs.
During the menstrual cycle, levels of FSH and LH fluctuate. As
the menstrual cycle progresses, levels of FSH rise, stimulating the
growth and development of the follicles. As the follicles mature,
they begin to produce estrogen, which causes a surge in LH levels.
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This LH surge triggers the release of the mature egg from the ovary,
a process known as ovulation.
In addition to FSH and LH, there are a number of other hormones
and inhibitors involved in the hormonal regulation of oogenesis.
One of these is inhibin, a hormone produced by the granulosa cells
within the follicles. Inhibin acts as an inhibitor of FSH, helping to
regulate the menstrual cycle.
Another hormone involved in the hormonal regulation of oogenesis is progesterone, a hormone produced by the corpus luteum,
a temporary endocrine gland that forms in the ovary after ovulation. Progesterone helps to prepare the uterus for pregnancy and
maintain pregnancy if it occurs.
25.4
The Menstrual Cycle
The menstrual cycle is a series of changes that occur in the female
reproductive system, leading to the release of an egg and the shedding of the uterine lining. The cycle is regulated by the interaction
of hormones produced by the hypothalamus, pituitary gland, and
ovaries.
The menstrual cycle is typically divided into three phases: the follicular phase, ovulation, and the luteal phase. In the follicular phase,
follicle-stimulating hormone (FSH) stimulates the growth and maturation of follicles in the ovaries. These follicles produce estrogen,
which thickens the uterine lining in preparation for a potential pregnancy.
Ovulation occurs when the mature egg is released from the ovary,
usually around day 14 of a 28-day cycle. Ovulation is triggered by a
surge in luteinizing hormone (LH) that is released by the pituitary
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gland.
During the luteal phase, the follicle that released the egg (now called
the corpus luteum) produces progesterone, which helps to thicken
and maintain the uterine lining. If the egg is not fertilized, the corpus
luteum eventually breaks down, causing a drop in hormone levels
that triggers the shedding of the uterine lining (menstruation).
Overall, the menstrual cycle is an important process in the female
reproductive system, leading to the release of an egg and the preparation of the uterine lining for potential pregnancy.
25.4.1 Hormonal Regulation of the Menstrual Cycle
The menstrual cycle is a complex process regulated by several hormones that work together to coordinate the cycle’s various stages,
such as follicular development, ovulation, and preparation of the
uterus for potential implantation of a fertilized egg. The menstrual
cycle is typically divided into four main phases: menstrual, follicular,
ovulation, and luteal. Each phase is regulated by a specific interplay
of hormones.
Menstrual phase (days 1-5): This is the first phase of the menstrual
cycle and begins on the first day of menstruation when the endometrial lining of the uterus is shed. The main hormones involved in
this phase are follicle-stimulating hormone (FSH) and luteinizing
hormone (LH), both released by the anterior pituitary gland. FSH
stimulates the growth of several ovarian follicles, while LH levels
remain relatively low.
Follicular phase (days 1-13): The follicular phase overlaps with the
menstrual phase and continues until ovulation occurs. The primary
hormone in this phase is FSH, which promotes the development of
multiple ovarian follicles, each containing an immature egg. One
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dominant follicle eventually outgrows the others and becomes the
mature follicle. As the dominant follicle grows, it releases increasing amounts of estrogen, which stimulates the thickening of the
endometrium in preparation for possible implantation.
Ovulation (day 14): Ovulation is triggered by a surge in LH, which
is stimulated by rising estrogen levels. This LH surge causes the
mature follicle to rupture and release the mature egg (ovum) into
the fallopian tube. The egg is then available for fertilization by sperm
for approximately 12-24 hours.
Luteal phase (days 15-28): After ovulation, the remnants of the ruptured follicle form the corpus luteum, a temporary endocrine structure that secretes progesterone and some estrogen. Progesterone
maintains the thickened endometrium and prevents the shedding
of the uterine lining. If fertilization occurs, the fertilized egg will travel
down the fallopian tube and implant into the endometrium, and
the corpus luteum continues to produce progesterone, supporting
early pregnancy. If fertilization does not occur, the corpus luteum
degenerates, leading to a decrease in progesterone and estrogen
levels, which in turn triggers the shedding of the endometrial lining
and the onset of menstruation, starting a new cycle.
The menstrual cycle is a complex interplay of hormones, with FSH
and LH released from the anterior pituitary gland and estrogen
and progesterone produced by the ovaries. These hormones work
together to regulate the development of follicles, ovulation, and the
preparation of the uterus for possible implantation of a fertilized
egg. Any disruptions in this delicate hormonal balance can lead to
irregularities in the menstrual cycle, impacting fertility and overall
reproductive health.
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Figure 121: The Hormonal Regulation of the Menstrual Cycle. (Credit:
openstax.org)
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