Lecture 8
The Structure and Function of Large Biological Molecules (Macromolecules) - Lecture 8
Introduction to Lipids
●​ Lipids are a diverse group of hydrophobic molecules.
●​ Unlike other large biological molecules, lipids do not form true polymers.
●​ The unifying feature of lipids:
○​ They mix poorly, if at all, with water.
●​ Composition of lipids:
○​ Primarily consist of hydrocarbon regions.
○​ The nonpolar C – H bonds in long hydrocarbon chains of fatty acids contribute to
their hydrophobic nature.
●​ Biologically significant lipids:
○​ Fats, phospholipids, and steroids.
Understanding Fats
●​ Definition of fats:
○​ Constructed from two smaller molecules: glycerol and fatty acids.
●​ Structure of glycerol:
○​ A three-carbon alcohol.
○​ Contains a hydroxyl (-OH) group attached to each carbon.
●​ Structure of fatty acids:
○​ Composed of a carboxyl group (-COOH) attached to a long carbon skeleton.
Key Questions about Lipids
1.​ Are lipids polymers?
○​ Answer: No.
2.​ What is the common feature in all lipids?
○​ Answer: Poor solubility in water.
3.​ Why are lipids hydrophobic?
○​ Answer: The C-H bonds in long hydrocarbon chains contribute to their
hydrophobicity.
4.​ What two molecules combine to form fats?
○​ Answer: Glycerol and fatty acids.
5.​ How many carbon atoms are in a glycerol molecule?
○​ Answer: 3.
6.​ What functional group does the glycerol molecule contain?
○​ Answer: Hydroxyl (-OH).
7.​ What functional group does a fatty acid molecule contain?
○​ Answer: Carboxyl (-COOH).
8.​ What reaction occurs when fatty acids combine with glycerol?
○​ Answer: Dehydration synthesis.
9.​ How many water molecules are lost when one fatty acid binds to glycerol?
○​ Answer: One.
10.​How many water molecules are lost when three fatty acids bind to glycerol?
○​ Answer: Three.
Formation of Triacylglycerol
●​ In a fat, three fatty acids are joined to glycerol through an ester linkage, forming a
triacylglycerol (or triglyceride).
●​ Fatty acids may be the same or differ in type (from two to three kinds).
●​ Diagram representation:
○​ Fatty acids are attached to all hydroxyl groups of glycerol (represent R as short
or long carbon chains).
Types of Fatty Acids
●​ Saturated fatty acids:
○​ Have no double bonds between carbon atoms.
○​ Saturated fats are derived from saturated fatty acids; they are solid at room
temperature.
○​ Most animal fats are saturated.
○​ Single bonds in the hydrocarbon chain increase flexibility and enable close
packing of molecules, resulting in solidity.
●​ Unsaturated fatty acids:
○​ One (monounsaturated) or more (polyunsaturated) double bonds present.
○​ Fats made from unsaturated fatty acids are called unsaturated fats (or oils) and
are liquid at room temperature.
○​ Plant and fish fats typically fall into this category.
○​ Cis form of double bonds:
○​ Reduces flexibility in the fatty acid hydrocarbon chain and prevents tight packing,
aiding liquid formation at room temperature.
Review Questions on Fatty Acids
1.​ What is the linkage that connects fatty acids with glycerol?
○​ Answer: Ester linkage.
2.​ Will all fatty acids connected to glycerol always be identical?
○​ Answer: No.
3.​ What distinguishes fat from fatty acids?
○​ Answer: Fat consists of glycerol and fatty acids, while fatty acids are long carbon
chains ending in carboxyl groups.
4.​ Define saturated and unsaturated fatty acids:
○​ Answer: Saturated have no double bonds; mono- unsaturated has one;
polyunsaturated has more than two double bonds.
5.​ What defines saturated and unsaturated fats?
○​ Answer: Saturated fats arise from saturated fatty acids, while unsaturated fats
arise from unsaturated fatty acids.
6.​ Reasons for state differences at room temperature:
○​ Answer: Saturated fats are solid due to their ability to tightly pack and are
formed by single bonds, whereas unsaturated fats remain liquid due to double
bonds inhibiting tight packing.
7.​ Examples:
○​ Saturated: Butter, lard; Unsaturated: Fish oil, plant oil.
Physiological Functions of Fats
●​ A diet high in saturated fats may lead to cardiovascular disease linked to plaque
deposits in arteries.
●​ Hydrogenation: The process of converting unsaturated fats to saturated fats by adding
hydrogen.
○​ Hydrogenated vegetable oils can also create unsaturated fats with trans double
bonds.
●​ Trans fats:
○​ Possibly more harmful than saturated fats concerning cardiovascular diseases.
●​ Main role of fats:
○​ Energy storage, found in adipose cells.
○​ Adipose tissue cushions vital organs and insulates the body.
Phospholipids
●​ Composition of phospholipids:
○​ Two fatty acids and a phosphate group attached to glycerol.
○​ The two fatty acids form hydrophobic tails and the phosphate group forms a
hydrophilic head.
●​ Assembly in water:
○​ Phospholipids self-assemble into bilayers when water is present.
○​ At cell surfaces, phospholipids arrange themselves into bilayers: hydrophobic
tails face inward, forming a boundary between the cell and its external
environment.
●​ Importance:
○​ Phospholipid bilayers are crucial components of cell membranes.
Structural Features of a Phospholipid
●​ Diagram highlights:
○​ Hydrophobic tails (fatty acids).
○​ Hydrophilic head (phosphate group, choline).
○​ Kink due to cis double bond in hydrocarbon chains.
Steroids
●​ Definition:
○​ Steroids are lipids with a carbon skeleton made of four fused rings.
●​ Cholesterol:
○​ A well-known steroid that is a component of animal cell membranes and a
precursor for synthesizing other steroids.
●​ Health implications:
○​ High levels of cholesterol in the blood may lead to cardiovascular diseases.
Review on Phospholipids and Steroids
1.​ Which fats are healthier?
○​ Answer: Unsaturated fats.
2.​ How to convert unsaturated fats to saturated fats?
○​ Answer: By adding hydrogen (Hydrogenation).
3.​ What are trans fats?
○​ Answer: A configuration where atoms are located in opposite planes around the
double bond; commonly found in processed foods, harmful to health.
4.​ Major functions of fats?
○​ Answer: Energy storage; adipose cells also serve vital organs and retain body
heat.
5.​ How do phospholipids become a bilayer and its significance?
○​ Answer: They self-assemble in water forming a bilayer, essential in cell
membranes.
6.​ Are steroids lipids?
○​ Answer: Yes, made of four fused carbon rings.
7.​ Important steroid example and its function?
○​ Answer: Cholesterol, crucial in cell membranes, and as a precursor for other
steroids.
Features of Proteins
●​ Proteins make up over 50% of the dry mass of most cells.
○​ They perform varied functions, including:
○​ Catalyzing chemical reactions.
○​ Defense mechanisms (e.g., antibodies).
○​ Storage and transport of substances.
○​ Cellular communication.
○​ Movement (contractile proteins).
○​ Structural support.
Types and Functions of Proteins
1.​ Enzymes:
○​ Function: Catalysts speeding up reactions without being consumed.
○​ Importance: Nearly every dynamic function depends on enzymes.
2.​ Hormonal proteins:
○​ Function: Coordinate organism activities.
○​ Example: Insulin, which regulates blood sugar concentration.
3.​ Receptor proteins:
○​ Function: Response to chemical stimuli.
○​ Example: Nerve cell receptors detect signaling molecules from other cells.
4.​ Contractile and motor proteins:
○​ Function: Movement.
○​ Examples: Motor proteins help with cilia and flagella movement; actin and myosin
enable muscle contraction.
5.​ Structural proteins:
○​ Function: Provide support.
○​ Examples: Keratin in hair and nails; silk fibers in spiders; collagen and elastin in
connective tissues.
Review Questions on Proteins
1.​ Name the proteins that accelerate reactions and their fate?
○​ Answer: Enzymes; not consumed after reaction.
2.​ Name proteins that work against antigens?
○​ Answer: Antibodies.
3.​ Examples of storage and transport proteins?
○​ Answer: Casein (milk protein), Hemoglobin (oxygen transport).
4.​ Examples of hormonal and structural proteins?
○​ Answer: Insulin (hormonal), Actin/Myosin (muscle contraction), Keratin
(structural), Collagen (connective tissue).
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Lecture 9
Lecture 9: The Structure and Function of Large Biological Molecules
What is an R Group?
●​ Definition: An “R” group is a placeholder for an attachment to a molecule, typically a
functional group.
●​ Context in Molecules: In molecules of the penicillin family, all molecules share the
same backbone or framework. The variations among these molecules arise from
different “R” groups attached to the backbone.
Protein Polymers
●​ Source of Proteins: Proteins are polymers constructed from a set of twenty amino acid
monomers.
●​ Amino Acids:
○​ Proteins are constructed from the same set of 20 amino acids.
○​ A biologically functional protein consists of one or more polypeptides.
●​ Definition of Amino Acids: Organic molecules characterized by the presence of amino
(–NH2) and carboxyl (–COOH) groups; their properties vary due to different side chains
called R groups.
Polypeptides
●​ Definition of Polypeptides: Unbranched polymers formed by amino acids.
●​ Linkage of Amino Acids: Amino acids are linked by covalent bonds known as peptide
bonds.
●​ Polypeptide Length: Polypeptides can range from a few to more than 1,000 monomers
in length.
●​ Polypeptide Ends: Each polypeptide has a unique linear sequence of amino acids,
defining a carboxyl end (C-terminus) and an amino end (N-terminus).
Four Levels of Protein Structure
1.​ Primary Structure:
○​ Definition: Unique sequence of amino acids that makes up the protein, similar to
the order of letters in a word.
○​ Determinants: Governed by inherited genetic information.
2.​ Secondary Structure:
○​ Definition: Coils and folds formed in the polypeptide chain, often stabilized by
hydrogen bonds.
3.​ Tertiary Structure:
○​ Definition: The overall 3D shape of the protein determined by interactions among
various side chains (R groups).
4.​ Quaternary Structure:
○​ Definition: Formed when a protein consists of multiple polypeptide chains bonded
together.
Structural Compatibility in Antibodies
●​ Context: The structure of antibodies is compatible with the structure of antigens (foreign
molecules that provoke an immune response).
Sickle-Cell Disease
●​ Definition: An inherited blood disorder caused by a single amino acid substitution in the
protein hemoglobin, changing glutamate (Glu) to valine (Val).
●​ Impact: This small change leads to abnormal hemoglobin molecules causing red blood
cells to aggregate and deform into a sickle shape, impairing oxygen transport.
Impacts on Protein Structure
●​ Denaturation: The process whereby physical and chemical conditions result in the loss
of a protein’s native structure, rendering it biologically inactive.
○​ Indicators: Factors such as pH alterations, salt concentration changes,
temperature variations, or other environmental influences can induce
denaturation.
○​ Reversibility: Sometimes reversible upon removal of denaturing agents, though
not always guaranteed.
○​ Example: Scrambling an egg denatures ovalbumin while preserving its primary
structure.
●​ Health Impact: Extremely high fevers can denature blood proteins, leading to potential
fatal outcomes.
Importance of Protein Folding
●​ Functionality: Proper protein folding is essential for function, structure, stability, and
localization within cells.
●​ Diseases Associated with Misfolding: Alzheimer’s, Parkinson’s, and mad cow disease
are linked to misfolded proteins.
●​ **Techniques for Structure Determination:
○​ X-ray crystallography
○​ Nuclear magnetic resonance (NMR) spectroscopy
○​ Bioinformatics approaches for predicting protein structure from amino acid
sequences.
Nucleic Acids
●​ Functions: Store, transmit, and assist in expressing hereditary information.
●​ Gene Definition: A segment of DNA guiding the synthesis of messenger RNA (mRNA).
●​ Structure of Nucleic Acids:
○​ Composed of monomers called nucleotides, which consist of a nitrogenous base,
a pentose sugar, and one or more phosphate groups.
●​ Types of Nucleic Acids:
○​ Deoxyribonucleic acid (DNA)
○​ Ribonucleic acid (RNA)
Gene Expression Process
1.​ DNA to mRNA: DNA directs the synthesis of mRNA, which takes place in the nucleus.
2.​ Polypeptide Production: The mRNA interacts with the cell's protein-synthesizing
machinery to produce a polypeptide in the cytoplasm.
3.​ Flow of Genetic Information: The flow can be summarized as DNA → RNA → Protein.
Components of Nucleic Acids
●​ Nucleotide Structure:
○​ Nucleotide = Nucleoside + Phosphate Group
○​ Nucleoside = Nitrogenous Base + Sugar
●​ Nitrogenous Bases: Two families categorized as:
○​ Pyrimidines: Cytosine, Thymine, Uracil (single six-membered ring).
○​ Purines: Adenine, Guanine (six-membered ring fused to a five-membered ring).
●​ Sugar Differences:
○​ DNA contains deoxyribose; RNA contains ribose.
Comparison of DNA and RNA
●​ Structure: DNA is double-stranded and forms a double helix, while RNA is
single-stranded and more variable in form.
●​ Base Pairing: In RNA, thymine is replaced by uracil, with adenine pairing with uracil.
●​ Polynucleotide Orientation: The two strands of DNA run in opposite 5′ → 3′
directions—termed antiparallel.
●​ Base Pairing Rules:
○​ Adenine (A) pairs with Thymine (T) in DNA.
○​ Guanine (G) pairs with Cytosine (C) in DNA.
Examples of Biological Molecules
●​ Monosaccharides: Glucose, Fructose
●​ Disaccharides: Lactose, Sucrose
●​ Polysaccharides:
○​ Cellulose (plants)
○​ Starch (plants)
○​ Glycogen (animals)
○​ Chitin (animals and fungi)
●​ Functions of Carbohydrates:
○​ Serve as fuel or carbon sources, can be converted to other molecules or
polymerized.
○​ Strengthen plant cell walls (cellulose), store glucose for energy (starch in plants,
glycogen in animals), and strengthen exoskeletons and fungal cell walls (chitin).
●​ Lipids: Triacylglycerols (fats or oils) as important energy sources; phospholipids form
cell membranes; steroids have various signaling roles.
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Lecture 10
Lecture 10
●​ Chapter 6: A Tour of the Cell
●​ Date: 09.19.25
Overview of Cells
●​ Definition: The cell is the basic unit of structure and function in organisms.
○​ Types of Organisms: Life exists as:
○​ Single-celled organisms (e.g., Paramecium, Yeast)
○​ Multicellular organisms (e.g., plants, animals)
●​ Key Cellular Structures:
○​ Mitochondria
○​ Chloroplast
○​ Nucleus
○​ Plasma membrane
○​ Ribosome
●​ Cellular Processes:
○​ DNA replication and transcription
○​ RNA translation
Investigating Cell Inner Workings
●​ Methods: Cell biologists investigate cells using microscopy and biochemistry due to the
small size of the inner structures.
Microscopy
●​ Principle: Microscopes visualize cells by passing visible light through specimens.
○​ Light Microscopes (LM): Light is passed through a specimen and refracted by
glass lenses. The image is magnified for viewing.
Important Parameters of Microscopy
1.​ Magnification: Ratio of an object's image size to its real size.
2.​ Resolution: Measure of clarity; minimum distance between two distinguishable points.
3.​ Contrast: Visible differences in brightness between dark and light areas of a sample.
●​ Resolution Capacity: The human eye resolves objects approximately $0.1 ext{ mm}$
(100 micrometers) in size.
Cell Organelles Review Questions
1.​ Q: Which organelle is responsible for energy processing? ​
A: Mitochondria and chloroplast.
2.​ Q: Which part of a cell interacts with environments? ​
A: Plasma membrane.
3.​ Q: Which organelle stores and transmits information? ​
A: Nucleus.
4.​ Q: Responsibility for protein synthesis? ​
A: Ribosome.
5.​ Q: Smallest resolvable object size by the human eye? ​
A: 0.1 mm or 100 µm.
6.​ Q: Can a light microscope determine protein structure? ​
A: No, an electron microscope is required for determining protein structure.
Types of Microscopy
●​ Various types of microscopes enhance visibility of cellular materials:
○​ Phase Contrast Microscopy: Amplifies differences in sample density,
enhancing contrast in unstained cells.
○​ Differential Interference Contrast (DIC): Utilizes optical modifications to
enhance differences in density, providing a 3-D appearance.
Fluorescent Microscopy
●​ Individual cell structures can be stained with fluorescent markers to capture images for
studying cell interiors.
●​ Types of Fluorescent Microscopy:
○​ Confocal Microscopy and Deconvolution Microscopy: Provide sharper 3-D
images of tissues and cells.
Electron Microscopy (EM)
●​ Purpose: Used to study subcellular structures.
●​ Types of EM:
1.​ Cryo-Electron Microscopy: Specimens are frozen rapidly at less than -160ºC,
and a beam of electrons passes through for 3-D imaging.
2.​ Scanning Electron Microscopy (SEM): Images surface structures. A beam of
electrons is directed onto the specimen for 3-D imaging.
3.​ Transmission Electron Microscopy (TEM): Examines sections of specimens to
get internal structures via electron beams.
Cell Fractionation
●​ Definition: A process to isolate organelles from cells.
●​ Homogenization: Breaks down the cell membrane to release cellular content into
solution for organelle function studies.
○​ Centrifugation: The nucleus can be isolated by centrifugation at 1000g.
Review Questions for Microscopy and Fractionation
1.​ Q: Which optical modification generates 3-D images in light microscopy? ​
A: DIC (Differential Interference Contrast).
2.​ Q: What fluorescence microscope is needed for sharper 3-D cell images? ​
A: Confocal and deconvolution microscopes.
3.​ True/False: Is a cryo-electron microscope sample frozen? ​
A: False.
4.​ True/False: Do scanning electron microscopes require frozen samples? ​
A: False.
5.​ Q: Centrifugal speed needed to isolate the nucleus? ​
A: 1000g.
Basic Features of Cells
●​ Cell Types: Two categories of cells exist:
○​ Prokaryotic Cells:
○​ No nucleus.
○​ DNA located in a nucleoid (unbound region).
○​ No membrane-bound organelles.
○​ Cytoplasm bound by plasma membrane.
○​ Generally smaller than eukaryotic cells.
○​ Eukaryotic Cells:
○​ DNA within a nucleus bounded by a double membrane.
○​ Membrane-bound organelles offer localized environments for various activities.
○​ Cytoplasm is the area between the plasma membrane and nucleus.
○​ Generally larger than prokaryotic cells.
○​ Plant and animal cells possess most of the same organelles.
Structure of the Cell Membrane
●​ Plasma Membrane Function: A selective barrier for passage of:
○​ Oxygen
○​ Nutrients
○​ Wastes
●​ Composition: Basic fabric consists of a double layer of phospholipids and lipids.
Characteristics of Prokaryotic Cells
●​ Key Features:
○​ Lack of nucleus.
○​ Unbound DNA in nucleoid.
○​ Absence of membrane-bound organelles.
○​ Cytoplasm enclosed by plasma membrane.
○​ Typically smaller than eukaryotic cells.
Review Questions about Cell Characteristics
1.​ Q: Is cytosol liquid? ​
A: No, it is semiliquid.
2.​ Q: Do prokaryotic cells contain ribosomes? ​
A: Yes.
3.​ Q: Are Bacteria and Archaea prokaryotes? ​
A: Yes.
4.​ True/False: Are fatty acids part of the phospholipid projecting outside the membrane? ​
A: No, they are sandwiched within the double layer.
5.​ Q: Region housing genetic material in prokaryotes? ​
A: Nucleoid.
Characteristics of Eukaryotic Cells
●​ Key Features:
○​ DNA is in a nucleus bounded by a double-layered membrane.
○​ Membrane-bound organelles facilitate diverse activities within the cell.
○​ Cytoplasm is the region between the plasma membrane and nucleus.
○​ Typically larger than prokaryotic cells.
○​ Both plant and animal cells share most organelles.
Lecture 11
Chapter 6: A Tour of the Cell
The Endomembrane System
●​ Definition: A collection of membranes of organelles inside and surrounding the
membrane of eukaryotic cells.
●​ Components:
1.​ Nuclear envelope
2.​ Endoplasmic reticulum (ER)
3.​ Golgi apparatus
4.​ Lysosomes
5.​ Vacuoles
6.​ Plasma membrane
●​ These components are either continuous or connected via transfer by vesicles.
The Endoplasmic Reticulum (ER): Biosynthetic Factory
●​ General Facts:
○​ Accounts for more than half of the total membrane in many eukaryotic cells.
○​ The ER membrane is continuous with the nuclear envelope.
●​ Regions of ER:
○​ Smooth ER:
○​ Characteristics: Lacks ribosomes.
○​ Functions: ​
I. Synthesizes lipids, including oils, steroids (e.g., sex hormones), and membrane
phospholipids. ​
II. Detoxifies drugs and poisons in liver cells by adding -OH groups to drugs,
making them water-soluble for bodily flush-out. ​
III. Stores calcium ions.
○​ Rough ER:
○​ Characteristics: Surface is studded with ribosomes.
○​ Functions: ​
IV. Bound ribosomes secrete glycoproteins (proteins covalently bonded to
carbohydrates). ​
V. Distributes transport vesicles, with secretory proteins surrounded by
membranes. ​
VI. Serves as a membrane factory for the cell.
The Golgi Apparatus: Shipping and Receiving Center
●​ Structure: Composed of flattened membranous sacs called cisternae.
●​ Functions: ​
I. Modifies products of the ER, such as proteins, and directs them to other destinations. ​
II. Manufactures certain macromolecules. ​
III. Sorts and packages materials into transport vesicles.
Lysosomes: Digestive Compartments
●​ Definition: A lysosome is a membranous sac of hydrolytic enzymes that can digest
macromolecules.
●​ Functionality:
○​ Lysosomal enzymes work best in the acidic environment inside the lysosome.
○​ Hydrolytic enzymes and lysosomal membranes are synthesized by rough ER,
transported to the Golgi apparatus for further processing.
○​ Some cells can engulf others via phagocytosis, forming food vacuoles that
lysosomes fuse with to digest contents.
○​ Also utilize enzymes to recycle the cell’s own organelles and macromolecules,
known as autophagy.
Vacuoles: Diverse Maintenance Compartments
●​ Definition: Large vesicles derived from the ER and Golgi apparatus.
●​ Types and Functions:
○​ Food vacuoles: Formed by phagocytosis.
○​ Contractile vacuoles: Found in freshwater protists, pump excess water out of
cells.
○​ Central vacuoles: Found in mature plant cells, containing a solution called sap.
○​ Function as the main repository of inorganic ions, including potassium and
chloride.
○​ Play a critical role in the growth of plant cells.
Mitochondria and Chloroplasts: Energy Transformation
●​ Function:
○​ Mitochondria are sites of cellular respiration, utilizing oxygen to generate ATP.
○​ Chloroplasts are found in plants and algae, acting as sites of photosynthesis.
○​ Peroxisomes are specialized oxidative organelles.
Evolutionary Origins of Mitochondria and Chloroplasts
●​ The Endosymbiont Theory:
○​ Suggests early eukaryotic ancestors engulfed oxygen-using, non-photosynthetic
prokaryotic cells, which became endosymbionts.
○​ These endosymbionts evolved into mitochondria and later chloroplasts.
●​ Supporting Similarities:
○​ Both are enveloped by a double membrane.
○​ Contain free ribosomes and circular DNA molecules.
○​ Grow and reproduce somewhat independently within cells.
Mitochondria: Chemical Energy Conversion
●​ Presence: Found in nearly all eukaryotic cells.
●​ Structure:
○​ Smooth outer membrane and an inner membrane folded into cristae.
●​ Functionality:
○​ The inner membrane creates two compartments: intermembrane space and
mitochondrial matrix.
○​ Some metabolic steps of cellular respiration are catalyzed in the mitochondrial
matrix.
○​ Cristae provide a large surface area for ATP synthesis enzymes.
Chloroplasts: Capture of Light Energy
●​ Structure and Composition:
○​ Contain green pigment chlorophyll and various enzymes for photosynthesis.
○​ Found in leaves and other green parts of plants and in algae.
○​ Structure includes thylakoids (membranous sacs stacked into a granum) and
stroma (internal fluid).
○​ Chloroplasts belong to a group of plant organelles called plastids.
Peroxisomes: Oxidation
●​ Definition: Specialized metabolic compartments bound by a single membrane.
●​ Functionality:
○​ Contain enzymes that remove hydrogen atoms from various substances and
transfer them to oxygen, forming hydrogen peroxide.
○​ Various functions include:
○​ Breaking fatty acids into smaller molecules for respiration.
○​ Detoxifying alcohol and other harmful compounds in the liver.
○​ Glyoxysomes, found in fat-storing tissues of plant seeds, convert fatty acids into
sugar to support the emerging seedling.
Roles of the Cytoskeleton: Support and Motility
●​ Function:
○​ Supports the cell and maintains its shape.
○​ Interacts with motor proteins to produce cell motility.
○​ Vesicles and organelles can travel along tracks provided by the cytoskeleton
using motor protein “feet.”
Components of the Cytoskeleton
●​ Types of Fibers:
○​ Microtubules (thickest)
○​ Microfilaments (also called actin filaments, thinnest)
○​ Intermediate filaments (middle range in diameter)
●​ Microtubules:
○​ Hollow rods, about 25 nm in diameter and 200 nm to 25 microns long.
○​ Constructed of tubulin dimers.
○​ Functions:
○​ Shape the cell.
○​ Guide movement of organelles.
○​ Separate chromosomes during cell division.
●​ Microfilaments (Actin Filaments):
○​ Solid rods about 7 nm in diameter, built from a twisted double chain of actin
subunits.
○​ Network supports the cell’s shape and forms a cortex just inside the plasma
membrane.
○​ Bundles of microfilaments make up the core of microvilli in intestinal cells.
○​ Function in cellular motility alongside myosin.
●​ Intermediate Filaments:
○​ Fibrous proteins coiled into cables of 8-12 nm.
○​ Maintain cell shape, anchor organelles, and form the nuclear lamina.
Extracellular Components and Connections Between Cells
●​ General Overview:
○​ Extracellular materials and structures coordinate cellular activities.
○​ Most cells synthesize and secrete materials outside the cell for various essential
functions.
●​ Cell Walls of Plants:
○​ An extracellular structure distinguishing plant cells from animal cells.
○​ Found in prokaryotes, fungi, and some protists.
○​ Protects the cell, maintains shape, and prevents excessive water uptake.
○​ Composed of cellulose fibers embedded in polysaccharides and proteins.
●​ The Extracellular Matrix (ECM) of Animal Cells:
○​ Animal cells lack cell walls but are covered by an elaborate ECM.
○​ ECM composed of glycoproteins (collagen, proteoglycans, and fibronectin).
○​ Fibronectin and other ECM proteins bind to integrins in the plasma membrane,
regulating behavior.
○​ Influences gene activity in the nucleus, with mechanical signaling possibly
occurring through cytoskeletal changes that activate chemical signals
Lecture 12
Lecture 12: Chapter 7 - Membrane Structure and Function
Overview of Plasma Membrane Functionality
●​ Role of Plasma Membrane: Acts as a selectively permeable barrier regulating inbound
and outbound traffic of molecules.
○​ Passive Transport: Some small molecules traverse the cell membrane without
energy expenditure; may sometimes require transport proteins.
○​ Active Transport: Involves energy and transport proteins for moving some small
molecules across the membrane.
○​ Bulk Transport: Large molecules utilize bulk transport mechanisms like
exocytosis (out of the cell) or endocytosis (into the cell).
Membrane Composition
●​ Cellular Membranes: Composed of a mosaic of lipids and proteins.
○​ Key Components: Phospholipids and proteins are the main components, with
carbohydrates also playing significant roles.
■​ Phospholipids:
■​ Amphipathic molecules with both hydrophobic ("water-fearing")
and hydrophilic ("water-loving") regions.
■​ Form a bilayer configuration: hydrophobic tails face inward,
hydrophilic heads face outward, interacting with water.
■​ Membrane Proteins:
■​ Generally amphipathic to facilitate function.
■​ Hydrophilic regions are oriented toward cytosol and extracellular
fluid; hydrophobic regions are embedded.
Organization and Dynamics of the Membrane
●​ Distribution of Proteins: Membrane proteins are not randomly distributed; they often
cluster to perform related functions.
●​ Fluid Mosaic Model:
○​ Illustrates the membrane as a mosaic of protein molecules floating in a fluid
phospholipid bilayer.
○​ Membrane structure is dynamic and selectively permeable.
Membrane Fluidity and Its Significance
●​ Mobility in Membranes:
○​ Membranes consist of molecules held predominantly by weak hydrophobic
interactions.
○​ Lipids and some proteins can shift sideways within the membrane, contributing to
fluidity.
●​ Fluidity Experiment:
○​ Conducted by Larry Frye and Michael Edidin to study membrane protein
movement by fusing mouse and human cells and observing markers under a
microscope.
○​ Conclusion: Demonstrated that some membrane proteins can move laterally
within the membrane plane.
Factors Affecting Membrane Fluidity
●​ Temperature Effects:
○​ As temperatures decrease, membranes transition from a fluid state to a solid
state.
○​ The solidification temperature varies based on lipid composition.
○​ Membranes with high levels of unsaturated fatty acids maintain greater fluidity
compared to those with saturated fatty acids.
●​ Cholesterol's Role:
○​ At warm temperatures (e.g., 37ºC), cholesterol restricts phospholipid movement,
whereas at cooler temperatures, it helps prevent close packing, maintaining
fluidity.
○​ Cholesterol can be described as a "fluidity buffer" due to its dual role.
Importance of Fluidity
●​ Functional Implications: Essential for membrane integrity and function; affects
permeability and the movement of transport proteins.
Types of Membrane Proteins
●​ Classification:
○​ Peripheral Proteins: Attach to the membrane's surface.
○​ Integral Proteins: Span across the membrane; include transmembrane proteins
that traverse the lipid bilayer.
■​ Integral proteins embed within the hydrophobic core and typically consist
of nonpolar amino acids, often arranged in alpha helices.
●​ Functions of Membrane Proteins:
○​ Transport: Facilitate the passage of molecules (channel proteins, pumps).
○​ Enzymatic Activity: Enzymes can catalyze reactions adjacent to the membrane.
○​ Signal Transduction: Receptors transmit signals from the extracellular
environment to the interior (e.g., binding of signaling molecules).
○​ Cell-Cell Recognition: Glycoproteins serve as markers for cellular identification.
○​ Intercellular Joining: Binding between membrane proteins of neighboring cells.
○​ Attachment to Cytoskeleton and ECM: Some proteins interface with the
cytoskeleton internally and the extracellular matrix externally to maintain cell
shape.
Synthesis and Orientation of Membrane Components
●​ Process Overview:
○​ Step 1: Synthesis of secretory proteins, membrane proteins, and lipids occurs in
the ER. Carbohydrates attach to proteins, forming glycoproteins.
○​ Step 2: Glycoproteins undergo modifications in the Golgi apparatus; glycolipids
are formed when lipids obtain carbohydrates.
○​ Step 3: Glycoproteins, glycolipids, and secretory proteins are transported to the
plasma membrane via vesicles.
○​ Step 4: Vesicles fuse with the plasma membrane, releasing secretory proteins
through exocytosis, positioning glycoproteins and glycolipids in the membrane.
Review Questions
●​ Q1: True/False: Large molecules pass through the membrane by passive transport. ​
Ans: False, large molecules require bulk transport methods.
●​ Q2: True/False: Active transport requires energy. ​
Ans: True.
●​ Q3: Define the transport process for large molecules. ​
Ans: Exocytosis (out) and endocytosis (in).
●​ Q4: What are the main components of the plasma membrane? ​
Ans: Proteins and phospholipids.
●​ Q5: Define amphipathic molecule with an example. ​
Ans: Molecules with both hydrophilic and hydrophobic regions, e.g., phospholipids.
●​ Q6: True/False: Most membrane proteins are randomly distributed. ​
Ans: False.
Lecture 13
Lecture 13: Membrane Structure and Function
Chapter 7: Overview of Membrane Structure and Function
●​ Membranes exhibit selective permeability; this means that some substances can cross
the membrane more easily than others.
●​ The fluid mosaic model explains how membranes regulate molecular traffic across the
membrane.
Permeability of the Lipid Bilayer
●​ Hydrophobic (nonpolar) molecules, such as hydrocarbons, O$2$, and CO$2$, can
dissolve in the lipid bilayer and pass through the membrane rapidly.
●​ Hydrophilic (polar) molecules, such as sugars, water, and ions, pass through the
membrane slowly, if at all.
●​ The question arises: What is the mechanism involved in passing these molecules
through the membrane?
Passive Transport
●​ Definition: Passive transport is the diffusion of a substance across a membrane with no
energy investment.
●​ Diffusion:
○​ Movement of particles of any substance so that they spread out evenly into the
available space.
○​ Although each molecule moves randomly, diffusion of a population of molecules
may be directional until they reach equilibrium.
○​ Chemical substances diffuse down their concentration gradient, from high
concentration to low concentration.
Mechanism of Diffusion
●​ Each substance moves down its own concentration gradient, unaffected by the
concentrations of other substances.
●​ Q1: What does membrane selectively permeable mean?
○​ Answer: While some substances pass through the membrane easily, others do
not.
●​ Q2: True/False: O$_2$ requires energy to pass through the membrane.
○​ Answer: False; O$2$ and CO$2$ do not require energy to pass through the
membrane.
●​ Q3: True/False: Diffusion continues until the solutes are distributed evenly.
○​ Answer: True.
●​ Q4: True/False: In diffusion, molecule movement is random.
○​ Answer: False.
●​ Q5: True/False: Diffusion occurs when there is no gradient of solutes across the
membrane.
○​ Answer: False.
●​ Q6: True/False: When two solutes are separated by a membrane, the diffusion of each
solute is dependent on the concentration of the other solute.
○​ Answer: False.
Mechanisms for Hydrophilic Solutes
●​ Small hydrophobic solutes pass through the cell membrane by passive diffusion
without requiring any energy.
Transport Proteins
Type 1: Channel Proteins
●​ Have a hydrophilic channel that can be used by certain molecules or ions as a tunnel.
●​ For example, aquaporin is composed of four polypeptide subunits, each forming a
channel for the passage of water, greatly increasing the rate of water molecules passing
through.
○​ Up to 3 billion water molecules pass through aquaporins per second.
●​ Hydrophilic substances cross membranes more quickly by passing through transport
proteins.
Type 2: Carrier Proteins
●​ Carrier proteins bind to molecules and change shape to shuttle them across the
membrane.
●​ For example, glucose carrier proteins only transport glucose and do not transport
fructose, a structural isomer of glucose.
Facilitated Diffusion
●​ Definition: In facilitated diffusion, transport proteins speed the passive movement of
molecules across the plasma membrane without requiring energy.
○​ Channel proteins provide corridors that allow a specific molecule or ion to cross
the membrane.
○​ Aquaporins facilitate the diffusion of water.
○​ Ion channels facilitate the transport of ions.
Hydrophilic Solutes and Transport
●​ Q1: True/False: Hydrophilic substances easily cross the membrane.
○​ Answer: False; they require transport proteins.
●​ Q2: How do hydrophilic solutes pass through the channel transport proteins?
○​ Answer: The channel protein has a hydrophilic region through which hydrophilic
solutes pass through the membrane.
●​ Q3: True/False: Aquaporin is a carrier protein.
○​ Answer: False.
●​ Q4: True/False: Aquaporin has three polypeptide chains.
○​ Answer: False; it has four polypeptide chains.
●​ Q5: True/False: Aquaporin changes its shape during water transportation.
○​ Answer: False.
●​ Q6: True/False: Glucose carrier is a channel transporter protein.
○​ Answer: False.
●​ Q7: True/False: Glucose carrier changes its shape while transporting glucose.
○​ Answer: True.
●​ Q8: True/False: Facilitated diffusion is energy-driven diffusion.
○​ Answer: False.
How Water Molecules are Transported
●​ Up to this point, the focus has been on the transport of solute molecules across the
membrane.
Osmosis and Water Balance
●​ Definition of Osmosis: Osmosis is the diffusion of free water across a selectively
permeable membrane.
●​ Free water molecules move from the region of lower solute concentration to the region
of higher solute concentration until equilibrium is reached.
Tonicity
●​ Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.
●​ Isotonic Solution:
○​ A solution is isotonic if its solute concentration is the same as that inside the cell.
○​ Water diffuses at the same rate in both directions, resulting in no net movement
of water across the membrane.
○​ Volume of a cell without a cell wall remains stable in an isotonic solution.
Hypertonic Solution
●​ A solution is hypertonic if its solute concentration is greater than that inside the cell.
○​ Net diffusion of water occurs from inside the cell to the surrounding solution,
causing cells without cell walls to lose water, shrivel, and potentially die.
Hypotonic Solution
●​ A solution is hypotonic if its solute concentration is less than that inside the cell.
○​ Net diffusion of water occurs from the surrounding solution to the inside of the
cell, resulting in cells without cell walls gaining water, swelling, and potentially
lysing (bursting).
Review on Osmosis and Tonicity
●​ Q1: True/False: Osmosis is diffusion of a solute.
○​ Answer: False; it is the diffusion of water.
●​ Q2: What happens to water molecules when a higher solute concentration region is
separated from a lower solute concentration region through a synthetic membrane in a
U-shaped tube?
○​ Answer: Free water molecules pass from the lower solute region to the higher
solute region.
●​ Q3: True/False: The level of water will be lower in the higher solute region than in the
lower solute region.
○​ Answer: False.
●​ Q4: True/False: In an isotonic solution, solute concentration inside the cell is higher than
in the surrounding area.
○​ Answer: False.
●​ Q5: When the concentration of solute inside the cell is higher than outside, this is known
as a** hypotonic** situation.
●​ Q6: What happens to a red blood cell (RBC) when immersed in a hypertonic solution?
○​ Answer: The RBC will shrivel.
●​ Q7: What happens to a plant cell with an intact cell wall in a hypertonic solution?
○​ Answer: The plant cell will lose water slowly.
Peripheral Proteins and Glycolipids
●​ Q1: What conclusion can be drawn about peripheral proteins from the diagram?
○​ Answer: They are located on the cytoplasmic side of the membrane and are
hydrophilic.
●​ Q2: What is a glycolipid and what role does it play?
○​ Answer: Glycolipids are lipids attached to carbohydrates, involved in cell-cell
recognition.
Active Transport
●​ Definition: Active transport uses energy to move solutes against their concentration
gradients.
●​ ATP vs. ADP: Active transport is an uphill process from regions of lower concentration
to regions of higher concentration.
●​ Passive Transport:
○​ Passive transport is a downhill process, moving substances from high to low
concentration without the need for energy.
Transport Mechanisms
●​ Example of Facilitated Diffusion: Movement of O$2$ and CO$2$.
●​ Example of Active Transport: The Na$^+$/K$^+$ pump which pumps K$^+$ ions into
the cell while moving Na$^+$ ions out.
Conclusion
●​ Knowledge of membrane structure and function is critical to understanding cellular
processes and maintaining homeostasis.
Acknowledgements
●​ Material adapted from Campbell Biology, Pearson Publication.
●​ Thank you!
Lecture 14
Chapter 7: Membrane Structure and Function
●​
Electrogenic Pumps
●​ Definition: An electrogenic pump is a transport protein that generates voltage across a
membrane, hence storing energy for cellular work.
●​ Main Electrogenic Pumps differ between organisms:
1.​ In Animals: The Sodium-Potassium Pump (Na+/K+ pump).
2.​ In Plants, Fungi, and Bacteria: The Proton Pump, which actively transports
hydrogen ions (H+) out of the cell, storing potential energy.
Cotransport: Coupled Transport by a Membrane Protein
●​ Cotransport: Occurs when the active transport of a solute indirectly drives the transport
of other substances.
1.​ The “downhill” diffusion of one solute is coupled to the “uphill” transport of a
second substance against its own concentration gradient.
●​ Examples:
1.​ In Plants: Proton pumps generate an H+ gradient across the cell membrane,
storing potential energy.
2.​ A cotransporter couples the movement of H+ back down its concentration
gradient to the active transport of sucrose into the cell. This process allows plants
to load sucrose into their veins for transport around the plant body.
Na+/K+ Pumps and Cellular Function
●​ Function: Na+/K+ pumps actively transport Na+ out of the cell to maintain the
electrochemical gradient.
●​ Cotransporter Connection: Animal cells utilize a cotransporter to couple the active
transport of glucose to the diffusion of Na+ into cells lining the intestine.
●​ Clinical Implication: In diarrhea, Na+ is expelled too rapidly for reabsorption, which can
lead to dangerous drops in sodium levels.
○​ Solution: Drinking a concentrated salt (NaCl) and glucose solution facilitates
uptake through Na+/glucose transporters in the intestine.
Bulk Transport Across the Plasma Membrane
●​ Mechanisms: Bulk transport occurs via exocytosis and endocytosis.
○​ Small Molecules and Water: Can enter or leave the cell through the lipid bilayer
or via transport proteins.
○​ Large Molecules: Such as polysaccharides and proteins, cross the membrane in
bulk inside vesicles.
Exocytosis
●​ Process: Transport vesicles migrate to the plasma membrane, fuse with it, and release
their contents outside the cell.
1.​ Commonly used by secretory cells to export products (e.g., pancreas cells
secrete insulin via exocytosis).
●​ Steps:
1.​ Secretory proteins in vesicle bud off from the Golgi apparatus.
2.​ Vesicle travels along microtubules to the plasma membrane.
3.​ Vesicle binds to the plasma membrane.
4.​ Two membranes fuse together, releasing proteins outside the cell.
5.​ Vesicle membrane becomes part of the plasma membrane.
Endocytosis
●​ Definition: Macromolecules are taken into the cell within vesicles formed by the plasma
membrane.
●​ Mechanism: The membrane forms a pocket that deepens and pinches off, creating a
vesicle around the material for transport.
●​ Types of Endocytosis:
1.​ Phagocytosis (“cellular eating”): Cells engulf particles by extending pseudopodia
to form food vacuoles, which are digested upon fusion with lysosomes.
2.​ Pinocytosis (“cellular drinking”): Involves engulfing extracellular fluid.
3.​ Receptor-Mediated Endocytosis: Specific uptake of molecules after binding to
receptors.
Chapter 11: Cell Communication
The Impala and Cell Signaling
●​ Scenario: When an impala senses a cheetah, the brain signals the adrenal glands to
release epinephrine into the blood, fueling a desperate flight.
●​ Cell Signaling Steps:
○​ Signal Reception: An epinephrine molecule binds to a receptor on a muscle cell.
○​ Signal Transduction: Relay molecules transmit the signal, activating an
enzyme.
○​ Cellular Response: The enzyme breaks down glycogen into glucose, providing
energy for leg muscles.
Quorum Sensing in Bacteria
●​ Definition: Allows bacteria to coordinate their behavior like multicellular organisms.
●​ Biofilm Formation: An aggregation of bacterial cells creating a protective environment
and providing nutrients.
○​ Example: Plaques on teeth; up to 80% of bacterial infections are caused by
biofilms.
Quorum Sensing Mechanism
●​ Involves Autoinducers: Molecules enabling bacteria to sense population density.
●​ Process: At low cell density, the inducer diffuses, resulting in low light emission; at high
density, the inducer activates luminescent systems across cells, resulting in high
luminescence.
Communication in Yeast
●​ Yeast Communication: Saccharomyces cerevisiae use quorum sensing to identify
mates for sexual reproduction; two mating types (a and α) communicate via secreted
factors.
○​ This leads to cellular fusion for mating.
Communication in Multicellular Organisms
●​ Complexity: In multicellular organisms, communication is more sophisticated compared
to unicellular methods.
○​ Signal Transduction Pathways: Modify cell behavior and gene expression.
○​ Types of Communication:
○​ Local Signaling: Involves direct communication through cell junctions or indirect
through signaling molecules traveling short distances.
○​ Long-distance Signaling: Utilizes molecules like hormones for signaling
between distant organs.