Anatomy and Physiology
Chapter 3 Notes – Cells
Cell Theory:
1. A cell is the basic structural and functional unit of living
organisms.
2. The activity of an organism depends both on the individual
and the collective activities of its cells.
3. According to the principal of complementarity, the
biochemical activities of cells are dictated by the relative
number of their specific subcellular structures.
4. Continuity of life has a cellular basis
The cell is the smallest living unit. It contains all the parts
necessary to survive in an ever changing world.
There are over 200 different types of cells in the human body.
Depending on the type, cells can range in size from 2
micrometers to over a meter in length. A cells shape reflects its
function.
All cells have the same basic parts and some common
functions:
3 main parts: the plasma membrane, cytoplasm, the nucleus.
Plasma Membrane Structure:
1. Encases the cell contents, mediates exchanges with the
extracellular environment and plays a role in cellular
communication.
2. The fluid mosaic model shows the plasma membrane as a
fluid bilayer of lipids (phospholipids, cholesterol and
glycolipids) within which proteins are inserted
3. Lipids have both hydrophilic (H2O loving) and hydrophobic
H2O avoiding) regions that organize their aggregation and
self-repair. The lipids form the structural part of the
plasma membrane.
4. Most proteins are integral trans-membrane proteins that
extend through the entire membrane, some are peripheral
proteins (append from the integral proteins)
5. Proteins are responsible for most specialized membrane
functions: enzymes, receptors, membrane transport.
External facing glycoproteins contribute to the glycocalyx
(sugar coating on the surface of the cell membrane which
is highly specific biological marker). Every cell has a
different pattern of sugar so cells can identify each other.
6. Microvilli are extensions of the plasma membrane
increasing its surface area for absorption.
7. Membrane junctions join cells together and may aid or
inhibit movement of molecules between or past cells.
Tight junctions are impermeable junctions; desmosomes
mechanically couple cells into a functional community; gap
junctions allow joined cells to communicate.
Plasma membrane functions:
1. Is selectively permeable barrier. Substances move across
the membrane by passive process, depends on the kinetic
energy of the molecules or on pressure gradients; by
active processes which depend on the use of ATP
2. Diffusion is the movement of molecules (kinetic energy)
down a concentration gradient. Fat soluble molecules can
diffuse directly through the membrane by dissolving in the
lipid
3. Facilitated diffusion is the passive movement of certain
solutes across the membrane either by binding with the
membrane carrier protein or by moving through the
membrane channel. (the carriers and channels are
selective)
4. Osmosis is the diffusion of a solvent (ie water) through a
selectively permeable membrane. Water diffuses through
membrane pores (aquaporins) or directly through the lipid
portion of the membrane from a solution of lesser
osmolarity to a solution of greater osmolarity.
5. The presence of a solute that is unable to permeate the
plasma membrane leads to changes in the cell tone that
may cause the cell to swell or shrink. Net osmosis stops
when the solute concentration on both sides of the
membrane reaches equilibrium
6. Solutions that cause a net loss of water from the cells are
hypertonic; those the cause a net gain in water is
hypotonic; and those causing neither gain or loss is
isotonic.
7. Filtration occurs when a filtrate is forced across a
membrane by hydrostatic pressure. It is non-selective and
limited only by pore size. Pressure gradient is driving force.
8. Active transport (solute pumping) depends on the carrier
protein and ATP. Substances are transported against the
concentration or electrical gradients.
9. Vesicular transport also requires ATP. Exocytosis ejects
substances (hormones, wastes, secretions) from the cell.
Endocytosis brings substances into the cell in protein
coated vesicles. If the substance is particulate the process
is called phagocytosis; if the substance is dissolved the
process is called pinocytosis.
End day 1
Generating and maintaining a Resting Membrane potential:
1. All cells in a resting stage exhibit a voltage across their
membrane called a resting membrane potential. Because
of the membrane’s potential, both concentration and
electrical gradients determine the ease of a ion’s
diffusion.
2. The resting membrane potential is generated by
concentration gradients of and differential permeability of
the plasma membrane to ions (particularly potassium).
Sodium is in high extra-cellular -> low intra-cellular
concentration, and the membrane is poorly permeable to
it. Potassium is in high concentration in the cell and low
concentration in the extra-cellular fluid. The membrane is
more permeable to Potassium than to sodium.
3. Essentially, the negative membrane potential is
established when the movement of K+ out of the cell
equals the movement of K+ into the cell. Sodium
movement contributes little to establishing the potential
of the membrane. The charge separation is maintained by
the sodium-potassium pump (maintains both the
membrane potential and osmotic balance)
Cell environment interactions:
1. Cells interact directly and indirectly with other cells.
Indirect interactions involve extracellular chemicals carried
in body fluids or forming part of the extracellular matrix.
2. Molecules of glycocalyx are intimately involved in the cellenvironment interactions. Most are cell adhesion
molecules (CAMs play a key role in embryonic
development and wound repair) or membrane receptors.
3. Activated membrane receptors act as catalysts, regulate
channels, or like G-protein-linked receptors, act through
second messengers such as cyclic AMP or Ca2+. Ligand
(signaling chemicals that bind specifically to plasma
membrane receptors) binding results in changes in the
protein structure or function within the target cell.
Cytoplasm:
1. The cellular region between the nuclear and plasma
membranes. Consists of the cytosol (fluid in cytoplasmic
environment), inclusions (nonliving nutrients stores – lipid
droplets, glycosomes – pigment granules, crystals, etc) and
cytoplasmic organelles.
2. Cytoplasm is the major functional area of the cell. These
functions are mediated by organelles.
3. Mitochondria: organelles limited by a double membrane,
are sites of ATP formation. Their internal enzymes carry
out the oxidative reactions of cellular respiration.
4. Ribosomes: composed of 2 subunits containing ribosomal
RNA (rRNA) and proteins, are the sites for protein
synthesis. They may be free or attached to membranes.
5. The Rough Endoplasmic Reticulum: is a ribosome studded
membrane system. Its cisternae act as sites for protein
modification. Its external face acts in phospholipid
synthesis. Vesicles pinched off from the ER transport the
proteins to other cell sites.
6. The Smooth Endoplasmic Reticulum: synthesizes lipid and
steroid molecules. It also acts in fat metabolism and in
drug detoxification. In muscle cells, it is a calcium ion
depot.
7. The Golgi Apparatus: is a membranous system close to the
nucleus that packages protein secretions for export,
packages enzymes into lysosomes for cellular use, and
modifies proteins destined to become part of cellular
membranes.
8. Lysosomes: are membranous sacs of acid hydrolases
packaged in the Golgi apparatus. Sites of intracellular
digestion, they degrade worn out organelles, and tissues
that are no longer useful and release ionic calcium from
bone.
9. Peroxisomes: are membranous sacs containing oxidase
enzymes that protect the cell from the destructive effects
of free radicals (highly reactive chemicals with unpaired
electrons that can scramble the structure of biological
molecules) and other toxic substances by converting them
first to hydrogen peroxide and then water.
10.
Cytoskeleton: (cell skeleton) a network of rods that
acts as the cells “bone’, “muscles”, and “ligaments” by
supporting the cellular structures and providing the
machinery to generate various cell movements. There are
3 types of rods: microtubules, microfilaments,
intermediate filaments.
a. Microtubules: hollow tubes made of spherical protein
subunits called tubulins. Most radiate from a small
region of cytoplasm near the nucleus called the
centrosome. They determine the shape of the cell as
well as the distribution of cellular organelles.
b. Microfilaments: the thinnest elements of the
cytoskeleton. Made of the protein actin (ray). each
cell has its own unique arrangement. No 2 cells are
alike. Most are involved in cell motility or changes in
cell shape. When combined with unconventional
myosin they generate contractile forces in a cell.
c. Intermediate filaments: constructed like woven rope
they are the most stable and permanent of the three
and have a high tensile strength. They attach to
desmosomes and their main function is to resist the
pulling forces exerted on the cell.
11.
Centrosomes and Centrioles: centrosomes act as a
microtubule organizing center. The granular matrix
contains centrioles (small barrel shaped organelles
oriented at right angles to one another). Their major
function is organizing the mitotic spindles in cell division.
Centrioles also form the bases of cilia and flagella (basal
bodies).
12.
Cilia and flagella: are whip like, motile cellular
extensions. Ciliary actions moves substances in one
direction across cell surfaces.
13.
Nucleus: the control center of the cell. It contains the
genes which hold the instructions to build nearly all the
body’s proteins. Some cells are multinucleate (multiple
nuclei, skeletal muscle cells). Mature red blood cells are
anucleate (without a nucleus). These cells cannot
reproduce so only live 3-4 months then start to
deteriorate. The nucleus is the largest of all organelles. It
has 3 recognizable regions or structures: the nuclear
envelope (membrane), nucleoi and chromatin.
a. Nuclear envelope: a double layer barrier separated
by fluid-filled space (similar to mitochondrial
membrane). The outer membrane is continuous with
the Rough ER of the cytoplasm and is studded with
ribosomes. The inner layer is lined by the nuclear
lamina (network of lamins – intermediate filaments)
that maintains the shape of the nucleus and acts as a
scaffold to organize the DNA. It is selectively
permeable
b. Nucleoli: they aren’t membrane bound. Typically
there are 1-2 nucleoli in a nucleus. Associated with
the synthesis of mRNA.
c. Chromatin: a system of bumpy threads that reside in
the nucleoplasm. Composed of about 30% DNA, 60%
globular histone protein and about 10% RNA chains.
Histones play an important role in gene regulation.
When the cells are about to divide, the chromatin
condenses to form chromosomes. END day2
Cell Growth/reproduction: Cell Life Cycle
The series changes a cell goes through from the time it is
formed until it reproduces, encompasses two major
periods: Interphase in which the cell grows and carries on
its usual activities, and Cell division (mitotic phase) during
which it divides into 2 cells.
Interphase: the period from cell formation to cell division.
This is the non-dividing phase of cell life cycle. Consists of
3 sub phases: G1, S1 and G2.
G1: the cell grows and the centriole replication begins
S1: DNA replicates
G2: the final preparation for cell division occurs
Many check points occur during interphase where the cell
gets signaled to continue through mitosis or to stop and is
prevented from continuing mitosis.
DNA replication: occurs before cell division; ensuring that
the daughter cells have identical genes. The DNA helix
unravels and each DNA nucleotide strand acts as a
template for the formation of a complementary strand.
Base pairing provides the guide for the proper positioning
of nucleotides. The daughter cells then have a complete
set of DNA identical to the parent cell made from one
“old” strand and one “new” strand.
Cell division: essential for body growth and repair, occurs
during the mitotic phase (M phase). Cell division is
stimulated by certain chemicals (including growth factors
and some hormones) and increasing cell size. Lack of
space and inhibitory chemicals deter cell division. Cell
division is regulated by cyclin-Cdk (cyclin dependent
kinases) complexes (ex. MPF, M-phase promoting factor)
This lets the cell know whether to proceed or not.
Events of Cell Division: Mitosis and Cytokinesis
Mitosis is described in 4 phases:
Prophase, Metaphase, Anaphase, Telophase. This all
actually flows together in a continuous process. In
humans this process usually takes about an hour or less
from start to finish.
Cytokinesis: the division of the cytoplasm begins during
late anaphase and is completed after mitosis ends. The
plasma membrane is drawn in at the center to form a
“cleavage furrow” by the activity of actin filaments. The
cell is pinched into 2 cells, each smaller than the mother
cell but genetically identical. These cells then enter into
interphase portion of the life cycle.
Protein Synthesis:
Cells are mini protein factories that synthesize a huge
variety of proteins that determine the chemical and
physical nature of cells – the whole body.
A gene is defined as a DNA segment that provides
instructions for the synthesis of one polypeptide chain.
Since the major structural materials of the body are
proteins, and all enzymes are proteins, this covers the
synthesis of all biological molecules.
The base sequence of exon (amino acid-specifying
informational sequences) DNA provides the information
for protein structure. Each 3 base sequence (triplet) calls
for a particular amino acid to be built into a polypeptide
chain.
The RNA molecules acting in protein synthesis are
synthesized on single strands of the DNA template. RNA
nucleotides are joined following the base pairing rules.
Ribosomal RNA (rRNA)forms part of the protein synthesis
sites; messenger RNA (mRNA) carries instructions for
making a polypeptide chain from the DNA to the
ribosomes; transfer RNA (tRNA) ferries amino acids to the
ribosomes and recognizes codons on the mRNA strand
specifying its amino acid.
Protein synthesis involves transcription: synthesis of a
complementary strand mRNA and translation: “reading”
of the mRNA by the tRNA and peptide bonding of the
amino acids into a polypeptide chain. Ribosomes
coordinate translation.