Chapter 3 Lecture Outline
I.
INTRODUCTION
A.
A
cell is the basic, living, structural, and functional unit of the body.
B.
Cytology
is the study of cell structure, and cell physiology is the study of cell
function.
II.
PARTS of a CELL
A.
A
generalized view of the cell is a composite of many different cells in the body
as seen in Figure 3.1. No single cell includes all of the features seen
in the generalized cell.
B.
The
cell can be divided into three principal parts for ease of study.
1.
Plasma (cell) membrane
2.
Cytoplasm
a.
Cytosol
b.
Organelles
(except for the nucleus)
3.
Nucleus
III. THE PLASMA MEMBRANE
A.
The
plasma membrane is a flexible, sturdy barrier that surrounds and contains the
cytoplasm of the cell.
1.
The
fluid mosaic model describes its structure (Figure 3.2).
2.
The
membrane consists of proteins in a sea of lipids.
B.
The
Lipid Bilayer
1.
The
lipid bilayer is the basic framework of the plasma
membrane and is made up of three types of lipid molecules: phospholipids,
cholesterol, and glycolipids (Figure 3.2).
2.
The
bilayer arrangement occurs because the lipids are amphipathic molecules. They have both polar (charged) and nonpolar (uncharged) parts with the polar “head” of the phospholipid pointing out and the nonpolar
“tail” pointing toward the center of the membrane.
C.
Arrangement of Membrane Proteins
1.
The
membrane proteins are divided into integral and peripheral proteins.
a.
Integral
proteins extend into or across the entire lipid bilayer
among the fatty acid tails of the phospholipid
molecules.
b.
Peripheral
proteins are found at the inner or outer surface of the membrane and can be
stripped away from the membrane without disturbing membrane integrity.
2.
Integral
membrane proteins are amphipathic.
a.
Those
that stretch across the entire bilayer and project on
both sides of the membrane are termed transmembrane
proteins.
b.
Many
integral proteins are glycoproteins.
3.
The
combined glycoproteins and glycolipids
form the glycocalyx which helps cells recognize one
another, adhere to one another, and be protected from digestion by enzymes in
the extracellular fluid.
D.
Functions
of Membrane Proteins
1.
Membrane
proteins vary in different cells and function as channels (pores),
transporters, receptors, enzymes, cell-identity markers, and linkers (Figure
3.3).
2.
The
different proteins help to determine many of the functions of the plasma
membrane.
E.
Membrane
Fluidity
1.
Membranes
are fluid structures, rather like cooking oil, because most of the membrane
lipids and many of the membrane proteins easily move in the bilayer.
2.
Membrane
lipids and proteins are mobile in their own half of the bilayer.
3.
Cholesterol
serves to stabilize the membrane and reduce membrane fluidity.
F.
Membrane
Permeability
1.
Plasma
membranes are selectively permeable, meaning that some things can pass through
and others cannot.
2.
The
lipid bilayer portion of the membrane is permeable to
small, nonpolar, uncharged molecules but impermeable
to ions and charged or polar molecules. It is also permeable to water.
3.
Transmembrane proteins that act as channels or transporters increase the
permeability of the membrane to molecules that cannot cross the lipid bilayer.
4.
Macromolecules
are unable to pass through the plasma membrane except by vesicular transport.
G.
Gradients
Across the Plasma Membrane
1.
A
concentration gradient is the difference in the concentration of a chemical
between one side of the plasma membrane and the other.
a.
Oxygen
and sodium ions are more concentrated outside the cell membrane with carbon
dioxide and potassium ions more concentrated inside the cell membrane (Figure
3.4a).
b.
The
inner surface of the membrane is more negatively charged and the outer surface
is more positively charged (Figure 3.4b). This sets up an electrical
gradient, also called the membrane potential.
2.
Maintaining
the concentration and electrical gradients are important to the life of the
cell.
3.
The
combined concentration and electrical gradients are called the electrochemical
gradient.
IV.
TRANSPORT
ACROSS THE PLASMA MEMBRANE
A.
Processes
to move substances across the cell membrane are essential to the life of the
cell.
1.
Some
substances cross the lipid bilayer while others cross
through ion channels.
2.
Transport
processes that move substances across the cell membrane are either active or
passive. (Figure 3.5)
a.
Three types of passive processes are
diffusion through the lipid bilayer, diffusion
through ion channels, and facilitated diffusion
b.
Active transport requires cellular
energy.
3.
Materials
can also enter or leave the cell through vesicle transport.
B.
Principles
of Diffusion
1.
Diffusion is the random mixing of particles
that occurs in a solution as a result of the kinetic energy of the particles.
(Figure 3.6)
2.
Diffusion rate across plasma
membranes is influenced by several factors: steepness of the concentration
gradient, temperature, size or mass of the diffusing substance, surface area,
and diffusion distance.
C.
Osmosis is the
net movement of a solvent through a selectively permeable membrane, or in
living systems, the movement of water (the solute) from an area of higher
concentration to an area of lower concentration across the membrane (Figure
3.7).
1.
Water
molecules penetrate the membrane by diffusion through the lipid bilayer or through aquaporins, transmembrane proteins that function as water channels.
2.
Water
moves from an area of lower solute concentration to an area of higher solute
concentration.
3.
Osmosis
occurs only when the membrane is permeable to water but not to certain solutes.
4.
Tonicity of a
solution relates to how the solution influences the shape of body cells.
a.
In
an isotonic solution, red blood cells maintain their normal shape (Figure
3.8a).
b.
In
a hypotonic solution, red blood cells undergo hemolysis
(Figure 3.8b).
c.
In
a hypertonic solution, red blood cells undergo crenation
(Figure 3.8c).
D.
Diffusion
Through the Lipid Bilayer
1.
Nonpolar, hydrophobic molecules such as respiratory gases, some
lipids, small alcohols, and ammonia can diffuse across the lipid bilayer.
2.
It
is important for gas exchange, absorption of some nutrients, and excretion of
some wastes.
E.
Diffusion
Through Membrane Channels
1.
Most
membrane channels are ion channels, allowing passage of small, inorganic ions
which are hydrophilic.
2.
Ion
channels are selective and specific and may be gated or open all the time (Figure
3.9).
F.
Facilitated
Diffusion
1.
In
facilitated diffusion, a solute binds to a specific transporter on one
side of the membrane and is released on the other side after the transporter
undergoes a conformational change.
G.
Active
Transport
1.
Active
transport is an energy-requiring process that moves solutes such as ions, amino
acids, and monosaccharides against a concentration
gradient.
2.
Primary
Active Transport
a.
In
primary active transport, energy derived from ATP changes the shape of a
transporter protein, which pumps a substance across a plasma membrane against
its concentration gradient.
b.
The
most prevalent primary active transport mechanism is the sodium ion/potassium
ion pump (Figure 3.11).
3.
Secondary
Active Transport
a.
In
secondary active transport, the energy stored in the form of a sodium or hydrogen ion concentration gradient is used to
drive other substances against their own concentration gradients.
b.
Plasma
membranes contain several antiporters and symporters powered by the sodium ion gradient (Figure
3.12).
H.
Transport
in Vesicles
1.
A
vesicle is a small membranous sac formed by budding off from an existing
membrane.
2.
Two
types of vesicular transport are endocytosis and exocytosis.
A.
Endocytosis
In endocytosis,
materials move into a cell in a vesicle formed from the plasma
membrane.
1) Receptor-mediated endocytosis is the selective uptake of large
molecules and particles by cells (Fig 3.13).
a)
The
steps of receptor-mediated endocytosis
includes binding, vesicle formation, uncoating,
fusion and endosome formation, recycling of
receptors, degradation in lysosomes.
b)
Viruses
can take advantage of this mechanism to enter cells.
2) Phagocytosis is the ingestion of solid particles
(Figure 3.14).
3)
Pinocytosis is the ingestion of extracellular
fluid (Figure 3.15).
B.
Exocytosis
In exocytosis,
membrane-enclosed structures called secretory
vesicles that form inside the cell fuse with the plasma membrane and release
their contents into the extracellular fluid (Figures
3.13 through 3.15).
In transcytosis,
vesicles undergo endocytosis on one side of a cell,
move across the cell, and then undergo exocytosis on
the opposite side.
I.
Table 3.1
summarizes the processes by which materials are transported into and out of
cells.
V.
CYTOPLASM
A.
Cytosol, the intracellular fluid, is the semifluid
portion of cytoplasm that contains inclusions and dissolved solutes (Figure
3.1).
1.
Cytosol is composed mostly of water, plus proteins, carbohydrates,
lipids, and inorganic substances.
2.
The
chemicals in cytosol are either in solution or in a
colloidal (suspended) form.
3.
Functionally,
cytosol is the medium in which many metabolic
reactions occur.
B.
Organelles
1.
Organelles
are specialized structures that have characteristic shapes and perform specific
functions in cellular growth, maintenance, and reproduction.
2.
The
Cytoskeleton
a.
The
cytoskeleton is a network of several kinds of protein filaments that
extend throughout the cytoplasm and provides a structural framework for the
cell (Figure 3.16).
b.
It
consists of microfilaments, intermediate filaments, and microtubules.
1)
Most
microfilaments are composed of actin and function in
movement and mechanical support.
2)
Intermediate
filaments are composed of several different proteins and function in support
and to help anchor organelles such as the nucleus (Figure 3.16).
3)
Microtubules
are composed of a protein called tubulin and help
determine cell shape and function in the intracellular transport of organelles
and the migration of chromosome during cell division. (Figure 3.16)
3.
Centrosomes are dense areas of cytoplasm
containing the centrioles, which are paired
cylinders arranged at right angles to one another, and serve as centers for
organizing microtubules in interphase cells and the
mitotic spindle during cell division. (Figure 3.17)
4.
Cilia
and Flagella
a.
Cilia are
numerous, short, hairlike projections extending from
the surface of a cell and functioning to move materials across the surface of
the cell (Figure. 3.18).
b.
Flagella are
similar to cilia but are much longer; usually moving an entire cell. The only
example of a flagellum in the human body is the sperm cell tail (Figure 3.18).
5.
Ribosomes
a.
Ribosomes are tiny spheres consisting of ribosomal RNA and
several ribosomal proteins; they occur free (singly or in clusters) or together
with endoplasmic reticulum (Fig 3.19).
b.
Functionally,
ribosomes are the sites of protein synthesis.
6.
Endoplasmic
Reticulum
a.
The
endoplasmic reticulum (ER) is a network of membranes that form flattened
sacs or tubules called cisterns (Figure 3.20).
b.
Rough ER is
continuous with the nuclear membrane and has its outer surface studded with ribosomes.
c.
Smooth ER
extends from the rough ER to form a network of membrane tubules but does not
contain ribosomes on its membrane surface.
d.
The
ER transports substances, stores newly synthesized molecules, synthesizes and
packages molecules, detoxifies chemicals, and releases calcium ions involved in
muscle contraction.
7.
Golgi Complex
a.
The
Golgi complex consists of four to six
stacked, flattened membranous sacs (cisterns) referred to as cis, medial, and trans (Figure
3.21).
b.
The
principal function of the Golgi complex is to
process, sort, and deliver proteins and lipids to the plasma membrane, lysosomes, and secretory vesicles
(Figure 3.22).
8.
Lysosomes
a.
Lysosomes are membrane-enclosed vesicles that form in the Golgi complex and contain powerful digestive enzymes
(Figure 3.23).
b.
Lysosomes function in intracellular digestion, digestion of worn-out
organelles (autophagy), digestion of cellular
contents (autolysis) during embryological development, and extracellular
digestion.
9.
Perioxosomes
a.
Peroxisomes are similar in structure to lysosomes,
but are smaller.
b.
They
contain enzymes (e.g., catalase) that use molecular
oxygen to oxidize various organic substances.
10. Proteosomes
a.
Proteosomes are structures that destroy
unneeded, damaged, or faulty proteins.
b.
They contain proteases which cut
proteins into small peptides.
c.
Proteosomes are thought to be a factor in
several diseases.
11. Mitochondria
a.
The
mitochondrion is bound by a double membrane. The outer membrane is
smooth with the inner membrane arranged in folds called cristae
(Figure 3.24).
b.
Mitochondria
are the site of ATP production in the cell by the catabolism of nutrient
molecules.
c.
Mitochondria
self-replicate using their own DNA.
d.
Mitochondrial DNA (genes) is usually inherited
only from the mother.
A.
The
nucleus is usually the most prominent feature of a cell (Figure 3.25).
B.
Most
body cells have a single nucleus; some (red blood cells) have none, whereas
others (skeletal muscle fibers) have several.
C.
The
parts of the nucleus include the nuclear envelope (which is perforated by
channels called nuclear pores), nucleoli, and genetic material (DNA),
D.
Within
the nucleus are the cell’s hereditary units, called genes, which are
arranged in single file along chromosomes.
1.
Each
chromosome is a long molecule of DNA that is coiled together with several
proteins (Figure 3.26).
2.
Human
somatic cells have 46 chromosomes arranged in 23 pairs.
E.
The
various levels of DNA packing are represented by nucleosomes,
chromatin fibers, loops, chromatids, and chromosomes.
F.
The
main parts of a cell and their functions are summarized in Table 3.2.
VII.
PROTEIN
SYNTHESIS
A.
Much
of the cellular machinery is devoted to synthesizing large numbers of diverse
proteins.
1.
The
proteins determine the physical and chemical characteristics of cells.
2.
The
instructions for protein synthesis are found in the DNA in the nucleus.
VIII.
CELL
DIVISION
A.
Cell
division is the process by which cells reproduce themselves. It consists of nuclear
division (mitosis or meiosis) and cytoplasmic
division (cytokinesis).
1.
Cell division
that results in an increase in body cells is called somatic cell division
and involves a nuclear division called mitosis, plus cytokinesis.
2.
Cell
division that results in the production of sperm and eggs is called reproductive
cell division and consists of a nuclear division called meiosis plus
cytokinesis.
B.
The
Cell Cycle in Somatic Cells
1.
The
cell cycle is an orderly sequence of events by which a cell duplicates
its contents and divides in two. It consists of interphase
and the mitotic phase (Figure 3.31).
2.
Interphase
a.
During
interphase the cell carries on every life
process except division. Interphase consists of three
phases: G1, S and G2 (Figure 3.31).
1)
In
the G1 phase, the cell is metabolically active, duplicating its
organelles and cytosolic components except for DNA.
2)
In
the S phase, chromosomes are replicated (Figure 3.32).
3)
In
the G2 phase, cell growth continues and the cell completes its
preparation for cell division.
b.
A
cell in interphase shows a distinct nucleus and the
absence of chromosomes (Figure 3.33a).
3.
Mitotic
Phase
a.
The
mitotic phase consists of mitosis (or nuclear division) and cytokinesis
(or cytoplasmic division).
b.
Nuclear
division: mitosis
1)
Mitosis is the
distribution of two sets of chromosomes, one set into each of two separate
nuclei.
2)
Stages
of mitosis are prophase, metaphase, anaphase, and telophase.
a)
During
prophase, the chromatin condenses and shortens into chromosomes (Figure
3.33b). The nuclear envelope “disappears”, the nucleolus “disappears, and the
spindle forms as the centrioles move to the poles of
the cell.
b)
During
metaphase, the centromeres line up at the exact center of the mitotic
spindle, a region called the metaphase plate or equatorial plane region (Figure
3.33c).
c)
Anaphase is
characterized by the splitting and separation of centromeres
and the movement of the two sister chromatids of each
pair toward opposite poles of the cell (Figure 3.33d).
d)
Telophase begins as soon as chromatid
movement stops; the identical sets of chromosomes at opposite poles of the cell
uncoil and revert to their threadlike chromatin form, centrioles
move to center of cell, a new nuclear envelope forms, new nucleoli appear, and
the new mitotic spindle eventually breaks up.
c.
Cytoplasmic Division: Cytokinesis
1)
Cytokinesis is the division of a parent cell’s cytoplasm and
organelles. The process begins in late anaphase or early telophase
with the formation of a cleavage furrow (Figure 3.33 e).
2)
When
cytokinesis is complete, interphase
begins (Figure 3.33f).
D.
Control
of Cell Destiny
1.
The
three possible destinies of a cell are to remain alive and functioning without
dividing, to grow and divide, or to die.
2.
Maturation
promoting factor (MPF) induces cell division.
3.
Cell
death, a process called apoptosis, is triggered either from outside the
cell or from inside the cell due to a “cell-suicide” gene.
4.
Necrosis is a
pathological cell death due to injury.
D.
Tumor-suppressor
genes can produce proteins that normally inhibit cell division resulting in the
uncontrollable cell growth known as cancer.
IX.
CELLULAR
DIVERSITY
A.
Not
all cells look alike, nor do they perform identical functional roles in the
body.
B.
The
shapes of cells vary considerably (Figure 3.34).