Anatomy and Physiology
Cells
Introduction
• There are 75 trillion cells that make up the
human body.
• Cells of different tissues have much in
common but vary in size and shape.
• Typically, their shapes make possible their
functions. Ex: nerve cells have long,
threadlike extensions that transmit
bioelectrical impulses.
Composite Cell
• A “typical” cell consists of three major
parts--the nucleus, the cytoplasm, and the
cell membrane.
• The nucleus is usually in the center of the
cell and is surrounded by a thin nuclear
envelope.
• Cytoplasm surrounds the nucleus and is
encircled by the thinner cell membrane
(plasma membrane).
A Typical Cell
Organelles
• Within the cytoplasm are specialized
structures called cytoplasmic organelles,
which are suspended in a liquid called
cytosol.
• Organelles (little organs) perform specific
functions which are directed by the nucleus.
• The cell membrane determines which
substances enter or leave the cell.
Cell Membrane
• The cell membrane regulates movement of
substances in and out of the cell and is the
site of much biological activity.
• Molecules that are part of the cell
membrane receive stimulation from outside
the cell and transmit it into the cell, a
process called signal transduction.
• The cell membrane also holds cells together.
Cell Membrane Characteristics
• The cell membrane is extremely thin but
flexible and elastic.
• The cell membrane is selectively permeable
(semipermeable) in that it controls which
substances enter and exit.
Cell Membrane Structure
• A cell membrane is composed mainly of
lipids, proteins, and some carbohydrates.
• It is a bilayer of phospholipid molecules.
• Each phospholipid molecule includes a
phosphate group and two fatty acids bound
to a glycerol molecule.
• The lipid molecules can move around
forming a soft and flexible fluid film.
Cell Membrane Structure
Membrane Solubility
• Because the membrane’s interior consists of
fatty acids, it is oily. Only substances
soluble in lipids can pass through this layer.
• O2 and CO2 can pass through easily but
water-soluble molecules, such as amino
acids, sugars, proteins, nucleic acids, and
various ions, cannot pass through.
• Cholesterol molecules help to stabilize the
membrane.
Membrane Proteins
• Membrane proteins are classified according
to their positions within the membrane.
• Membrane-spanning (trans-membrane)
proteins extend through the lipid bilayer and
may protrude from 1 or both faces.
• Peripheral membrane proteins are
associated with one side of the
bilayer.Membrane proteins also vary in
shape--globular or elongated.
Membrane Protein Functions
• Some proteins form receptors on the cell
surface that bind incoming hormones or
growth factors, starting signal transduction.
• Others transport ions or molecules across
the cell membrane.
• Some membrane proteins form selective
channels that allow only particular ions to
enter or leave.
Membrane Proteins Illustrated
More Protein Functions
• Proteins that protrude from the inner face of
the cell anchor it to the cytoskeleton (rods
and tubules that support the inner cell).
• Proteins that extend from the outer surface
mark the cell as part of a particular tissue or
organ--important identification for the
immune system.
• CAM (cellular adhesion molecule)
determines a cell’s interactions with other
cells.
CAM
Cytoplasm
• Cytoplasm contains networks of membranes
and organelles suspended in a clear liquid
called cytosol.
• Cytoplasm also includes many protein rods
and tubules that form a framework called a
cytoskeleton.
• Cell activities occur mainly in the
cytoplasm, where nutrients are received,
processed, and used.
Cytoplasmic Organelles
• Endoplasmic
Reticulum
• Ribosomes
• Golgi Apparatus
• Mitochondria
• Lysosomes
• Peroxisomes
• Microfilaments and
microtubules
• Centrosome
• Cilia and Flagella
• Vesicles
• Cell Nucleus
• nucleolus
• chromatin
Organelles
Endoplasmic Reticulum
• Endoplasmic Reticulum (ER) is composed
of membrane-bound, flattened sacs,
elongated canals, and fluid-filled bubblelike sacs called vesicles.
• ER provides a vast tubular network that
transports molecules from one cell part to
another.
Functions of ER
• ER participates in synthesis of
protein and lipid molecules.
• These molecules may leave the
cell as secretions, or be used
within the cell to produce new
ER or cell membranes as the
cell grows.
Two Types of ER
• Rough ER contains ribosomes on its
surface; smooth ER is ribosome-free.
• The ribosomes of rough ER are sites or
protein synthesis. The proteins may then
move through ER tubules to the Golgi
apparatus for further processing.
• Smooth ER contains enzymes important in
lipid synthesis.
Smooth and Rough ER
Ribosomes
• All ribosomes are composed of protein and
RNA molecules.
• Many ribosomes are attached to ER and
others are scattered throughout the
cytoplasm.
• Ribosomes provide enzymes as well as a
structural support for the RNA molecules
that come together as the cell synthesizes
proteins from amino acids.
Ribosomes
Golgi Apparatus
• The Golgi apparatus is composed of a stack
of about 6 flattened, membranous sacs.
• This organelle refines, packages, and
delivers proteins synthesized on ribosomes
associated with the ER.
• Proteins arrive at the Golgi apparatus
enclosed in vesicles composed of the ER
membrane.
Golgi Apparatus
Golgi Function
• Vesicles arriving at the Golgi apparatus fuse at the
innermost end, which is specialized to receive
glycoproteins.
• As the glycoproteins pass from layer to layer of
the Golgi, they are modified chemically.
• When they reach the outermost layer, they are
packaged in bits of Golgi membrane, which bud
off and form transport vesicles.
• Vesicles may move to the cell membrane and
release its contents to the outside or be used within
the cell--vesicle trafficking.
Vesicle Formation
Mitochondria
• Mitochondria are elongated, fluid-filled sacs
that have an outer and inner layer.
• The inner layer folds extensively to form
partitions called cristae.
• Cristae contain enzymes that control
chemical reactions that release energy from
glucose.
• Mitochondria are the major sites for cellular
respiration--the power house of the cell.
More Mitochondria
• Mitochondria contain their own DNA-much like that of bacteria.
• According to the endosymbiont theory,
mitochondria are the remnants of once freeliving bacteria-like cells that were
swallowed by more complex primitive cells.
• You can only inherit mitochondria from
your mother.
Lysosomes
• Lysosomes, the ‘garbage disposals of the
cell’, are tiny membranous sacs containing
powerful enzymes that break down nutrient
molecules or foreign particles.
• Certain white blood cells can engulf
bacteria which are then digested by the
lysosomal enzymes.
• Lysosomes also destroy worn cellular parts.
Peroxisomes
• Peroxisomes, membranous sacs abundant in
liver and kidney cells, house enzymes that
catalyze a variety of biochemical reactions,
including synthesis of bile acids;
detoxification of hydrogen peroxide;
breakdown of certain lipids and rare
biochemicals; and detoxification of alcohol.
Microfilaments and Microtubules
• Microfilaments
and microtubules
are 2 types of thin,
threadlike strands
within the
cytoplasm that
form the
cytoskeleton.
Cytoskeleton
• Microfilaments are
tiny rods of actin
protein that form
meshworks or
bundles.
• They provide cell
motility.
• In muscle cells,
microfilaments form
myofibrils to help
muscle cells contract.
• Microtubules are long,
slender tubes
composed of globular
tubulin proteins .
• Usually 2-3 times
larger than
microfilaments,
microtubules are
arrayed in a pattern
called 9 + 2.
9 + 2 Arrangement
Centrosome
• The centrosome is a nonmembranous
structure near the nucleus of animal cells.
• It consists of 2 hollow cylinders called
centrioles, made of microtubules.
• The centrioles lie at right angles to each
other. During mitosis, they help distribute
chromosomes to newly forming cells.
Centrosome and Centrioles
Cilia and Flagella
• Cilia and flagella are motile extensions
from the surfaces of certain cells, made of
microtubules in a 9 + 2 arrangement.
• Their main difference is in length.
• Cilia is a tiny, hairlike structure attached
beneath the cell membrane that moves in a
‘to-and-fro’ pattern.
• Flagella are longer than cilia. Usually a cell
may have only 1 cilia which moves in a
whip-like motion.
Cilia and Flagella
Vesicles
• Vesicles are membranous sacs formed by
part of the cell membrane folding inward
and pinching off.
• Vesicles that hold food or water are called
vacuoles.
• The Golgi apparatus and ER also form
vesicles that play a role in secretion.
Formation of Vacuoles
Cell Nucleus
• The nucleus houses the genetic material
(DNA), which directs all cell activity.
• It is a large spherical structure enclosed in
a double-layered lipid nuclear envelope.
• The nuclear envelope has protein channels
called nuclear pores that allow certain
molecules to exit the nucleus.
• A nuclear pore is not just a hole, but a
complex opening formed from 100+
proteins.
Within the Nucleus
1. Nucleolus=“little nucleus”-a
small, dense body composed
largely of RNA and protein. It
has no membrane. Ribosomes
form in the nucleolus, then
migrate through nuclear pores to
the cytoplasm.
2. Chromatin=loosely coiled fibers
of protein and DNA. When the
cell begins to divide, chromatin
fibers coil tightly into rodlike
chromosmes.
Movement Through Cell Membranes
•
The cell membrane is a selective barrier
that controls which substances enter and
leave the cell.
1. Passive mechanisms do not require
energy: diffusion, facilitated diffusion,
osmosis, and filtration.
2. Active mechanisms use cellular energy:
active transport, endocytosis, and
exocytosis.
Passive Mechanisms: Diffusion
• Diffusion is the process by which molecules or
ions scatter or spread spontaneously from regions
of higher concentrations to regions of lower
concentrations.
• Molecules move at random and mix molecules
together.
• Equilibrium occurs when the molecules are
equally mixed. Movement still occurs but there is
no change in concentration.
Diffusion and Equilibrium
Examples of Diffusion at Work
• Oxygen molecules diffuse through cell
membranes and enter cells if these
molecules are more highly concentrated on
the outside than on the inside.
• Carbon dioxide molecules also diffuse
through cell membranes in the same way.
• Diffusion enables oxygen and carbon
dioxide molecules to be exchanged
between the air and blood in the lungs and
between the blood and cells.
Dialysis
• Dialysis uses diffusion to separate smaller
molecules from larger ones in a liquid.
• The artificial kidney used a process called
hemodialysis to filter wastes from the blood, much
as a real kidney would.
• To remove blood urea, the dialyzing fluid must
have a lower urea concentration than the blood;
glucose concentration in the fluid must match the
blood to prevent from losing glucose.
Facilitated Diffusion
• Facilitated diffusion occurs in most cells
when a special protein carrier molecule
attached to the surface of the cell membrane
helps another substance move across the
membrane.
• Once inside the cell, the molecule is released.
Again, the material moves from areas of high
concentration to areas of low concentration.
Facilitated Diffusion
• Facilitated diffusion
proteins change shape
when combined with their
substrate. When the
substrate is released, it
changes back to its original
form.
• Examples: insulin allows
for facilitated diffusion of
glucose.
Osmosis: A Special Case of
Diffusion
• Osmosis is the diffusion of water only, from
an area of high water concentration to an
area of low water concentration.
• *In solutions, a higher concentration of
solute means a lower concentration of water
because the solute takes up space that could
be occupied by water molecules.
Osmotic Pressure
• The ability of osmosis to generate enough
pressure to lift a volume of water is called
osmotic pressure.
• The greater the concentration of nonpermeable
solute particles in a solution, the lower the
water concentration of that solution and the
greater the osmotic pressure.
• Water always tends to diffuse toward solutions
of greater osmotic pressure.
Three Types of Solution
• Since cell membranes are permeable to water,
water equilibrates by osmosis and the
concentration of water and solutes everywhere in
body fluids is the same.
• Any solution that has the same osmotic pressure as
body fluids is called isotonic.
• Solutions with a higher osmotic pressure than
body fluids are called hypertonic.
• A hypotonic solution has a lower osmotic pressure
than body fluids.
A Cell’s Reaction to Different
Solutions
• A cell placed in an
isotonic solution will
have no net gain of
water.
• A cell placed in a
hypertonic solution
will , it will lose water.
• A cell in a hypotonic
solution will gain
water.
Filtration
• Molecules pass through membranes by diffusion
or osmosis because of random movements.
The process of filtration forces molecules through
membranes.
• Filtration is commonly used to separate solids
from water. Tissue fluid forms when water and
small dissolved substances are forced out through
the thin, porous walls of blood capillaries.
• Larger particles, such as blood proteins, are left
inside.
Filtration
• In the body, tissue fluid forms when water
and small dissolved substances are forced out
through the thin, porous walls of blood
capillaries, but larger particles, blood
proteins, are left inside.
• The force comes from
blood pressure generated
mostly by heart action.
Active Mechanisms
• When particles move from areas of lower
concentration to one of higher
concentration, energy is required.
• This energy comes from cellular
metabolism and specifically from a
molecule called ATP, adenosine
triphosphate.
Active Transport
• Active transport is a process that moves particles
through membranes from a region of lower
concentration to a region of higher concentration.
• Sodium ions can diffuse passively into cells
through protein channels but active transport
continually moves sodium ions through cell
membranes to the outside where the concentration
is higher.
• Equilibrium is never reached.
Active Transport Facts
• Active transport used specific carrier
molecules in cell membranes and may use
up to 40% of the cell’s energy to actively
transport particles.
• The carrier molecules are proteins with
binding sites that combine with the particles
being transported.
Active Transport
• When the carrier protein
combines with the
substrate, it changes shape
which moves the particle
through the membrane.
• Once inside, the particle is
released and the carrier
protein goes back to its
original shape.
• Particles that are actively
transported across cell
membranes include
sugars, amino acids, and
• sodium, potassium,
calcium, and hydrogen
ions.
Endocytosis and Exocytosis
• Two processes use cellular energy to move
substances into (endocytosis) or out of
(exocytosis) a cell without crossing the cell
membrane.
Endocytosis
• In endocytosis,
molecules too large for
diffusion or active
transport may be
conveyed within a
vesicle formed by
pinching in of the cell
membrane.
Exocytosis
• In exocytosis, the reverse
process secretes a
substance stored in a
vesicle from the cell.
• Nerve cells use exocytosis
to release the
neurotransmitter
chemicals that signal other
nerve cells, muscle cells,
or glands.
Three Forms of Endocytosis
1. Pinocytosis=cell drinking
2. Phagocytosis=cell eating
3. Receptor-mediated endocytosis for
specific molecules
Pinocytosis
• Pinocytosis takes in droplets of liquid from the
surroundings as a small portion of the cell
membrane indents.
• A vesicle forms which detaches from the
surface and moves into the cytoplasm.
Phagocytosis
• Phagocytosis is when the cell takes in
solids.
• Certain white blood cells are called
phagocytes because they engulf
bacteria.
• Once inside the cell, a lysosome then
combines with the vesicle and
digestive enzymes decompose the
contents.
Receptor-Medicated Endocytosis
• Receptor-mediated
endocytosis contain
proteins that extend
through a portion of the
cell membrane to the outer
surface.
• These receptors bind only
with specific molecules
(ligands) which can be
transmitted across.
• Ex: cholesterol
Transcytosis
• Transcytosis combines endocytosis and
exocytosis to selectively and rapidly
transport a substance or particle from one
end of a cell to the other.
• Transcytosis moves substances across
barriers formed by tightly connected cells.
• HIV uses transcytosis to corss lining cells in
the anus and female reproductive tract.
Transcytosis
The Cell Cycle
• Cell cycle—well-ordered sequence of
events between the time a cell divides to
form 2 daughter cells and the time those
daughter cells divide.
• The cell cycle alternates between M phase,
or dividing phase, and Interphase, the nondividing phase.
Cell Cycle
Division of the Cell
• The M phase is the shortest part of the cell
cycle and the phase during which the cell
divides, includes:
1.Mitosis—division of the nucleus.
2.Cytokinesis—division of the cytoplasm.
Mitosis
• Cells undergo mitosis as they approach the
maximum cell size at which they can work
efficiently.
• The 4 phases of mitosis include:
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Prophase: The first phase of Mitosis
• During prophase, the longest phase, the
nuclear membrane disappears, the sister
chromatids are clearly visible, and
centrioles replicate.
• Sister chromatids are exact copies of each
other and are held together by a centromere.
Prophase
In the nucleus:
.
Nucleoli disappear.
The 2 identical sister
chromatids are joined at the
centromere.
In the cytoplasm:
Mitotic spindle forms
between the 2 centrosomes.
Centrosomes move apart.
Metaphase
• Centrosomes are positioned at
opposite poles of the cell
• Chromosomes move to the
metaphase plate (middle)
• Centromeres of all chromosomes
are aligned on the metaphase plate.
• Kinetochores of sister chromatids
face opposite poles.
• Entire structure formed by
nonkinetochore microtubules plus
kinetochore microtubules is called
the spindle.
Anaphase
• Characteristized by movement.
• Sister chromatids split apart into
separate chromosomes and move
toward opposite poles.
• Move centromere first (V-shape)
• Kinetochore microtubules shorten as
chromosomes approach the poles.
• The poles move farther apart,
elongating the cell
Telophase
• Nonkinetochore microtubules further
elongate the cell.
• Daughter nuclei begin to form at the
poles.
• Nuclear envelopes form around the
chromosomes.
• Nucleoli reappear
• Chromatin fiber of each chromosome
uncoils and chromosomes become less
distinct.
Cytokinesis
• By the end of
telophase:
• Mitosis is complete.
• A cleavage furrow
forms and separates
the cell into 2
daughter cells.
Control of the Cell Cycle
• Enzymes control the cell cycle.
• Certain enzymes are necessary to begin and
drive the cell cycle and other enzymes
control the cycle through the phases.
Cancer
• Occasionally, cells lose control of the cell cycle
resulting in uncontrolled dividing of the cells.
This abnormal growth is called a tumor.
• If a tumor becomes malignant, the result is cancer.
• This loss of control may be caused by
environmental factors or by changes in enzyme
production.
Genes that Cause Cander
• Oncogenes activate other genes that
increase cell division rate.
• Tumor suppressor genes normally hold
mitosis in check.
• When tumor suppressor genes are removed
or inactivated, cancer may result.
Causes of Cancer
• Both genetic and environmental factors are
involved.
• Environmental influences include cigarette
smoke, air/water pollution, and exposure to
UV radiation from the sun.
• Cancer may also be caused by viral
infections that damage genes.
Stem and Progenitor Cells
• In 1855, German physiologist Rudolph Virchow
stated that all cells come from preexisting cells.
• Cells that retain the ability to divide repeatedly
allow for continual growth and renewal.
• A stem cell divides mitotically to produce either 2
daughter cells, or one daughter cell and one stem
cell.
Stem and Progenitor Cells
• A cell that is partially specialized is an
intermediate between a stem cell and a
differentiated cell is called a progenitor cell.
• A progenitor is said to be ‘committed’
because its daughter cells can become any
of a restricted number of cell types.
Totipotent vs Pluripotent
• Stem and progenitor cells are described in
terms of their potential to become different
types of cells.
• Totipotent mans that they can give rise to
every cell type.
• Pluripotent means that can they can follow
several different pathways but not all of
them.
Totipotent
Pluripotent