THE CELLULAR LEVEL OF ORGANIZATION
A.
GENERALIZED ANIMAL CELL
It is at the cellular level or organization that activities essential to life occur and
where disease processes originate.
A cell is the basic, living, structural, and functional unit of the body.
All animal cells have four anatomical features in common. Name them.
1.
2.
3.
4.
Plasma (cell) membrane
Cytosol
Organelles
Inclusions
Define each of the following terms:
Plasmalemma -- The plasmalemma (cell or plasma membrane) is the
outer limiting membrane that separates the cell’s internal
components from the extracellular material and external
environment.
Cytoplasm -- Cytoplasm is the term used for all cellular material between
the plasma membrane and the nucleus of the cell.
Cytosol -- Cytosol is the thick, semi-fluid portion of the cytoplasm called
the intracellular fluid. It contains soluble proteins, enzymes, ions,
and nutrients, and suspends the organelles.
Organelles -- Organelles are highly organized structures with characteristic shapes that are highly specialized for specific activities.
Inclusions -- Inclusions are temporary structures suspended or stored
within the cytoplasm that contain secretions and storage products
of the cell.
B.
PLASMA (CELL) MEMBRANE
1.
MEMBRANE CHEMISTRY AND ANATOMY
a.
MEMBRANE LIPIDS
b.
MEMBRANE PROTEINS
What is the function of the cell membrane?
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It is the thin barrier that separates the cell’s internal fluid
from the external fluid. It serves as the gatekeeper that
regulates passage of substances into and out of the cell.
Describe the fluid mosaic model of the membrane structure.
The fluid mosaic model of membrane structure describes the
molecular arrangement of the cell membrane. The
membrane is a mosaic of protein molecules “floating” like
icebergs in a “sea” of lipid molecules.
Describe the three types of membrane lipids.
1.
2.
3.
Phospholipids (75%) -- hydrophilic “head” and hydrophobic
“tails”
Glycolipids (5%) -- role obscure (cell adhesion, recognition)
Cholesterol (20%) -- membrane strength and steroid
hormone precursor
Compare integral membrane proteins with peripheral membrane
proteins.
Integral proteins extend across the lipid bilayer into the
extracellular fluid on one side and into the intracellular fluid
on the other. They have a number of functions
Peripheral proteins do not extend across the lipid bilayer but
rather are loosely attached to either the inner or outer
surfaces of it. Their roles in cell function remain obscure.
Describe the functions of the following membrane proteins:
Channels (pore) -- allows a specific substance to
move through water-filled passageways. Most cell
membranes include specific channels for several
ions. Most common are for Na+ and K+
Transporter (carrier) -- transports specific substances
across membrane by changing shape. For example,
amino acids, needed to synthesize new proteins,
enter body cells via transporter.
Receptor -- recognizes a specific ligand and alters the cell’s
functions in some way. For example, this is a classic
mechanism by which protein-based hormones affect
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cells.
Enzyme -- catalyzes reactions inside or outside (depending
upon which direction the active site faces) the membrane. *ex. small intestinal enzymes for digestion of
food)
Cytoskeleton anchor -- anchors filaments and tubules of the
cytoskeleton inside cell membrane to provide structural stability and shape for the cell. May also participate in movement of the cell.
Cell identity markers -- distinguish your cells from anyone
else’s (unless you are an identical twin) and from
microbial cells. An important class of such markers is
the histocompatibility (MHC) proteins.
2.
MEMBRANE PHYSIOLOGY
a.
COMMUNICATION
b.
ELECTROCHEMICAL GRADIENT
c.
SELECTIVE PERMEABILITY
List the three major functions of the cell membrane.
1.
2.
3.
Communication
Electrochemical gradient
Selective permeability
With what must cells of the body be able to communicate?
The cell, via the plasma membrane, communicates with
other body cells, foreign cells, and ligands (neurotransmitters, enzymes, hormones, nutrients, and antibodies) in
the extracellular fluid.
Describe the chemical and electrical components of the
electrochemical gradient formed by the cell membrane.
The chemical portion of the gradient is formed because the
membrane maintains very different chemical compositions in
the cytosol and the extracellular fluid.
*The major extracellular cation is Na+ and the major
anion is Cl*The major intracellular cation is K+ and the major
anions are organic phosphates and amino acids.
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The electrical gradient arises because the inside of the
membrane is more negatively charged than the outside. As
a result, there is a voltage (voltage is electrical potential or
potential difference as expressed in volts.) called the
membrane potential across the membrane.
The third major function of the cell membrane is to provide
selective permeability. What is this?
The plasma membrane regulates the entry and exit of
materials by permitting certain substances to pass freely
while restricting the passage of others. This property is
called selective permeability.
A membrane is said to be permeable to a substance if...
It allows that substance free passage into and out of the cell.
Permeability of a membrane is dependent upon four factors that
relate to membrane structure. List and describe these factors.
1.
Solubility in lipids -- Substances that dissolve in lipid pass
easily across the membrane while water-soluble substances
do not. For example: oxygen is fat-soluble and crosses the
membrane easily; glucose is water -soluble and does not
cross the membrane.
2.
Size -- Ions and molecules less than 8 Angstroms in
diameter can cross the cell membrane freely. Substances
larger than 8 Angstroms are restricted from crossing the
membrane because they are too large to pass between the
molecules that form the membrane itself.
3.
Charge -- The membrane is impermeable to all charged ions
and molecules. However, the overall negative membrane
potential of most cells aids the influx of cations and hinders
the influx of anions.
4.
Presence of specific channels or transporters -- Channels
increase membrane permeability for some substances by
providing specific routes of entry or exit. This is done by
either attaching to and shuttling the substances through the
channel or by simply letting it pass through the channel’s
water-filled space.
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Why are the mechanisms that enable substances to move across
cell membranes essential to life?
Without these mechanisms the cell would be unable to
procure oxygen, nutrients, etc., or rid itself of carbon dioxide
and other waste products of metabolism.
C.
MOVEMENT OF MATERIALS ACROSS THE PLASMA MEMBRANES
Compare passive processes with active processes.
Mechanisms that move materials across the cell membrane without the
use of cellular energy are called passive processes.
Mechanisms that move material across the plasma membrane with the
use of cellular energy are called active processes.
List the four passive processes.
1.
2.
3.
4.
Simple diffusion
Osmosis
Filtration
Facilitated diffusion
List the four active processes
1.
2.
3.
4.
1.
Primary active transport
Secondary active transport
Endocytosis
Exocytosis
PASSIVE PROCESSES
a.
SIMPLE DIFFUSION
Describe the process of diffusion. What is the concentration
gradient? How do substances move from one area to another?
Passive processes depend upon pressure or concentration
differences (gradients).
Simple diffusion -- Since all substances have kinetic energy,
they are constantly moving about, colliding into one another
and then moving off into various directions.
This random mixing of ions and molecules in a solution due
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to their kinetic energy is called diffusion. (Latin “diffusus”= to
spread out)
If the substance is present in high concentration in one area,
the difference in concentration between the two areas is said
to be the concentration gradient.
When two such areas are connected (Ex. by water or air)
more particles diffuse from the area of high concentration
than from the area of low concentration. There is, therefore,
net diffusion, from the area of high concentration to the area
of low concentration.
Substances undergoing net diffusion (from high to low
concentration) are said to move down or with their
concentration gradient.
After a period of time, the particles become evenly
distributed throughout the connected areas and a uniform
concentration or equilibrium is reached so that no more net
diffusion occurs.
b.
OSMOSIS
Describe the process of osmosis. What is an osmotic gradient?
Distinguish between isotonic, hypotonic, and hypertonic solutions.
Osmosis is the net movement of water through a selectively
permeable membrane.
In osmosis, water moves across the membrane from an
area of higher water concentration to an area of lower water
concentration until a uniform water concentration is reached.
For this to occur, there must be a solute in the water to
which the membrane is impermeable; therefore, no solute
can cross the membrane to satisfy the diffusion gradient.
This movement of water through the selectively permeable
membrane generates a pressure called the osmotic
pressure.
To maintain normal shape, a cell must be bathed in an
isotonic solution. This is a solution in which the total
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concentrations of water molecules and impermeable solutes
are the same on both sides of the membrane.
Under these conditions, while the membrane remains
impermeable to almost all solutes, water molecules enter
and leave the cell at the same rate, so that there is no net
osmosis, allowing the cell to maintain its normal shape and
volume.
A hypotonic solution is one that has fewer solutes than cells
have. A cell placed in such a solution will have a net
osmosis into the cell, causing it to swell and burst.
A hypertonic solution has a greater concentration of solutes
that do cells. A cell placed in such a solution will have a net
osmosis out of the cell, causing the cell to shrink (crenate).
c.
FILTRATION
Describe the process of filtration.
In this passive process, water and solutes move across a
cell membrane due to gravity or hydrostatic (water) pressure.
Such movement is always from an area of high filtration
pressure to an area of low filtration pressure.
Example: movement across the capillary walls due largely to
blood hydrostatic pressure.
d.
FACILITATED DIFFUSION
What is facilitated diffusion? What three factors determine its rate?
Some substances are too large to pass through membrane
channels and too lipid-insoluble to diffuse through the lipid
bilayer.
Such substances may move through the membrane with the
aid of transporter proteins, moving down their concentration
gradient to a region of lower concentration.
This process is called facilitated diffusion. It can be much
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faster than a simple diffusion and depends on three factors:
(1)
concentration gradient
(2)
number of transporters available
(3)
rate at which transporter and substrate combine
2.
ACTIVE PROCESSES
What are active processes? Describe a primary active transport process
by discussing the Na-K ATPase membrane pump.
When cells actively participate in moving substances across
membranes, they must expend energy by splitting the ATP
molecule.
Using the released energy from ATP, the cell then moves
substances against their concentration gradients, from an area of
low concentration to and area of higher concentration.
Primary active transport -- Energy from ATP is used to directly
move substances across the membrane by changing the shape of
transport (pump) proteins in the membrane.
Na+ in the cytosol binds to the pump protein. Remember that Na+
is the major extracellular cation whereas K+ is the major intracellular anion
Na+ binding triggers the breakdown of ATP and the attachment of
the released high-energy phosphate group to the pump protein.
This changes the shape of the protein so that the Na+ ions are
pushed through the membrane and expelled from the cell.
K+ binding to the transport protein triggers release of the
phosphate molecule. This again causes the shape of the transport
protein to change. As the pump returns to its original shape the K+
is pushed through the membrane and into the cytosol. At this point
the pump is ready to bind Na+ again and renew the process.
Describe secondary active transport. How is it different from primary
active transport?
The energy stored in ion gradients drives substances across the
membrane. For example, using primary active transport, a nerve
cell forms 14x more Na+ outside the cell than inside. If channels
for Na+ open, the Na+ ions move into the cell by diffusion.
Energy was required to create the diffusion gradient, therefore this
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is considered to be a type of active transport.
What is exocytosis?
Exocytosis is an active transport process whereby the cell expends
energy to actively discharge large substances from the cell.
Example: secretion from glands.
What is endocytosis? Name and define the three types of this process.
Endocytosis is the process by which large substances are actively
taken into the cell.
1.
2.
3.
Phagocytosis -- “cell eating”
Pinocytosis -- “cell drinking”
Receptor-mediated endocytosis -- specific ligands are taken
into the cell by attaching to membrane receptors and being
carried into the cell.
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