The Structure and Transport of
Materials across - Cell
Membranes
The cell regulates its internal environment. Its internal
environment is very different from the external environment that it
is bathed in. It uses cell walls and cell membrane to control its
environment. The membrane is semi-permeable (or selectively
permeable) meaning some molecules can cross the membrane and
other cannot.
Most cells except for animal cells have cell walls. They are
composed of carbohydrates depending on the kingdom. Cell walls
are dead and are used to prevent the cells from bursting and give
them structures. Cell walls are porous and allow most everything to
cross- The cell membrane is the barrier that regulates what "get
into and out of" the cell.
All cells have membranes surrounding them and many of their
organelles. The basic structure is a bilayer of phospholipids with
proteins inserted in the layer. This model is the fluid mosaic layer
of cell membranes.
All cells have membranes
surrounding them and many
of their organelles. The basic
structure is a bilayer of
phospholipids with proteins
inserted in the layer. This
model is the fluid mosaic
layer of cell membranes. The
charge phosphate group is the
hydrophilic head and the
hydrocarbon tails are
hydrophobic tails.
Very seldom in illustrating phospholipids, are all the atoms shown.
Instead they are illustrated as such-
The cell membrane has two layers of phospholipids as shown below.
The hydrophilic heads are facing an aqueous environment and the
hydrophobic tails are facing one another
The phospholipids are capable of horizontal movement but very
little vertical movement. Proteins can also exhibit lateral movement.
When the bilayer contains more unsaturated phospholipids, the
membrane becomes more fluid. When preparing for winter, many
organisms incorporate more unsaturated phospholipids.
A word about cholesterol - It is found in the cell
membrane of animals but not plants. It
functions in the following:
1. It can weakly bind to hydrocarbon tails making
it more difficult for smaller molecules to cross
membrane.
2. If the phospholipids are saturated, it prevents
them from being packed too closely, making the
membrane more fluid.
3. However - if the phospholipids are unsaturated
there are kinks in the tails and the cholesterol
molecules can fill in and anchor them.
Proteins are "stuck" in the membrane like a mosaic. They can be
lying on the surface in an integral part of the membrane. Proteins
can be on just the surface (extrinsic) or embedded in the membrane
(intrinsic). It is the different proteins that are responsible for the
uniqueness of different membranes (plasma, eukaryotic,
prokaryotic, organelle etc.)
Function of Membrane Proteins
1. Transport Proteins or permeases transport molecules across the
membrane. (Aquaporins are channels used to move water across)
2. Enzyme-Some proteins in the membrane may expose their active
site to speed up a chemical reaction.
3. Receptor site-Ex. Insulin never goes into a cell but binds to a
receptor site on the cell membrane
4. Cell to cell recognition.
5. Intercellular joining
6. Attachment to the cytoskeleton and extracellular matrix (ECM
only in animal cells)
The plasma membrane is
increased and decreased
with the interaction of
vesicles. Vesicles that are
bringing material to the
membrane to be secreted
will increase the surface
area of the plasma
membrane. Through the
process of endocytosis, the
surface area of the cell
membrane can be
decreased.
The cell membrane is said to be semi-permeable or
selectively permeable because certain molecules can
cross the membrane and others can not. Those that
can cross the membrane are said to be permeable
and those that cannot are said to be impermeable
Passive diffusion- the molecules (CO2, O2, etc) involved are
permeable to the membrane. Predicting the net movement is as if
there were no membrane at all. The molecules move from high to
low concentration.
In the first system, molecule A is more
concentrated on side 2 than side 1. Molecule
A is permeable to the membrane. The net
movement of molecule A is from side 2-->1.
At some point in time the concentration of
molecule A on side 1 equals that of A on side
2. Equilibrium is reached. The movement
of molecule A across the membrane is equal
in both directions.
The net movement is from the side from the side with the high
concentration of A to the side with the low concentration of A.
Facilitated diffusion- smaller molecules(glucose, amino acids etc.)
are impermeable to the membrane. Because of that, a transport
protein is needed to transport the molecule across the membrane.
Predicting the net movement is like passive diffusion. The net
movement of the molecules move from an area of high
concentration to an area of low concentration.
The protein carriers (permeases)
may take the form of a channel
that allow certain molecules with
a certain shape to enter the
cytoplasm and others may take
the form of a carries protein that
changes conformational shape to
move the molecules across.
Facilitated diffusion and passive
diffusion are also called passive
transport as no energy in the
form of ATP is needed.
Active Transport- molecules are impermeable to the
membrane and the molecules are moving against a
concentration gradient (from low to high) Because of
that, a transport protein and ATP is needed.
First example is an electrogenic
pump which is active transport
that is goes against a concentration gradient AND a charge
gradient. The second example is
cotransport involving an
electrogenic pump and then the
build up of H+ ions drives the
cotransport of sucrose across the
membrane by facilitated
diffusion.
Osmosis-is the special case of the movement of water. When things
are impermeable to the membrane and not in equilibrium, water
will move across the membrane to help reach equilibrium. "Water
likes to dilute".
Hypertonic-An excess of solutes in the surrounding water that
are unable to cross the membrane. Extreme- Submerging cells
in syrup. These cells tend to lose water to the environment
until the concentration of solutes inside equals that outside the
cell. Then the movement of water will be same in both
directions. Plasmolysis is the plasma membrane pulling away
from plant cell walls.
Hypotonic-A solution that has fewer solutes. ExtremeSubmerging cells in pure water. Water crosses into the cell
until the concentration of solutes inside equals that outside the
cell, however, animal cell may not be able to accommodate the
water and will burst. Plant cells become turgid because of a
back pressure from the cell wall. Then the movement of water
will be same in both directions.
Isotonic- Two solutions that has equal amounts of solutes on
both sides of the membrane. Water moves across the
membrane at the same rate.
Endocytosis is the movement of larger particles into the cell by use of
membrane vesicles. Three types
Phagocytosis-cell eating
Pinocytosis-cell drinking
Receptor mediated- Receptors on the outside of cell membrane
allows for the attachment of a particular molecule. When a certain
number of receptor sites are filled, endocytosis occurs.
Exocytosis- is the movement of larger particles out of the cell by
membrane vesicles