Biology Revision; Cell Physiology

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Cell Physiology
The structure of the cell surface membrane;
The phospholipid bilayer is the basic structure of the cell surface
membrane. There are also cholesterol molecules in among the
hydrocarbon tails. Proteins are attached to the bilayer (extrinsic),
embedded into one layer (intrinsic) or span both layers (intrinsic
and transmembrane). The phospholipids in the cell membrane are
constantly moving while the proteins are scattered among them, so
that the structure proposed is called the fluid-mosaic model.
The membrane also contains polysaccharides bound either to the
proteins (glycoproteins) or to lipids (glycoproteins). The
polysaccharides are on the outer face only, where they form a fringe
called the glycocalyx.
The two hydrophobic tails turn away from the solution and join with
other hydrophobic tails, whereas the hydrophilic heads readily mix
with the surrounding aqueous medium.
Some proteins are attached to the outer layer (extrinsic) and some
are attached to the inner layer (embedded intrinsic). Some intrinsic
proteins expand across the bilayer (transmembrane).
Fluidity of cell membranes;
The more phospholipids with unsaturated hydrocarbon chains
there are the more fluid in the membrane. Kinks in the tails
prevent them from packing close together, so more movement is
Phospholipids with longer hydrocarbon chains will decrease the
fluidity of the membrane, since attractive forces among the tails
will be greater.

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Fluidity is influenced by temperature. The membrane at higher
temperatures is more fluid and less fluid at lower temperatures
as the phospholipid bilayer `freezes' into a `gel' state.
Cholesterol acts as a temperature stability buffer. At high
temperatures, cholesterol provides additional binding forces
and so decreases membrane fluidity. At low temperatures,
cholesterol keeps the membrane in a fluid state by preventing the
phospholipid from packing too close together and' freezing'.…read more

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Adding water=higher solute potential
Adding salts=lower solute potential
Pressure potential this is the effect of pressure on the solution.
A turgid cell results in higher pressure on the cell
wall. A plasmolysed cell results in lower pressure on
the cell wall (=0). Pressure potential is usually a
positive value.
NB// as more water enters a cell, pressure potential will restrict the
entry of water.
Water potential this is the tendency of a solution to take in water.…read more

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The vacuole pushes against the cell wall.
Why does the plant cell not burst?
Cell wall for protection.
If water leaves a plant cell, it becomes plasmolysed-what does this
The cell membrane shrinks away from the cell wall.
When does incipient plasmolysis occur?
Water potential inside the cell=water potential outside the cell
(isotonic solution).
Diffusion of gases from a high concentration to a lower concentration
e.g. Diffusion of gases in lungs.

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The substrate binds to carrier proteins
The protein changes shape, releasing the substance on the other
side of the membrane.
What would an increase in temperature do to the rate of active
Increase-more energy would be available for the carriers to change
What organelle is in large quantities in cells involved in active
Mitochondria-supply ATP needed for active transport.
Pinocytosis smaller molecules taken into a cell.
Phagocytosis larger molecules taken into a cell.…read more


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