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Cell Membranes & Organisations
(a) the principal components of the plasma membrane and understand the fluid-mosaic model
Fluid Mosaic Model
• Singer & Nicholson (1972) proposed this model for the structure of the cell membrane
• ‘Fluid’
Because phospholipid molecules within a layer can move relative to each other
• ‘Mosaic’
Because the proteins within the phospholipid layer are of different sizes and
shapes, forming different patterns
...
g
...
g
...
g
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Functions of Plasma Membrane

Factors affecting cell permeability:

1
...

3
...

5
...
Heat
2
...
Ethanol

Regulating uptake of nutrients
Regulating secretion of chemicals
Cell recognition (using glycolipids & glycoproteins)
Separating the inside of cell from the outside
Separating one cell from another
...

• Water soluble substances (glucose, ions, all polar molecules) cannot pass through the
hydrophobic fatty acid tails and so must use intrinsic proteins to pass though
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5

7
Intrinsic proteins
L

1
...
g
...

2
...
Binding of the molecules changes the shape of the protein moving the substance into or
out of the cell
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The hydrophilic phosphate heads
of the phospholipids form the
outer and
inner surface of the cell
membrane
...

Extrinsic proteins
Found on either outer surface of
the bilayer
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Factors affecting cell permeability:
1
...
pH
3
...


Between temperatures of O°C and 45°C,
membranes are partially permeable
...
The more fluid the
membrane is, the more substances it allows
through
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Water inside the cell
expands, putting pressure on the
membrane, and transport proteins
deform (due to denaturing of
proteins) so they can't control what
enters or leaves the cell
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This
means that if you place a cell in ethanol, its membrane will become permeable and allow
substances to leak into and out of the cell
...
If the
ethanol concentration is high enough, enough
phospholipids will dissolve to cause the plasma
membrane to disintegrate completely which will
kill the cell
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Diffusion
• Diffusion is the movement of molecules from a region of higher concentration to a region of
low concentration down a concentration gradient
• It is a passive process and so requires no energy from ATP
• Simple diffusion occurs through the phospholipid bilayer
Rate of uptake
*

↑

Respiratory inhibitor added

3

#
Concentration difference across membrane

Diffusion rate is increased by:
• higher concentration gradient
• thinner membrane
shorter diffusion
distance
• larger surface area
• smaller molecules
• being non-polar or fat soluble
• increased temperature

The concentration on one side of
membrane increases, there is directly
proportional increase in the rate of
diffusion
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2
...

All the same rules apply, the only difference is that the substances enter the cell through
protein channels
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This is a passive process; therefore, the
respiratory inhibitor has no effect
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Channel proteins
• channel proteins are molecules with pores lined with
polar groups
• as the channels are hydrophilic, ions, being watersoluble, can pass through
• the channels open and close according to the needs
of the cell
2
...


Channel Protein

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