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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
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g
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g
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g
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Functions of Plasma Membrane

Factors affecting cell permeability:

1
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3
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5
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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
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2
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Binding of the molecules changes the shape of the protein moving the substance into or
out of the cell
...


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
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pH
3
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Between temperatures of O°C and 45°C,
membranes are partially permeable
...
The more fluid the
membrane is, the more substances it allows
through
...
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
...
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 Proteins Carrier Proteins
Pore

Allow diffusion, facilitated diffusion
Only transport down a concentration
gradient

Example of facilitated diffusion:
Co-transport

Solute does not bind to transport
protein
Does not change shape
Only transport water-soluble
molecules
Rapid transport: 10^8 ions per
second

No pore
Allow diffusion, facilitated
diffusion, active transport
Can transport molecules against
concentration gradient
Solute binds to transport protein
on one side of membrane and
are released on the other
Changes shape
Transport soluble and insoluble
molecules
Slower transport: 10^4 ions
per second

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3
...

• Water potential (ψ) is a measure of energy of water molecules and the tendency of water molecules to
move
measured in kPa
• Solute potential is the osmotic strength of the solution
...

Cells can burst in hypotonic and shrink in hypertonic solutions due to
osmosis
...
g
...


e
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Red blood cells

The balance of these forces determines the water
potential of the cell and whether water moves in or out
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*

...


Cell wall

Hypotonic

Hypotonic

·

ir

Plant cell

ψ = -ve

Solution has lower ψ outside than inside
cell
water moves out of the cell by

Because of this, protoplast pushes outwards against the cell
wall
since cellulose makes cell wall strong, instead of

Turgor pressure prevents any more water from
entering the cell by osmosis

ψ = -ve

ψ = -ve

Because the ψ outside the cell is greater than inside,
water enters the cell by osmosis
causes the
hydrostatic pressure inside the cell to increase

internal

solution ψ =
even more -ve

Hypertonic

Pure water
ψ=0
ψ=0

bursting, the plant cell becomes (turgid) firm
pressure called ‘turgor pressure’
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Pure water has highest water potential, so
ψ is greater outside than inside
water
molecules move inside cell by osmosis
movement inside molecules causes
pressure to increase (hydrostatic
pressure)
Cell membrane not strong enough to withstand
increase in hydrostatic pressure inside cell
causes cell membrane to rupture and burst
In the case of a red blood cell, this is
known as ‘haemolysis’

Water will move out of the cell by
osmosis
cell membrane draws away
from cell wall

cell shrinks

Hypertonic

4
...
This stop production of
ATP
prevents active transport and no energy produced so cell dies
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phagocytosis
2
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1

Exocytosis (secretion) • vesicle (formed from Golgi body)
moves towards cell membrane
• vesicle fuses with cell membrane
• vesicle contents empty out of cell
ATP is required to move the vesicles,
so this is known as an active transport

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Specified Practical work:
1
...
e
...

• Dependent variable (the variable being measured, whose
value depends on the independent variable)
i
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mean % change in mass of potato chip
...
e
...

Length of time left in the sucrose solution
i
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Volume of water on the surface of the potato chip
i
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Volume of sucrose solution
i
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Same type and age of potato
i
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Conclusion
The more concentrated the sucrose
solution the more negative the mean
% change in mass of the potato chip
(mass is lost)
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Water moves out of the
potato chip by osmosis from an area
of high Ψ to an area of low Ψ, down its
water potential gradient into the
surrounding solution
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Determination of solute potential by measuring the degree of incipient
plasmolysis
...
e
...

• Dependent variable (the variable being measured, whose
value depends on the independent variable)
i
...
the % of plasmolysed cells
...
e
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8 × 0
...
e
...
e
...
e
...
e
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Investigation into the permeability of cell membranes using beetroot
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e
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e
...
e
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e
...
e
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e
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e
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This is because the
proteins in the membrane denature as the heat damages
the bonds in their tertiary structure
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● At low temperatures, phospholipids have little energy and
are packed closely together to make the membrane rigid
...

NB: At very low temperatures, ice crystals can form which
pierce the cell membrane and increase the permeability
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Higher
concentrations of ethanol will cause more disruption to the
membrane and more gaps will form
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