Cell membranes
AS Biology: BY1
What are membranes?
Membranes cover the surface of every cell,
and also surround most organelles within
cells. They have a number of
functions, such as:
 keeping all cellular components
inside the cell
 allowing selected molecules to move in and out of the cell
 isolating organelles from the rest of the cytoplasm,
allowing cellular processes to occur separately.
 a site for biochemical reactions
 allowing a cell to change shape.
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Cell Membrane
Phospholipid bilayer + protein + glycoprotein
 Appears as a double line on electron
microscope (about 7- 8 nm wide)
 Selectively permeable

Cell membrane appearance
intracellular space (blue)
1st cell
membrane
1 light layer =
phospholipid tails
2 dark layers:
phospholipid heads
2nd cell membrane
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Lipids
1) Phospholipids
•
Arranged in bilayer (hydrophilic heads to
outside, hydrophobic tails inside)
• Act as a barrier between 2 aqueous
environments preventing most water soluble
molecules from passing through
•
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Lipids
2) Cholesterol- between the tails
• Helps strengthens the membrane
• Regulates sideways movement by holding
some phospholipid tails together making it
less fluid
• Prevents polar molecules from passing
though
Cholesterol is also important in
keeping membranes stable at
normal body temperature – without
it, cells would burst open.
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The fluid mosaic model
The freeze-fracture images of cell membranes were further
evidence against the Davson–Danielli model.
They led to the
development of
the fluid mosaic
model, proposed
by Jonathan
Singer and Garth
Nicholson in 1972.
P-ace
protein
This model suggested that proteins are found within,
not outside, the phospholipid bilayer.
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Exploring the fluid mosaic model
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Proteins in membranes
• Proteins are extrinsic (one layer) or intrinsic (both
layers)
1)
Integral (or intrinsic, or
transmembrane) proteins
span the whole width of the
membrane
carbohydrate
chain
integral protein
- Make pores/hydrophilic
channels through the
membrane for diffusion of
polar molecules
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Proteins
2) Peripheral (or extrinsic) proteins are confined to the
inner or outer surface of the membrane.
• used in cell recognition on the outside
• attach to the cytoskeleton on the inside to maintain cell
shape
• Can be enzymes for reactions on either side (eg: small
intestine)
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Proteins
3) glycoproteins – (proteins with attached
carbohydrate chains)
-act as receptors for hormones or
neurotransmitters for cell recognition.
-Form H- bonds with the surrounding water
to help stabilize the membrane
-Aid in adhesion to other
cells.
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Membrane fluidity
It is important that a cell membrane maintains its fluidity
otherwise the cell would not be able to function.
A fluid membrane is needed for many processes, such as for:

the diffusion of substances across the membrane

membranes to fuse, e.g. a
vesicle fusing with the cell
membrane during exocytosis

cells to move and change
shape, e.g. macrophages
during phagocytosis.
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Factors affecting membrane fluidity
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Integral proteins
Many integral proteins are carrier molecules or channels.
These help transport substances,
such as ions, sugars and amino
acids, that cannot diffuse across
the membrane but are still vital to
a cell’s functioning.
Other integral proteins are receptors
for hormones and neurotransmitters,
or enzymes for catalyzing reactions.
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Peripheral proteins
Peripheral proteins may be free on the membrane surface or
bound to an integral protein.
Peripheral proteins on the
extracellular side of the
membrane act as receptors
for hormones or
neurotransmitters, or are
involved in cell recognition.
Many are glycoproteins.
Peripheral proteins on the cytosolic side of the membrane are
involved in cell signalling or chemical reactions. They can
dissociate from the membrane and move into the cytoplasm.
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Membrane models: true or false?
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Components of the membrane
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Functions of membrane components
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Cell membranes
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