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CELL MEMBRANE STRUCTURE
BIOLOGICAL AND PHYSICAL PROPERTIES OF MEMBRANES
What do membranes look like?

What do internal membranes look like?

EM of Golgi body

What do membranes look like in more detail?
The early endosome:
• Receives material internalised from plasma membrane
• Sorts this to different destinations
o Back to the plasma membrane
o Transfers to other organelles such as the Golgi
o Delivers contents to lysosomes

Cell membrane structure

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CELL MEMBRANE STRUCTURE
Membrane features
• Sheet-like structure
• Thickness 6-10nm
• Tendency to form closed boundaries
• Non-covalent assemblies
• Asymmetric
• Fluid
• Mostly electrically polarised – the inside is negative
• Purpose:
o Encloses cell
o Defines boundaries
o Maintains essential differences between cytosol and extracellular
environment
o In eukaryotic cells, PM maintains essential differences between cytosol and
internal environment of organelle
• Ion gradients across membranes are established by specialised membrane proteins
and are used to synthesise ATP, drive the transmembrane movement of selected
solutes or, in the case or nerve and muscle cells, to produce and transmit electrical
signals
• Proteins in membrane act as sensors of external signals – cell can change behaviour
in response to environmental cues – receptors transfer information rather than
molecules across the membrane
Membrane composition
• Composed of lipids, proteins and carbohydrates – thin film of lipid (fatty) and protein
molecules held together mainly by non-covalent interactions
• Lipids (hydrophobic and hydrophilic areas – lead to formation of closed sheets) –
form lipid bilayer – provides basic fluid structure of membrane and serves as
relatively impermeable barrier to passage of most water-soluble molecules
• Proteins span the lipid bilayer – transmembrane proteins – and mediate all other
functions of the membrane e
...
transporting specific molecules across, catalysing
other membrane-associated reactions such as ATP synthesis
...
About 30% of the proteins encoded in an animal cell’s genome are membrane
proteins
...

• Phospholipids
o Charged
o Neutral
Cell membrane structure

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CELL MEMBRANE STRUCTURE
• Sphingolipids
• Glycolipids
• Steroids
Phospholipids
Phospholipids are the most abundant membrane lipids
...


What drives formation of lipid bilayers?
• Need to consider the hydrophobic effect
o Water is a highly disordered, polar polymer
▪ Entropy is high
o Fluid lipids are highly disordered – entropy is high
• Free energy ΔG = ΔH – T
...
ΔS
• ΔH = water can’t realise full hydrogen bonding potential – enthalpy increases
• ΔS = water “structure” changes – increased “order” – entropy decreases
• This has a major influence
• Free energy becomes positive so the reaction is not spontaneous
...

Why do phospholipids form a bilayer?
• Consider a phospholipid monolayer
...

o This is energetically unstable
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o Non-polar lipid tails face each other
...

o This is energetically stable
...

• Bilayer curves round to seal ends – this is energetically favourable
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Suspend in aqueous medium
a
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Diameter = variable
2
...
Inject through needle into aqueous solution
b
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Suspension of phospholipid in mixed solvent
a
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Diameter = 1μm

Cell membrane structure

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CELL MEMBRANE STRUCTURE

Membranes are varied structures
There is a huge variety of membrane lipids
...
Eukaryotic cells
contain especially large amounts of cholesterol – up to one molecule for
every phospholipid
▪ Cholesterol is a sterol
▪ Rigid ring structure – single polar hydroxyl group and non-polar
hydrocarbon chain attached
▪ Cholesterol molecules orient themselves in the bilayer with hydroxyl
group close to the polar head groups of adjacent phospholipid
molecules
Phospholipids spontaneously form bilayers
• Form bilayers spontaneously in aqueous environments due to shape and amphiphilic
nature
• Lipid molecules aggregate to bury hydrophobic hydrocarbon tails in the interior and
expose hydrophobic heads to the water – this is the energetically the most
favourable environment
o Form spherical micelles – with the tails inwards
o Form bilayers – double-layered sheets – hydrophobic tails sandwiched in
between the hydrophilic heads
• Phospholipids are cylindrical so spontaneously form bilayers in aqueous
environments
o This can also provide a self-healing property
▪ A small tear in the bilayer creates a free edge with water
▪ The lipids rearrange spontaneously to eliminate the free edge
• The only way for a bilayer to avoid having edges is to close in on itself and form a
sealed compartment (i
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cells)
The lipid bilayer is a two-dimensional fluid
• Studies in the 1970s showed that individual lipid molecules can diffuse freely within
the lipid bilayers
• Two types of preparation used:
Cell membrane structure

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CELL MEMBRANE STRUCTURE
1
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Planar bilayers
• Also called black membranes
• Formed across a hole in a partition between two aqueous
compartments
Various techniques can be used to measure the motion of individual lipid molecules and
their components
...
Temperature will be lower if chain is shorter or have double
bonds
o Shorter chains reduces tendency of chains to interact with one another
o Cis-double bonds – kinks – more difficult for chains to pack together –
membrane remains fluid at lower temperatures
• Bacteria / yeast / other organisms in fluctuating heat – change composition of fatty
acid lipids to maintain a relatively constant fluidity
• Cholesterol modulates the properties of lipid bilayers
o When mixed with phospholipids, it enhances the permeability-barrier
properties of the lipid bilayer
o Inserts into the bilayer with its hydroxyl group close to the polar head groups
of the phospholipids – rigid, plate-like steroid rings interact with – and
partially immobilise – regions of the hydrocarbon chains closest to the polar
head groups
Cell membrane structure

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CELL MEMBRANE STRUCTURE
▪

By decreasing the mobility of the first few –CH2 groups of the
hydrocarbon chains of the phospholipid molecules, cholesterol makes
the lipid bilayer less deformable in this region and thereby decreases
the permeability of the bilayer to small water-soluble molecules
o Although cholesterol tightens the packing of the lipids in a bilayer, it does not
make membranes any less fluid
o At high concentrations, e
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in eukaryotic cells, cholesterol also prevents the
hydrocarbon chains from coming together and crystallising
APPROXIMATE LIPID COMPOSITIONS OF DIFFERENT CELL MEMBRANES
PERCENTAGE OF TOTAL LIPID BY WEIGHT
Red blood
Mitochondrion
Liver cell
LIPID
cell
(inner and
Endoplasmic
E
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g
...
g

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