2. Why do cells need membranes?
• Control entry and exit of substances
• Contain the cell contents and organelles;
compartmentalise individual cell
processes
• Communication with external
environment
• Site of chemical reactions
• Allows cell to change shape
• Transport and packaging within a cell
3. What are cell membranes made
of?
• Most of the cell membrane is
impenetrable to water soluble
substances – it is composed of LIPIDS
• Water-soluble substance and other
molecules can pass into the cell through
passageways composed of PROTEINS
4. Fats and Water
• These two substances do not
mix
• Water is a polar molecule
(the oxygen end is slightly
negative and the hydrogen
end slightly positive)
• Fats are non-polar and do not
form hydrogen bonds with
water
• Fats are said to be
hydrophobic and lie on the
surface of the water to
reduce the surface area in
contact between the fat and
the water
5. Dietary lipids…
• The fats that make up the cell membrane are
different in structure to the dietary fats we have
come across before
• Dietary fats are composed of one glycerol backbone
with three fatty acids attached to it
• The fatty acids are non-polar and do not form
hydrogen bonds with water so they are insoluble
6. Lipids in the cell membrane
• These are
phospholipids
• They are composed of
a glycerol backbone,
two fatty acid chains
and a phosphate group
• The fatty acid chains
are non-polar
• The phosphate group
is polar
7. Phospholipids
• They are usually drawn
like this
• The head (phosphate
group) is hydrophilic
• The tail (fatty acids) are
hydrophobic
8. And if you shake them
up in water?....
A MICELLE forms!
What happens if you place them
in water?....
9. So how are fats arranged in the
cell membrane?...
10. The Fluid Mosaic Model…
• The phospholipid bilayer
is not a fixed structure
• Although the bilayer will
always remain arranged
in this configuration,
the individual
phospholipids are able
to move, hence why it is
also known as the fluid
mosaic model
• The phospholipids
rotate on their axis
• They can ‘swap' places
with the phospholipid
next to them
• They can occasionally
‘swap’ places with the
phospholipid directly
opposite them, though
this is rare
All of these movements increase the fluidity of the
membrane
11. What about the proteins in the
membrane?
• Some proteins are transmembrane – they span the
width of the phospholipid bilayer
• Others are fixed into the bilayer on one side only;
these are peripheral proteins
• The proteins have several different functions
12. Transmembrane Proteins
• Water soluble substances pass
through proteins channels in the
bilayer
• These channels can be
permanent or temporary
• Gated channels allow selected
molecules through: conditions
within the cell determine
whether they are open or closed
• Other channels allow substances
to pass through freely in both
directions
• Some channels are active carrier
systems which use energy to
transport substances across
(e.g. the Sodium-Potassium
pump)
13. Peripheral proteins
• Glycoproteins, found on the cells
surface, are protein molecules
with a carbohydrate group
attached
• These allow cells to communicate
with one another
• Individual groups of cells have
their own glycoproteins,
recognisable by the immune
system
• Proteins on the cells surface
also act as receivers for
incoming messages, such as
hormones
• Intracellular cell membranes
may also contain enzymes for
cell surface reactions
