Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
76
Intrinsic proteins have hydrophobic and hydrophilic
regions. They stay in the membrane because the
hydrophobic regions, made from hydrophobic amino
acids, are next to the hydrophobic fatty acid tails and
are repelled by the watery environment either side of
the membrane. The hydrophilic regions, made from
hydrophilic amino acids, are repelled by the hydrophobic
interior of the membrane and therefore face into the
aqueous environment inside or outside the cell, or line
hydrophilic pores which pass through the membrane.
Most of the intrinsic protein molecules float like mobile
icebergs in the phospholipid layers, although some are
fixed like islands to structures inside or outside the cell
and do not move about.
A second type of protein molecule is the extrinsic
protein (or peripheral protein). These are found on the
inner or outer surface of the membrane. Many are bound
to intrinsic proteins. Some are held in other ways – for
example, by binding to molecules inside or outside the cell,
or to the phospholipids.
All the proteins referred to from now on in this chapter
are intrinsic proteins.
Many proteins and lipids have short, branching
carbohydrate chains attached to that side of the molecule
which faces the outside of the membrane, thus forming
glycoproteins and glycolipids, respectively.
The total thickness of the membrane is about 7 nm
on average. Molecules of cholesterol are also found in the
membrane.
Roles of the components of cell
membranes
We have seen that cell membranes contain several different
types of molecule. There are three types of lipid, namely
phospholipids, cholesterol and glycolipids. There are also
proteins and glycoproteins. Each of these has a particular
role to play in the overall structure and function of the
membrane.
Phospholipids
As explained on pages 73–76, phospholipids form the
bilayer, which is the basic structure of the membrane.
Because the tails of phospholipids are non-polar, it is
difficult for polar molecules, or ions, to pass through
membranes, so they act as a barrier to most water-soluble
substances. For example, water-soluble molecules such as
sugars, amino acids and proteins cannot leak out of the
cell, and unwanted water-soluble molecules cannot enter
the cell.
Some phospholipids can be modified chemically to
act as signalling molecules. They may move about in the
phospholipid bilayer, activating other molecules such
as enzymes. Alternatively, they may be hydrolysed to
release small, water-soluble, glycerol-related molecules.
These diffuse through the cytoplasm and bind to specific
receptors (page 78). One such system results in the
release of calcium ions from storage in the ER, which in
turn brings about exocytosis of digestive enzymes from
pancreatic cells as described on page 87.
Cholesterol
Cholesterol is a relatively small molecule. Like
phospholipids, cholesterol molecules have hydrophilic
heads and hydrophobic tails, so they fit neatly between the
phospholipid molecules with their heads at the membrane
surface. Cell surface membranes in animal cells contain
almost as much cholesterol as phospholipid. Cholesterol
is much less common in plant cell membranes and absent
from prokaryotes. In these organisms, compounds very
similar to cholesterol serve the same function.
At low temperatures, cholesterol increases the fluidity
of the membrane, preventing it from becoming too
rigid. This is because it prevents close packing of the
phospholipid tails. The increased fluidity means cells
can survive colder temperatures. The interaction of the
phospholipid tails with the cholesterol molecules also
helps to stabilise cells at higher temperatures when the
membrane could otherwise become too fluid. Cholesterol
is also important for the mechanical stability of
membranes, as without it membranes quickly break and
cells burst open. The hydrophobic regions of cholesterol
molecules help to prevent ions or polar molecules from
passing through the membrane. This is particularly
important in the myelin sheath (made up of many layers of
cell surface membrane) around nerve cells, where leakage
of ions would slow down nerve impulses.
Glycolipids, glycoproteins and proteins
Many of the lipid molecules on the outer surfaces of
cell surface membranes, and probably all of the protein
molecules, have short carbohydrate chains attached
to them. These ‘combination’ molecules are known
as glycolipids and glycoproteins, respectively. The
carbohydrate chains project like antennae into the watery
fluids surrounding the cell, where they form hydrogen
bonds with the water molecules and so help to stabilise
the membrane structure. The carbohydrate chains form
a sugary coating to the cell, known as the glycocalyx.
In animal cells, the glycocalyx is formed mainly from
glycoproteins; in plant cells it mainly comprises glycolipids.