Cambridge International A Level Biology Revision Guide

Chapter 4: Cell membranes and transport
glycolipid
glycolipid
carbohydrate part part of of glycoprotein
outside outside
inside
protein glycoprotein phospholipid channel protein
protein glycoprotein cholesterol phospholipid transport protein inside
Figure 4.4 An artist’s impression of the fluid mosaic model of membrane structure.
length: the longer the tail, the less fluid the membrane.
As temperature decreases, membranes become less fluid,
but some organisms which cannot regulate their own
temperature, such as bacteria and yeasts, respond by
increasing the proportion of unsaturated fatty acids in
their membranes.
Two types of protein are recognised, according to their
position in the membrane.
Proteins that are found embedded within the
membrane, such as those in Figure 4.5, are called intrinsic
proteins (or integral proteins). Intrinsic proteins may
be found in the inner layer, the outer layer or, most
commonly, spanning the whole membrane, in which
case they are known as transmembrane proteins.
In transmembrane proteins, the hydrophobic regions
which cross the membrane are often made up of one or
more α-helical chains.
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hydrophilic head
hydrophobic tails
phospholipid
bilayer
branching carbohydrate
attached to a protein to
form a glycoprotein
branching carbohydrate attached
to a lipid to form a glycolipid
transport protein (channel or carrier
protein) has hydrophilic interior for
ions and hydrophilic molecules
outer surface
inner surface
one phospholipid
molecule
cholesterol
in both layers
proteins
Figure 4.5 Diagram of the fluid mosaic model of membrane structure.