Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Figure 4.3 Scanning electron micrograph of a cell surface
membrane. The membrane has been prepared by freezefracturing,
which has split open the bilayer. The P-face is the
phospholipid layer nearest the inside of the cell and shows
the many protein particles embedded in the membrane. The
E-face is part of the outer phospholipid layer (× 50 000).
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Figure 4.2 Phospholipids in water: a spread as a single layer
of molecules (a monolayer) on the surface of water b forming
micelles surrounded by water c forming bilayers d bilayers
forming membrane-bound compartments.
Structure of membranes
The phospholipid bilayer is visible using the electron
microscope at very high magnifications of at least
× 100 000 (Figure 1.23 on page 17). The double black line
visible using the electron microscope is thought to show
the hydrophilic heads of the two phospholipid layers;
the pale zone between is the hydrophobic interior of the
membrane. The bilayer (membrane) is about 7 nm wide.
Membranes also contain proteins. These can be seen in
certain electron micrographs, such as Figure 4.3.
In 1972, two scientists, Singer and Nicolson, used
all the available evidence to put forward a hypothesis
for membrane structure. They called their model the
fluid mosaic model. It is described as ‘fluid’ because
both the phospholipids and the proteins can move about
by diffusion. The phospholipid bilayer has the sort of
fluidity we associate with olive oil. The phospholipids
move sideways, mainly in their own layers. Some of
the protein molecules also move about within the
phospholipid bilayer, like icebergs in the sea. Others
remain fixed to structures inside or outside the cell.
The word ‘mosaic’ describes the pattern produced by
the scattered protein molecules when the surface of the
membrane is viewed from above.
Figures 4.4 and 4.5 are diagrams of what we imagine a
membrane might look like if we could see the individual
molecules.
Features of the fluid mosaic model
The membrane is a double layer (bilayer) of phospholipid
molecules. The individual phospholipid molecules move
about by diffusion within their own monolayers.
The phospholipid tails point inwards, facing each
other and forming a non-polar hydrophobic interior. The
phospholipid heads face the aqueous (water-containing)
medium that surrounds the membranes.
Some of the phospholipid tails are saturated and some
are unsaturated. The more unsaturated they are, the more
fluid the membrane. This is because the unsaturated fatty
acid tails are bent (Figure 2.11, page 36) and therefore
fit together more loosely. Fluidity is also affected by tail