Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
78
Distances travelled may be short, as with diffusion
within one cell, or long, as with long-distance transport
in blood (animals) or phloem (plants). There are usually
many components and different mechanisms along the
route. Signalling includes both electrical and chemical
events and their interactions with each other – for
example, the events associated with the nervous and
hormonal systems in animals. These events involve a wide
range of molecules produced by cells within the body
(e.g. hormones and neurotransmitters) as well as outside
stimuli (e.g. light, drugs, pheromones and odours).
The cell surface membrane is a critical component
of most signalling pathways because it is a barrier to the
movement of molecules, controlling what moves between
the external and internal environments of the cell. In
a typical signalling pathway, molecules must cross or
interact with cell surface membranes.
Signalling molecules are very diverse. If they are
hydrophobic, such as the steroid hormones (e.g. oestrogen),
they can diffuse directly across the cell surface membrane
and bind to receptors in the cytoplasm or nucleus.
QUESTION
4.1 Why does the cell surface membrane not provide a
barrier to the entry of hydrophobic molecules into
the cell?
More commonly, the signalling molecule is watersoluble.
In this case, a typical signalling pathway starts
with the signal arriving at a protein receptor in a cell
surface membrane. The receptor is a specific shape which
recognises the signal. Only cells with this receptor can
recognise the signal. The signal brings about a change
in the shape of the receptor, and since this spans the
membrane, the message is in effect passed to the inside of
the cell (signal transduction). Changing the shape of the
receptor allows it to interact with the next component of
the pathway, so the message gets transmitted.
This next component is often a ‘G protein’, which
acts as a switch to bring about the release of a ‘second
messenger’, a small molecule which diffuses through the cell
relaying the message. (G proteins are so-called because the
switch mechanism involves binding to GTP molecules. GTP
is similar to ATP, but with guanine in place of adenine.)
Many second messenger molecules can be made in
response to one receptor molecule being stimulated. This
represents an amplification (magnification) of the original
signal, a key feature of signalling. The second messenger
typically activates an enzyme, which in turn activates
further enzymes, increasing the amplification at each
stage. Finally, an enzyme is produced which brings about
the required change in cell metabolism.
The sequence of events triggered by the G protein is
called a signalling cascade. Figure 4.7 is a diagram of
a simplified cell signalling pathway involving a second
messenger. Examples of such a pathway involving the
hormones adrenaline and glucagon are discussed in
Chapter 14.
signal
protein
receptor
G protein
activation
second messenger –
small, soluble, signalling
molecules spread
through the cell, greatly
amplifying the signal
enzyme –
makes
second
messenger
activated enzyme
other activated enzymes
response: secretion
transcription
movement
metabolic change
cell surface
membrane
signalling
cascade with
amplification
at each stage
Figure 4.7 A simplified cell signalling pathway involving a
second messenger.
Besides examples involving second messengers, there
are three other basic ways in which a receptor can alter the
activity of a cell:
■■
■■
opening an ion channel, resulting in a change
of membrane potential (e.g. nicotine-accepting
acetylcholine receptors, Chapter 15)
acting directly as a membrane-bound enzyme
(e.g. insulin receptor)