Cambridge International A Level Biology Revision Guide

Chapter 15: Coordination
The morning-after pill
This form of birth control is intended to be taken after
a woman has had unprotected sexual intercourse and
thinks that she might be pregnant. It might be taken by a
woman who forgot to take her oral contraceptive pill, or
if a condom broke, or by someone who was raped, as well
as by a woman who simply did not take any precautions
to prevent pregnancy. It works for up to 72 hours after
intercourse, not just the ‘morning after’.
The pill contains a synthetic progesterone-like
hormone. If taken early enough, it reduces the chances of
a sperm reaching and fertilising an egg. However, in most
cases, it probably prevents a pregnancy by stopping the
embryo implanting into the uterus.
Control and coordination in
plants
Plants, like animals, have communication systems that
allow coordination between different parts of their bodies.
They too must respond to changes in their external and
internal environments, as we saw in Chapter 14. Most
plant responses involve changing some aspect of their
growth to respond to factors such as gravity, light and
water availability. Plants can also respond fairly quickly
to changes in carbon dioxide concentration, lack of water,
grazing by animals and infection by fungi and bacteria.
Some of these responses are brought about by quick
changes in turgidity, as happens when stomata respond to
changes in humidity, carbon dioxide concentration and
water availability.
Electrical communication in plants
Plant cells have electrochemical gradients across their cell
surface membranes in the same way as in animal cells.
They also have resting potentials. As in animals, plant
action potentials are triggered when the membrane is
depolarised. In at least some species, some responses to
stimuli are coordinated by action potentials. The ‘sensitive
plant’, Mimosa, responds to touch by folding up its leaves.
Microelectrodes inserted into leaf cells detect changes
in potential difference that are very similar to action
potentials in animals. The depolarisation results not from
the influx of positively charged sodium ions, but from the
outflow of negatively charged chloride ions. Repolarisation
is achieved in the same way by the outflow of potassium
ions. Plants do not have specific nerve cells, but many of
their cells transmit waves of electrical activity that are
very similar to those transmitted along the neurones
of animals. The action potentials travel along the cell
membranes of plant cells and from cell to cell through
plasmodesmata that are lined by cell membrane (Figure
1.28, page 20). The action potentials generally last much
longer and travel more slowly than in animal neurones.
Many different stimuli trigger action potentials in
plants. Chemicals coming into contact with a plant’s
surface trigger action potentials. For example, dripping a
solution of acid of a similar pH to acid rain on soya bean
leaves causes action potentials to sweep across them. In
potato plants, Colorado beetle larvae feeding on leaves
have been shown to induce action potentials. No-one
knows what effect, if any, these action potentials have, but
it is thought that they might bring about changes in the
metabolic reactions taking place in some parts of
the plant.
The Venus fly trap is a carnivorous plant that obtains a
supply of nitrogen compounds by trapping and digesting
small animals, mostly insects. Charles Darwin made
the first scientific study of carnivorous plants describing
Venus fly traps as ‘one of the most wonderful plants in
the world’. The specialised leaf is divided into two lobes
either side of a midrib. The inside of each lobe is often red
and has nectar-secreting glands around the edge to attract
insects. Each lobe has three stiff sensory hairs that respond
to being deflected. The outer edges of the lobes have stiff
hairs that interlock to trap the insect inside. The surface
of the lobes has many glands that secrete enzymes for the
digestion of trapped insects. The touch of a fly or other
insect on the sensory hairs on the inside of the folded
leaves of the Venus fly trap stimulates action potentials
that travel very fast across the leaf causing it to fold over
and trap the insect (Figure 15.35).
The deflection of a sensory hair activates calcium ion
channels in cells at the base of the hair. These channels
open so that calcium ions flow in to generate a receptor
potential. If two of these hairs are stimulated within a
period of 20 to 35 seconds, or one hair is touched twice
within the same time interval, action potentials travel
across the trap. When the second trigger takes too long to
occur after the first, the trap will not close, but a new time
interval starts again. If a hair is deflected a third time then
the trap will still close. The time between stimulus and
response is about 0.5 s. It takes the trap less than 0.3 s to
close and trap the insect.
The lobes of the leaf bulge upwards when the trap is
open. They are convex in shape. No one is quite sure how
the trap closes, but as Darwin noticed, the lobes rapidly
change into a concave shape, bending downwards so the
trap snaps shut. This happens too fast to be simply the
result of water movement from the cells on the top of the
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