Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
Stomata close when the hydrogen ion pump proteins
stop and potassium ions leave the guard cells and enter
neighbouring cells. Now there is a water potential gradient
in the opposite direction, so water leaves the guard cells
so that they become flaccid and close the stoma. Closing
of the stomata has significant effects on the plant. It
reduces the uptake of carbon dioxide for photosynthesis
and reduces the rate of transpiration. As transpiration is
used for cooling the plant and also for maintaining the
transpiration stream that supplies water and mineral
ions to the leaves, stomatal closure only occurs when
reducing the loss of water vapour and conserving water is
the most important factor. In conditions of water stress,
the hormone abscisic acid (ABA) is produced in plants to
stimulate stomatal closure.
Abscisic acid and stomatal closure
Abscisic acid has been found in a very wide variety of
plants, including ferns and mosses as well as flowering
plants. ABA can be found in every part of the plant, and is
synthesised in almost all cells that possess chloroplasts or
amyloplasts (organelles like chloroplasts, but that contain
large starch grains and no chlorophyll).
One role of ABA is to coordinate the responses
to stress; hence it is known as a stress hormone. If a plant
is subjected to difficult environmental conditions, such as
very high temperatures or much reduced water supplies,
then it responds by secreting ABA. In a plant in drought
conditions, the concentration of ABA in the leaves can rise
to 40 times that which would normally be present. This
high concentration of ABA stimulates the stomata to close,
reducing the loss of water vapour from the leaf.
If ABA is applied to a leaf, the stomata close within just
a few minutes. Although it is not known exactly how ABA
achieves the closure of stomata, it seems that guard cells
have ABA receptors on their cell surface membranes, and
it is possible that when ABA binds with these it inhibits
the proton pumps to stop hydrogen ions being pumped
out. ABA also stimulates the movement of calcium ions
into the cytoplasm through the cell surface membrane and
the tonoplast (membrane around the vacuole). Calcium
acts as a second messenger to activate channel proteins to
open that allow negatively charged ions to leave the guard
cells. This, in turn, stimulates the opening of channel
proteins which allows the movement of potassium ions out
of the cells. At the same time, calcium ions also stimulate
the closure of the channel proteins that allow potassium
ions to enter. The loss of ions raises the water potential
of the cells, water passes out by osmosis, the guard cells
become flaccid and the stomata close.
QUESTION
14.12 a Describe the mechanism of stomatal opening.
b Explain when it is an advantage for plants to
close their stomata.
c Outline how abscisic acid functions to
stimulate the closure of stomata.
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Summary
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Mammals keep their internal environment relatively
constant, so providing steady and appropriate
conditions within which cells can carry out their
activities. This is known as homeostasis. Homeostatic
balance requires receptors that detect changes in
physiological factors such as the temperature, water
potential and pH of the blood.
Effectors are the cells, tissues and organs (including
muscles and glands) that carry out the functions
necessary to restore those factors to their set points.
Homeostatic control systems use negative feedback
in which any change in a factor stimulates actions by
effectors to restore the factor to its set point.
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Thermoregulation involves maintaining a constant core
body temperature. The hypothalamus is the body’s
thermostat monitoring the temperature of the blood
and the temperature of the surroundings. It controls
the processes of heat production, heat loss and heat
conservation to keep the core body temperature close
to its set point.
Excretion is the removal of toxic waste products of
metabolism, especially carbon dioxide and urea. The
deamination of excess amino acids in the liver produces
ammonia, which is converted into urea, the main
nitrogenous waste product. Urea is excreted in solution
in water, as urine.