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Module 1. Communication and Homeostasis.
The need for communication,
Keeping cells active
All living things need to maintain a certain limited set of conditions inside their cells. This is because the
cellular activities rely on the action of enzymes. You will recall from your AS work that enzymes need a
specific set of conditions in which to work efficiently. These include a suitable temperature, a suitable
p.H., an aqueous environment that keeps the substrate and products in solution and freedom from toxins
and excess inhibitors.
Stimulus and response to external environments
All living organisms have an external environment that consists of the air, water or soil around them .
This external environment will change. As it changes it may place stress on the living organism. For
instance, a cooler environment will cause greater heat loss. If the organism is to remain and survive, the
changes in the environment must be monitored and the organism must change its behaviour or
physiology to reduce the stress. The environmental change is a stimulus and the way in which the
organism changes its behaviour or physiology is its response.
The environment may change slowly, for example as the seasons pass or because of global warming.
This change will elicit a gradual response. For example arctic fox (Alopex Iagopus) has a much thicker
white coat in winter and a thinner grey/brown coat in summer. The different coats adapt it to the
However, the environment may change much more quickly. The changes from day to night or walking
from bright sunlight into an unlit room are rapid changes. Again, the change (a stimulus) must be
monitored and the organism must respond to the change.
Stimulus and response to internal environments
Most multicellular organisms have a range of tissues and organs. Many of the cells and tissues are not
exposed to the external environment. they are protected by epithelial tissues and organs such as skin or
bark. In animals the internal calls and tissues are bathed in tissue fluid. This is the environment of the
As cells undergo their various metabolic activities they use up substrates and produce products. Some
of these products may be unwanted or may even be toxic. These substances diffuse out of the cells into
the tissue fluid. Therefore, the activities of the cells alter their own environment. One waste product is
carbon dioxide. If this is allowed to build up in the tissue fluid outside the cells it could disrupt the action
of enzymes by changing the p.H. of the environment around the cell.
It is clear that accumulation of excess water or toxins in this internal environment must act as a
stimulus to cause removal of these wastes so that the cells can survive. This stimulus may act directly
on the cells which respond by reducing the their activities so that less waste is produced. However, this
response may not be good for the whole organism.
Maintaining the internal environment of the cells
The composition of the tissue fluid is maintained by the blood. Blood flows throughout the body and
transports substances to and from the cells. Any wastes or toxins accumulating in the tissue fluid are
likely to enter the blood and be carried away. In order to prevent their accumulation in the blood they
must be removed from the body, a process called excretion.
It is important that the concentrations that the concentrations of wastes and all substances in the blood
are monitored closely. This ensures that the body does not excrete too much of any useful substances
but removes enough of the wastes to maintain good health. It also ensures that all of the cells of the
body are supplied with the substrates they need.
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A multicellular organism is more effective than a singlecelled organism, as its cells can be
differentiated. This means that its cells can be specialised to perform particular functions. Groups of
cells specialised in this way form tissues and organs. As a result the cells that monitor the blood may
be in a different part of the body and well away from the cells that release a substance into the blood or
well away from the organ that removes that substance from the blood.…read more