Homeostasis revision

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1.1 The characteristics of life and the need for communication in multicellular organisms
The characteristics of life
Scientists have long debated what the criteria for life are, and there is still no definite answer. Take, for example, viruses ­
which to this day scientists have still not agreed on whether or not they are to be considered life forms. Some examples
of characteristics of life include:
homeostasis adaptation
organisation response to stimuli
metabolism reproduction
The process known as homeostasis is vital to life because it is essential that the right internal conditions are maintained
within an organism, so that they are sufficient for enzymes to work efficiently, as enzymes are vital to maintaining life as
they are involved in all major metabolic life reactions.
Animals are an example of a multicellular organism. There is a division of labour between different tissues and organs
(and cells) which keeps the organism functioning. Stimulus and response is required, to allow the different tissues to
communicate with each other.
Multicellular organisms require communication as they are differentiated and have this division of labour, in order to
coordinate the functions of the different systems. It provides a link between the receptor, detecting a stimulus, and the
effector being triggered to carry out an appropriate response.
Stimulus and response
The body must respond to external stimuli (such as changes in temperature, light, sound, taste, etc) as well as internal
stimuli (such as changes in body temperature, blood sugar levels and presence of pathogens). The communication which
takes place between the detection of a stimulus and the implementation of the response relies on two systems: the
nervous system and the endocrine system (hormonal system).
Nervous communication uses signals carried by nerves throughout the central nervous system (CNS), in the form of an
electrical impulse. The endocrine system uses hormones, which travel around in the bloodstream and trigger a response
when they bind with target cells or target tissues.
The term homeostasis describes the process of maintaining a constant internals environment despite external changes
which may be taking place.
Negative feedback
One mechanism, called negative feedback, operates by detecting the external change (stimulus), communicating with
other cells, and reversing the change (response). The diagram outlines negative feedback response for a change in
receptors detect coordination effector reverses
increase in decrease in
temperature change of signal the change temperature
optimum returns to optimum
temperature temperature
OR decrease in increase in
receptors detect coordination effector reverses
temperature change of signal the change

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Positive feedback
The other type of feedback, which is much less common than negative feedback, is called positive feedback. This type of
feedback usually doesn't lead to homeostasis, as the effector actually increases the change detected by the receptor, and
so it can be seen as a `vicious cycle' approach. With positive feedback, the change is amplified, which is usually harmful to
the organism. One example of this is, again, using temperature.…read more


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