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Unit 4 - Homeostasis
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Annotated notes and lecture slides from the fourth unit of grade 12 biology, homeostasis.
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Unit 4 - Biology Notes, HOMEOSTASIS
HOMEOSTASIS the maintenance of relatively
stable internal conditions within an organism.
• All living things maintain an internal environment quite different from their
surroundings. All biological functions depend upon having a membrane whose proteins
are all functioning properly. Proteins are highly sensitive to physical factors such as
temperature and pressure as well as chemical factors such as pH, O2 & CO2
concentrations and nutrient & waste concentrations. The membranes and proteins must
remain in operating condition.
sodium
• Homeostasis is necessary because the environment of the organism, both internal
simportant and external is constantly changing.
or the
unction • Homeostatic control is very precise in birds and mammals. E.g. control over
fneurons glucose, O2, CO2, Na+, Ca++, H2O, etc.
• Living systems must respond to stress (a physical, mental or emotional pressure).
• External stresses include light, temperature, predators, lack of food, etc.
• Internal stresses (usually due to metabolic activities) include elimination of waste,
repair of worn out tissue, aging, etc.
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, HOW IS HOMEOSTASIS ACHIEVED?
To maintain cells, tissues and entire organisms within their biological
tolerance limits, various mechanisms have evolved. These may be
• structural: the animal or plant has particular physical features which
help its survival in an otherwise hostile environment.
• functional: the metabolism of the animal or plant is able to adjust to
changes in conditions as they are detected.
• behavioural: the actions and interactions of the individual, either alone
or with others, help it to survive in its particular environment.
Homeostasis is really the combined result of all of these, a failure of any one
of them can result in the death of an individual.
damage to To maintain homeostasis, the body must recognize the stresses and
eurons pain respond to them. The stress provides the stimulus which allows the
or't be felt body to respond to an appropriate manner after detection by the
moredamaged body’s receptors. Examples of these receptors can be the eye, the
or becaused skin (touch, pain, hot, cold, pressure), ears, etc. These receptors then
pass the stress signal or stimulus into the body so that some action or
response can occur.
\ The stress provides the stimulus and by applying the appropriate response, the system
exercises control (response).
me
an
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, FEEDBACK MECHANISMS
Feedback mechanisms are the general
mechanism of nervous or hormonal
regulation in animals. Essentially,
feedback occurs when the response to a
stimulus has an effect of some kind on
the original stimulus. The nature of the
response determines how the feedback is
'labelled'.
Negative feedback is when the response
diminishes the original stimulus.
Positive feedback is when the response
enhances the original stimulus.
Negative feedback is most common in biological systems.
Examples of this are:
• Blood glucose concentrations rise after a sugary
meal (the stimulus), the hormone insulin is released and it
speeds up the transport of glucose out of the blood and
into selected tissues (the response), so blood glucose
concentrations decrease (thus decreasing the original
stimulus).
• Exercise creates metabolic heat which raises the
body temperature (the stimulus), cooling mechanisms
such as vasodilatation (flushed skin) and sweating begin
(the response), body temperature falls (thus decreasing
the original stimulus).
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