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Homeostasis
Tuesday, 13 February 2024

07:56

To function efficiently, organisms have control systems to keep their internal conditions near
constant
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Information about conditions inside the body and the external surroundings are detected by
sensory cells
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For a cell,
its immediate environment is the tissue fluid that surrounds it
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◊ water potential – if the water potential decreases, water may move out of cells by osmosis,
causing metabolic reactions in the cell to slow or stop; if the water potential increases,
water may enter the cell causing it to swell and maybe burst
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◊ pH – enzyme activity is influenced by pH; the pH of cytoplasm is between 6
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0 and if
it fluctuates outside this range, enzymes will function less efficiently and will be denatured
at extreme values of pH
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Homeostatic Control
Most control mechanisms in living organisms use a negative feedback control loop to maintain
Homeostatis Page 1

Most control mechanisms in living organisms use a negative feedback control loop to maintain
homeostatic balance
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The receptor detects stimuli that are involved with the condition (or physiological factor) being
regulated
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These receptors send information about the changes they detect through the nervous system to
a central control in the brain or spinal cord
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The central control instructs an effector to carry out an action, which is called the output
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The homeostatic mechanisms in mammals require information to be transferred between
different parts of the body
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Sometimes control mechanisms do not respond via negative feedback
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This is sensed by carbon dioxide receptors, which cause the breathing rate to increase
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This is an example of a positive feedback
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Excretion
Many of the metabolic reactions occurring within the body produce unwanted substances
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The removal of these unwanted products of metabolism is
known as excretion
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These are carbon dioxide and urea
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The waste
Homeostatis Page 2

Carbon dioxide is produced continuously by cells that are respiring aerobically
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Gas exchange occurs within the lungs, and carbon dioxide diffuses from the blood into the
alveoli; it is then excreted in the air we breathe out
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It is produced from excess amino acids and is transported from the
liver to the kidneys, in solution in blood plasma
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Deamination
If more protein is eaten than what is needed, the excess cannot be stored in the body
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To make use of this energy, the liver removes the amine groups in a process known as
deamination
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These combine to produce ammonia (NH3)
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Ammonia is a very soluble and highly toxic compound
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g
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However, in terrestrial animals such as humans, ammonia increases the Ph in cytoplasm which
then interferes with metabolic processes such as respiration and with cell signalling in the
brain
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Several reactions, known as the urea cycle, are involved in combining ammonia and
carbon dioxide to form urea
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Urea is the main nitrogenous excretory product of humans
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A substance called creatine is made in the liver, from certain amino acids
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However, some is converted to creatinine and excreted
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Homeostatis Page 3

of purines from nucleotides, not from amino acids
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All of the urea made each day must be
excreted, or its concentration in the blood would build up and become dangerous
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Homeostatis Page 4



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