AQA A2 BIOLOGY UNIT 5: Homeostasis

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  • Created on: 18-04-14 14:25
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The maintenance of a constant internal environment
Homeostasis involves maintaining the chemical make-up, volume and other features of blood and
tissue fluid within restricted limits.
The internal environment is made up of tissue fluids that bathe each cell, supplying nutrients and
removing wastes.
Homeostasis ensures all the cells of the body are in an environment that meets their needs and allows
them to function normally despite external changes.
Homeostasis is important for the proper functioning of organisms for the following reasons:
Temperature If body temperature is too high, enzymes may become denatured
The enzymes molecules vibrate too much which breaks the hydrogen bonds that
hold them in their 3D shape
The shape of the enzymes active site is changed and it no longer works as a
This means metabolic reactions are less efficient
If body temperature is too low, enzyme activity is reduced, slowing down the
rate of metabolic reactions
The highest rate of enzyme activity happens at their optimum temperature
pH If blood pH is too high or too low enzymes become denatured
The hydrogen bonds that hold them in their 3D shape are affected so the shape
of the enzymes active site is changed and it no longer works as a catalyst
This means metabolic reactions are less efficient
The highest rate of enzyme activity happens at their optimum pH ­ usually
around pH 7, but some work best at other pHs
Water Changes to the water potential of the blood and tissue fluids may cause cells to
potential shrink and expand as a result of water leaving or entering by osmosis
In both instances the cells cannot operate normally
The maintenance of a constant blood glucose concentration is essential in
ensuring a constant water potential
A constant blood glucose concentration also ensures a reliable source of glucose
for respiration by cells
If blood glucose is too high the water potential of the blood is reduced to a point
where water molecules diffuse out of cells into the blood by osmosis
This can causes the cells to shrivel up and die
If blood glucose is too low, cells are unable to carry out normal activites because
there isn't enough glucose for respiration to provide energy

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The control of any self-regulating system involves a series of stages that feature:
The set point which is the desired level at which the system operates which is
monitored by the...
Receptor which detects any deviation from the set point and informs the...
Controller which coordinates information from various receptors and sends
instructions to an appropriate...
Effector which brings about the changes needed to return the system to the set
point ­ this return to normality creates a...…read more

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Temperature Homeostasis: Thermoregulation
One of the most important examples of homeostasis is the regulation of body temperature.
Body temperature needs to be regulated to keep enzymes working close to their optimum
temperature and to prevent them from denaturing.
There are basically two ways of doing this: mammals and birds can generate their own heat and are
called endotherms; while all other animals rely on gaining heat from their surroundings, so are called
ectotherms.…read more

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Temperature Homeostasis in Endotherms
Endotherms gain most of their heat from internal metabolic activities.
Their body temperature remains relatively constant despite fluctuations in the external temperature.
The core body temperature is between 35-44Oc.
This range is a compromise between having a higher temperature at which enzymes work more rapidly
and the amount of energy needed to maintain that higher temperature.
Endothermic animals use behaviour to maintain a constant temperature ­ they use a wide range of
physiological mechanisms.…read more

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Regulation of body temperature in endotherms is an example of homeostasis.
The stimulus is detected by thermoreceptors which pass the information to a coordinator ­ the
hypothalamus, in the brain which then causes an effector ­ the skin, to produce the appropriate
response.…read more

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Glucose is the transport carbohydrate in animals, and its concentration in the blood affects every cell in
the body.
The brain in particular can only respire glucose (not lipids) but it doesn't store glycogen.
Very low concentrations of glucose (hypoglycaemia) will cause brain cells to die and very high
concentrations (hyperglycaemia) will lower the blood water potential and kill cells by dehydration. The
concentration of glucose in the blood is therefore strictly controlled within the range 80-100 mg.…read more

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The Role of the Pancreas in Regulating Blood Glucose
The pancreas is a large, pale-coloured gland that is situated in the upper abdomen, behind the stomach.
It produces enzymes: protease, amylase and lipase for digestion
It produces hormones: insulin and glucagon for regulating blood glucose
The pancreas is made up largely of the cells that produce its digestive enzymes.
Scattered throughout these cells are groups of hormone producing cells known as islets of Langerhans.…read more

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Role of Adrenaline in regulating the blood glucose level
Adrenaline is a hormone that is secreted from your adrenal glands.
It's secreted when there's a low concentration of glucose in your blood.
Adrenaline raises the blood glucose by:
· Activating an enzyme that causes the breakdown of glycogen to glucose in the liver (glycogenolysis)
· Inactivating an enzyme that synthesises glycogen from glucose (glycogenesis)
These two hormones are antagonistic, which means that they have opposite effects on blood glucose.…read more

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Diabetes is a chronic disease in which a person is unable to metabolise carbohydrates, especially
glucose, properly.
Types of Diabetes
Diabetes is a metabolic disorder cause by an inability to control blood glucose levels due to a lack of
the hormone insulin of a loss of responsiveness to insulin.…read more


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