Biol. Unit 5, Chapter 13 - Homeostasis

A summary of the homeostasis chapter of unit 5 biology.

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  • Created on: 07-01-13 20:04
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Ajay Rai
Regulation of blood glucose:
Glucose is an important contributor to the water potential of blood and tissue fluid. The
pancreas plays a vital role in regulating blood glucose as it acts as both a receptor and an
effector. There are two distinct types of cells in the pancreas, in the islets of Langerhans region,
alpha () cells and beta () cells. cells secrete insulin and cells secret glucagon.
cells are receptors that detect a decrease in blood glucose concentration. They respond by
secreting glucagon. Glucagon activates enzymes in cells that convert glycogen to glucose,
glycogenolysis. Glucose diffuses into the blood, increasing the blood glucose concentration to
normal levels. ­ negative feedback. If glycogen supplies deplete, glucose can be formed from
pyruvate by gluconeogenesis. ­ the reverse of glycolysis but with different enzymes. During
starvation body proteins are broken down into amino acids which are used to produce pyruvate
for use in gluconeogenesis. This process ensures blood glucose doesn't fall below critical
cells are also receptors but they respond to an increase in blood glucose concentration. They
respond by secreting insulin. This travels in the blood and mainly affects muscle cells, liver cells
and adipose (fat storage). Insulin increases the rate at which these cells absorb glucose. It
stimulates glucose carrier proteins in cytoplasm to move to the cell surface membrane. Insulin
also activates enzymes in the liver to convert glucose to glycogen, glycogenesis. Insulin
secretion therefore causes a decrease in blood glucose concentration to bring it to normal levels.
negative feedback.
Insulin and glucose have opposite effects and are therefore antagonistic.
Adrenaline, another hormone, is secreted by the adrenal glands, two small glands just above the
kidneys. It stimulates glycogenolysis, similar to glucagon. Adrenaline and glucagon don't enter
target cells they form weak bonds with receptor sites on the cell membrane. They act via a
`second messenger' model of hormonal control the hormone acts as the first messenger, going
from the endocrine gland to the target cell, binding to a receptor on the target cell membrane.
The binding stimulates cAMP, a second chemical messenger, inside the target cell. cAMP
activates specific enzymes that intitiate glycogenolysis.
When control of blood glucose fails, diabetes can develop. Type I occurs well cells in the islets of
Langerhans are damaged and cannot produce insulin. Type II occurs when cells in the body
become resistant to the effects of insulin. If too much insulin is injected, a meal is skipped or
strenuous exercise is carried out, blood glucose can fall too low. This is hypoglycaemia. The
opposite of this causes blood glucose to increase too much, causing hyperglycaemia.
Temperature regulation in mammals:
The hypothalamus is the body's thermostat, and has two regions.

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Heat loss centre: a group of cells at the front of the hypothalamus that increase the blood flow
to the skin's surface if body temperature increases. They also initiate sweating.
Heat gain centre: a group of cells at the rear of the hypothalamus that initiates responses to
prevent the body cooling down too much.…read more

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are not cold blooded animals. An active reptile therefore may use less than a tenth as much
metabolic energy as an endotherm.
Most energy used for muscular activity in reptiles comes from anaerobic metabolism as opposed
to aerobic in endotherms. Glycogen is immediately available within muscles to facilitate bursts of
activity.…read more


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