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Organisms respond to changes in their internal and external
environments (AQA A2 Biology) PART 8 of 9 TOPICS
Control of blood glucose concentration:
Factors that affect glucose concentration are food, medication, activity, biological, environmental and
Insulin is released from the beta cells of the islets of Langerhans in the pancreas when high levels of
glucose are detected. It binds insulin receptors of the liver which stimulates the conversion of glucose
to glycogen (glycogenesis) and glucose to fat. This causes the level of glucose to drop. If the blood
glucose concentration becomes too low, it is detected by the alpha cells of the islets of Langerhans in
the pancreas. These produce glucagon which binds to receptors of the liver cells. This stimulates the
conversion of glycogen to glucose (glycogenolysis) and using other biological molecules such as amino
acids and glycerol to convert them into glucose (gluconeogenesis). This will cause an increase in the
level of glucose. This whole process is known to be multiple feedback as the levels of glucose fluctuate
around the normal level.
Insulin that is secreted by the beta cells also binds to insulin receptors of body cells. This increases the
rate of absorption of glucose into the body cells particularly muscle cells and is used for respiration.
The adrenal glands are found on top of each kidney and it consists of the adrenal cortex and adrenal
Adrenal cortex: This releases cortisol which is secreted when glycogen stores run out. It
stimulates gluconeogenesis which increases glucose levels quickly.
Adrenal medulla: This releases adrenaline and causes glycogenolysis. NB: Adrenal medulla
may be abbreviated into just medulla. However it may get confusing with the medulla
oblongata in the brain which is also abbreviated into medulla. It is better to learn adrenal
medulla than just medulla.
How does adrenaline cause glycogen to turn into glucose: It is not a one step process which is as
follows. Adrenaline, the first messenger, binds to receptors on the cell surface membrane to create
hormone-receptor complex. The formation of the complex causes the enzyme adenylate
cyclase/adenylyl cyclase to be activated. NB: Some textbooks and websites may say that a protein is
activated by the complex which in turn stimulates the enzyme adenylate cyclase/adenylyl cyclase.
The protein does not need to be known for AQA. It catalyses the conversion of ATP into cyclic AMP
which is the second messenger. NB: Cyclic AMP can be abbreviated into cAMP. If you want to use
this make sure you know that the little c stands for cyclic. Cyclic AMP activates an enzyme, Protein
Kinase A/Protein Kinase, in the cell which starts a cascade of reactions. The last reaction to occur is
the conversion of glycogen into glucose (glycogenolysis).
Diabetes/Diabetes mellitus is a disorder where the blood glucose levels are not regulated properly.
There are two types:
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Type 1/insulin-dependent: This occurs suddenly in childhood. The body is unable to produce
any insulin and is thought to be because the beta cells of the islets of Langerhans in the
pancreas have been targeted by the immune system. The levels of glucose can be controlled
by regular injections with insulin and the careful management of diet and exercise.
Type 2/insulin-independent: This occurs mainly in people over the age of 40 however it is
becoming increasingly more common in adolescents too.…read more