Easy to understand and detailed summary of AQA A Level Biology Unit 6 (Organisms respond to changes in their internal and external environments) section B- Homeostasis.
Negative Feedback → When any deviations from the normal values are restored to
their original level. This involves the nervous system and hormones.
Positive feedback → Deviation from the normal e.g. contractions
Control of blood glucose:
● Blood glucose increases following the ingestion of food or drink containing
carbohydrates.
● Blood glucose falls following exercise or if you have not eaten.
● Adrenaline is released by the adrenal gland when the body anticipates
danger, resulting in more glucose being released from the stores of glycogen
in the liver.
● The pancreas detects changes in blood glucose levels.
● It contains endocrine cells in the Islets of Langerhans which release the
hormones insulin and glucagon to restore normal glucose levels.
Beta cells release insulin. → Liver cells become more permeable to
glucose glucose and enzymes are activated to convert
↑ glucose to glycogen.
Detected by the beta cells in ↓
islets of Langerhans (pancreas). Glucose is removed from the blood and
Blood glucose levels decrease. ↑
↓ Glycogen is hydrolysed to glucose
Detected by the alpha cells in and more glucose is released back
islets of Langerhans (pancreas). into the blood.
↓ ↑
Alpha cells release glucagon and
the adrenal glands release → Second messenger model occurs
adrenaline. to activate enzymes to
h hydrolyse glycogen.
, Key words:
Glycogenesis → The process of excess glucose being converted to glycogen when
the blood glucose is higher than normal, mainly occurring in the liver.
Glycogenolysis → The hydrolysis of glycogen back into glucose in the liver when the
blood glucose levels are lower than normal.
Gluconeogenesis → The process of creating glucose from non-carbohydrate stores
in the liver. This occurs when all glycogen has been hydrolysed into glucose but the
body still needs more glucose.
Action of Insulin:
● Beta cells in the islets of Langerhans detect when blood glucose levels are
too high and secrete insulin in response.
● Insulin will decrease blood glucose levels by:
1. Attaching to receptors on the surface of target cells. This will change the
tertiary structure of the channel proteins so more glucose is being absorbed
by facilitated diffusion.
2. More protein carriers are incorporated into cell membranes so more glucose
is absorbed from the blood into the cells. Insulin binds to the insulin receptor
on the membrane of liver cells and an intracellular chemical is released. This
causes vesicles containing glucose protein channels to fuse with the cell
membrane.
3. Activating enzymes used in converting glucose to glycogen, resulting in
glycogenesis in the liver.
Action of Glucagon:
● Alpha cells in the islets of Langerhans detect when blood glucose levels are
too low and secrete glucagon in response.
● Glucagon will increase blood glucose levels by:
1. The second messenger model.
2. Activating enzymes involved in converting glycerol and amino acids into
glucose.
The Second Messenger Model:
● Glucagon binds to glucagon receptors
● Once bound, it changes the shape of the enzyme adenyl cyclase and
activates it.
● Activated adenyl cyclase converts ATP into cyclic AMP (cAMP) which
is the ‘second messenger.’
● cAMP activates the protein kinase enzyme which catalyses the
hydrolysis of glycogen into glucose.
Role of Adrenaline:
● Secreted by the adrenal glands when the blood glucose level is too low.
● Adrenaline will increase blood glucose levels by the second messenger model
by activating adenyl cyclase.
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