6.4.2 Control of blood glucose concentration

Question 1

The factors that influence blood glucose concentration.

Answer 1

Glucose is added to the blood from
1. absorption in the ileum
2. glycogenolysis in liver and muscle cells
3. gluconeogenesis in liver cells

Glucose is removed from the blood by:
1. respiring cells
2. glycogenesis in liver and muscle cells

Question 2

Insulin

Answer 2

Produced by beta cells in the islets of Langerhans in response to high blood glucose concentration.

Insulin attaches to specific receptors on the plasma membrane of target cells (liver and muscle), triggering an increase in the uptake of glucose by stimulating the inclusion of glucose transport proteins in the plasma membranes of those target cells (increasing their permeability to glucose), and by activating enzymes involved in the conversion of glucose to glycogen (glycogenesis).

Question 3

Glucagon

Answer 3

Produced by alpha cells in the islets of Langerhans in response to low blood glucose concentration.

Glucagon attaches to receptors on the surfaces of target cells (liver), activating enzymes involved in the conversion of glycogen to glucose (glycogenolysis) and activating enzymes involved in the conversion of glycerol and amino acids into glucose (gluconeogenesis).

Question 4

Adrenaline

Answer 4

Produced by adrenal glands in response to low blood glucose concentration.

Adrenaline attaches to receptors on the surfaces of target cells (liver), activating enzymes involved in the conversion of glycogen to glucose (glycogenolysis).

Question 5

The second messenger model

Answer 5

The second messenger model applies to the hormones glucagon and adrenaline only.

In the second messenger model a hormone (either glucagon or adrenaline) binds to its specific receptor on the plasma membrane of a liver cell, causing the release of a G protein. The G protein binds to the membrane-bound enzyme adenylate cyclase, causing it to change tertiary structure and so become activated.

The activated adenylate cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP), which is released into the cytoplasm. cAMP binds to protein kinase enzymes, activating them.

The activated protein kinase enzymes go on to activate other enzymes - these other enzymes are involved in glycogenolysis. (protein kinases actually activate glycogen phosphorylases, because the first step in glycogenolysis is the phosphorylation of glycogen terminals, making them more reactive).

This metabolic pathway is an example of a cascade - a cascade amplifies the original signal, in this case resulting in the release of more glucose per hormone.

Question 6

Diabetes

Answer 6

Type l diabetics do not produce insulin (often because their pancreatic beta cells were destroyed by their own T lymphocytes, usually during childhood) whereas Type 2 diabetics do produce insulin but have cells which do not respond to insulin (Type 2 diabetes is generally later onset, and has associated risk factors, notably poor diet and lack of exercise).

Type 1 diabetics need regular and frequent insulin injections, and need to manage their carbohydrate intake to avoid sudden rises in blood glucose, whilst Type 2 diabetics need to eat healthily and exercise more.