When blood glucose concentration increases above the optimum concentration (90mg 100cm −3 ), insulin returns the level to normal through negative feedback. The steps involved are:

When blood glucose concentration decreases below the optimum concentration (90mg 100cm −3 ), glucagon, like insulin, returns the level to normal through negative feedback. The steps involved are:

Controlling Blood Glucose Concentration

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Insulin

When blood glucose concentration increases above the optimum concentration (90mg 100cm⁻³), insulin returns the level to normal through negative feedback. The steps involved are:

1) Detection by beta cells

High blood glucose concentration is detected by the beta (β) cells in the pancreas.
Beta cells are located in the islets of Langerhans.

2) Secretion of insulin

Beta cells respond to high blood glucose concentration by secreting a hormone called insulin into the blood.
Insulin travels in the blood to the liver and muscle cells.

3) Binding to muscle cells

Insulin binds to receptors on the muscle cell membranes.
The muscle cells insert more glucose channel proteins in the cell membrane. This causes:
- The rate of uptake of glucose by muscle cells to increase.
- The rate of respiration in the muscle cells to increase.

4) Glycogenesis

Insulin binds to receptors on the liver cell membranes.
The liver cells produce enzymes that convert glucose to glycogen.
Glycogen is stored in the liver cells' cytoplasm.
This process is called glycogenesis.

Importance of insulin

The role of insulin in lowering blood glucose concentration is important for maintaining an optimum blood water potential.
If blood glucose levels were not reduced by insulin, the blood water potential would decrease.
Water in the cells in the body would diffuse out, causing the cells to shrink and die.

Glucagon

When blood glucose concentration decreases below the optimum concentration (90mg 100cm⁻³), glucagon, like insulin, returns the level to normal through negative feedback. The steps involved are:

1) Detection by alpha cells

Low blood glucose concentration is detected by the alpha (α) cells in the pancreas.
Alpha cells are located in the islets of Langerhans.

2) Secretion of glucagon

Alpha cells respond to low blood glucose concentration by secreting a hormone called glucagon into the blood.
Glucagon travels in the blood to the liver cells.

3) Glycogenolysis

Glucagon binds to receptors on the liver cell membranes.
The liver cells produce enzymes that convert glycogen to glucose.
This process is called glycogenolysis.

4) Gluconeogenesis

Binding of glucagon to liver cell membranes also causes the release of enzymes that form glucose from glycerol and amino acids.
This process is called gluconeogenesis.

5) Rate of respiration

Glucagon also slows the respiration rate in cells.
Slowing respiration slows the rate at which glucose is used up.

Importance of glucagon

The role of glucagon in increasing blood glucose concentration is important for survival.
If blood glucose levels were not increased by glucagon, there would not be enough glucose available for respiration.
If there is not enough glucose for respiration, there will be no energy available for survival.

Adrenaline

Adrenaline is a hormone that is secreted in response to low blood glucose concentration. It is also released during exercise and in times of stress. The steps of the adrenaline response are:

1) Secretion of adrenaline

Adrenaline is secreted from the adrenal gland in response to low blood glucose concentration, exercise and stress.

2) Binding to liver cells

Adrenaline binds to receptors on the liver cell membrane.
Adrenaline induces two reactions in the liver cells:
- Activation of glycogenolysis (glycogen → glucose).
- Inhibition of glycogenesis (glucose → glycogen).
Adrenaline also promotes secretion of glucagon from the pancreas and inhibits secretion of insulin.