Controlling Blood Glucose
Biology: Cambridge International A Level, topic 14.4.2
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.
- When glucose diffuses from a high concentration outside the cell to a lower concentration inside the beta cell, the glucose is metabolised, producing ATP.
- When ATP concentration increases, potassium ion channels close. This lowers the negativity inside the cell, changing the potential over the membrane.
- The potential change triggers the opening of voltage-gated calcium ion channels.
- Calcium in the beta cell triggers the vesicles containing insulin to move to the cell surface membrane to be secreted by exocytosis.
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.
Test yourself on Controlling Blood Glucose
Stages in Action of Insulin to Decrease Blood Glucose Levels
When insulin binds to receptors on the muscle cells, what two processes take place?
What is the process that converts glucose to glycogen in the liver cells called?
Which of the following statements about insulin are true?
Where are the alpha cells located?