Control of blood glucose concentration: Students should be able to explain how insulin controls blood glucose (sugar) levels in the body.

The idea

Blood glucose must stay within a narrow range to supply cells with fuel without damaging them. After a meal, glucose absorbed from the small intestine raises blood glucose. The pancreas detects this rise and secretes insulin into the blood. Insulin travels to target cells, mainly liver and muscle, where it increases the rate at which glucose is taken up from the blood and converted into glycogen for storage. As cells remove glucose, blood glucose falls, so the pancreas reduces insulin secretion. This is negative feedback: the response cancels the original change.

For example: After a carbohydrate-rich meal blood glucose might rise from about 5 mmol/dm³ towards 8 mmol/dm³; insulin restores it towards the set point.

What earns the marks

• The pancreas detects a rise in blood glucose concentration and secretes insulin into the bloodstream.
• Insulin is a hormone that travels in the blood plasma to target cells such as liver and muscle cells.
• Insulin increases the uptake of glucose from the blood into cells and increases the conversion of glucose to glycogen for storage.
• As glucose is removed from the blood, blood glucose concentration falls, which reduces insulin secretion.
• The fall in blood glucose is an example of negative feedback because the response reverses the original increase.
• Glycogen is the storage polysaccharide in liver and muscle; it is not the same as the hormone glucagon.

Common mistakes and how to put them right

• Writing that insulin is an enzyme that digests glucose.

  Put it right: Insulin is a hormone that changes cell activity, and glucose is not digested in the blood.

• Stating that insulin converts glucose into glucagon.

  Put it right: Glucose is stored as glycogen, while glucagon is a separate hormone that raises blood glucose.

• Saying the liver produces insulin.

  Put it right: The pancreas produces insulin, and the liver is a target organ that stores glucose as glycogen.

Exam tips

• Use the sequence stimulus, hormone, target, response, negative feedback when explaining the control pathway.
• Name the pancreas, insulin, liver or muscle, glucose and glycogen explicitly, because vague terms such as 'it controls sugar' do not show understanding.
• Link the fall in blood glucose back to reduced insulin secretion to demonstrate negative feedback rather than describing only a one-way process.