Control of blood glucose concentration: (HT only) If the blood glucose concentration is too low, the pancreas produces the hormone glucagon that causes glycogen to be converted into glucose and released into the blood.

The idea

When blood glucose falls below the normal level, the pancreas detects this and secretes glucagon into the blood. Glucagon travels to the liver, where it stimulates the breakdown of glycogen into glucose. The glucose then leaves the liver cells and enters the blood, so blood glucose concentration rises back towards normal. This is an example of negative feedback: the original change is reversed. Glycogen is a storage polysaccharide made of many glucose units; glucagon is a hormone, not an enzyme, and it does not directly convert glycogen. Instead, it triggers the cells to carry out the conversion. In a person without diabetes, this response helps keep glucose steady between meals.

What earns the marks

• State that a fall in blood glucose concentration is detected by the pancreas, which acts as the receptor and coordinator.
• State that the pancreas produces and secretes the hormone glucagon into the blood when blood glucose is too low.
• State that glucagon causes glycogen to be converted into glucose; glycogen is the storage form of glucose in the liver.
• State that the glucose produced is released into the blood, so blood glucose concentration increases.
• Explain that this is negative feedback because the response reverses the original fall in blood glucose.
• Distinguish glucagon from insulin: insulin lowers blood glucose, whereas glucagon raises it, and both are hormones secreted by the pancreas.

Common mistakes and how to put them right

• Saying that glucagon is an enzyme that digests glycogen.

  Put it right: Glucagon is a hormone that stimulates cells to break down glycogen; enzymes such as glycogen phosphorylase carry out the reaction inside cells.

• Confusing glucagon with glycogen.

  Put it right: Glucagon is a hormone; glycogen is a storage carbohydrate made of glucose units.

• Saying that glucagon is released when blood glucose is too high.

  Put it right: Glucagon is released when blood glucose is too low; insulin is released when it is too high.

• Writing that glucose is converted into glycogen when glucagon acts.

  Put it right: Glucagon causes glycogen to be converted into glucose, the opposite direction.

Exam tips

• Use the sequence: low blood glucose → pancreas detects → glucagon secreted → glycogen converted to glucose → glucose released into blood → blood glucose rises.
• Name the hormone and the storage molecule precisely; do not use 'sugar hormone' or 'glycogen hormone'.
• If asked to explain negative feedback, state the original change and how the response reverses it.
• Link the site of action to the liver, where glycogen is broken down to release glucose into the blood, rather than saying glucagon acts everywhere.