(HT only) Students should be able to explain how glucagon interacts with…

The idea

Blood glucose is controlled by two pancreatic hormones acting antagonistically in a negative feedback cycle. When blood glucose rises after a meal, insulin is secreted by the pancreas; it increases the rate at which glucose is taken up by cells and converted to glycogen in the liver and muscles, so glucose falls. When blood glucose falls, for example after fasting, glucagon is secreted; it stimulates the liver to break down glycogen into glucose, which is released into the blood, so glucose rises. Because each hormone reverses the change that triggered the other, the level oscillates around a set point. A useful method is to trace one loop: stimulus, receptor, hormone, effector (e.g. the liver), response, correction.

What earns the marks

• Insulin is secreted by the pancreas when blood glucose concentration is too high.
• Insulin increases the uptake of glucose by cells and stimulates the conversion of glucose to glycogen in the liver and muscles.
• Glucagon is secreted by the pancreas when blood glucose concentration is too low.
• Glucagon stimulates the liver to convert glycogen back into glucose, which is released into the blood.
• The two hormones are antagonistic: insulin lowers blood glucose while glucagon raises it.
• The cycle is negative feedback because the response reverses the original change and returns blood glucose towards its normal level.
• A correct sequence links the stimulus to the hormone, the effector organ and the resulting change in blood glucose.

Common mistakes and how to put them right

• Stating that glucagon is produced by the liver.

  Put it right: Glucagon is secreted by the pancreas and acts on the liver.

• Claiming that glucagon converts glucose into glycogen.

  Put it right: Glucagon stimulates the breakdown of glycogen into glucose.

• Describing the cycle as positive feedback.

  Put it right: It is negative feedback because the response opposes the initial change.

• Omitting the role of the liver as the effector organ for glucagon.

  Put it right: Name the liver explicitly.

Exam tips

• Use the phrase 'blood glucose concentration' rather than 'sugar level' when explaining the feedback loop.
• Write the loop in order: stimulus, hormone, effector, response, correction, so the examiner can follow your reasoning.
• When asked to explain, include both hormones and state clearly which one acts when glucose is high and which acts when it is low.
• Link each hormone to its effect on liver cells, because the liver is the key effector for glucagon.