AQA GCSE Biology: Blood Glucose and Diabetes

The concentration of glucose in the blood is controlled by hormones from the pancreas. This guide explains how insulin lowers blood glucose, how glucagon raises it, and what goes wrong in the two types of diabetes. It is a clear example of hormonal negative feedback.

Controlling blood glucose

The pancreas monitors and controls the concentration of glucose in the blood.

When blood glucose is too high:

• the pancreas detects the rise and secretes insulin;
• insulin makes glucose move from the blood into the liver and muscle cells;
• in these cells, excess glucose is converted into glycogen for storage;
• blood glucose falls back towards normal.

When blood glucose is too low (Higher tier only):

• the pancreas detects the fall and secretes glucagon;
• glucagon makes the liver break glycogen back down into glucose;
• glucose is released into the blood;
• blood glucose rises back towards normal.

Because insulin and glucagon have opposite effects, they form a negative feedback loop that keeps blood glucose steady.

Two warnings about the words themselves, because this is where most marks go missing on this topic. Glycogen is the store; glucagon is the hormone. They differ by two letters and an examiner cannot give you the benefit of the doubt. And insulin does two things, not one: it moves glucose into cells and has the liver and muscles store the excess as glycogen. An answer with only the storage half is only half the answer.

It helps to picture the graph a question will show you. After a meal, blood glucose rises; insulin is secreted and the line comes back down, usually dipping slightly below normal before settling. Between meals it drifts down, glucagon is secreted, and the line comes back up. The level is never flat, because a correction can only begin after the change has been detected.

Type 1 diabetes

In type 1 diabetes, the pancreas does not produce enough insulin, so blood glucose is uncontrolled and rises too high. It is treated with regular insulin injections to bring blood glucose down when it rises, for example after a meal.

Injections rather than tablets, and that is not an arbitrary detail. Insulin is a protein, so swallowing it would mean digesting it: the protease enzymes in the stomach and small intestine would break it into amino acids before it ever reached the blood.

The dose has to be matched to what is about to be eaten, which is why people with type 1 diabetes count carbohydrate. Too little insulin and blood glucose stays high; too much and it falls too low.

Type 2 diabetes

In type 2 diabetes, the pancreas still makes insulin, but the body's cells no longer respond to it properly, so blood glucose is uncontrolled. A major risk factor is obesity. It is treated with a carbohydrate-controlled diet and regular exercise, which uses glucose in respiration and helps to lower blood glucose.

The distinction between the two types is the whole of this section: type 1 is a supply problem and type 2 is a response problem. That is why injecting insulin is the answer to one and not the first answer to the other, and why diet and exercise help with type 2 in a way they cannot with type 1.

Obesity is a risk factor, not a cause. It makes type 2 diabetes considerably more likely without guaranteeing it, and plenty of people develop it without being obese. Questions that supply data about the two often turn on exactly that distinction, which the health and non-communicable disease guide covers in full.

How this fits together

Blood glucose control is one of the endocrine system's key jobs. In Triple award biology, the body also balances its water and waste, which is the subject of the water and nitrogen balance guide.