Control of blood glucose: insulin and glucagon

What you'll learn

This revision guide covers everything you need to know about how the body controls blood glucose concentration for AQA GCSE Biology. You'll understand the role of the pancreas, how insulin and glucagon work as antagonistic hormones, and the differences between Type 1 and Type 2 diabetes. This topic frequently appears in Paper 2 (Biology 2) worth 4-6 marks.

Key terms and definitions

Homeostasis — the regulation of internal conditions within a cell or organism to maintain optimal conditions for enzyme action and cellular function, in response to internal and external changes.

Insulin — a hormone produced by the pancreas that reduces blood glucose concentration by causing glucose to move from the blood into cells and be converted to glycogen for storage in the liver and muscles.

Glucagon — a hormone produced by the pancreas that increases blood glucose concentration by stimulating the liver to convert glycogen back into glucose and release it into the blood.

Glycogen — a storage polymer of glucose found in liver and muscle cells; it is insoluble and therefore does not affect the water potential of cells.

Negative feedback — a control mechanism in which a change in a condition triggers a response that counteracts the initial change, returning the system to its optimum level.

Type 1 diabetes — a disorder in which the pancreas fails to produce sufficient insulin, usually developing in childhood and requiring insulin injections to control blood glucose.

Type 2 diabetes — a disorder in which body cells no longer respond properly to insulin produced by the pancreas, often linked to obesity and controlled by diet and exercise.

Pancreas — an organ that monitors and controls blood glucose concentration by secreting the hormones insulin and glucagon into the bloodstream.

Core concepts

Why blood glucose control matters

Blood glucose concentration must be carefully controlled because:

• Glucose is the primary respiratory substrate for cells, particularly brain cells which cannot use alternative fuels
• Too high a concentration (hyperglycaemia) causes water to leave cells by osmosis, damaging them
• Too low a concentration (hypoglycaemia) means cells cannot release enough energy for vital processes
• The normal blood glucose concentration is approximately 90 mg per 100 cm³ of blood (about 5 mmol/dm³)

After eating a meal rich in carbohydrates, blood glucose concentration rises. During exercise or between meals, blood glucose concentration falls. The body must respond to both situations.

The role of the pancreas

The pancreas is both an endocrine and exocrine gland located near the stomach. Its endocrine function involves:

• Continuously monitoring blood glucose concentration
• Detecting changes away from the optimum level
• Secreting appropriate hormones directly into the bloodstream
• Producing insulin when blood glucose is too high
• Producing glucagon when blood glucose is too low

The pancreas contains specialised cells called islets of Langerhans. Within these structures:

• Beta cells produce and secrete insulin
• Alpha cells produce and secrete glucagon

These hormones travel in the blood plasma to target organs throughout the body.

How insulin reduces blood glucose concentration

When blood glucose concentration is too high (for example, after eating):

1. The pancreas detects the rise in blood glucose
2. Beta cells in the pancreas secrete insulin into the blood
3. Insulin travels in the blood to target organs (mainly the liver and muscles)
4. Insulin binds to receptors on cell membranes
5. This causes glucose to move from the blood into body cells (particularly muscle cells where it is used for respiration)
6. Insulin also stimulates liver and muscle cells to convert excess glucose into glycogen for storage (a process called glycogenesis)
7. Blood glucose concentration returns to normal
8. The pancreas detects this and reduces insulin secretion

This is a negative feedback mechanism because the response (lowering glucose) counteracts the original stimulus (high glucose).

How glucagon raises blood glucose concentration

When blood glucose concentration is too low (for example, during exercise or fasting):

1. The pancreas detects the fall in blood glucose
2. Alpha cells in the pancreas secrete glucagon into the blood
3. Glucagon travels in the blood to the liver
4. Glucagon binds to receptors on liver cell membranes
5. This stimulates liver cells to break down glycogen stores into glucose (a process called glycogenolysis)
6. The liver releases glucose into the bloodstream
7. Blood glucose concentration returns to normal
8. The pancreas detects this and reduces glucagon secretion

Glucagon only affects the liver, not muscle cells. This is because only liver cells have the necessary receptors and enzymes to respond to glucagon by releasing glucose into the blood.

Insulin and glucagon as antagonistic hormones

Insulin and glucagon are described as antagonistic because they have opposite effects:

• Insulin decreases blood glucose concentration
• Glucagon increases blood glucose concentration

They work together in a coordinated way to maintain blood glucose within narrow limits. When one hormone's secretion increases, the other's decreases. This allows precise control through negative feedback.

The graph of blood glucose over time shows fluctuations around the optimum, with insulin and glucagon secretion alternating to keep levels stable. This is more efficient than a single hormone trying to control the system alone.

Type 1 diabetes

Type 1 diabetes is a disorder in which the pancreas produces little or no insulin. Key facts:

Cause:

• The body's immune system attacks and destroys beta cells in the pancreas (an autoimmune condition)
• The exact trigger is unknown but may involve genetic and environmental factors
• Not caused by diet or lifestyle

Characteristics:

• Usually develops in childhood or adolescence (though can occur at any age)
• Symptoms include excessive urination, extreme thirst, weight loss, and fatigue
• Blood glucose concentration can rise to dangerously high levels after eating
• Without treatment, glucose appears in urine (normally it's reabsorbed by kidneys)

Treatment:

• Regular insulin injections throughout the day (or via an insulin pump)
• Injections must be timed with meals
• Blood glucose concentration must be monitored several times daily using finger-prick tests or continuous monitors
• Diet must be carefully controlled, limiting simple carbohydrate intake
• Regular exercise helps cells absorb glucose from the blood

Why insulin must be injected: Insulin is a protein hormone. If taken orally, it would be digested by protease enzymes in the stomach and small intestine, breaking it down into amino acids before it could enter the bloodstream. Therefore it must be injected subcutaneously (under the skin) directly into tissue fluid where it can enter the blood.

Type 2 diabetes

Type 2 diabetes is a disorder in which body cells no longer respond properly to insulin produced by the pancreas (insulin resistance). The pancreas may also become less effective at producing insulin over time.

Risk factors:

• Obesity (particularly excess abdominal fat)
• Lack of physical exercise
• Diet high in simple carbohydrates and processed foods
• Family history (genetic predisposition)
• Age (risk increases with age, though increasingly affecting younger people)
• Ethnicity (higher risk in people of African-Caribbean, South Asian, and Hispanic descent)

Characteristics:

• Usually develops in adults over 40, but increasingly diagnosed in younger people
• Develops gradually, sometimes with no obvious symptoms initially
• Blood glucose concentration remains elevated
• Cells cannot absorb glucose efficiently despite insulin being present

Treatment:

• Lifestyle changes are the first line of treatment
• Eating a balanced diet low in simple sugars and high in complex carbohydrates and fibre
• Regular exercise to improve insulin sensitivity and help control body mass
• Weight loss if the person is obese
• Medication may be prescribed if lifestyle changes are insufficient
• Some people eventually require insulin injections if beta cells become exhausted

Prevention: Unlike Type 1, Type 2 diabetes can often be prevented or delayed through:

• Maintaining a healthy body mass
• Regular physical activity (at least 150 minutes of moderate exercise per week)
• Eating a balanced diet rich in vegetables, whole grains, and lean proteins
• Limiting intake of sugary drinks and processed foods high in simple carbohydrates

The rising incidence of Type 2 diabetes globally, particularly in developed nations and increasingly in Caribbean countries, is linked to increased obesity rates and sedentary lifestyles. This creates significant healthcare costs and reduces quality of life.

Comparing Type 1 and Type 2 diabetes

Worked examples

Example 1: Explaining negative feedback (4 marks)

Question: Explain how the control of blood glucose concentration is an example of negative feedback.

Mark scheme answer:

• Blood glucose concentration is monitored by the pancreas (1 mark)
• If blood glucose rises, the pancreas secretes insulin / if blood glucose falls, the pancreas secretes glucagon (1 mark)
• Insulin causes glucose to be taken up by cells and converted to glycogen / glucagon causes glycogen to be converted to glucose (1 mark)
• This returns blood glucose to normal, which is detected by the pancreas, reducing hormone secretion (1 mark)

Examiner tip: Notice this answer identifies the receptor (pancreas), the response (hormone secretion), the effect (on glucose/glycogen), and the reversal that completes the feedback loop.

Example 2: Comparing the two types of diabetes (6 marks)

Question: Compare and contrast Type 1 and Type 2 diabetes.

Mark scheme answer:

Similarities:

• Both result in inability to control blood glucose concentration (1 mark)
• Both can be treated with careful diet (1 mark)

Differences:

• Type 1: pancreas doesn't produce insulin; Type 2: cells don't respond to insulin (1 mark)
• Type 1 usually develops in children/young people; Type 2 usually in older adults (1 mark)
• Type 1 is not linked to obesity; Type 2 is often linked to obesity/poor diet (1 mark)
• Type 1 requires insulin injections; Type 2 can often be controlled with diet and exercise alone (1 mark)

Examiner tip: "Compare and contrast" questions require you to identify both similarities AND differences. Make your points clearly paired to show the contrast.

Example 3: Explaining insulin's mechanism (3 marks)

Question: Describe how insulin reduces blood glucose concentration.

Mark scheme answer:

• Insulin is secreted by the pancreas when blood glucose is high (1 mark)
• Insulin causes glucose to move from blood into (muscle) cells (1 mark)
• Insulin stimulates liver/muscle cells to convert glucose to glycogen for storage (1 mark)

Examiner tip: For "describe" questions, focus on the sequence of events. Don't explain why—just state what happens.

Common mistakes and how to avoid them

• Confusing which hormone does what: Remember INsulin lets glucose IN to cells (lowers blood glucose), while glucagon has a G for Glycogen breakdown and Glucose release (raises blood glucose).

• Saying insulin "breaks down glucose": Insulin doesn't break down glucose—it causes glucose to be converted to glycogen (storage) or used in respiration. Glucagon breaks down glycogen, not glucose.

• Forgetting that glucagon only affects the liver: Students often write that glucagon acts on all body cells. Only liver cells have the receptors and enzymes to respond to glucagon by releasing glucose into the blood.

• Not explaining the negative feedback loop fully: You must state that the response reverses the original change AND that this is detected, stopping further hormone secretion. Don't just describe what insulin or glucagon does in isolation.

• Confusing cause and treatment of Type 1 and Type 2: Type 1 is NOT caused by poor diet—it's an autoimmune condition. Type 2 is strongly linked to obesity but Type 1 is not. Be precise about which diabetes type you're discussing.

• Writing that insulin can be taken as tablets: Insulin is a protein and would be digested if swallowed, so it must be injected. This is a common exam question worth 1-2 marks.

Exam technique for "Control of blood glucose: insulin and glucagon"

• Command word awareness: "Explain" questions require you to give reasons or mechanisms (use "because," "this causes," "as a result"). "Describe" questions need you to state what happens without explaining why. "Compare" requires both similarities and differences.

• Use correct terminology: Write "blood glucose concentration" not just "sugar levels." Use "secreted by the pancreas" not "released" or "made." Use "glycogen" not "glucose stores." Precision earns marks.

• Structure longer answers logically: For 4-6 mark questions on negative feedback or diabetes, use the format: detection → response → effect → return to normal. This ensures you cover all mark scheme points.

• Link causes to effects explicitly: Don't assume the examiner will make the connection. Write "Insulin causes glucose to move into cells, which reduces blood glucose concentration" rather than just "Insulin moves glucose into cells."

Quick revision summary

Blood glucose concentration is controlled by the pancreas through negative feedback. When blood glucose rises, the pancreas secretes insulin, which causes cells to absorb glucose and the liver to convert glucose to glycogen, lowering blood glucose. When blood glucose falls, the pancreas secretes glucagon, which causes the liver to break down glycogen to glucose, raising blood glucose. Type 1 diabetes results from the pancreas not producing insulin and requires insulin injections. Type 2 diabetes results from cells not responding to insulin and is linked to obesity; it's controlled through diet and exercise.