AQA A-level Biology: Homeostasis and Control of Blood Glucose (7402) – Revision Notes

These notes condense sections 3.6.4.1 and 3.6.4.2 of the AQA AS and A-level Biology specification for first teaching 2015 (A-level Biology 7402), both A-level only, plus Required practical 11. Full explanations and worked examples are in the study guide. Test yourself with the practice set, use the AQA A-level Biology hub and the printable checklist, and try the free diagnostics to find weak spots.

3.6.4.1 Homeostasis and feedback

Homeostasis: physiological control systems in mammals keep the internal environment within restricted limits.

Variable	Why it must stay stable
Core temperature	Enzyme activity. Too cool: fewer collisions and enzyme-substrate complexes. Too warm: hydrogen bonds break, active site changes, denaturation
Blood pH	Changes the charges on R groups, disrupting ionic and hydrogen bonds, so the active site changes shape
Blood glucose	Supply of respiratory substrate (low glucose starves cells, especially neurones); water potential of blood (high glucose lowers it, so water leaves cells by osmosis)

Negative feedback: a change is detected, a response reverses it and the system returns to its original level.

Steps in a negative feedback loop:

1. Receptor detects a departure from the set point.
2. Coordinator (nervous or endocrine system) processes the information.
3. Effector acts to reverse the change.
4. The stimulus shrinks, so the response is switched off.

Separate mechanisms for each direction (for example insulin against glucagon, vasodilation against vasoconstriction) give a greater degree of control because the level can be corrected from either side.

Positive feedback: a change causes more of the same change. Examples are the opening of voltage-gated sodium ion channels and oxytocin release in labour. It moves the system away from its starting point, so it is not homeostatic.

	Negative feedback	Positive feedback
Effect of response	Reverses the change	Increases the change
Result	Stable value near a set point	Rapid, runaway change
Role	Homeostasis	Amplification of a process

How to interpret a feedback data set:

1. Identify the stimulus and the response.
2. Ask: does the response oppose the stimulus (negative) or add to it (positive)?
3. Quote values from the data, including a percentage change if useful.
4. Say what is kept constant, if anything.

3.6.4.2 Blood glucose control

What raises blood glucose: absorption of glucose after digestion, glycogenolysis, gluconeogenesis. What lowers it: respiration of glucose in cells, glycogenesis.

Liver processes

Process	Change	Effect on blood glucose
Glycogenesis	Glucose to glycogen	Lowers
Glycogenolysis	Glycogen to glucose	Raises
Gluconeogenesis	Glycerol and amino acids to glucose	Raises

Hormones

Hormone	Source	Trigger	Actions
Insulin	Beta cells, islets of Langerhans	High blood glucose	Binds to surface receptors; more glucose channel proteins in surface membranes; activates enzymes for glucose to glycogen
Glucagon	Alpha cells, islets of Langerhans	Low blood glucose	Binds to surface receptors; activates enzymes for glycogen to glucose and for glycerol and amino acids to glucose
Adrenaline	Adrenal glands	Stress or danger	Binds to surface receptors; activates enzymes for glycogen to glucose

Second messenger model (adrenaline and glucagon)

1. Hormone (first messenger) binds to receptor on the cell surface membrane.
2. Adenylate cyclase is activated.
3. Adenylate cyclase makes cyclic AMP (cAMP) from ATP; cAMP is the second messenger.
4. cAMP switches on protein kinase.
5. Protein kinase activates enzymes that convert glycogen to glucose.
6. The signal is amplified at each stage.

Memory line: hormone, receptor, adenylate cyclase, ATP to cAMP, protein kinase, enzymes.

Diabetes

	Type I	Type II
Cause	Immune system destroys beta cells; little or no insulin	Target cell receptors lose responsiveness to insulin, sometimes with inadequate insulin secretion
Typical risk factors	Not linked to lifestyle	Obesity, diet high in refined carbohydrate and fat, inactivity, age, family history
Control	Insulin injections, monitoring, managing carbohydrate in the diet	Manipulating the diet, exercise, weight loss; sometimes drugs or insulin

Evaluating positions (health advisers and the food industry on type II diabetes)

• Health advisers: link a diet high in sugar, fat and energy, and low activity, to obesity and type II diabetes. They recommend labelling, lower sugar and more exercise. Evidence is mostly from population studies, which show correlation.
• Food industry: emphasises personal choice and balance, and points to reformulation and labelling. It has a commercial interest.
• To evaluate: judge sample size, correlation against cause, funding and who gains.

Required practical 11

1. Make a dilution series of glucose solution from a stock, using a measured volume of stock and distilled water.
2. React each with the same volume of Benedict’s reagent, heated for the same time at the same temperature (a typical method; follow your school’s version).
3. Remove the precipitate, then measure with a colorimeter using a red filter, zeroed on distilled water.
4. Plot absorbance against concentration to give the calibration curve.
5. Read the unknown “urine” sample’s concentration from the curve, within the range of the standards.

Volume of stock = (required concentration × final volume) ÷ stock concentration. With Benedict’s reagent, absorbance falls as glucose concentration rises, because the blue colour is used up. If the sample was diluted before testing, multiply the answer by the dilution factor.

Worked reminder: stock 100 mmol dm⁻³, make 20 cm³ of 25 mmol dm⁻³. Stock volume = 25 × 20 ÷ 100 = 5.0 cm³, plus 15.0 cm³ of water.

Worked reminders

Percentage change: (new − original) ÷ original × 100. Skin blood flow of 6 units rising to 27 units is (27 − 6) ÷ 6 × 100 = 350%.

Mean rate: change in glucose ÷ time. A fall from 7.8 to 4.9 mmol dm⁻³ over 90 minutes is 2.9 ÷ 90 = 0.032 mmol dm⁻³ min⁻¹.

Reading a calibration curve: a line of best fit with absorbance = 0.62 − 0.00245 × concentration gives, for an absorbance of 0.37, a concentration of (0.37 − 0.62) ÷ (−0.00245) = about 102 mmol dm⁻³.

Answer frame for a glucose control question:

1. State the stimulus and the receptor (beta cells for a rise, alpha cells for a fall).
2. Name the hormone and say it is carried in the blood.
3. State what it binds to on target cells.
4. Give the enzymes or channels affected and the process (glycogenesis, glycogenolysis or gluconeogenesis).
5. State the effect on blood glucose and that the stimulus then declines.

Must-know distinctions

• Glycogenesis (make glycogen) versus glycogenolysis (break glycogen down) versus gluconeogenesis (make glucose from non-carbohydrates).
• First messenger (the hormone) versus second messenger (cAMP).
• Insulin binds to a surface receptor and never enters the cell.
• Type I (too little insulin) versus type II (poor response to insulin).
• Negative feedback (corrects) versus positive feedback (amplifies).

Quick self-test

1. Define negative feedback.
2. Give two reasons why blood glucose concentration must be kept within limits.
3. Name the three processes carried out by the liver in blood glucose control.
4. Which cells in the pancreas secrete insulin?
5. How does insulin increase the uptake of glucose by target cells?
6. Name the enzyme that produces the second messenger and state the reaction it catalyses.
7. Name the enzyme activated by cAMP.
8. A blood glucose reading fell from 9.0 to 5.4 mmol dm⁻³ after a meal. Calculate the percentage fall.
9. Calculate the volume of stock needed to make 20 cm³ of a 25 mmol dm⁻³ solution from a 100 mmol dm⁻³ stock.
10. A “urine” sample diluted 1 in 2 reads as 87 mmol dm⁻³ on a calibration curve. What was the concentration of the original sample?
11. State the cause of type I diabetes.
12. Name one example of positive feedback.

Answers

1. A change is detected and a response reverses it, restoring the system to its original level.
2. Glucose is the respiratory substrate, so low levels starve cells of ATP; a high level lowers the water potential of the blood, so water leaves cells by osmosis.
3. Glycogenesis, glycogenolysis and gluconeogenesis.
4. Beta cells of the islets of Langerhans.
5. It binds to receptors, which causes more glucose channel proteins to be included in the surface membrane.
6. Adenylate cyclase converts ATP to cyclic AMP.
7. Protein kinase.
8. (9.0 − 5.4) ÷ 9.0 × 100 = 40%.
9. 25 × 20 ÷ 100 = 5.0 cm³ (plus 15.0 cm³ distilled water).
10. 87 × 2 = 174 mmol dm⁻³.
11. Destruction of beta cells by the immune system, so little or no insulin is made.
12. Entry of sodium ions opening more voltage-gated sodium channels, or oxytocin release during labour.

Where marks are usually lost

• Writing “blood sugar is controlled” without naming the receptor, the hormone and the effector.
• Saying glucose is “stored” without naming glycogen and the liver.
• Describing the second messenger sequence out of order, or leaving out ATP to cAMP.
• Confusing the first messenger (adrenaline, glucagon) with the second messenger (cAMP).
• Forgetting to multiply by the dilution factor in the practical.
• Reading a calibration curve outside the range of the standards.
• Saying negative feedback “stops” a change rather than reversing it and returning to the set point.
• Giving only one side in an evaluation of health advisers and the food industry.

Official syllabus

AQA AS and A-level Biology specification for first teaching 2015, published by AQA (sections 3.6.4.1 and 3.6.4.2, with Required practical 11).