A Level Biology: Homeostasis — Practice Questions (Cambridge 9700)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Each question practises a skill tested in the June 2024 Paper 42. After each answer there is an examiner insight or a tip and, where one matches, the real question to try next.

---

Questions

1. A hiker walks for five hours on a hot day without drinking. Describe how the hiker’s body detects the change in their blood and causes more antidiuretic hormone (ADH) to enter the blood. [3]

2. A volunteer drank 1 dm³ of water. Over the next 45 minutes, the concentration of ADH in her blood fell and her rate of urine production rose from 1.6 cm³ min⁻¹ to 6.9 cm³ min⁻¹. Calculate the percentage increase in her rate of urine production. Give your answer to three significant figures. [2]

3. After a person drinks a large volume of water, the concentration of ADH in their blood falls. Explain how this fall in ADH concentration leads to the production of a larger volume of more dilute urine. [4]

4. A student does not eat for several hours. Her blood glucose concentration falls from 5.0 mmol dm⁻³ to 4.2 mmol dm⁻³, then stays close to this value.

(a) Blood glucose concentration is brought back towards a set point whenever it changes. Name this type of control mechanism.

(b) Explain how the student’s body stops her blood glucose concentration falling any further. [5]

5. A man with diabetes wakes up and, at 7 a.m., dips a urine test strip into a sample of the first urine he passes that morning. The strip contains the enzymes glucose oxidase and peroxidase and a colourless compound, and it turns a dark colour. At the same time, a biosensor shows that his blood glucose concentration is within the normal range.

(a) Explain how the enzymes on the strip cause the colour change.

(b) Suggest why the urine test and the biosensor seem to give different results. [5]

6. During a drought, the roots of a plant release abscisic acid (ABA), which travels to the leaves. Explain how ABA causes the stomata to close. [4]

---

Answers

1. Osmoreceptors in the hypothalamus [1] detect a decrease in the water potential of the blood [1]. They send nerve impulses to the posterior pituitary gland, which releases (secretes) ADH into the blood [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 42, Question 1(a)): Say “water potential of the blood”, not “water level” or “water concentration”. Name the posterior pituitary: some answers needed to specify posterior, and a few wrongly gave the anterior pituitary.

Source for the examiner insights on this page: Cambridge International AS & A Level Biology 9700 June 2024 Principal Examiner Report for Teachers, Paper 9700/42 section, paraphrased.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 1(a).

2. Increase = 6.9 − 1.6 = 5.3 cm³ min⁻¹. Percentage increase = 5.3 ÷ 1.6 × 100 [1] = 331.25 = 331% (3 s.f.) [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 42, Question 1(b)(i)): The most common mistake was dividing by the wrong value. Divide the change by the starting value (here 1.6), and round as the question asks.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 1(b)(i).

3. Any four: less ADH binds to receptors on the cell surface membrane of collecting duct cells [1]; so there is less activation of the cascade of reactions (via cAMP), and fewer vesicles containing aquaporins fuse with the luminal membrane (aquaporins are taken back into the cytoplasm in vesicles) [1]; the luminal membrane has fewer aquaporins, so it is less permeable to water [1]; less water is reabsorbed from the filtrate in the collecting duct by osmosis into the tissue fluid and blood [1]; so more water stays in the filtrate, giving a larger volume of more dilute urine [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 42, Question 1(b)(ii)): Strong answers started with ADH binding to its receptor on the cell surface membrane and followed the chain through to vesicles with aquaporins fusing with the luminal membrane. Weaker answers said water is “absorbed” without making clear that reabsorption into the blood increases, and the most frequent error was describing the graph instead of explaining it.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 1(b)(ii).

4. (a) Negative feedback [1].

(b) Any four: α cells in the islets of Langerhans of the pancreas detect the fall in blood glucose and secrete glucagon [1]; glucagon binds to receptors on liver cells [1]; this activates a second messenger (cAMP) and an enzyme cascade inside the cells [1]; glycogenolysis: glycogen is broken down to glucose [1]; gluconeogenesis: glucose is made from non-carbohydrates such as amino acids and glycerol [1]; glucose is released into the blood, so the concentration rises back towards the set point [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 42, Question 10): Many wrote “homeostasis” when the answer was negative feedback. The report also asked for care when spelling similar names. For example (our illustration), glucagon, glycogen and glucose are easily confused, and a wrong spelling can turn one term into another.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 10.

5. (a) Glucose oxidase catalyses the oxidation of glucose by oxygen, forming gluconic acid and hydrogen peroxide [1]. Peroxidase catalyses a reaction in which hydrogen peroxide oxidises the colourless compound (a chromogen) into a coloured product [1]. The more glucose there is, the darker the colour, which is compared with a colour chart [1].

(b) Any two: the urine was made over several hours during the night and stored in the bladder, so it shows glucose that entered the filtrate earlier [1]; his blood glucose must have been high earlier (above the concentration at which all filtered glucose can be reabsorbed), so glucose was left in the urine [1]; the biosensor measures the blood glucose concentration at the moment of testing, which has since fallen back into the normal range [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 42, Question 10(c)(i)): Some candidates struggled to spell gluconic acid, so learn it carefully.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 42, Question 10(c).

6. Any four: ABA binds to receptors on the cell surface membrane of guard cells [1]; this causes calcium ions to enter the cytoplasm, acting as a second messenger [1]; ion channels open and potassium ions (and anions) leave the guard cells [1]; the water potential of the guard cells rises, so water leaves by osmosis [1]; the guard cells lose turgor (become flaccid) and the stoma closes, reducing water loss by transpiration [1].

Tip: Follow the chain in order: hormone → receptor → ions move → water potential changes → water moves → guard cells change shape.

---

Where marks are usually lost

• Writing “water level” or “water concentration” instead of water potential.
• Dividing by the wrong value in a percentage change calculation.
• Describing a data trend when the command word is “explain”.
• Saying water is “absorbed” without saying more water is reabsorbed into the blood.
• Naming homeostasis when the question asks for the mechanism (negative feedback).