OxfordAQA International AS and A-level Biology: Digestion, absorption and cholera (9610) – Practice Questions

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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover sections 3.2.2 and 3.2.3 of Unit 2 in the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, for International A-level exams May/June 2018 onwards. All data are invented for practice. Revise first with the study guide and revision notes. Course pages: hub, checklist and diagnostics.

Questions

1. Name the part of the human digestive system described in each case.

(a) It stores faeces before egestion. [1] (b) It is where pancreatic juice and bile salts enter the small intestine. [1] (c) It is the main site of absorption of the products of digestion. [1]

2. Explain the difference between:

(a) digestion and absorption [2] (b) assimilation and egestion [2]

3. Describe how the starch in a bread roll is digested to glucose. Name each enzyme and where it acts. [4]

4. A polypeptide contains 120 amino acids. An endopeptidase hydrolyses it at 5 places.

(a) Calculate how many peptide bonds the chain contained and how many remain after the endopeptidase has acted. [2] (b) Explain why exopeptidases digest the fragments faster than they digest the original chain. [2] (c) Name the type of enzyme in the ileum cell-surface membrane that hydrolyses dipeptides. [1]

5. Milk containing an indicator that changes colour as pH falls was mixed with lipase. The colour changed after 190 s. With bile salts added to an identical mixture, it changed after 70 s.

(a) Calculate the rate of each reaction as 1000 ÷ time, to 3 significant figures. [2] (b) Explain why the pH fell. [1] (c) Explain the effect of bile salts. [3]

6. A micrograph shows a villus 57 mm long. Its real length is 0.76 mm.

(a) Calculate the magnification. [2] (b) Model the villus as a cylinder of radius 0.07 mm and height 0.8 mm. Calculate its curved surface area in mm², to 2 significant figures. [2] (c) Calculate how many times larger this is than the area of the flat circle of the same radius, to 3 significant figures. [1] (d) Explain how the capillary network inside a villus helps absorption. [1]

7. Glucose uptake by pieces of ileum was measured in three conditions. In normal saline, uptake was 42 units. Without sodium ions in the saline, it was 6 units. With an inhibitor of the sodium-potassium pump, it was 9 units.

(a) Calculate the percentage fall in uptake in each experimental condition, to 3 significant figures. [2] (b) Use co-transport to explain why the inhibitor reduced uptake. [3]

8. Describe the route taken by a fatty acid from the lumen of the ileum to a lacteal. [4]

9. Cholera is caused by a bacterium that secretes a toxin in the small intestine.

(a) Explain how the toxin leads to severe diarrhoea. [4] (b) A patient of mass 60 kg produces 800 mL of watery stool per hour for 6 hours. Calculate the volume lost and the loss as a percentage of body mass, taking 1 L as 1 kg. [2]

10. An oral rehydration solution (ORS) contains glucose at 75 mmol per litre and sodium ions at 75 mmol per litre.

(a) Explain why this ORS rehydrates a cholera patient better than water alone. [4] (b) Calculate the mass of glucose in 1 litre. The molar mass of glucose is 180 g per mol. [2]

11. A trial compared a standard ORS with a new rice-based ORS in two groups of 120 children with watery diarrhoea.

	Standard ORS	New ORS
Mean stool output (mL per kg)	156	129
Mean duration of diarrhoea (h)	62	51

(a) Calculate the percentage reduction in mean stool output with the new ORS, to 3 significant figures. [2] (b) Evaluate the evidence that the new ORS is better. [3] (c) Discuss the ethical issues in running this trial. [5]

Answers

1. (a) Rectum [1] (b) Duodenum [1] (c) Ileum [1] Examiner insight: Spell the names exactly; “small intestine” is too vague when a question asks for a specific part.

2. (a) Digestion is the hydrolysis of large molecules by enzymes into smaller ones [1]; absorption is the movement of the products across the gut wall into the body [1]. (b) Assimilation is the use of absorbed substances to become part of cells [1]; egestion is the removal of undigested food as faeces [1]. Examiner insight: Each term needs its own definition; saying the two terms “are different” does not show what either one means.

3. Salivary amylase hydrolyses starch to maltose in the mouth [1]; pancreatic amylase continues this in the duodenum [1]; a membrane-bound maltase in the ileum epithelial cell membrane [1] hydrolyses maltose to glucose [1]. Examiner insight: Naming an enzyme without its site, or “amylase” without a product, usually leaves marks unclaimed.

4. (a) Bonds = 120 − 1 = 119 [1]; remaining = 119 − 5 = 114 [1]. (b) The endopeptidase creates more ends [1]; exopeptidases act only at ends, so more points of attack speed hydrolysis [1]. (c) Dipeptidase [1] Examiner insight: Show the subtraction in part (a); the “minus one” step is the commonest slip.

5. (a) 1000 ÷ 190 = 5.26 [1]; 1000 ÷ 70 = 14.3 [1]. (b) Lipase hydrolyses triglycerides to fatty acids, which are acidic [1]. (c) Bile salts emulsify the lipid into smaller droplets [1]; surface area increases [1]; more lipase can bind at once, so the reaction is faster [1]. Examiner insight: Say “emulsify” and “surface area”; “breaks down fat” describes what lipase does, not bile salts.

6. (a) 57 ÷ 0.76 [1] = ×75 [1]. (b) 2πrh = 2 × π × 0.07 × 0.8 [1] = 0.35 mm² [1]. (c) πr² = 0.0154 mm²; ratio = 0.352 ÷ 0.0154 = 22.9 [1]. (d) It carries absorbed substances away, keeping a steep concentration gradient [1]. Examiner insight: Write the formula and substitution; clear working lets a reader see your method even if the final figure slips.

7. (a) No sodium: (42 − 6) ÷ 42 × 100 = 85.7% [1]; inhibitor: (42 − 9) ÷ 42 × 100 = 78.6% [1]. (b) The pump uses ATP to remove sodium ions from the cell [1]; the inhibitor leaves sodium ion concentration inside high, so the gradient from lumen to cell falls [1]; less sodium enters through the co-transporter, so less glucose is carried in [1]. Examiner insight: Link the pump to the sodium ion gradient before mentioning glucose; answers that jump straight to glucose miss the chain of reasoning.

8. Fatty acid is carried in a micelle with bile salts to the epithelial surface [1]; it diffuses across the cell-surface membrane because it is lipid-soluble [1]; inside the cell it is rebuilt into triglyceride and packaged into a chylomicron [1]; the chylomicron leaves by exocytosis and enters the lacteal [1]. Examiner insight: A route question rewards a correct order; a list of terms in the wrong order loses the sequence marks.

9. (a) The toxin increases chloride ion secretion into the lumen [1]; sodium ions follow, so the lumen ion concentration rises [1]; lumen water potential becomes lower than that of the cells and blood [1]; water moves out by osmosis, down the water potential gradient, giving watery stools [1]. (b) 0.8 × 6 = 4.8 L [1]; 4.8 ÷ 60 × 100 = 8.0% [1]. Examiner insight: Use water potential, not “water concentration”, and say “by osmosis” so the direction is clear.

10. (a) Glucose and sodium ions are absorbed together by co-transport [1]; the toxin does not stop this [1]; absorbing the solutes lowers the water potential in the cells and blood [1]; water follows by osmosis [1]. (b) 0.075 mol × 180 [1] = 13.5 g [1]. Examiner insight: Convert mmol to mol before multiplying; the factor of 1000 is a frequent slip.

11. (a) (156 − 129) ÷ 156 × 100 [1] = 17.3% [1]. (b) Both outcomes are lower for the new ORS: output by 27 mL per kg, and duration by 11 h, which is 17.7% shorter [1]; the groups are equal and fairly large (120 each) [1]; but only means are given, with no spread or statistical test, so a difference due to chance cannot be ruled out [1]. (c) Informed consent from parents or guardians [1]; the right to withdraw [1]; the control group must receive the standard ORS, not an untreated placebo [1]; ethics committee approval, with risks weighed against benefits [1]; children are vulnerable, and the benefit should be shared afterwards [1]. Examiner insight: An evaluation scores for using the figures and for a limit on the evidence; a general statement that the new ORS “works” does not.

Where marks are usually lost

• Giving an enzyme with no substrate or product.
• Describing bile salts as enzymes.
• Forgetting that a chain of n amino acids has n − 1 bonds.
• Not showing the formula and substitution in a calculation.
• Leaving out the sodium-potassium pump in co-transport.
• Mixing up micelles and chylomicrons.
• Writing “water concentration” instead of water potential.
• Giving ethics answers as a list of words with no reason.

Next steps

• Revisit the revision notes and the study guide.
• Tick off the checklist and visit the course hub.
• Take the free 10-minute diagnostics.
• Or book a free trial class.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International A-level exams May/June 2018 onwards, published by OxfordAQA. Sections 3.2.2.1 to 3.2.3.2.