AQA A-Level Biology: Organisms Exchange Substances With Their Environment (7402) – 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 section 3.3 (Organisms exchange substances with their environment, 3.3.1 to 3.3.4) of the AQA AS and A-level Biology specification, version 1.5, for the A-level Biology (7402) course. Read the study guide first. Course hub: AQA A-level Biology. Printable list: checklist. To find weak spots, use the free diagnostics.

Questions

1. Two cubes of agar have sides of 3 mm and 6 mm. (a) Calculate the surface area to volume ratio of each. [2] (b) A spherical single-celled organism has a diameter of 40 µm. Its surface area to volume ratio is 6 divided by the diameter. Calculate the ratio and explain why the organism needs no specialised gas exchange organ. [3]

2. Animal A has a mass of 20 g and uses 36 cm³ of oxygen per hour. Animal B has a mass of 400 g and uses 120 cm³ per hour. (a) Calculate the oxygen used per gram per hour by each animal. [2] (b) Explain the difference using surface area to volume ratio. [2] (c) Describe two adaptations, each a change to body shape or a body system, that help a large organism exchange substances. [2]

3. (a) Describe how oxygen reaches the muscle cells of a flying insect. [2] (b) Explain the compromise between gas exchange and water loss in insects. [2] (c) Explain how the stomata and mesophyll of a dicotyledonous leaf allow gas exchange. [2] (d) Explain how one feature of a xerophyte reduces water loss and state the cost to gas exchange. [2]

4. In a fish gill, water passes over the lamellae while blood flows through them in the opposite direction. (a) Name this arrangement and explain how it helps oxygen uptake. [2] (b) If blood and water flowed in the same direction, the two would reach about 50% saturation and then stop exchanging. Explain why. [1] (c) In a counter-current gill, water entering at 100% saturation leaves at 10% saturation. Calculate the percentage of oxygen removed and how many percentage points more this is than the parallel-flow value. [2]

5. (a) A resting person has a tidal volume of 0.50 dm³ and breathes 12 times per minute. Calculate the pulmonary ventilation rate. [1] (b) During exercise the rate is 45 dm³ per minute and the tidal volume 2.5 dm³. Calculate the breathing rate. [2] (c) Describe three features of the alveolar epithelium that favour gas exchange. [3]

6. (a) Explain how the intercostal muscles and diaphragm cause inhalation. [4] (b) In pulmonary fibrosis lung tissue is thickened and scarred. Explain how this reduces gas exchange. [2]

7. A study followed adults for 10 years. The numbers of lung disease cases per 1000 people were: non-smokers 12; 1 to 10 cigarettes per day 31; 11 to 20 per day 58; over 20 per day 94. (a) Calculate how many times greater the incidence is in the 11 to 20 group than in non-smokers. [1] (b) Evaluate whether the data show that smoking causes lung disease. [4]

8. (a) Describe how proteins are digested to amino acids. [3] (b) Explain how amino acids are absorbed by co-transport. [3] (c) Explain the roles of bile salts and micelles in the digestion and absorption of lipids. [3]

9. The table shows percentage saturation of haemoglobin from two animals at different partial pressures of oxygen (pO2).

pO2 (kPa)	2	4	6	8	10	13
Adult human (%)	20	53	80	91	95	97
Animal X (%)	40	75	92	96	98	98

(a) Calculate the difference in saturation at 4 kPa. [1] (b) Suggest an advantage to animal X of its haemoglobin. [2] (c) Explain the S-shape of the adult curve. [2] (d) Explain how raised carbon dioxide at a muscle alters oxygen release. [2]

10. Pressures in the left side of a heart (kPa) at four times are: 0.05 s, atrium 0.8, ventricle 0.3, aorta 10.5; 0.15 s, atrium 0.6, ventricle 4.0, aorta 10.0; 0.25 s, atrium 0.4, ventricle 15.0, aorta 14.0; 0.45 s, atrium 0.5, ventricle 1.0, aorta 11.0. (a) State the times at which the atrioventricular valve is closed and explain your choice. [2] (b) Explain why the semilunar valve is open at 0.25 s. [1] (c) Stroke volume is 70 cm³ and heart rate 72 beats per minute. Calculate cardiac output in dm³ per minute. [2] (d) Cardiac output rises to 7.2 dm³ per minute at a stroke volume of 80 cm³. Calculate the heart rate. [2]

11. At the arteriole end of a capillary, hydrostatic pressure is 4.3 kPa; at the venule end it is 1.9 kPa. The pull of the plasma proteins is 3.3 kPa throughout. Ignore pressure outside the capillary. (a) Calculate the net pressure at each end, stating the direction of fluid movement. [2] (b) Explain the formation and return of tissue fluid. [2] (c) Explain how the structure of an artery and a vein suits each function. [3]

12. (a) Explain how water rises in the xylem. [3] (b) A ring of bark and phloem is removed from a stem. Describe and explain the expected result. [2] (c) A tracer moves 36 cm along a stem in 30 minutes. Calculate the speed in mm per second. [1] (d) Give one piece of evidence for and one against the mass flow hypothesis. [2]

Answers

1. (a) 3 mm cube: surface area 54 mm², volume 27 mm³, ratio 2 : 1 [1]. 6 mm cube: 216 mm² and 216 mm³, ratio 1 : 1 [1]. (b) Ratio = 6 / 40 = 0.15 per µm [1]. The ratio is large, so oxygen diffuses across the surface fast enough [1] to supply the small volume of cytoplasm [1]. Examiner insight: Show surface area and volume before dividing.

2. (a) A: 36 / 20 = 1.8 cm³ per g per hour [1]. B: 120 / 400 = 0.3 cm³ per g per hour [1]. (b) A is smaller, so it has the larger surface area to volume ratio and loses heat faster [1]; it needs a higher metabolic rate per gram, using six times as much oxygen [1]. (c) Any two: flattened body shape gives a short diffusion distance [1]; a circulatory system carries substances around the body [1]. Examiner insight: Compare oxygen per gram, not total, to show the ratio link.

3. (a) Air enters through spiracles and passes along tracheae [1] to tracheoles reaching the muscle cells, where oxygen diffuses in [1]. (b) Open spiracles let oxygen in but water vapour out [1]; closing them reduces water loss but also restricts gas exchange [1]. (c) Stomata open so gases diffuse into air spaces [1]; the mesophyll cells’ large wet surface lets gases dissolve and exchange [1]. (d) Sunken stomata or hairs trap humid air, lowering the water vapour gradient [1]; the cost is a longer diffusion path for CO2 [1]. Examiner insight: A compromise answer states both the benefit and the cost.

4. (a) Counter-current flow [1]; blood always meets water with a higher oxygen concentration, so a gradient remains along the whole lamella [1]. (b) Concentrations equalise partway along, so there is no gradient for further diffusion [1]. (c) 100 - 10 = 90% removed [1]; 90 - 50 = 40 percentage points more [1]. Examiner insight: Say “opposite directions” explicitly.

5. (a) 0.50 × 12 = 6.0 dm³ per minute [1]. (b) Rate = 45 / 2.5 [1] = 18 per minute [1]. (c) Any three: one cell thick, so a short diffusion distance [1]; close to a dense capillary network, keeping a gradient [1]; very many alveoli give a large area [1]. Examiner insight: Pair each feature with its effect.

6. (a) External intercostals contract, raising the ribs [1]; the diaphragm contracts and flattens [1]; thoracic volume increases [1]; pressure falls below atmospheric so air flows in [1]. (b) The thicker tissue lengthens the diffusion path [1], so less oxygen crosses per unit time [1]. Examiner insight: Name the pressure change; “air is sucked in” does not explain why.

7. (a) 58 / 12 = 4.8 times [1]. (b) Incidence rises with cigarettes smoked, a trend that supports a link [1]. Correlation alone does not prove cause [1]; age or pollution might differ between groups [1]. A known mechanism, such as smoke damaging lung tissue, strengthens the case [1]. Examiner insight: A balanced evaluation ends with a judgement.

8. (a) Endopeptidases hydrolyse peptide bonds inside the chain [1]; exopeptidases remove terminal amino acids [1]; membrane-bound dipeptidases split dipeptides into amino acids [1]. (b) Sodium ions are actively transported out of the epithelial cell, lowering the internal concentration [1]; sodium enters down its gradient through a carrier that brings in an amino acid too [1]; the amino acid then leaves by facilitated diffusion into blood [1]. (c) Bile salts emulsify lipids, giving droplets with more surface for lipase [1]; lipase produces monoglycerides and fatty acids [1]; these join micelles, which carry them to the membrane where they diffuse in [1]. Examiner insight: Name sodium ions; co-transport with no ion gradient described is incomplete.

9. (a) 75 - 53 = 22 percentage points [1]. (b) It is more saturated at low pO2, so it loads oxygen where oxygen is scarce, for example at a placenta [1]; X is 75% saturated at 4 kPa against 53% for the adult [1]. (c) The first oxygen changes the shape of haemoglobin [1], making the next ones bind more easily until saturation nears [1]. (d) The curve moves right (Bohr effect) [1], so haemoglobin releases more oxygen at the same pO2 [1]. Examiner insight: State the direction of the shift and its effect separately.

10. (a) 0.15, 0.25 and 0.45 s [1], because ventricular pressure exceeds atrial pressure at those times [1]. (b) Ventricular pressure (15.0) exceeds aortic (14.0), forcing the valve open [1]. (c) 70 × 72 = 5040 cm³ = 5.04 dm³ per minute [1]; unit conversion [1]. (d) 7200 / 80 = 90 beats per minute [1]; conversion from dm³ shown [1]. Examiner insight: Quote the two pressures as figures when comparing them.

11. (a) Arteriole end: 4.3 - 3.3 = 1.0 kPa outward [1]. Venule end: 1.9 - 3.3 = -1.4 kPa, so inward [1]. (b) Net outward pressure forms tissue fluid [1]; plasma proteins lower water potential, so water returns by osmosis at the venule end [1]. (c) Artery: thick elastic and muscular wall withstands and smooths high pressure [1]. Vein: thin wall and wide lumen suit low pressure [1]; valves stop backflow [1]. Examiner insight: The sign of each net value shows the direction.

12. (a) Transpiration at the leaf lowers pressure, so water is pulled up under tension [1]; hydrogen bonds make water molecules cohere [1] and adhere to the walls, keeping the column continuous [1]. (b) Sugars accumulate above the ring [1] because phloem transport is interrupted, while the xylem keeps working [1]. (c) 360 mm / 1800 s = 0.2 mm per second [1]. (d) For: sap exudes from cut phloem under pressure, or a tracer moves faster than diffusion alone allows [1]. Against: sieve plates would be expected to slow bulk flow, or translocation stops when respiration is inhibited, which pressure flow alone does not explain [1]. Examiner insight: Convert units before dividing; a speed in the wrong unit can lose the final mark.

Where marks are usually lost

• Leaving out units or conversions in ventilation and cardiac output.
• Describing co-transport without sodium ions.
• Judging risk factors by correlation alone.
• Confusing xylem (tension) with phloem (pressure flow).

Next steps

• Review the revision notes and study guide.
• Go to the AQA A-level Biology hub or print the checklist.
• Take the free diagnostics.
• Book a free trial class.

Frequently asked questions

Which section of the AQA specification do these questions cover? They cover section 3.3, Organisms exchange substances with their environment: exchange surfaces, digestion, haemoglobin, the heart and plant transport.

What units should you check in ventilation and cardiac output calculations? Check the units of each value and convert where needed. Leaving out units or conversions is a common error.

What should you revise before attempting these questions? Work through the study guide and revision notes for section 3.3 first, then use the questions to test recall and application.

Are these AQA past paper questions? No. They are original questions with worked answers. Use them alongside official AQA past papers.

Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), version 1.5, published 26 November 2021 by AQA. This page covers section 3.3.