A Level Biology: Cell membranes and transport — 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 22. After each answer there is an examiner insight or a tip and, where one matches, the real question to try next.

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Questions

1. Epidermal cells in the roots of a rice plant absorb phosphate ions from the soil water, even when the concentration of phosphate ions inside the cells is much higher than in the soil water. The phosphate ions enter through a membrane protein that carries hydrogen ions (protons) and phosphate ions across the membrane together. Describe, with reasons, the sequence of events that allows these cells to take up phosphate ions from the soil water. [4]

2. Thin slices of barley root were placed in two flasks of aerated potassium chloride solution at 25 °C. The potassium ion concentration of the solution was 0.5 mmol dm⁻³ and stayed almost constant throughout. The potassium ion concentration inside the root cells was measured at the start, after 2 hours and after 4 hours. Flask X (control): 4 mmol dm⁻³, 20 mmol dm⁻³, 36 mmol dm⁻³. Flask Y was treated in the same way, except that cyanide, which stops ATP production in respiration, was added immediately after the 2-hour reading: 4 mmol dm⁻³, 20 mmol dm⁻³, 18 mmol dm⁻³. Using the data, explain what these results suggest about how potassium ions are taken up by barley root cells. [4]

3. Cells in a salivary gland release large quantities of the enzyme amylase.

(a) Name the process by which amylase leaves the cells.

(b) Describe this process and explain why it needs a supply of ATP. [4]

4. Plant cell P has a water potential of −580 kPa. The cell next to it, cell Q, has a water potential of −340 kPa. State the direction of net water movement between the two cells and explain your answer. [2]

5. Two neighbouring cells in a leaf are connected by many plasmodesmata. Suggest why glucose can pass from one of these cells into the other without using any membrane transport protein. [2]

6. Oxygen molecules cross a phospholipid bilayer easily, but sodium ions do not. Explain this difference and name the type of membrane protein that allows sodium ions to diffuse across the membrane. [3]

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Answers

1. ATP is hydrolysed to provide energy for a proton pump, which actively transports hydrogen ions out of the cell [1] into the cell wall (apoplast) just outside the cell surface membrane [1]. This builds up a high concentration of hydrogen ions outside the cell, a proton (electrochemical) gradient [1]. Hydrogen ions then diffuse back into the cell down their gradient through the cotransporter protein (facilitated diffusion) [1], and phosphate ions are carried in with them (cotransport) [1], so phosphate ions move against their own concentration gradient [1]. (Any four.) The phosphate ions are not pumped directly: the energy comes from the hydrogen ion gradient.

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 22, Question 4(c)(i)): Many answers were clear and sequential, but some missed out where the pumped protons end up (the cell wall or apoplast). Some wrongly called the movement of the transported solute active transport; only the protons pass through the cotransporter by facilitated diffusion.

Source for the examiner insights on this page: Cambridge International AS & A Level Biology 9700 June 2024 Principal Examiner Report for Teachers, Paper 9700/22 section, paraphrased.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 22, Question 4(c)(i).

2. The results suggest uptake is by active transport [1]. By 2 hours the cells contain 20 mmol dm⁻³, 40 times the 0.5 mmol dm⁻³ outside, so potassium ions are taken up against their concentration gradient, which diffusion cannot do [1]. Between 2 and 4 hours the control rose by 16 mmol dm⁻³ (to 36 mmol dm⁻³), but with cyanide the concentration did not rise and fell by 2 mmol dm⁻³ (to 18 mmol dm⁻³), so uptake stopped [1]. Cyanide stops respiration producing ATP, and ATP is needed by the carrier proteins (pumps) that move the ions in [1]. The small fall suggests some potassium ions leak out by diffusion down their gradient once the pumps stop [1]. (Any four; at least one comparison with the control must use data with units.)

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 22, Question 4(e)): The strongest answers compared each experiment with the control and suggested how each factor had its effect. Weaker answers only described the results, or compared the experiments with each other. Many could have earned more marks by quoting data with units.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 22, Question 4(e).

3. (a) Exocytosis [1].

(b) Amylase is packaged into vesicles from the Golgi body, which are moved along the cytoskeleton (microtubules) to the cell surface membrane [1]. The vesicle membrane fuses with the cell surface membrane, releasing the amylase outside the cell [1]. ATP is needed to move the vesicles and for membrane fusion [1].

Tip: Exocytosis and endocytosis are both “bulk transport” and both need ATP. Say where the vesicle comes from and what it fuses with.

4. Water moves from cell Q to cell P [1], because water moves by osmosis from a higher (less negative) water potential to a lower (more negative) water potential across partially permeable membranes, and −340 kPa is higher than −580 kPa [1].

Tip: The number closer to zero is the higher water potential. Always name both cells and give the direction, rather than only saying “down the gradient”.

5. Plasmodesmata are strands of cytoplasm passing through the cell walls, so the cytoplasm of the two cells is continuous [1]. Glucose can therefore diffuse down its concentration gradient through the plasmodesmata without having to cross a cell surface membrane [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 22, Question 4(c)(ii)): Most candidates identified plasmodesmata or a concentration gradient. Incorrect answers described movement through membrane proteins, often calling it facilitated diffusion.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 22, Question 4(c)(ii).

6. Sodium ions are charged (and surrounded by water molecules), so they cannot pass through the hydrophobic core made by the fatty acid tails of the phospholipids [1]. Oxygen molecules are small and non-polar, so they dissolve in and diffuse through the hydrophobic core [1]. Sodium ions diffuse through channel proteins [1].

Tip: Link each substance to the hydrophobic interior of the bilayer. Size alone does not explain it, because sodium ions are small too.

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Where marks are usually lost

• Saying the cotransported solute is “actively transported” through the cotransporter, instead of explaining that protons diffuse back in and carry the solute with them.
• Describing results from a data set without comparing each result with the control or quoting values with units.
• Forgetting that the number closer to zero is the higher water potential.
• Treating movement through plasmodesmata as if it needs membrane proteins.
• Explaining membrane permeability by size alone, without mentioning charge or the hydrophobic core.