Question 1
Paper 1A style
Which substance crosses a lipid bilayer without membrane proteins most rapidly by simple diffusion?
- Sodium ions
- Glucose
- Oxygen
- Amino acids
Show mark scheme for question 1
Answer: C – oxygen is small and non-polar, so it passes between the phospholipids; ions and polar molecules do not readily enter the hydrophobic core [1]
Question 2
Paper 1A style
Which statement distinguishes active transport by pump proteins from facilitated diffusion through channel proteins?
- Only active transport is selective for specific particles
- Only active transport can move particles against a concentration gradient
- Only facilitated diffusion involves integral membrane proteins
- Only facilitated diffusion depends on the hydrophobic core of the membrane
Show mark scheme for question 2
Answer: B – pumps use energy from ATP to move particles against a concentration gradient; both processes are selective and both use integral proteins [1]
Question 3
Paper 2A style
Draw a labelled two-dimensional diagram of the fluid mosaic model of membrane structure.
Show mark scheme for question 3
- phospholipid bilayer drawn with two layers, heads facing outwards and tails inwards [1]
- hydrophilic regions and hydrophobic region correctly labelled [1]
- integral protein shown embedded in / spanning the bilayer [1]
- peripheral protein shown attached to one surface only [1]
- glycoprotein with carbohydrate chain shown on the extracellular side only [1]
- cholesterol shown within the hydrophobic region, between the hydrocarbon tails [1]
- Labels must point clearly to the structure; do not accept proteins drawn on top of the bilayer as integral
- max 4
Question 4
Paper 1B style
Frog (Xenopus) egg cells normally have very few aquaporins in their plasma membrane. In an experiment, one group of egg cells was injected with mRNA coding for an aquaporin and a control group was injected with water. All the cells were then placed in a dilute solution and their volume was measured. The data are for practice. (a) Calculate the percentage increase in volume of the aquaporin cells after 180 s. [1] (b) Explain the difference between the two groups. [3] (c) Suggest why the volume of the control cells still increased slightly. [1]
| Time / s | Control cells | Aquaporin cells |
|---|---|---|
| 0 | 1.00 | 1.00 |
| 60 | 1.01 | 1.10 |
| 120 | 1.02 | 1.21 |
| 180 | 1.03 | 1.32 |
| 240 | 1.04 | Cells burst |
Show mark scheme for question 4
- (a) (1.32 − 1.00) ÷ 1.00 × 100 = 32 % [1]
- (b) aquaporin mRNA is translated, so aquaporins are inserted into the plasma membrane [1]
- (b) aquaporins increase the permeability of the membrane to water [1]
- (b) solute concentration inside the cell is higher than in the dilute solution, so water enters by osmosis [1]
- (b) water enters faster, so volume increases faster / cells burst [1]
- (b) max 3
- (c) water can still cross the bilayer slowly by diffusing between phospholipids / a few aquaporins are present OWTTE [1]
Question 5
Paper 2B style
Explain how the structure of the plasma membrane allows it to control the movement of substances into and out of cells.
Show mark scheme for question 5
- phospholipids form a bilayer with a hydrophobic (hydrocarbon) core [1]
- core has low permeability to ions / polar molecules / large molecules, so it acts as a barrier [1]
- small non-polar molecules such as O₂ / CO₂ cross by simple diffusion between phospholipids [1]
- simple diffusion is not selective / depends only on size and hydrophilic or hydrophobic properties [1]
- channel proteins allow facilitated diffusion of specific ions [1]
- channels can be gated / opened or closed to control permeability [1]
- aquaporins allow rapid movement of water by osmosis [1]
- pump proteins use energy from ATP to move specific particles against a concentration gradient [1]
- the proteins present determine which particles cross, giving selective permeability [1]
- max 6