IB DP Biology: Membranes and membrane transport (DP Biology) – 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 – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover section B2.1 (Membranes and membrane transport) of the IB Biology guide, first assessment 2025, for DP Biology. Questions 1 to 9 are SL and HL. Questions 10 to 12 test additional higher level content and are labelled HL only.

Read the study guide and revision notes first if you need to. See also the IB DP Biology hub, the printable checklist and the free 10-minute diagnostics.

Questions

1. Describe how phospholipid molecules are arranged when they form a bilayer in water, and explain why they arrange themselves in this way. [3]

2. Explain why oxygen can cross a membrane without help from a protein, whereas a sodium ion cannot. [3]

3. Distinguish between integral and peripheral proteins in a membrane, and give one function of a membrane protein. [3]

4. Using the idea that particles move randomly, explain why water shows net movement from a dilute solution to a more concentrated solution across a membrane that is impermeable to the solute. [4]

5. Cylinders of beetroot were weighed, left in sucrose solutions for one hour, blotted dry and weighed again.

Sucrose (mol dm⁻³)	0.10	0.20	0.30	0.40
Initial mass (g)	3.20	3.15	3.18	3.22
Final mass (g)	3.52	3.21	3.04	2.90

(a) Calculate the percentage change in mass at 0.20 mol dm⁻³. [2] (b) Using a straight line between 0.20 and 0.30 mol dm⁻³, estimate the concentration giving zero change in mass. [2] (c) Explain the loss of mass at 0.40 mol dm⁻³. [2]

6. Compare facilitated diffusion with active transport. [4]

7. An electron micrograph shows a membrane. The width of the membrane on the image is 21 mm and the magnification is ×3 000 000. Calculate the actual width of the membrane in nanometres. [3]

8. Outline the structure of the fluid mosaic model of the plasma membrane, as you would show it in a labelled two-dimensional drawing. [5]

9. State where the carbohydrate part of a glycoprotein is found in a plasma membrane, and give two roles of glycoproteins and glycolipids. [3]

10. (HL only) The percentage of unsaturated fatty acid chains in a fish’s membrane phospholipids was measured after acclimation at four temperatures.

Temperature (°C)	5	15	25	35
Unsaturated chains (%)	62	55	47	38

(a) Calculate the percentage decrease in unsaturated chains from 5 °C to 35 °C, relative to the value at 5 °C. [2] (b) Explain the advantage of the pattern shown, in terms of membrane fluidity. [3] (c) Explain the role of cholesterol in animal cell membranes at high and low temperatures. [2]

11. (HL only) A nerve cell has a sodium-potassium pump that completes 2.4 × 10⁶ cycles per minute.

(a) Calculate the net number of positive charges moved out of the cell per minute. [2] (b) Calculate the number of ATP molecules used per second, assuming one ATP per cycle. [2] (c) Explain how the pump helps to produce a membrane potential. [2]

12. (HL only) Epithelial cells of the small intestine absorb glucose using a sodium-dependent glucose cotransporter.

(a) Explain why this is indirect active transport. [3] (b) The transporter carries 2 sodium ions with each glucose molecule. 600 glucose molecules enter. Calculate the ATP needed by sodium-potassium pumps to remove the sodium, assuming one ATP per cycle. [3] (c) State the type of gating of the nicotinic acetylcholine receptor and of the sodium channels in an axon. [2]

Answers

1. Phospholipids form two layers, with the hydrophilic heads facing the water on the outside [1] and the hydrophobic fatty acid tails pointing inwards, facing each other [1]. The tails are shielded from water, which makes the arrangement stable [1] [3] Examiner insight: A good answer names both the arrangement and the reason. Saying only “heads out, tails in” describes without explaining, and explanation is what the command word asks for.

2. Oxygen is small and non-polar [1], so it dissolves in the hydrophobic hydrocarbon core and passes between phospholipids [1]. A sodium ion is charged and hydrophilic, so the core repels it and it cannot cross without a protein [1] [3] Examiner insight: Refer to both particles. An answer about oxygen alone leaves the contrast unexplained.

3. Integral proteins lie within one lipid layer or span both [1]; peripheral proteins are attached to a surface and do not enter the core [1]. Any one function, for example a channel, a pump, a receptor or an enzyme [1] [3] Examiner insight: “Integral proteins go through the membrane” is incomplete, since some are embedded in one layer only. Describe the position, not just the span.

4. Water molecules move randomly in both directions across the membrane [1]. The membrane is impermeable to the solute, so only water crosses [1]. The more concentrated side has a lower proportion of water molecules, so fewer cross from it than arrive from the dilute side [1]. Net movement is therefore towards the higher solute concentration [1] [4] Examiner insight: Name the direction using solute concentration. “Towards more water” or “towards less water” is ambiguous and tends to lose credit.

5. (a) Change = 3.21 − 3.15 = 0.06 g [1]. 0.06 ÷ 3.15 × 100 = +1.9% [1] (b) The percentage change at 0.30 is (3.04 − 3.18) ÷ 3.18 × 100 = −4.4% [1]. Concentration = 0.20 + 0.10 × 1.9 ÷ (1.9 + 4.4) = 0.23 mol dm⁻³ [1] (c) The solution outside has a higher solute concentration than the cell contents [1], so there is net movement of water out of the cells by osmosis [1] [6] Examiner insight: Write out both the subtraction and the division. A percentage change needs its sign, and a bare mass change in grams does not answer a question asking for percentage.

6. Both move specific particles through a membrane protein [1] and are selective [1]. Facilitated diffusion moves particles down a concentration gradient using no ATP, through a channel [1], while active transport uses ATP in a pump and can move particles against a gradient [1] [4] Examiner insight: A comparison needs at least one shared feature and one difference, each stated outright. Describing each process in separate paragraphs without linking them does not compare.

7. Actual size = image size ÷ magnification [1]. 21 mm = 21 × 10⁶ nm, so 21 × 10⁶ ÷ 3 000 000 [1] = 7 nm [1] [3] Examiner insight: Convert units before dividing. Dividing 21 by 3 000 000 without converting units gives a meaningless number, and the unit must be written.

8. Two layers of phospholipids with hydrophilic heads outwards and hydrophobic tails inwards [1]. Integral proteins embedded in or across the bilayer [1]. Peripheral proteins on a surface [1]. Cholesterol among the phospholipids and glycoproteins with carbohydrate chains on the outer surface [1]. Hydrophilic and hydrophobic regions labelled [1] [5] Examiner insight: Draw tails as two lines from each head and keep carbohydrate chains on the outside. Unlabelled drawings are hard to credit.

9. The carbohydrate is on the extracellular (outer) side of the membrane [1]. Roles: cell recognition [1] and cell adhesion [1] [3] Examiner insight: “Cell communication” is vaguer than recognition. Use the terms in the syllabus statement.

10. (a) Decrease = 62 − 38 = 24 percentage points [1]. 24 ÷ 62 × 100 = 38.7% [1] (b) At low temperature, more unsaturated chains are present [1]. Their kinks prevent tight packing, so the bilayer stays fluid and flexible [1]. At higher temperatures, more saturated chains make the membrane stronger [1] (c) At high temperatures cholesterol restricts the movement of phospholipids, which stabilises the membrane [1]; at low temperatures it prevents the tails packing closely, so the membrane does not stiffen [1] [7] Examiner insight: In data questions, use the numbers and link them to the mechanism. For cholesterol, state what it does at both ends of the temperature range.

11. (a) Each cycle moves 3 Na⁺ out and 2 K⁺ in, a net of one positive charge out [1]. 2.4 × 10⁶ cycles give 2.4 × 10⁶ net positive charges per minute [1] (b) 2.4 × 10⁶ ÷ 60 [1] = 4.0 × 10⁴ ATP per second [1] (c) More positive charge leaves than enters, so the inside becomes negative relative to the outside [1], and the pump also keeps sodium low and potassium high inside, producing concentration gradients for ions [1] [6] Examiner insight: Mention the unequal ratio when explaining membrane potential. Saying only “it pumps ions” does not link to charge.

12. (a) Sodium ions move into the cell down their gradient through the cotransporter [1]. Glucose is carried in with them even against its gradient [1]. The ATP is used by a different pump, which keeps the sodium gradient, so ATP is not used directly by the cotransporter [1] (b) Sodium = 2 × 600 = 1200 ions [1]. Each cycle removes 3, so 1200 ÷ 3 [1] = 400 ATP [1] (c) The nicotinic acetylcholine receptor is neurotransmitter-gated [1]; the axon sodium channels are voltage-gated [1] [8] Examiner insight: For indirect active transport, say where the energy comes from. “Uses sodium” alone does not say that ATP was spent elsewhere.

Where marks are usually lost

• Describing an arrangement without giving the reason when “explain” is the command word.
• Saying osmosis moves water “to where there is less water” instead of towards the higher solute concentration.
• Leaving out the fact that the membrane is impermeable to the solute.
• Omitting the sign or the unit in a percentage change or magnification answer.
• Forgetting to convert millimetres to nanometres before dividing.
• Writing that facilitated diffusion needs ATP.
• Putting the carbohydrate of a glycoprotein on the cytoplasmic side.
• Treating cholesterol as only a stiffener (HL only).
• Giving the sodium-potassium ratio as 2 out, 3 in (HL only).

Next steps

• Revision notes for this unit
• Study guide for this unit
• IB DP Biology hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, section B2.1.