IB Biology 2025 · Theme B · Form and function

B2.1 Membranes and membrane transport. Practice questions with markscheme.

46 original IB-style questions on B2.1, written from the 2025 guide: 20 multiple-choice, 16 short-answer, 5 data-based, 2 drawing, 2 extended-response part, 1 labelling. Below is a 21-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

10 statements at SL and HL, 7 additional higher level.

B2.1.1SL / HL Lipid bilayers as the basis of cell membranes
B2.1.2SL / HL Lipid bilayers as barriers
B2.1.3SL / HL Simple diffusion across membranes
B2.1.4SL / HL Integral and peripheral proteins in membranes
B2.1.5SL / HL Movement of water molecules across membranes by osmosis and the role of aquaporins
B2.1.6SL / HL Channel proteins for facilitated diffusion
B2.1.7SL / HL Pump proteins for active transport
B2.1.8SL / HL Selectivity in membrane permeability
B2.1.9SL / HL Structure and function of glycoproteins and glycolipids
B2.1.10SL / HL Fluid mosaic model of membrane structure
B2.1.11HL Relationships between fatty acid composition of lipid bilayers and their fluidity
B2.1.12HL Cholesterol and membrane fluidity in animal cells
B2.1.13HL Membrane fluidity and the fusion and formation of vesicles
B2.1.14HL Gated ion channels in neurons
B2.1.15HL Sodium–potassium pumps as an example of exchange transporters
B2.1.16HL Sodium-dependent glucose cotransporters as an example of indirect active transport
B2.1.17HL Adhesion of cells to form tissues

In the bank for B2.1

20 multiple-choice
16 short-answer
5 data-based
2 drawing
2 extended-response part
1 labelling
17 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

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The practice paper

Take it on screen → Build a fresh paper Paper code BbB-EAAACAAAABQAOq89

Biology · topic quiz

Standard level · topic practice, not an exam format

30 minutes21 marks

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Covers B2.1 Membranes and membrane transport

Name:

Why do the phospholipids of a membrane spontaneously arrange as a bilayer in water?

[1]

Enzymes assemble the bilayer from phospholipids
The cell wall forces them into position
ATP is used to hold the tails in place
Hydrophobic tails turn inward, away from the water

A printed two-dimensional diagram of the fluid mosaic model shows phospholipids in two neat rows with proteins set between them. Which property of a real membrane must such a diagram misrepresent?

[1]

The hydrophilic phosphate heads face the aqueous solutions on each side
Integral proteins extend across the whole width of the phospholipid bilayer
Phospholipids and many proteins move continuously within the plane of the bilayer
Cholesterol molecules are positioned between the hydrophobic hydrocarbon tails

Where are the carbohydrate chains of the glycoproteins and glycolipids of the plasma membrane located?

[1]

Embedded in the hydrophobic core of the bilayer
On the cytoplasmic (inner) face
Equally on both faces
On the extracellular (outer) face only

Distinguish between facilitated diffusion and active transport as mechanisms of membrane transport.

[3]

Oxygen enters an actively respiring muscle cell and carbon dioxide leaves it. Outline how these two gases cross the plasma membrane.

[3]

The diagram shows a section of a plasma membrane. Identify the structures labelled I–IV.

[4]

II.

III.

IV.

Beetroot cells contain a red pigment inside the vacuole. A student hypothesises that increasing the concentration of ethanol (an organic solvent) in the surrounding water increases the permeability of beetroot cell membranes. More pigment released gives a higher absorbance reading.

(a)Design an investigation to test this hypothesis. Include the independent, dependent and controlled variables and a suitable control.[6]

(b)Explain, in terms of membrane structure, why a high concentration of ethanol is expected to increase pigment release.[2]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-EAAACAAAABQAOq89

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. B2.1.1 [1]

D: self-assembly driven by the amphipathic nature of phospholipids; no energy input or machinery needed;

2. B2.1.10 [1]

C: a printed diagram is static, so the lateral movement that makes the membrane fluid cannot be shown; the arrangement of heads and tails, the span of integral proteins and the position of cholesterol are all represented correctly;

Design move: the figure as a reasoning object — the neat rows are the visually salient feature and are precisely what is false.

3. B2.1.9 [1]

D: the membrane is asymmetric: carbohydrate groups face outward (the glycocalyx), suiting their roles in cell recognition and adhesion;

4. B2.1.6-B2.1.7 [3 max]

facilitated diffusion is passive / needs no ATP, whereas active transport requires energy from ATP;
facilitated diffusion moves particles down their concentration gradient, whereas active transport moves them against the gradient;
facilitated diffusion uses channel or carrier proteins, whereas active transport uses pump proteins;
facilitated diffusion continues until equilibrium, whereas active transport can build/maintain concentration differences, OWTTE;

Points must be explicitly comparative. Award converse statements.

5. B2.1.3 [3]

by simple diffusion;
the molecules are small and non-polar / uncharged, so they pass (between the phospholipids) through the hydrophobic core of the bilayer;
no membrane protein / channel / carrier is needed;
no energy / ATP is used / it is a passive process;
each gas moves down its concentration gradient: oxygen is at a lower concentration inside (because it is used in respiration) and carbon dioxide is at a higher concentration inside (because it is produced), OWTTE;
the net movement results from the random movement of the molecules;

Do not accept 'osmosis'. Do not accept 'facilitated diffusion' or reference to channels for these gases.

6. B2.1.4 [4]

I — glycoprotein / carbohydrate chain attached to a protein;
II — phospholipid (bilayer);
III — integral / transmembrane protein;
IV — peripheral protein;

Accept "channel protein" for III only if drawn spanning the bilayer.

7. B2.1.5

(a) [6]
- independent variable: concentration of ethanol surrounding the beetroot (e.g. 0, 10, 20, 30, 40 %);
- dependent variable: pigment released, measured as absorbance of the surrounding solution using a colorimeter;
- method: cut equal-sized beetroot discs/cubes, wash off surface pigment, place equal numbers in equal volumes of each ethanol concentration for a fixed time, then measure absorbance;
- control: discs in water with 0 % ethanol (to show pigment release without solvent);
- controlled variables: size/surface area and number of discs, volume of solution, temperature, time, beetroot source/variety;
- repeat each concentration ≥3 times and calculate a mean, OWTTE;
Award for IV, DV+measurement, method, control, controlled variables, replication.
(b) [2]
- ethanol dissolves in / disrupts the (hydrophobic) phospholipid bilayer / dissolves membrane lipids;
- the membrane becomes more permeable / develops gaps, so pigment leaks out of the vacuole and cell, OWTTE;