IB Biology SL B2.1 Membranes and membrane transport Questions

Practise IB Biology SL B2.1 by linking membrane structure and selective permeability to diffusion, osmosis, transport proteins and ATP-driven active transport.

52 questions in this syllabus scope, with question parts counted together.

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam analysis

SL · 2010–2025
Papers with this topic50 / 141in the analysed sample
Latest appearanceNovember 2025
Tagged question parts70
Paper types5

Most tested objectives

  1. B2.1.10—Fluid mosaic model13 papers
  2. B2.1.6—Channel proteins10 papers
  3. B2.1.1—Lipid bilayers as basis9 papers
  4. B2.1.4—Integral and peripheral proteins7 papers
  5. B2.1.7—Pump proteins6 papers

Common question formats

  • Structured response
  • Comparison
  • Process explanation
  • Definition or recall
  • Diagram interpretation
  • Graph interpretation

Recent exam appearances

November 2025Paper 2 SL · TZ3Q3(a), 3(c)[ 4 ]
November 2025Paper 1A SL · TZ1Q3, 9[ 2 ]
May 2025Paper 2 SL · TZ1Q4(c)[ 2 ]

141 papers analysed · Tagging incomplete

Exam points

  • Identify amphipathic phospholipid bilayers, explain head–tail orientation in water and relate the hydrophobic core to selective barriers and compartment separation.
  • Define simple diffusion as passive net movement down a concentration gradient without ATP or transport proteins, including small non-polar molecules crossing the bilayer.
  • Distinguish integral and peripheral membrane proteins and identify their channel, pump, receptor, enzyme, adhesion and recognition roles.
  • Define osmosis, predict water movement from solute or water-potential differences and explain how aquaporins increase water permeability.
  • Explain facilitated diffusion through selective channel proteins as passive movement down a gradient, using size, charge and polarity to explain selectivity.
  • Explain active transport as ATP-powered movement against a concentration gradient using selective pump proteins.
  • Identify glycoproteins and glycolipids as membrane components carrying extracellular carbohydrate chains and relate them to cell recognition and adhesion.
  • Use the fluid mosaic model to locate hydrophilic heads, hydrophobic tails, integral proteins and mobile membrane components in a selective membrane.

Covered syllabus points

B2.1.1—Lipid bilayers as basisB2.1.2—Bilayers as barriersB2.1.3—Simple diffusionB2.1.4—Integral and peripheral proteinsB2.1.5—Osmosis and aquaporinsB2.1.6—Channel proteinsB2.1.7—Pump proteinsB2.1.8—Selectivity in permeabilityB2.1.9—Glycoproteins and glycolipidsB2.1.10—Fluid mosaic model

Exam points

  • Identify phospholipids as amphipathic molecules with hydrophilic phosphate heads and hydrophobic fatty-acid tails.
  • Explain how phospholipids form a continuous, closed bilayer in water, with hydrophilic heads facing water and hydrophobic tails directed inwards.
  • Identify phospholipid bilayers in plasma membranes, vesicles and nuclear envelopes, and distinguish them from a nucleoid.

Question 1

[Maximum number: 1]

The diagram shows the fluid mosaic model of cell membranes. Which labelled regions are hydrophilic?

Figure for Question 1 — IB Biology SL
A

I and II

B

I and III

C

II and IV

D

III and IV

Exam points

  • Explain why the hydrophobic fatty-acid core restricts diffusion of ions and large or hydrophilic molecules across a phospholipid bilayer.
  • Use membrane examples to explain how bilayers separate aqueous compartments while protein-mediated routes control entry and exit.

No public sample questions are available for this syllabus point yet.

Exam points

  • Define simple diffusion as passive net movement of particles down a concentration gradient using random kinetic motion, without ATP or transport proteins.
  • Explain how a steeper concentration gradient increases the rate of simple diffusion until net movement stops at equilibrium.
  • Identify small non-polar molecules such as oxygen and carbon dioxide that cross directly through the phospholipid bilayer by simple diffusion.

Question 1

[Maximum number: 1]

Apparatus was set up as shown to collect data.

Figure for Question 1 — IB Biology SL

The graph shows the results after 47 minutes of data collection.

Figure for Question 1 — IB Biology SL

What causes the rates to differ?

I. Different concentration gradients at the start
II. Diffusion of sugar is initially greater in Y than in X
III. The systems are reaching equilibrium over time

A

I only

B

II only

C

I and III only

D

II and III only

Exam points

  • Distinguish integral proteins embedded in the bilayer from peripheral proteins at its surface in membrane diagrams.
  • Match membrane proteins to functions such as channels, pumps, receptors, enzymes, adhesion and cell recognition.

Question 1

[Maximum number: 4]

Describe the functions of proteins in cell membranes.

Exam points

  • Define osmosis as passive net water movement across a partially permeable membrane from low to high solute concentration.
  • Predict water movement in soil-to-root and frog-egg contexts from solute or water-potential differences.
  • Compare aquaporin and control frog-egg graphs to explain faster water uptake, cell swelling and bursting.

Question 1

[Maximum number: 5]

In the early 1990s, investigations involving egg cells from the African clawed frog (Xenopus laevis) led to the discovery of membrane channel proteins called aquaporins.

In one investigation, scientists injected 10 frog egg cells with a solution containing mRNA encoding for an aquaporin, and another 10 frog egg cells with distilled water (control cells). All the frog egg cells were then incubated in a hypotonic solution. The volume of the frog egg cells was then measured during a period of five minutes.

The graph shows the mean results of the investigation.

Figure for Question 1 — IB Biology SL

Question (a)

(a)

State the direction of net movement of water across the plasma membrane of all the frog egg cells between 0 and 3 minutes.

[ 1 ]

Question (b)

(b)

Distinguish between the results shown for the frog egg cells with aquaporin mRNA and the control egg cells.

[ 2 ]

Question (c)

(c)

Explain the role of aquaporins in the movement of water across cell membranes.

[ 2 ]

Exam points

  • Explain why polar amino acids line hydrophilic channels that carry ions or polar solutes.
  • Explain channel-mediated facilitated diffusion as selective, passive, down-gradient movement without ATP to equilibrium.
  • Interpret chloride graphs showing facilitated-diffusion rate rising before it plateaus.

Question 1

[Maximum number: 1]

What explains the movement of glucose molecules down a concentration gradient across the cell surface membrane?

A

They can diffuse between phospholipids due to their flexibility.

B

They are actively transported by protein pumps due to their size.

C

They move through hydrophilic channels because they are polar.

D

They dissolve in the phospholipid bilayer because they are not charged.

Exam points

  • Explain active transport as ATP-powered movement of specific ions or molecules against concentration gradients.
  • Describe pump proteins as selective carriers that change conformation during ATP-powered transport.
  • Interpret intracellular/extracellular K+ and Na+ patterns as evidence for pump-maintained gradients.

Question 1

[Maximum number: 1]

The table shows concentrations of potassium ions and sodium ions maintained inside and outside human cells.

Table for Question 1 — IB Biology SL

What explains these concentrations?

A

Active transport using a protein pump

B

Facilitated diffusion using a vesicle

C

Simple diffusion using a protein channel

D

Osmosis using an aquaporin

Exam points

  • Compare simple and facilitated diffusion by solute polarity/size and need for a membrane protein.
  • Distinguish active transport from passive routes: it uses ATP to move molecules against a concentration gradient.
  • Use molecule-route tables to pair oestradiol with the bilayer and reject mismatched protein routes.

Question 1

[Maximum number: 1]

The following electron micrograph shows part of a palisade mesophyll cell. Which of the labelled structures controls the exchange of substances to and from the cell?

Figure for Question 1 — IB Biology SL

Exam points

  • Identify glycoproteins and glycolipids by their short carbohydrate chains on the extracellular surface of the membrane.
  • Explain how the glycocalyx supports cell recognition, adhesion, signalling and interactions with water.

Question 1

[Maximum number: 1]

Which part of the membrane allows cell recognition?

Figure for Question 1 — IB Biology SL

Exam points

  • Label a fluid mosaic membrane with hydrophilic heads, hydrophobic tails, integral proteins, peripheral proteins and other embedded components.
  • Explain fluidity as movement of phospholipids and proteins within the bilayer, and describe the mosaic distribution of membrane proteins, glycoproteins and glycolipids.
  • Compare the Singer–Nicolson model with Davson–Danielli using evidence about protein placement, and identify freeze-fracture/etching in membrane electron micrographs.

Question 1

[Maximum number: 1]

The diagram shows a portion of fluid mosaic membrane.

Figure for Question 1 — IB Biology SL

What are the membrane structures X and Y ?

X

Y

glycoprotein

integral protein

integral protein

peripheral protein

channel protein

integral protein

glycoprotein

peripheral protein