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IB Biology · Theme B: Form and function · SL and HL

B2.1 Membranes and membrane transport

A one-page summary of B2.1 Membranes and membrane transport, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.

Open the B2.1 revision slides Practise B2.1 in the app

Guiding questions

How do molecules of lipid and protein assemble into biological membranes?

What determines whether a substance can pass through a biological membrane?

What B2.1 covers

The membrane: structure and selective permeability

Membrane fluidity and specialised transport

B2.1 Membranes and membrane transport: summary

Bilayer & barrier

  • Amphipathic phospholipids self-assemble tails-inward in water.
  • Hydrophobic core blocks ions, polar and large molecules.

Passive movement

  • Simple diffusion: O₂/CO₂ through the bilayer, down gradient.
  • Facilitated diffusion: channels/carriers, down gradient, saturates.

Osmosis & active transport

  • Osmosis: water toward higher solute via aquaporins.
  • Pumps: ATP moves particles against their gradient.

Membrane parts

  • Integral (embedded) vs peripheral (surface) proteins.
  • Glycoproteins/glycolipids tag the extracellular surface.

Fluidity HL

  • Unsaturated tails (kinks) increase fluidity; saturated pack tight.
  • Cholesterol buffers fluidity high and low temperatures.

Specialised transport HL

  • Na⁺/K⁺ pump (3 out : 2 in) is electrogenic; powers cotransport.
  • Gated channels fire neurons; CAMs build tissues.

Key terms

Phospholipid bilayer
Two layers of phospholipids, tails inward — the basis of cell membranes.
Amphipathic
Having both a hydrophilic and a hydrophobic region, like a phospholipid.
Simple diffusion
Net movement of small non-polar particles down their gradient through the bilayer.
Osmosis
Net movement of water across a partially permeable membrane toward higher solute concentration.
Facilitated diffusion
Passive movement down a gradient through a channel or carrier protein.
Active transport
Movement against a gradient by a pump, using energy such as ATP.
Glycoprotein / glycolipid
Membrane protein or lipid with a carbohydrate chain, used in recognition.
Fluid mosaic model
Membrane as a fluid bilayer with a mosaic of proteins drifting laterally.
Cholesterol HL
Steroid between phospholipid tails that buffers membrane fluidity in animal cells.
Endo-/exocytosis HL
Vesicle formation and fusion that move materials into and out of cells.
Gated ion channel HL
Channel opened by a voltage change or by a chemical signal binding.
Cell-adhesion molecule HL
Membrane protein that binds cells together at cell–cell junctions.

Sample exam questions

Three of the 52 multiple-choice questions for B2.1. Try each one before opening the answer.

Question 1. A phospholipid is described as amphipathic because it has:

  1. Equal numbers of hydrophilic and hydrophobic regions on both ends
  2. Two hydrophilic heads and one hydrophobic tail
  3. The ability to bond covalently with both water and lipids simultaneously
  4. A hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails
Show the answer

Answer: D. Amphipathic means a molecule has both hydrophilic and hydrophobic regions. In phospholipids, the phosphate-containing head is polar/hydrophilic and faces the aqueous environment, while the two fatty acid tails are nonpolar/hydrophobic and face the interior of the bilayer.

Question 2. The fluid mosaic model describes the cell membrane as:

  1. A rigid, static layer of phospholipids with proteins fixed on the surface
  2. A phospholipid bilayer with embedded proteins that can move laterally within the plane of the membrane
  3. A solid barrier composed entirely of proteins with lipid molecules trapped inside
  4. A single layer of phospholipids with proteins attached only to the inner surface
Show the answer

Answer: B. The fluid mosaic model (Singer and Nicolson, 1972) describes the membrane as a dynamic phospholipid bilayer in which both lipids and proteins can move laterally ('fluid'), with diverse proteins embedded throughout ('mosaic'). This fluidity is essential for membrane function.

Question 3. A red blood cell placed in distilled water will:

  1. Remain unchanged because the membrane is impermeable to water
  2. Swell and eventually burst (haemolyse) because water enters by osmosis — the cytoplasm has a higher solute concentration than distilled water
  3. Actively pump water out to maintain its volume
  4. Shrink (crenate) because water leaves the cell by osmosis
Show the answer

Answer: B. Distilled water has zero solutes — it is hypotonic relative to the RBC cytoplasm. Water enters the cell by osmosis, moving from lower solute concentration (outside) to higher solute concentration (inside). The cell swells until the membrane ruptures (haemolysis).

Linking questions

Questions that connect B2.1 to other parts of the course, the kind that come up in Paper 2.

Practise B2.1

52 quiz questions10 data questions9 exam questionsmarkschemes included

Study notes, every question and full markschemes for B2.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.

Practise B2.1 in the app Revision slides