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Learn9700Paper 2 · AS Level Structured QuestionsCell membranes and transportTransport across membranes
Cell membranes and transport · Subtopic 2 of 2

Transport Across Membranes

Diffusion: net movement down a concentration gradient (passive). Facilitated diffusion: polar molecules and ions through channel or carrier proteins (passive). Osmosis: net movement of water from higher to lower water potential (ψ) across a partially permeable membrane; pure water ψ = 0, solutions negative. Plant cells: ψ=ψs+ψp\psi=\psi_s+\psi_p; in concentrated solutions they become plasmolysed; in water, turgid. Animal cells burst (lysis) or shrink (crenation). Active transport moves substances against a gradient using ATP and carrier proteins; co-transport (e.g. Na⁺ and glucose in the ileum) uses a gradient set up by active transport. Endocytosis/exocytosis move bulk material in vesicles. Rate of diffusion increases with surface area, concentration gradient and temperature, and decreases with distance.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI11AQA 7402 Paper 1≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe passive and active transport mechanisms.
  • Use water potential to explain osmosis.
  • Explain co-transport.
  • Relate SA:V ratio to diffusion.
Pause and think

Why do red blood cells burst in pure water but plant cells don't?

Section 2 of 14

Core idea

Start hereSalt and slugs

Sprinkle salt on a slug and it shrivels. Water leaves its cells by osmosis.

The big idea

Passive transport goes down gradients; active transport uses ATP to go against them. Water follows water potential.

Water potential of a plant cell
  1. ψ=ψs+ψp\psi=\psi_s+\psi_p
    Solute and pressure components.
  2. ψ=−800+300\psi=-800+300
    ψs negative, ψp positive in a turgid cell.
  3. ψ=−500 kPa\psi=-500\ \text{kPa}
    Add.
Ways across a membrane

Choose by gradient and molecule.

Worked exampleAgar cubes

SA : V for cubes of side 1 mm and 4 mm?

Goal
Ratios.
Strategy
6/x.
  1. 6/1=6 mm−16/1=6\ \text{mm}^{-1}
    Small cube.
  2. 6/4=1.5 mm−16/4=1.5\ \text{mm}^{-1}
    Large cube: slower to colour throughout.
Answer6 and 1.5 mm⁻¹.
✓ CheckLarger cube, smaller ratio ✓.

Summary

  • ψ from high to low.
  • Active transport needs ATP.
  • Co-transport uses Na⁺ gradient.
Key words
Water potential
Tendency of water to move; pure water = 0 kPa.
Co-transport
Movement of one substance coupled to another moving down its gradient.
HistoryOsmosis named. Jean-Antoine Nollet observed osmosis in 1748 using a pig bladder stretched over a jar of alcohol.
Fun factSalting slugs. Salt lowers the water potential on a slug's skin, drawing water out by osmosis.
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