Explain how the cell membrane controls the movement of substances into and out of a cell. In your answer, include reference to diffusion, osmosis and active transport.

Eduqas A-Level Biology — 1.3 Cell membranes and transport · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Red blood cells are surrounded by blood plasma. The plasma contains glucose, oxygen, carbon dioxide and various salts at different concentrations to those inside the red blood cells.

Model answer (5 marks)

The cell membrane is selectively permeable, allowing only certain substances to pass through.

Diffusion is the movement of substances from a region of high concentration to a region of low concentration across the membrane.

Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration across a selectively permeable membrane.

Active transport moves substances from a region of low concentration to a region of high concentration (against the concentration gradient).

Active transport requires energy (from respiration), whereas diffusion and osmosis do not.

Examiner tips

  • Use the word ‘selectively permeable’ early to show understanding of membrane control.
  • Show each transport type with its direction and energy requirement.
  • Mention that diffusion and osmosis are passive, active transport is active.
  • Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Confusing diffusion with osmosis or vice‑versa.
  • Forgetting that active transport needs energy.
  • Using vague phrases like ‘passive transport’ without specifying diffusion or osmosis.

Mark scheme (5 marks)

  1. The cell membrane is selectively permeable, allowing only certain substances to pass through
  2. Diffusion is the movement of substances from a region of high concentration to a region of low concentration across the membrane
  3. Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration across a selectively permeable membrane
  4. Active transport moves substances from a region of low concentration to a region of high concentration (against the concentration gradient)
  5. Active transport requires energy (from respiration), whereas diffusion and osmosis do not

Key terms in this question

cell membrane · diffusion · osmosis · active transport

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