The cell membrane is described as a selectively permeable barrier. Explain how the cell membrane controls the movement of substances into and out of the cell, including 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).

A student places a plant cell into a solution that has a lower water concentration than the cell's cytoplasm. The cell membrane plays a key role in determining what enters and leaves the cell.

Model answer (5 marks)

The cell membrane is selectively permeable, so it only allows certain substances to pass.
1. Diffusion: substances move from high to low concentration across the membrane.
2. Osmosis: water moves from high to low water concentration across a selectively permeable membrane.
3. In the given situation, the external solution has lower water concentration, so water leaves the cell by osmosis.
4. Active transport: substances are moved against a concentration gradient (low to high) and requires energy (ATP).

Examiner tips

  • Use the word ‘selectively permeable’ first. Explain each process (diffusion, osmosis, active transport) with direction of movement. Link the scenario to osmosis. Show that active transport requires energy and moves against a gradient.

Common mistakes

  • Confusing diffusion with osmosis. Forgetting that active transport needs ATP. Not stating the direction of water movement in the scenario.

Mark scheme (5 marks)

  1. The cell membrane is selectively permeable, meaning it only allows certain substances to pass through
  2. Diffusion moves substances from an area of high concentration to an area of low concentration across the membrane (down a concentration gradient)
  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. In the scenario described, water would leave the cell by osmosis because the external solution has a lower water concentration than the cytoplasm
  5. Active transport moves substances against a concentration gradient (from low to high concentration) and requires energy

Key terms in this question

cell membrane · selectively permeable · diffusion · osmosis · active transport

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