The cell membrane is described as selectively permeable. Explain how the cell membrane controls the movement of substances into and out of the cell, and why this selective permeability is important for the cell.

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).

Model answer (5 marks)

The cell membrane is a selectively permeable barrier.
1. It allows essential substances such as glucose and ions to enter the cell by facilitated diffusion or active transport.
2. It prevents the loss of essential molecules and the entry of harmful substances by restricting their passage.
3. It contains receptor proteins that bind signalling molecules, triggering transport processes.
4. Thus selective permeability maintains the cell’s internal environment and protects against toxins, keeping the cell functional.

Examiner tips

  • Use the phrase ‘selectively permeable barrier’ first. Mention both passive and active transport. Include receptor proteins for signalling. Show why it keeps internal conditions stable.

Common mistakes

  • Confusing ‘selectively permeable’ with ‘impermeable’. Forgetting to mention receptor proteins or signalling. Lacking explanation of why it is important.

Mark scheme (5 marks)

  1. The cell membrane acts as a selectively permeable barrier
  2. The cell membrane controls the movement of substances into the cell
  3. The cell membrane controls the movement of substances out of the cell
  4. The cell membrane has receptor molecules on its surface for cell-to-cell signalling
  5. Selective permeability is important because it maintains the correct internal conditions / prevents harmful substances from entering or essential substances from being lost

Key terms in this question

cell membrane · selectively permeable

Related

More Cell membranes and transport questions

▶ Try answering this question with AI marking (free) →