Explain how the structure of the cell surface membrane is related to its function in controlling the movement of substances into and out of the cell.

IB DP Biology Standard Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The phospholipid bilayer has a hydrophobic core that blocks water‑soluble or charged molecules, so only lipophilic substances can diffuse freely.

Integral proteins (channels or carriers) span the bilayer and provide specific routes for ions or polar molecules that cannot cross the lipid core.

Because the bilayer is fluid, these proteins can move laterally, allowing the cell to regulate transport dynamically.

Glycoproteins and glycolipids on the outer surface act as receptors, recognising specific molecules and triggering selective uptake such as receptor‑mediated endocytosis.

Examiner tips

  • Use the exact terms – phospholipid bilayer, hydrophobic core, integral proteins, fluid‑mosaic model, glycoproteins/glycolipids.
  • Show the link between structure (hydrophobic core) and function (selective permeability).
  • Mention both passive (diffusion) and active (carrier/channel) transport.
  • Include receptor‑mediated uptake to cover all four points.

Common mistakes

  • Confusing the bilayer with the cytoskeleton; the bilayer is the barrier, not the scaffold.
  • Omitting the fluid‑mosaic model or the role of lateral protein movement.
  • Using vague terms like ‘membrane proteins’ without specifying channels or carriers.

Mark scheme (4 marks)

  1. The phospholipid bilayer forms a hydrophobic core/barrier that restricts the passage of water-soluble (polar/charged) molecules or ions.
  2. Intrinsic/integral (channel or carrier) proteins span the bilayer and allow specific substances to cross that cannot pass through the phospholipid bilayer directly.
  3. The fluid/flexible nature of the bilayer (fluid-mosaic model) allows proteins to move laterally, enabling dynamic regulation of transport.
  4. Glycoproteins / glycolipids act as receptor or recognition molecules on the outer surface, enabling cell signalling or selective uptake via receptor-mediated processes such as endocytosis.

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