The cell membrane is described as selectively permeable and controls the movement of substances into and out of the cell. Explain how the structure and properties of the cell membrane allow it to carry out this function, and describe two ways in which substances can move across the cell membrane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cells need to take in useful substances such as glucose and oxygen, and remove waste products such as carbon dioxide. The cell membrane plays a key role in controlling this movement.
Model answer (5 marks)
The cell membrane is a phospholipid bilayer with embedded proteins, giving it selective permeability. The hydrophilic heads face the aqueous surroundings while the hydrophobic tails form an interior that blocks most ions and large molecules, allowing only small, non‑polar substances to diffuse freely.
Receptor proteins on the surface bind specific signalling molecules, creating a controlled response that can open or close transport channels.
Substances cross the membrane mainly by:
1. Diffusion – small, non‑polar molecules move down their concentration gradient.
2. Osmosis – water moves across the bilayer from high to low water concentration.
3. Active transport – energy‑dependent pumps move ions against their gradient.
Receptor proteins on the surface bind specific signalling molecules, creating a controlled response that can open or close transport channels.
Substances cross the membrane mainly by:
1. Diffusion – small, non‑polar molecules move down their concentration gradient.
2. Osmosis – water moves across the bilayer from high to low water concentration.
3. Active transport – energy‑dependent pumps move ions against their gradient.
Examiner tips
- Show the bilayer structure and explain why it blocks ions; mention receptor proteins for signalling; list diffusion, osmosis and active transport as the three main mechanisms; keep answer concise to fit 5 marks.
Common mistakes
- Confusing diffusion with osmosis; not recognising the role of the phospholipid bilayer; over‑describing the membrane without linking to selective permeability.
Mark scheme (5 marks)
- The cell membrane is selectively permeable, meaning it only allows certain substances to pass through
- The cell membrane has receptor molecules on its surface for cell-to-cell signalling
- Diffusion — substances move from an area of high concentration to an area of low concentration across the membrane
- Osmosis — water moves across the selectively permeable membrane from a region of high water concentration to a region of low water concentration
- Active transport — substances are moved against the concentration gradient (from low to high concentration) using energy
Key terms in this question
cell membrane · selectively permeable
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Cell membranes and transport questions
- A student places pieces of fresh liver into solutions of different pH values and…
- Red blood cells need glucose for cellular respiration. Explain how glucose moves…
- Osmosis is an important process that occurs across cell membranes. Explain what …
- Explain how a rise in temperature above an enzyme's optimum affects the rate of …