Explain how the phospholipid bilayer and membrane proteins together enable a cell to control the movement of ions across its plasma membrane.
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 the free diffusion of ions, which are charged and polar.
Ion‑selective channel proteins form hydrophilic pores that allow specific ions to move across the membrane by passive diffusion, following their concentration or electrochemical gradient.
Carrier proteins or pumps bind ions and, using ATP, actively transport them against their gradient, moving ions into regions of higher concentration.
By varying the number, type and gating of these membrane proteins, the cell can control which ions move, when they move and in what quantity, thereby regulating ion flux across the plasma membrane.
Ion‑selective channel proteins form hydrophilic pores that allow specific ions to move across the membrane by passive diffusion, following their concentration or electrochemical gradient.
Carrier proteins or pumps bind ions and, using ATP, actively transport them against their gradient, moving ions into regions of higher concentration.
By varying the number, type and gating of these membrane proteins, the cell can control which ions move, when they move and in what quantity, thereby regulating ion flux across the plasma membrane.
Examiner tips
- Use the phrase "hydrophobic core" to show understanding of the bilayer’s barrier function.
- Mention both passive (channels) and active (pumps) transport and the role of ATP.
- Explain that gating and protein regulation give the cell control over ion movement.
- Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing the bilayer with the membrane proteins; forgetting that the bilayer itself blocks ions.
- Lumping all transport into one type (e.g., only channels) and ignoring pumps.
- Using vague terms like "control" without explaining how gating or ATP use gives control.
Mark scheme (4 marks)
- The phospholipid bilayer is hydrophobic in its core / non-polar interior, which prevents free diffusion of ions (charged/polar particles) through the bilayer.
- Channel proteins / ion channels provide hydrophilic pores that allow specific ions to move across the membrane by diffusion (down their concentration gradient / electrochemical gradient).
- Carrier proteins / pumps can actively transport ions against their concentration gradient using ATP, allowing the cell to move ions to where they are in higher concentration.
- The number, type, and gating (opening/closing) of membrane proteins can be regulated, giving the cell control over which ions move and when / in what quantity.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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