Explain how the structure of channel proteins and carrier proteins differs and how this relates to their different roles in membrane transport.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Channel proteins form a continuous, hydrophilic pore or tunnel that spans the lipid bilayer, allowing ions or small polar molecules to pass directly through the membrane.
Carrier proteins possess a specific binding site for a particular solute; binding induces a conformational change that moves the solute across the membrane.
Because channel proteins do not change shape, they only mediate passive, facilitated diffusion down a concentration gradient.
Carrier proteins, by contrast, can undergo conformational changes to transport solutes against a gradient (active transport) or down a gradient (facilitated diffusion).
Carrier proteins possess a specific binding site for a particular solute; binding induces a conformational change that moves the solute across the membrane.
Because channel proteins do not change shape, they only mediate passive, facilitated diffusion down a concentration gradient.
Carrier proteins, by contrast, can undergo conformational changes to transport solutes against a gradient (active transport) or down a gradient (facilitated diffusion).
Examiner tips
- Mention the pore/tunnel structure of channel proteins and the binding site of carrier proteins. Explain the role of conformational change for carriers but not for channels. State that carriers can mediate active transport, channels cannot. Use terms: facilitated diffusion, active transport, concentration gradient.
Common mistakes
- Confusing the direction of transport for carriers (saying they only move down a gradient). Forgetting to mention the conformational change in carrier proteins. Using vague language like "different shapes" without linking to transport mechanism.
Mark scheme (4 marks)
- Channel proteins form a continuous, hydrophilic pore/tunnel through the membrane
- Channel proteins allow facilitated diffusion of ions or small polar molecules down their concentration gradient without a conformational change
- Carrier proteins have a binding site that binds a specific solute, causing a conformational change (change in shape) that moves the solute across the membrane
- Carrier proteins can facilitate active transport (using ATP) as well as facilitated diffusion, whereas channel proteins only allow passive transport
Key terms in this question
channel protein · carrier protein
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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