Explain how membrane proteins facilitate the transport of glucose into a cell that lacks glucose transporters.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Glucose is a polar, hydrophilic molecule that cannot diffuse freely across the phospholipid bilayer. Some cells rely entirely on protein-mediated transport to obtain glucose from the extracellular fluid.
Model answer (4 marks)
Glucose cannot cross the hydrophobic core of the phospholipid bilayer by simple diffusion because it is polar and hydrophilic.
Cells lacking glucose transporters use integral membrane proteins – either channel proteins or carrier proteins – to provide a hydrophilic pathway through the membrane.
Facilitated diffusion via a carrier protein involves the protein binding glucose on one side of the membrane, changing conformation, and releasing it on the other side.
Glucose moves down its concentration gradient (from high to low concentration) without ATP expenditure, until equilibrium is reached.
Cells lacking glucose transporters use integral membrane proteins – either channel proteins or carrier proteins – to provide a hydrophilic pathway through the membrane.
Facilitated diffusion via a carrier protein involves the protein binding glucose on one side of the membrane, changing conformation, and releasing it on the other side.
Glucose moves down its concentration gradient (from high to low concentration) without ATP expenditure, until equilibrium is reached.
Examiner tips
- Use the word ‘facilitated diffusion’ and mention the protein type (carrier or channel).
- Show the step of conformational change and that no ATP is used.
- State the direction of movement is down the concentration gradient.
- Mention equilibrium is reached.
Mark scheme (4 marks)
- Glucose cannot cross the hydrophobic core of the phospholipid bilayer by simple diffusion because it is polar/hydrophilic.
- Channel proteins or carrier proteins (intrinsic/integral membrane proteins) provide a hydrophilic pathway through the membrane.
- Facilitated diffusion via carrier proteins involves a conformational change in the protein that transfers glucose from one side of the membrane to the other.
- Net movement of glucose occurs down its concentration gradient (from high to low concentration) without the expenditure of ATP, until equilibrium is reached.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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