Explain why glucose can be absorbed from the small intestine into the blood by active transport, even when the concentration of glucose in the blood is already higher than in the small intestine.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
After a meal, glucose is produced in the small intestine by the digestion of carbohydrates. The glucose must be moved from the small intestine into the bloodstream.
Model answer (4 marks)
1. Active transport moves glucose against its concentration gradient, from the dilute intestinal lumen to the more concentrated blood.
2. This process requires energy, which is supplied by ATP generated in cellular respiration.
3. Diffusion cannot be used because glucose would have to move from a lower to a higher concentration, which is thermodynamically unfavourable.
4. By using active transport, all glucose produced can be absorbed into the bloodstream, preventing loss in faeces and ensuring efficient utilisation.
2. This process requires energy, which is supplied by ATP generated in cellular respiration.
3. Diffusion cannot be used because glucose would have to move from a lower to a higher concentration, which is thermodynamically unfavourable.
4. By using active transport, all glucose produced can be absorbed into the bloodstream, preventing loss in faeces and ensuring efficient utilisation.
Examiner tips
- Use the phrase "against its concentration gradient" to show understanding of active transport.
- Mention ATP or cellular respiration as the energy source.
- Explain why diffusion is unsuitable – glucose would move against the gradient.
- Show the benefit – complete absorption of glucose.
Common mistakes
- Confusing passive diffusion with active transport.
- Forgetting to mention the energy requirement (ATP).
- Claiming diffusion can move glucose against the gradient.
Mark scheme (4 marks)
- Active transport moves substances against the concentration gradient / from a more dilute to a more concentrated solution
- Active transport requires energy from respiration
- Diffusion cannot be used because glucose would need to move against its concentration gradient
- This allows all available glucose to be absorbed / ensures no glucose is wasted / lost in faeces
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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