Explain how glucose molecules move from the small intestine into the blood, and why this movement can continue even when the concentration of glucose in the blood is 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 absorbed from the small intestine into the bloodstream. Initially, glucose moves by diffusion, but active transport is also involved in absorption.
Model answer (5 marks)
Glucose first diffuses down its concentration gradient from the lumen of the small intestine into the enterocyte and then into the bloodstream.
Diffusion occurs because, after a meal, the glucose concentration in the intestinal lumen is higher than that in the blood.
When the blood glucose concentration rises, glucose can no longer move by diffusion alone. Active transport then takes over.
Active transport uses the sodium–glucose linked transporter (SGLT) in the apical membrane of the enterocyte.
The transporter couples glucose uptake to the inward sodium gradient, which is maintained by the Na⁺/K⁺‑ATPase that hydrolyses ATP, allowing glucose to be moved against its concentration gradient into the blood.
Diffusion occurs because, after a meal, the glucose concentration in the intestinal lumen is higher than that in the blood.
When the blood glucose concentration rises, glucose can no longer move by diffusion alone. Active transport then takes over.
Active transport uses the sodium–glucose linked transporter (SGLT) in the apical membrane of the enterocyte.
The transporter couples glucose uptake to the inward sodium gradient, which is maintained by the Na⁺/K⁺‑ATPase that hydrolyses ATP, allowing glucose to be moved against its concentration gradient into the blood.
Examiner tips
- Mention both diffusion and active transport, linking each to the concentration gradient. Include the sodium–glucose linked transporter and the Na⁺/K⁺‑ATPase. Show that ATP is the energy source. Keep the answer concise and use the exact terminology.
- common_mistakes
- :
- Confusing diffusion with active transport or omitting the sodium–glucose linked transporter. Failing to state that ATP is required for the Na⁺/K⁺‑ATPase. Using vague terms like "carrier" without specifying the SGLT or the Na⁺/K⁺‑ATPase.
Mark scheme (5 marks)
- Glucose initially moves by diffusion / down a concentration gradient from the small intestine into the blood
- Diffusion occurs because the concentration of glucose in the small intestine is higher than in the blood (at first)
- Active transport is used to move glucose from the small intestine into the blood against the concentration gradient
- Active transport requires energy (from respiration / ATP)
- Active transport uses carrier proteins / protein molecules in the cell membrane to move glucose
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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