Explain how the concentration gradient of glucose across the plasma membrane of intestinal epithelial cells is maintained to allow continuous absorption by facilitated diffusion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Intestinal epithelial cells absorb glucose from the lumen of the small intestine. Glucose moves into these cells down its concentration gradient through carrier proteins, and is then transported into the blood on the other side of the cell.
Model answer (4 marks)
1. Glucose enters the intestinal epithelial cell by facilitated diffusion through GLUT transporters on the apical membrane, moving down its concentration gradient.
2. Inside the cell glucose is immediately transported into the blood across the basolateral membrane, keeping the intracellular glucose concentration low.
3. The sodium‑potassium pump on the basolateral membrane actively exports Na⁺, maintaining a low intracellular Na⁺ concentration.
4. The low intracellular Na⁺ drives Na⁺‑glucose co‑transport (SGLT) on the apical membrane, coupling Na⁺ movement down its gradient to glucose uptake against its gradient, so the lumen always has a higher glucose concentration than the cell.
2. Inside the cell glucose is immediately transported into the blood across the basolateral membrane, keeping the intracellular glucose concentration low.
3. The sodium‑potassium pump on the basolateral membrane actively exports Na⁺, maintaining a low intracellular Na⁺ concentration.
4. The low intracellular Na⁺ drives Na⁺‑glucose co‑transport (SGLT) on the apical membrane, coupling Na⁺ movement down its gradient to glucose uptake against its gradient, so the lumen always has a higher glucose concentration than the cell.
Examiner tips
- Use the exact terms GLUT, SGLT, basolateral, apical; show the sequence of transport steps; link the pump to Na⁺ gradient and co‑transport; keep answer concise and to the point.
Common mistakes
- Confusing facilitated diffusion with active transport for the apical step; not mentioning the basolateral removal of glucose; omitting the role of the Na⁺‑K⁺ pump in maintaining the Na⁺ gradient.
Mark scheme (4 marks)
- Glucose moves into the epithelial cell by facilitated diffusion through carrier proteins (GLUT transporters) down its concentration gradient.
- Glucose is continuously removed from the cell into the blood (capillary) on the basolateral side, keeping intracellular glucose concentration low.
- The sodium-potassium pump (on the basolateral membrane) actively transports sodium ions out of the cell, maintaining a low intracellular sodium concentration.
- The low intracellular sodium concentration drives sodium-glucose co-transport (SGLT) on the apical membrane, coupling sodium movement down its gradient to glucose uptake against its concentration gradient when needed, so the lumen always has a higher glucose concentration relative to the cell.
Key terms in this question
facilitated diffusion · concentration gradient
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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