# Explain how the concentration gradient of glucose across the plasma membrane of intestinal epithelial cells is maintained to allow continuous absorption by facilitated diffusion.

> IB DP Biology Standard Level (2023 syllabus) — B2.1 Membranes and membrane transport · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Glucose moves into the epithelial cell by facilitated diffusion through carrier proteins (GLUT transporters) down its concentration gradient.
2. Glucose is continuously removed from the cell into the blood (capillary) on the basolateral side, keeping intracellular glucose concentration low.
3. The sodium-potassium pump (on the basolateral membrane) actively transports sodium ions out of the cell, maintaining a low intracellular sodium concentration.
4. 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

- [facilitated diffusion](https://www.gradenine.co.uk/glossary/facilitated-diffusion)
- [concentration gradient](https://www.gradenine.co.uk/glossary/concentration-gradient)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-concentration-gradient-of-9d4bea95) · Published by Druglandscape Ltd.