Sodium-glucose cotransport uses the movement of sodium ions down their electrochemical gradient to move glucose into epithelial cells against its concentration gradient. It is indirect active transport and operates in both the small intestine and the proximal convoluted tubule of the kidney.
The mechanism
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A sodiumtopotassium pump in the basolateral membrane uses ATP to pump sodium ions out of the epithelial cell and potassium ions into it. This maintains a low sodium ion concentration inside the cell.
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Sodium ions therefore have an electrochemical gradient from the lumen or filtrate into the cell. In the apical membrane, a sodium-dependent glucose cotransporter binds sodium ions and glucose.
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The cotransporter changes conformation and moves both substances into the epithelial cell. Sodium ions move down their electrochemical gradient, providing the energy needed to move glucose against its concentration gradient.
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Glucose then leaves through a glucose carrier in the basolateral membrane by facilitated diffusion and enters tissue fluid and nearby capillaries. Sodium ions are pumped out again, maintaining the gradient.
| Location | Biological role |
|---|---|
| Small-intestine epithelium | Glucose is absorbed from the intestinal lumen into epithelial cells and then transported into the blood. |
| Proximal convoluted tubule of the nephron | Glucose is reabsorbed from the filtrate into epithelial cells and returned to the blood, preventing its loss in urine under normal conditions. |
A common misconception is that the cotransporter itself hydrolyses ATP. It does not: ATP is used by the sodiumpotassium pump, while the cotransporter uses energy stored in the sodium electrochemical gradient. This is why the process is described as indirect rather than direct active transport.
Exam technique
This is AHL content in B2.1. In an explain question, link ATP use to the sodium gradient, cotransport into the cell, and glucose movement into the blood. Always identify the apical and basolateral membranes and avoid stating that glucose enters by simple diffusion.