Glucose is not actively reabsorbed; instead, it's reabsorbed using a secondary active transport mechanism. Here's how it works:
Secondary Active Transport of Glucose:
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Sodium-Glucose Cotransporters (SGLTs): Glucose reabsorption primarily occurs in the proximal convoluted tubule of the nephron in the kidneys. It's facilitated by SGLT proteins, which are located on the apical membrane of the epithelial cells lining the tubule.
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Coupled Transport with Sodium: SGLTs don't directly use ATP. Instead, they utilize the sodium gradient established by the Na+/K+ pump (a primary active transport system) on the basolateral membrane. Sodium ions (Na+) move down their concentration gradient into the epithelial cell, which provides the energy needed for glucose to move into the cell against its concentration gradient. This is therefore secondary active transport.
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Concentration Inside the Cell: This process increases the concentration of glucose inside the epithelial cell.
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Passive Transport out of the Cell: According to the reference provided, glucose, along with other substances, diffuses out of the epithelial cell down their concentration gradients using passive transporters. This movement occurs at the basolateral membrane on the blood-facing side of the cell and allows glucose to enter the blood capillaries.
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Reabsorption into the Blood: The reabsorbed glucose then enters the blood capillaries, preventing its loss in the urine.
Key Points:
- No Direct ATP Use: Glucose reabsorption relies on the sodium gradient, not direct ATP hydrolysis by SGLTs.
- Secondary Active: The transport of glucose is secondary to the active transport of sodium.
- Epithelial Cells: Reabsorption occurs through the epithelial cells of the proximal tubule.
- Blood Absorption: The final step involves glucose moving from these cells to the blood.
- Passive diffusion out: Glucose diffuses out of the cell down a concentration gradient through passive transporters at the basolateral membrane into the blood capillaries
| Transport Stage | Mechanism | Energy Source | Location |
|---|---|---|---|
| Sodium entry into epithelial cell with glucose | Secondary Active Transport | Na+ Gradient | Apical Membrane |
| Glucose Exit from Epithelial cell into Blood | Facilitated Diffusion | Concentration Gradient | Basolateral Membrane |
