The big idea: A cotransporter moves two things at once: Na⁺ down its gradient and glucose against its gradient. The energy comes from the sodium gradient, not directly from ATP.
The points to remember
- The sodium-potassium pump uses ATP to keep the Na⁺ concentration low inside the cell.
- So Na⁺ tends to diffuse into the cell from outside.
- A sodium-glucose cotransporter lets Na⁺ in only together with glucose.
- Na⁺ moving down its gradient pulls glucose in against its gradient.
- The cotransporter uses no ATP itself: this is indirect active transport.
Remember it as: The pump pays; the cotransporter rides.
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The big idea: Two organs need to take up almost all the glucose they meet, even when there is more inside the cells than outside.
Small intestine
- Cells lining the villi absorb glucose from digested food
- Even the last traces are taken up
Kidney nephron
- Cells of the tubule reabsorb glucose from the filtrate
- Normally none is lost in urine
Real example: oral rehydration salts: Oral rehydration drinks for diarrhoea contain both salt and glucose. Sodium-glucose cotransport takes them in together, and water follows by osmosis.
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How it is asked: Expect an Explain on why the process is called indirect active transport, or data on glucose uptake with and without sodium.
IB-style questionExplain[3 marks]
Explain why the uptake of glucose by sodium-glucose cotransporters is described as indirect active transport.
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