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Glucose appears in the urine when blood glucose is very high because:
AThe kidney stops filtering blood entirely
BCarrier proteins for glucose are finite
CGlucose cannot dissolve in urine at all
DChannel proteins reject glucose always
Answer & Solution
Correct answer: B. Carrier proteins for glucose are finite
1. Glucose is filtered in the kidney and then reabsorbed in another part of it.
2. Reabsorption depends on carrier proteins that bind glucose one molecule at a time.
3. There are only a finite number of carrier proteins for glucose.
4. If more glucose is present than the proteins can handle, the excess is not transported and it is excreted in the urine.
5. Filtration itself does not stop, so option A misplaces the bottleneck.
_Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 5 'Structure and Function of Plasma Membranes', sections 5.1-5.4_
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