Apentix/AP Biology/Unit 2: Cell Structure and Function/Question
AP Biologyeasymcq1 pt

Water crosses the membranes of kidney tubule cells far faster than simple diffusion through lipids allows. Which explanation accounts for the speed?

A.Aquaporin channels provide polar passageways through which water moves down its gradient without ATP
B.The sodium-potassium pump drags water along with the sodium it exports
C.Kidney cells build unusually thin membranes that water penetrates easily
D.A water-specific ATPase pumps water inward against its gradient

Explanation

Core Concept

Aquaporins are channel proteins whose narrow interiors line water molecules up and pass them in single file along the osmotic gradient. Because the channel is passive, flux rises dramatically while the cell spends no energy. The pore's constricted, charged throat excludes ions such as hydronium, so the high throughput stays specific to water.

Correct Answer

AAquaporin channels provide polar passageways through which water moves down its gradient without ATP

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