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Aquaporins are membrane proteins that

  • A.pump sodium out of the cell
  • B.bind hormones at the surface
  • C.anchor the cytoskeleton to the membrane
  • D.allow water to cross rapidly

Answer: D

They form narrow channels selective for water and exclude ions, which is why they speed osmosis without dissipating the cell's electrical gradients. Kidney tubule cells are especially rich in them, which is how urine is concentrated.

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