[Campbell Biology P.181] Aquaporins are crucial for rapid water transport across cell membra... | Practice Question

Aquaporins are crucial for rapid water transport across cell membranes. Based on the text, aquaporins exclude hydronium ions H3O+ but allow the passage of glycerol a three-carbon alcohol and water H2O , even though H3O+ is closer in size to water than glycerol is. What is the most probable basis for this selective permeability?

  • A: Aquaporins primarily select molecules based on their exact molecular size, allowing only molecules smaller than H3O+.
  • B: The hydrophobic interior of the aquaporin channel repels charged ions like H3O+, while allowing uncharged polar molecules like water and glycerol.
  • C: Aquaporins have a specific binding site for water and glycerol, but not for H3O+.
  • D: The selectivity is primarily due to the energy investment required for H3O+ transport, which aquaporins do not provide.

Explanation

The text implies that the hydrophobic interior of the lipid bilayer impedes charged molecules. Aquaporins, despite forming a hydrophilic channel, are highly selective. The exclusion of charged ions like H3O+, while allowing uncharged polar molecules water, glycerol even if they are larger, suggests that the channel's interior properties are designed to repel charges, possibly through specific amino acid residues lining the pore that interact unfavorably with charged species, ensuring only uncharged polar molecules can pass.