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Aquaporins are integral proteins of the plasma membrane whose role is to allow the passage of water molecules down its concentration gradient.

Recent observations have shown an interesting arrangement of charged amino acid residues along the inner surface of the channel, resulting in the entry of water via its oxygen then flipping to lead with its hydrogens half way through the channel.

How might this explain aquaporin’s specificity for water and not cations or protons?


Cations such as $K^+$ and $Na^+$ would be repelled at the opening of the channel.


Water gains access because it is electrically neutral.


Protons cannot cross since they do not interact with water.


Water, being a small polar molecule, is able to interact with both negative and positive residues in the channel.

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