Why Do Aquaporins Allow Water But Not Protons to Cross Membranes?

💡 The Answer

• Aquaporins form narrow pores that permit single-file water passage.
• A conserved NPA motif creates a positive electrostatic barrier that repels protons.
• Water molecules flip orientation as they move through the channel.
• This prevents proton leakage, which would disrupt cellular pH gradients.
• Defective aquaporins cause kidney and eye diseases.