Question

Explain how aquaporin is selective for water and does not allow ions to diffuse through.

Answer 1

Aquaporins allow the transport of water rapidly between the cells through the channels which are also called water channels. Aquaporins are integral membrane proteins which forms pores in the membrane of the cells. Aquaporins regulate the volume and the osmotic pressure of the cells. In some cases, aqaporins allow small solutes to move across the membrane. The water pores are impermeable to ions. Polar and charged molecules require special pathways for the transport across the membrane. The hydrophobic tails of lipid molecules create energetic barrier which is against their diffusion. Aquaporin acts as passive pores allowing permeation of water and small substrates. The pore size is not so large to accomodate more than one water molecule, thus water is transferred in a single file. Based on the hydrophobicity of the aquaporin amino acid sequences, each monomer aqaporin folds to form a water channel. The water molecules orient themselves in the electric field having less electrostatic repulsion, thus passes through the narrow channel. While, the charged ions are stopped and repelled by the positive charge formed at the center of the channel. Hence, aquaporin doesnot allow ions to pass through the membrane.