Aquaporins

The lipid bilayer that surrounds most cells is an integral part of how life developed. As the name suggests, a bilayer is composed of two different layers of molecules, in this phospholipids. The phopholipid has polar phosphate head and a nonpolar fatty acid tail. The two phosphate heads are pointed toward each other on the inside of the membrane and the fatty acid tails point outwards toward the extracellular environment and inward towards the intracellular matrix. However, this makes it difficult for polar molecules to cross the boundary. Special transmembrane transport proteins are needed to transport different molecules. In the case of water, a critical molecule for life and one that is polar, it falls to the aquaporins to be the transport molecule. Water transports across membranes through osmosis, the diffusive transport of water, but due to the high permeability of epithelial cells, it was suspected that there were additional molecules helping in the transport of water. Some cells leak water and water still moves by osmosis, but cells with aquaporins transport water much more rapidly. Aquaporins are selective in their transport of water molecules in and out of the cell, and do not allow for the passage of other solutes and ions.