What are aquaporins and their function in cells?

Aquaporins are proteins that form channels in cell membranes, allowing water molecules to move in and out of cells.

Aquaporins, also known as water channels, are integral membrane proteins from a larger family of major intrinsic proteins (MIP) that form pores in the membrane of biological cells. They are a vital component of cells, particularly in relation to osmosis and homeostasis. Their primary function is to facilitate the transport of water between cells.

Water molecules normally cross cell membranes slowly because they are polar molecules, which means they have a slight electrical charge. This charge can cause them to be repelled by the middle layer of the cell membrane, which is made of fatty acid molecules. However, aquaporins provide a pathway through which water can move more quickly across the cell membrane.

Aquaporins are selective for water, meaning that they generally prevent ions and other small molecules from crossing the membrane along with water. This selectivity is due to the specific structure of the aquaporin channels. Each aquaporin molecule has a narrow central channel that water molecules can pass through in single file. The walls of this channel are made of amino acids that interact favourably with water, helping to draw the water molecules through.

The presence and abundance of aquaporins in a cell membrane can affect the cell's water permeability and therefore its ability to absorb and release water. This can be particularly important in cells that need to rapidly adjust their volume, such as red blood cells, or in cells that are involved in water transport, such as kidney cells.

In summary, aquaporins play a crucial role in maintaining the water balance in cells, allowing them to function properly. They are a fascinating example of how the structure of a protein can be perfectly suited to its function.