Water is the most essential molecule of life. Because of water’s physical properties, living organisms depend on it to survive. Water’s neutral pH and polarity make it a universal solvent for many of the solutes that life depends on, such as essential ions, some vitamins, and sugars that supply energy. Although living organisms are largely dependant on water to survive, water is not readily accessible to all parts of organisms. Due to the nonpolar region of cell membranes, water may have difficulty passing through those membranes to dissolve the essential nutrients needed for life’s processes. Fortunately, cell membranes come equipped with proteins called aquaporins that help water pass through cell membranes and help organisms carry out life’s processes.
Cell membranes are largely made up of a group of lipids called phospholipids. Phospholipids consist of a polar (hydrophilic) “head” group where the phosphate group is located and two nonpolar (hydrophobic) fatty acid “tail” groups. The fatty acid tails are typically made up of long hydrocarbon chains. Due to this ampipathic nature of phospholipids, the head groups align themselves with other head groups and the tail groups align themselves with other tail groups. This provides the most energetically favorable orientation. In most cell membranes, there are two layers of phospholipids. The head groups are orientated in the inside and outside of the membrane, leaving the tail groups to form a “core” within the membrane. Because of this hydrophobic core, simple diffusion of water molecules through the membrane can be difficult.
Because of the difficulty of simple diffusion of water molecules across cell membranes, a group of proteins, called aquaporins, aid in the movement of water across membranes. Aquaporins consist of a series of six alpha helices from the same polypeptide chain. These alpha helices gather around to form the shape of a tunnel that goes through the membrane. This tunnel does not allow all substances to pass through; just water. This is because of a cluster of amino acids inside the tunnel called the ar/R (aromatic/arginine) filter. These amino acids select for water by binding to the individual molecules and excluding anything else, especially ions that may disturb the osmosis of the cell. These amino acids are often found in the narrowest part of the tunnel, which allows them to select individual water molecules to bind with the positively charged argenine amino acids.
A gating mechanism of aquaporins is also discovered in plant cells. The gating of aquaporins is necessary to keep cells hydrated during periods of drought. The mechanism in some aquaporins involves the dephosphorylation of certain serine residues, causing the protein to change shape. This change in shape causes the tunnel of the protein to close and not allow any water to pass through.
Aquaporins are necessary for water transport in plants, but they can also be found in human cells. Although most of the aquaporins discovered in humans were found in the kidneys, the existence of more of these proteins is expected. Aquaporins can be used to treat some human medical disorders, such as brain edema after stroke and dry eye syndrome. One of the causes of brain edema is by the swelling of damage tissue in the brain after a stroke. The swelling can be a result of an excess accumulation of water in the intracellular or extracellular spaces of the brain. Dry eye syndrome can be caused by the inability of the lacrimal gland to produce the tears needed to aid in eye lubrication. Further study of aquaporins may help scientists discover ways to increase or decrease the flow of water through the protein and help treat these disorders.
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