What is the importance of aquaporin membrane channels?
There are a dozen genes encoding aquaporins in humans, each channel having its particular location in the body. Genetic deficiency resulting in a lack of one of the aquaporins can have significant consequences. For example, one of the aquaporins occurs in the part of the kidney where water is removed from the developing urine and returned to the blood. When this aquaporin is absent, there is more water in urine and the person urinates more and more often; bedwetting is a common result. In plants, aquaporins occur in the cell membrane and tonoplast (membrane that encloses the vacuole). In the cell membranes of root cells, aquaporins form channels for water on its way from the soil to the interior tissues of the root, from which it is transported to the rest of the plant. In the vacuole, aquaporin channels are important in the organelle swelling, resulting in turgor pressure, which is important in plant organ structure and plant cell expansion. All of these aquaporins, and those of other organisms, share a common structure that spans the cell membrane and has a channel through which water molecules pass in single file.
Future directions
A fascinating role for aquaporins is not just in the transport of water across membranes, but in the accompanying transport of substances dissolved in it. For example, in plants boron is an important nutrient, and it does not diffuse efficiently across membranes. Soil water containing boron ions passes readily into plants through aquaporin channels. Even gases such as CO2 may pass in and out of cells via aquaporins while dissolved in water. Efforts are under way to insert aquaporins into synthetic membranes used for industrial applications. Because aquaporins allow only water and low concentrations of ions to pass through, such membranes could be used to purify contaminated fresh water, or to desalinate water, which as you know has very high ion concentrations. Aquaporins are expressed in skin, where they play a role in keeping the skin moist. This is also a goal of some cosmetics (moisturizers) and has led to a search for, and patenting of, small molecules that stimulate aquaporins. These molecules are components of some widely marketed skin creams.