Cotransporter

Cotransporters are a subcategory of membrane transport proteins (transporters) that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient. They enable cotransport (secondary active transport) and include antiporters and symporters. In general, cotransporters consist of two out of the three classes of integral membrane proteins known as transporters that move molecules and ions across biomembranes. Uniporters are also transporters but move only one type of molecule down its concentration gradient and are not classified as cotransporters.

Cotransporters are capable of moving solutes either up or down gradients at rates of 1000 to 100000 molecules per second. They may act as channels or transporters, depending on conditions under which they are assayed. The movement occurs by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient. Some studies show that cotransporters can function as ion channels, contradicting the classical models. For instance the wheat HKT1 transporter shows two modes of transport by the same protein.

Cotransporters can be classified as antiporters and symporters. Both utilize electric potential and/or chemical gradients to move protons and ions against their concentration gradient. In plants the proton is considered a secondary substance and high proton concentration in the apoplast powers the inward movement of certain ions by symporters. A Proton gradient moves the ions into the vacuole by proton-sodium antiporter or the proton-calcium antiporter. In plants, sucrose transport is distributed throughout the plant by the proton-pump where the pump, as discussed above, creates a gradient of protons so that there are many more on one side of the membrane than the other. As the protons diffuse back across the membrane, the free energy liberated by this diffusion is utilized to co-transport sucrose. In mammals, glucose is transported through sodium dependent glucose transporters, which use energy in this process. Here, since both glucose and sodium are transported in the same direction across the membrane, they would be classified as symporters. The glucose transporter system was first hypothesized by Dr. Robert K. Crane in 1960, this is discussed later in the article.

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