Answer: Countertransport
Explanation:
Cotransport is a type of active transport of substances through a biological membrane. It is characterized by the simultaneous transport of two or more substances, hence its name, and is common in small organic molecules, such as glucose or amino acids, especially in epithelia of absorption and secretion, such as the small intestine and renal tubules. The cotransport can be of two types, simporte or antiport.
In both cases, one of the substances moves in favor of its gradient and the other against it. In electrochemical gradients there is potential energy stored and when a substance moves in favor of its gradient this energy is released.
In contransport, the transmembrane protein responsible for co-transportation, is able to use the energy stored in the electrochemical gradient of a substance to move the other substance against its gradient. The electrochemical gradient from which the energy for transport comes is maintained by ATPases, which consume ATP. Since the co-transporter does not directly consume ATP, the co-transport is classified as secondary active transport, while the ATPases perform primary active transport. So, ATPases consume energy to create a gradient and then co-carriers use the energy from the gradient to transport another substance.
In the case of Countertransport, the cell transports two types of molecules in the opposite direction across the membrane. Countertransport is a type of antiport exchange. The key point regarding countertransport is that it transports ions or molecules in opposite directions. Thus, one molecule leaves the cell while the other enters at the same time through the membrane. In addition, this is a type of secondary active transport that uses electrochemical gradient to drive the movements (as it was explained before). Countertransport can mediate the exchange of the same solutes or different solutes. The sodium-calcium exchanger, the Na+ / H+ exchanger and the Cl- / bicarbonate exchanger are examples of countertransport.
