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In this process, the substances are transported with energy consumption and may occur from the lowest to the highest concentration (against the gradient of concentration).
This gradient can be chemical or electrical, as in ion transport. Active transport acts as a “revolving door”. The molecule to be transported binds to the carrier molecule (membrane protein) as an enzyme binds to the substrate. The carrier molecule spins and releases the charged molecule on the other side of the membrane.
Turns again to the starting position. THE sodium and potassium pump binds on an ion Na+ on the inner face of the membrane and releases it on the outer face. There, it binds to a K ion+ and releases it on the inner face. Energy for active transport comes from hydrolysis of ATP.
Many membranes hitchhike with other substances or ions to get in or out of cells using the same “transport vehicle.” That's what happens, for example, with sugar molecules that get into cells against their concentration gradient. In the previous item, the sodium / potassium pump expels sodium ions from the cell, at the same time that it enters the potassium ions, using the same carrier protein (the same ion channel), with energy expenditure. of sodium within the cell becomes low, which induces these ions to return to the cell interior.
At the same time, sugar molecules, whose concentration within the cell is high, take advantage of sodium intake and "accompany" it to the intracellular environment.
This simultaneous transport occurs with the participation of a membrane protein. “Co-carrier” which, while favoring the return of sodium ions to the cell, also lets in sugar molecules whose concentration in the cell is high.
Note that the energy used in this type of transport is indirectly from that generated in the active transport of sodium / potassium ions.