It must work against the gradient.
is the biological equivalent of swimming upstream. It requires energy (usually in the form of ATP) and specialized protein pumps to move molecules against their concentration gradient—from low to high concentration. examples of active transport
In the microscopic world of cells, nature has a preference for moving things from high to low concentration (a process called passive transport ). But what happens when a cell desperately needs a substance that is scarce on its side but abundant on the other? It must work against the gradient