To this point, the transport processes we have discussed have all been passive processes in which the solute, or the water, movement has been down a concentration gradient with no input of energy required. However, there are times when it is important for the cell to be able to move solutes against their concentration gradient. Just like moving water from the first to the top floor of a high-rise building, these processes require an energy source. Processes that require energy are called active transport processes.