In order to control its internal environment, a cell must often expend energy to bring substances into or out of the cell. Energy is required in active transport, processes in which a cell moves a substance across a membrane from a region of lower concentration to a region of higher concentration. In other words, the cell moves the substance against its concentration gradient. In contrast, passive transport (not shown here) occurs when a substance moves from a region of higher concentration to a region of lower concentration; the substance rushes into or out of the cell by diffusion and requires no input of energy to do so.
When a cell expends ATP directly during active transport, the process is called primary active transport. Using another energy source, such as the potential energy stored in an ion gradient, is secondary active transport.
When a cell transports a substance against its concentration gradient, the cell must expend energy. The direct use of ATP in fueling transport across a membrane is primary active transport. The sodium-potassium pump that is found in all animal cells provides an example of primary active transport. The pump delivers sodium and potassium ions across the membrane, pumping both ions against their concentration gradients and consuming ATP in the process. Although this pumping process requires energy, it builds an electrochemical gradient of ions that the cell can later tap as a potential energy source. In secondary active transport, the cell creates a passageway for sodium ions to flow down the sodium ion concentration gradient—a process that releases energy and in so doing fuels the transport of another substance against its concentration gradient.
Textbook Reference: Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients