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.
An animal cell maintains internal ion concentrations that are high in potassium but low in sodium. On the outside of the cell, the concentrations are reversed: high sodium and low potassium. These concentration differences represent electrochemical gradients across the membrane, which the cell can later tap for energy.
Generally, one would expect such concentration gradients to dissipate, with a net flow of ions moving from a higher concentration on one side of the membrane to a lower concentration on the other. However, a membrane protein, called the sodium-potassium pump, keeps the ion gradients intact. The ions are actively pumped against their concentration gradients.
Primary active transport involves the direct hydrolysis of ATP, which provides the energy required for transport. In the first step of the cycle, three sodium ions and one ATP molecule bind to the pump.
ATP is used to phosphorylate the pump, causing the pump to change shape and deliver the sodium ions to the opposite side of the membrane. At the same time, ion-binding sites open up on the extracellular face of the pump.
On the extracellular side of the membrane, the pump picks up two potassium ions. As the potassium ions bind, the pump releases its inorganic phosphate. The release causes the pump to change shape again and deliver the potassium ions to the inside of the cell, completing the cycle.
In secondary active transport, a substance such as glucose is pumped from a region of lower concentration to a region of higher concentration. This process requires energy, because glucose molecules are transported against their concentration gradient.
The energy that drives glucose across a membrane against its concentration gradient does not come directly from ATP. Rather, it comes from the energy stored in a sodium ion gradient (which was created using ATP). Because ATP does not fuel the pump directly, this process is called secondary active transport.
To pump glucose against its concentration gradient, the pump takes up both sodium and glucose from outside of the cell and then changes shape, depositing both substances inside the cell. A pump that transports two substances in the same direction is called a symport protein.
The sodium ions that enter the cell are later returned to the outside by the action of the sodium-potassium pump. This process, called primary active transport, creates sodium and potassium ion gradients at the expense of ATP.
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.