13.4: Primary Active Transport
In contrast to passive transport, active transport involves a substance being moved through membranes in a direction against its concentration or electrochemical gradient. There are two types of active transport: primary active transport and secondary active transport. Primary active transport utilizes chemical energy from ATP to drive protein pumps embedded in the cell membrane. With energy from ATP, the pumps transport ions against their electrochemical gradients—a direction they would not normally travel by diffusion.
Relationship between Concentration, Electrical, and Electrochemical Gradients
To understand the dynamics of active transport, it is important to first understand electrical and concentration gradients. A concentration gradient is a difference in the concentration of a substance across a membrane or space that drives movement from areas of high concentration to areas of low concentration. Similarly, an electrical gradient is the force resulting from the difference between electrochemical potentials on each side of the membrane that leads to the movement of ions across the membrane until the charges are similar on both sides of the membrane. An electrochemical gradient is created when the forces of a chemical concentration gradient and electrical charge gradient are combined.
One important transporter responsible for maintaining the electrochemical gradient in cells is the sodium-potassium pump. The primary active transport activity of the pump occurs when it is oriented such that it spans the membrane with its extracellular side closed, and its intracellular region open and associated with a molecule of ATP. In this conformation, the transporter has a high affinity for sodium ions normally present in the cell in low concentrations, and three of these ions enter into and attach to the pump. Such binding allows ATP to transfer one of its phosphate groups to the transporter, providing the energy needed to close the pump's intracellular side and open the extracellular region.
The change in conformation decreases the pump's affinity for sodium ions—which are released into the extracellular space—but increases its affinity for potassium, allowing it to bind two potassium ions present in low concentration in the extracellular environment. The extracellular side of the pump then closes, and the ATP-derived phosphate group on the transporter detaches. This enables a new ATP molecule to associate with the pump's intracellular side, which opens and allows the potassium ions to exit into the cell—returning the transporter to its initial shape beginning the cycle again.
Due to the pump's primary active transport activity, there is an imbalance in the distribution of ions across the membrane. There are more potassium ions inside the cell and more sodium ions outside the cell. Therefore, the inside of the cell is more negative than the outside. An electrochemical gradient is generated as a result of the ion imbalance. The force from the electrochemical gradient then propels the reactions of secondary active transport. Secondary active transport, also known as co-transport, occurs when a substance is transported across a membrane as a result of the electrochemical gradient established by primary active transport without requiring additional ATP.