The active transport mechanism commonly involves the movement of substance that are unable to freely cross the cell membrane but are important for cell function. These molecules typically have a small mass, such as ions and amino acids. However, other mechanisms can also involve the transport of larger molecules, such as glucose.
In some kinds of active transport, specific carrier proteins undergo phosphorylation by ATP hydrolysis. When the carrier protein binds its target, the ATP transfers a phosphate to the carrier protein, changing the shape of the carrier protein. The changed shape gives the target molecule access to the other side of the membrane, and the target molecule is then released. This is the general mechanism used to transport many amino acids and ions across the membrane. In cotransport, the movement of one substance with its gradient releases energy used to move another substance against its gradient in a coupled reaction. This process is used to move many larger molecules, such as sugars.
Understanding the Sodium-Potassium Pump
Because sodium (Na+) and potassium (K+) ions are constantly moving in and out of a cell, the cell relies on the use of a sodium-potassium pump (Na+/K+ pump) to maintain the ideal concentration of sodium and potassium in living cells. This is important for control of cell volume, pH, and nutrient balance of the cell. The pump also helps generate voltages across the cell membrane. These membrane voltages are crucial for the function of some cells, such as nerve cells.
The transport protein involved in the Na+/K+ pump has binding sites for three Na+ ions and two K+ ions. The protein also has a binding site for ATP. Although the function of the pump is cyclical, the discussion will begin with the Na+/K+ pump open to the inside of the cell. In this shape, it has a high affinity for Na+ ions, so three will bind.
The binding of sodium ions to the carrier protein triggers ATP hydrolysis. Hydrolysis involves the breakdown of ATP to adenosine diphosphate (ADP) and inorganic phosphate (a salt of phosphoric acid). The phosphate stays bound to the protein pump, and the energy released by hydrolysis is used to power the pump.
Energy from hydrolysis causes the pump to physically change its shape, opening toward the outside of the cell rather than inside the cell. At this point the pump has a low affinity for Na+ ions, so the three bound Na+ ions are released outside the cell.
When open to the extracellular space, the pump has a high affinity for K+ ions , so two K+ ions will bind to the pump. Binding changes the shape of the protein again and triggers the removal of the phosphate group that is attached to the pump.The pump has changed back to its original form and opens toward the cell's interior. The pump no longer has a strong affinity for the K+ ions, causing the two attached K+ ions to dislodge from the pump. These K+ ions are released inside the cell. The cycle will then continue when three Na+ ions bind again to the protein.