ATP uses free energy created by hydrolysis to drive endergonic reactions through phosphorylation. The phosphorylation process involves transferring the ATP's phosphate to a protein or molecule, such as the sodium-potassium pump. The transfer is considered an endergonic reaction because the protein pump absorbs energy from ATP.
The process starts with an exergonic reaction, as energy is released by ATP through hydrolysis. A cell protein such as the sodium-potassium pump then uses this energy to move ions across a cellular membrane, driven by phosphorylation. In this example, three sodium ions are moved out of the cell, while two potassium ions are moved into the cell. This entire process is referred to as energy coupling because an exergonic reaction ultimately results in an endergonic reaction.
Another example of energy coupling involves ATP and glucose. In metabolism, energy from ATP phosphorylates glucose, which allows glucose to work with enzymes. Once phosphorylated, glucose is converted to fructose with the help of the enzymes. This process also involves energy coupling because ATP's exergonic reaction leads to the phosphorylation of glucose, an endergonic reaction.
ATP utilizes energy coupling because it is a highly unstable molecule. If ATP is not used in energy coupling, the free energy generated by hydrolysis is lost to the environment as heat.