The metabolism is made up of anabolic and catabolic processes. Anabolic processes are endergonic, meaning they are not spontaneous, while catabolic processes are exergonic and spontaneous. So how do endergonic reactions occur then? There is metabolic coupling between biochemical pathways so that energy released from an exergonic reaction is used to provide energy needed for an endergonic reaction. This exchange of energy happens in two forms: phosphate groups and electrons. Certain molecules with phosphate groups have high free energies, so the removal of a phosphate group by hydrolysis has a largely negative change is free energy. Phosphorylation, on the other hand, requires energy input. The magnitude of free energy available in the bond is the phosphoryl group transfer potential, and molecules with larger phosphoryl group transfer potentials can phosphorylate molecules with lower potential. In other words, energy released by hydrolysis is used to power other reactions. An example is the coupling that occurs to power muscle contractions. Creatine phosphate is hydrolyzed to form creatine, and this energy is used to phosphorylate ADP to ATP. ATP is the main energy currency molecule in nearly all cells.
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