Final answer:
The addition of a phosphate to a molecule like ADP to form ATP requires energy due to the inherent instability and repulsion between negatively charged phosphate groups.
Step-by-step explanation:
When phosphate is added to a molecule, it requires energy because phosphate groups carry a negative charge and repel one another, especially in series as seen in adenosine diphosphate (ADP) and adenosine triphosphate (ATP). This series arrangement creates an unstable molecule, meaning that when a phosphate group is removed (dephosphorylation), energy is released. This is important in cellular processes, where ATP acts as the body's fundamental molecule of energy transfer.
The classic example of this process is seen during glycolysis, where ATP is used to phosphorylate glucose, which in turn can be metabolized for immediate energy or stored as glycogen. In biochemical reactions, ATP donates a phosphate group to become ADP, releasing energy from the broken phosphate bonds. The reaction can go in the reverse direction, too, where ADP gets rephosphorylated to ATP, but this requires reinvestment of energy.