All cells require a supply of chemical energy for carrying out the activities that keep them alive.
The energy required for cellular activities is provided directly by molecules of adenosine triphosphate (ATP).
ATP is made of one adenosine molecule and three phosphate groups, called Pi for short.
Each molecule of ATP stores a small quantity of chemical energy. This energy can be released by breaking down ATP into adenosine diphosphate (ADP) and a phosphate group.
Energy is required to regenerate molecules of ATP that have been broken down. ATP is regenerated by joining a molecule of ADP to a phosphate group.
The breakdown and regeneration of ATP can be summarised by the diagram below.