ATP is a molecule that can be recycled by cells. Recycling molecules enables the cell to conserve energy that might otherwise be expending in the building of new substances.
The process of generating or regenerating a molecule of ATP is called phosphorylation. ATP is generated by cells in three different ways:
Substrate-level phosphorylation - This process involves the anaerobic transfer of high energy phosphate groups from organic compounds to ATP or other molecules. Substrate-level phosphorylation takes place during anaerobic metabolic pathways such as glycolysis and the Krebs Cycle reactions.
Oxidative phosphorylation - This process occurs via a series of redox reactions that occur during respiration, involving inorganic final electron and hydrogen acceptors such as oxygen (O2), nitrate (NO3-), nitrite (NO2-) and sulphate (SO2-).
Photophosphorylation - This process, performed by photoautotrophs, involves the building of ATP molecules utilizing light energy to drive the process of adding a phosphate group to ADP.