Adenosine triphosphate or ATP is the means by which energy is transferred between cells. ATP is produced as a result of several biological processes taking place inside cells that include respiration, photosynthesis, fermentation, etc.
The syntesised ATP is then used as a source of energy by other cells.
When ATP was discovered and the role played by it in cellular metabolism understood, the production of ATP in mitochondria was assumed to be by substrate-level phosphorylation. Peter Dennis Mitchell identified the process of production of ATP by oxidative phosphorylation. He was able to show that there exists a potential difference between two sides of membranes; and that this leads to the movement of charged particles across the membrane. ATP formation is a result of this. This discovery is commonly known as the Mitchell Hypothesis of Chemiosmotic coupling.