An electron transport chain (also Oxyative phosphorylation) associates electron carriers (such as NADH and FADH2) and mediating biochemical reactions that produce adenosine triphosphate (ATP), which is a major energy intermediate in living organisms. Only two sources of energy are available to biosynthesize organic molecules and maintain biochemical and kinetic processes in living organisms: oxidation-reduction (redox) reactions and some forms of radiation, such as sunlight (used for photosynthesis). Organisms that use redox reactions to produce ATP are called chemotrophs. Organisms that use sunlight are called phototrophs. Both chemotrophs and phototrophs use electron transport chains to convert energy into ATP. This is achieved through a three-step process: