Definition of Electron transport chain

An electron transport chain associates electron carriers (such as NAD⁺ and FADH2) and mediating biochemical reactions that produce adenosine triphosphate (ATP), which is the energy currency of life. Only two sources of energy are available to living organisms: oxidation-reduction (redox) reactions and 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 utilize electron transport chains to convert energy into ATP. This is achieved through a three-step process:

Gradually sap energy from high-energy electrons in a series of individual steps

Use that energy to forcibly unbalance the proton concentration across the membrane, creating an electrochemical gradient 

Use the energy released by the drive to rebalance the proton distribution as a means of producing ATP.