Biochemistry. A sequence of electron carrier molecules (membrane proteins) through which electrons of initial high energy are converted to a lower energy state with the capture of the released energy as ATP, the main energy intermediate in living organisms. The major role of the electron transport chain is to extract energy via redox reactions in cellular respiration and photosynthesis. Electron transport chains occur in the inner mitochondrial membrane of eukaryotic cells (see mitochondrial respiratory chain) and in the cell membrane of bacteria to convert sugars into ATP in the cellular respiration. They are also found in the chloroplasts of photosynthetic organisms to convert energy of the sunlight into ATP.