This chapter considers the molecular mechanisms by which electron transport enables cells to generate the energy they need to survive. It describes how such systems operate in both mitochondria and chloroplasts. The chapter reviews the chemical principles that allow the transfer of electrons to release large amounts of energy. It traces the evolutionary pathways that gave rise to these mechanisms. The chapter looks at the general principles central to the generation of energy in all living things: the use of a membrane to harness the energy of moving electrons. The generation of ATP by oxidative phosphorylation differs from the way ATP is produced during glycolysis, in that it requires a membrane. Mitochondria, chloroplasts, and many prokaryotes generate energy by a membrane-based mechanism known as chemiosmotic coupling, which involves using an electrochemical proton gradient to drive the synthesis of ATP. Mitochondria produce most of an animal cell's ATP, using energy derived from oxidation of sugars and fatty acids.