ABSTRACT
Cells need energy for work including muscle contraction, biosynthesis, active transport across membranes and generation of heat. Energy is generated from metabolic fuels and from reduced molecules, which are oxidized to release energy. Oxidation involves removing electrons at high potential from the fuel molecules and transferring them to a lower potential, thus releasing energy. The removed electrons must be transferred to a suitable electron acceptor, which has to be transportable, soluble in water and generally available. Oxygen is not used until the end of the electron transport chain. The basic chemical currency of energy in all living cells consists of the two high-energy phosphate bonds contained in adenosine triphosphate (ATP). The immediate source of cellular energy is ATP, which can lose one phosphate group producing adenosine diphosphate and usable energy. The end product of glycolysis is pyruvate, which under aerobic conditions enters the citric acid cycle.
