Reactions on the left depict the breakdown of organic molecules into energy (stored as ATP), electrons (stored in NADH), and small carbonaceous molecules. These reactions are dominated by oxidations. The reactions on the right depict the re-combination of the small organic molecules into larger compounds that are characteristic of living material. These reactions are dominated by reductions, and they require energy input (as ATP) and electron input (as NADH). The top of the diagram represents reactions characteristic of heterotrophs. Chemo- and photoautotrophs derive energy from oxidation of inorganic compounds or light, respectively. Carbon dioxide serves as the carbon source for production of the small organic intermediates. Electrons are obtained from the inorganic molecule being oxidized or from water, in the case of phototrophs. Once ATP, NADH, and the carbon intermediates have been formed, the anabolic reactions proceed in the same manner as for the heterotrophs. Taken largely from Lynch and Poole.