Plants and cyanobacteria capture the light of the sun and utilize its energy to synthesize organic compounds from inorganic substances such as CO2, nitrate, and sulfate to synthesize their cellular material; they are photoautotrophic. In photosynthesis, photon energy splits water into oxygen and hydrogen, the latter bound as NADPH. This process, termed the light reaction, takes place in the photosynthetic reaction centers embedded in membranes. It involves the transport of electrons, which is coupled to the synthesis of ATP. NADPH and ATP are consumed in a so-called dark reaction to synthesize carbohydrates from CO2. The photosynthesis of plants and cyanobacteria created the biomass on earth, including the deposits of fossil fuels and atmospheric oxygen. Animals are dependent on the supply of carbohydrates and other organic compounds as food; they are heterotrophic. They generate the energy required for their life processes by oxidizing the biomass, which has first been produced by plants. When oxygen is consumed, CO2 is formed. Thus light energy captured by plants is the source of energy for the life processes of animals. In the process of biological oxidation, substrates such as carbohydrates are oxidized to form water and CO2. Biological oxidation can be seen as a reversal of the photosynthesis process. It evolved only after oxygen accumulated in the atmosphere during photosynthesis. Both biological oxidation and photosynthesis serve the purpose of generating energy in the form of ATP. Biological oxidation involves a transport of electrons through a mitochondrial electron transport chain, which is in part similar to the photosynthetic electron transport. The overall reaction of biological oxidation is equivalent to combustion of substrates. In contrast to technical combustion, biological oxidation proceeds in a sequence of partial reactions, which allows the utilization of the major part of the free energy for ATP synthesis. Light microscopic studies of many different cells revealed small granules, with an appearance similar to bacteria. Like plastids, mitochondria also form a separated metabolic compartment. The degradation of substrates to CO2 and hydrogen takes place in the mitochondrial matrix. NADH thus formed diffuses through the matrix to the mitochondrial inner membrane and is oxidized there by the respiratory chain. The respiratory chain comprises a sequence of redox reactions by which electrons are transferred from NADH to oxygen. As in the photosynthetic electron transport, the mitochondrial electron transport by the respiratory chain releases free energy, which is used to generate a proton gradient. This in turn drives the synthesis of ATP, which is exported from the mitochondria and provides the energy required for cellular metabolism. This process is universal and functions in the mitochondria of all eukaryotic cells.