Microbial Photosynthesis Introduction
Metabolism is the sum total of all biochemical reactions that take place in the cell with the involvement of flow of energy and the participation of variety of enzymes and proteins. Metabolism, in fact, represents the chemistry of life and can be divided into two major parts: catabolism and anabolism. Catabolism (G. cata = down, ballein = to through) represents the breakdown of more complex chemicals into smaller, simpler molecules resulting in the release of energy. Some part of this released energy is trapped and made available for cellular functions while the rest is released as heat.

The chemotrophic organisms have adopted two catabolic mechanisms for energy conservation: respiration and fermentation. In respiration, the energy is conserved by the process oxydative phosphorylation with the involvement of molecular oxygen or some other externally derived electron­-acceptor. Respiration, however, is of two different types, namely, aerobic and anaerobic. In aerobic respiration, the final electron acceptor is oxygen whereas the electron-acceptor in anaerobic respiration is more often inorganic (e.g., NO₃^-, SO₄²-, CO₂, Fe³⁺, SeO₄^2-, and many others), though organic electron-acceptors such as fumaric acid may also be used.