Simple organic molecules are used in all anabolic and catabolic processes as building blocks. On the other hand, the reactions associated with the transformations of simple organic molecules provide the transport of hydrogen ions and electron required for the synthesis of ATP. For example, in the process of internal respiration, two pyruvate molecules produced from a glucose molecule are used for the synthesis of acetyl-CoA that enters the Krebs cycle. Further transformations of the organic compounds (acetyl-CoA, oxaloacetate, isocitrate, alpha-ketoglutarate, succinate, fumarate, and malate) are accompanied with the release of protons and electrons that are carried by NADH and FADH2 to the respiratory chain resulting in the production of ATP. On the other hand, the organic compounds produced in the Krebs cycle can be used for the synthesis of certain amino acids. Fatty acids breakdown is also associated with the production of NADH and FADH2 whereas acetyl-CoA is a final product of the lipolysis. As a result, in a living cell, the transformations of simple organic molecules are directly associated with energy-generating processes, as well as the production of organic monomers used for the production of proteins, polysaccharides, and lipids.