text: The citric acid cycle (Krebs cycle) is a good example of an amphibolic pathway because it functions in both the degradative (carbohydrate, protein, and fatty acid) and biosynthetic processes. The citric acid cycle occurs on the cytosol of bacteria and within the mitochondria of eukaryotic cells. It provides electrons to the electron transport chain which is used to drive the production of ATP in oxidative phosphorylation. Intermediates in the citric acid cycle, such as oxaloacetate, are used to synthesize macromolecule constituents such as amino acids, e.g. glutamate and aspartate.
  The first reaction of the cycle, in which oxaloacetate (a four-carbon compound) condenses with acetate (a two-carbon compound) to form citrate (a six-carbon compound) is typically anabolic. The next few reactions, which are intramolecular rearrangements, produce isocitrate. The following two reactions, namely the conversion of D-isocitrate to α-Ketoglutarate followed by its conversion to succinyl-CoA, are typically catabolic. Carbon dioxide is lost in each step and succinate (a four-carbon compound) is produced.
  There is an interesting and critical difference in the coenzymes used in catabolic and anabolic pathways; in catabolism NAD+ serves as an oxidizing agent when it is reduced to NADH. Whereas in anabolism the coenzyme NADPH serves as the reducing agent and is converted to its oxidized form NADP.
  Citric acid cycle has two modes that play two roles, the first being energy production produced by the oxidative mode, as the acetyl group of acetyl-coA is fully oxidized to CO.  This produces most of the ATP in the metabolism of aerobic heterotrophic metabolism, as this energy conversion in the membrane structure (cytoplasmic membrane in bacteria and mitochondria in eukaryotes) by oxidative phosphorylation by moving electron from donor (NADH and FADH) to the acceptor O. Every cycle give 3 NADH, 1 FADH, CO and GTP. The second role is biosynthetic, as citric acid cycle regenerate oxaloacetate when cycle intermediates are removed for biosynthesis.