Metabolism. Metabolism is the sum total of all the chemical reactions that take place in a living organism. Metabolism consists of catabolism and anabolism. Catabolic reactions involve the breakdown of large molecules into smaller fragments. Anabolic reactions synthesize large molecules from smaller ones.

Mitochondria. Mitochondria are membrane-enclosed subcellular structures that are the site of energy production in the form of ATP molecules. Enzymes for both the citric acid cycle and the electron transport chain are housed in the mitochondria.

Important coenzymes. Three very important coenzymes involved in catabolism are NAD+, FAD, and CoA. NAD+ and FAD are oxidizing agents that participate in the oxidation reactions of the citric acid cycle. They transport hydrogen atoms and electrons from the citric acid cycle to the electron transport chain. CoA interacts with acetyl groups produced from food degradation to form acetyl CoA. Acetyl CoA is the "fuel" for the citric acid cycle.

Common catabolic pathway. The common catabolic pathway includes the reactions of the citric acid cycle and those of the electron transport chain and oxidative phosphorylation. The degradation products from all types of foods (carbohydrates, fats, and proteins) participate in these reactions.

Citric acid cycle. The citric acid cycle is a cyclic series of eight reactions that oxidize the acetyl portion of acetyl CoA, resulting in the production of two molecules of CO2. The complete oxidation of one acetyl group produces three molecules of NADH, one of FADH2, and one of GTP.

Electron transport chain. The electron transport chain is a series of reactions that passes electrons from NADH and FADH2 to molecular oxygen. Each electron carrier that participates in the chain has an increasing affinity for electrons. Upon accepting the electrons and hydrogen ions, the O2 is reduced to H2O.

Oxidative phosphorylation. In the electron transport chain, released energy is used to convert ADP to ATP. One molecule of NADH produces three molecules of ATP. FADH2, which enters the chain later than NADH, produces only two molecules of ATP.

Importance of ATP. ATP is the link between energy production and energy use in cells. The conversion of ATP to ADP powers life processes, and the conversion of ADP back to ATP regenerates the energy expended in cell operation.

Glycolysis. Glycolysis, a series of ten reactions that occur in the cytoplasm, is a process in which one glucose molecule is converted into two molecules of pyruvate. A net gain of two molecules of ATP and two molecules of NADH results from the metabolizing of glucose to pyruvate.

Fates of pyruvate. With respect to energy-yielding metabolism, the pyruvate produced by glycolysis can be converted to acetyl CoA under aerobic conditions or to lactate under anaerobic conditions. Some microorganisms convert pyruvate to ethanol, an anaerobic process.