Figure 1. Aerobic energy metabolism. Glycogen and triacyglycerol are broken down in the cytosol to glucose and free fatty acid. In the glycolytic pathway, glucose is oxidized to pyruvate. In the mitochondria, pyruvate is oxidized to acetyl-CoA, also the end product of -oxidation of fatty acids. Acetyl-CoA is completely oxidized in the citrate cycle. The energy released during transport of electrons from NADH or FADH₂ along the electron transport chain (bold arrow) is used for the synthesis of ATP.

Figure 2. Anaerobic metabolism in invertebrates. In the absence of oxygen, pyruvate is reduced to lactate or to an opine, which is formed in a reaction of an amino acid and pyruvate (see boxed structure). Alternatively, phosphoenolpyruvate reacts with carbon dioxide to oxaloacetate, which in turn is reduced to malate. Malate is both oxidized to pyruvate, and reduced to fumarate. Further reactions produce additional ATP and lead to various end products.

Figure 3. Symbiotic energy metabolism in the clam Calyptogena. Separate uptake mechanisms prevent the mixing of oxygen and hydrogen sulfide. Oxygen-rich sea water enters the gills through the siphon, while sulfide-rich vent water is taken up through the animal's foot. A transport protein delivers hydrogen sulfide to sulfide-oxidizing bacteria inhabiting the gills. Sulfide oxidation leads to the production of ATP, which is used by the bacteria to fix carbon dioxide and to produce carbohydrates. The organic matter produced by the bacteria serves as metabolic fuel for the clam.