Types of MetabolismWhile in theory there are many pathways cells could utilize to metabolize their food, in nature, especially in animals, only a few pathways are used. While rather complex, these pathways vary little from organism to organism. Two major pathways are generally available to heterotrophic animal cells during metabolism: aerobic respiration and fermentation. Aerobic respiration is an oxygen requiring series of reactions, and is necessary in all vertebrates. Fermentation does not require oxygen, and while most vertebrates can perform fermentation reactions, it serves them mainly in emergencies when extra energy is needed. Only organisms such as yeast and some bacteria can live entirely by engaging in fermentation.
While many molecules may be metabolized (monosaccharides, amino acids, fatty acids, etc.), most cells prefer glucose as their source of fuel. It is therefore convenient to examine glucose metabolism as a model for the overall process, while keeping in mind that other molecules may also be used. Whichever pathways are ultimately utilized to harvest energy from glucose, they always involve the oxidation of glucose and always begin with a series of reactions called glycolysis. If the aerobic pathway is used, glycolysis is followed by two processes: the Krebs cycle, a cyclic series of reactions, and the electron transport chain (ETC), where much ATP is synthesized. Letâ€™s look at each of these processes in more detail.