Types of Respiration

Based on whether molecular oxygen is used, respiration is classified into two broad types: aerobic respiration, which requires oxygen, and anaerobic respiration, which does not. The two pathways differ dramatically in efficiency, end products, and which cell compartments they use -- a difference summarised across six points of comparison, from the organisms that use each pathway to the number of ATP molecules ultimately produced (see the comparison table in the notes). Aerobic respiration is by far the more energy-efficient pathway (yielding around 36 ATP per glucose molecule in plants) and is used by all higher organisms, while anaerobic respiration, limited mostly to yeast and certain bacteria, yields only about 2 ATP per glucose molecule and produces alcohol or lactic acid rather than fully oxidising the substrate to CO2 and water. A combined flow diagram of both pathways shows glucose first being split by glycolysis into two molecules of pyruvic acid -- the one step both pathways share -- after which the aerobic branch proceeds through the link reaction, Krebs c …