Anaerobic Respiration

Anaerobic respiration is fundamentally less efficient than aerobic respiration because it never completely oxidises the respiratory substrate: only a limited number of ATP molecules are generated per glucose molecule (see the comparison tables in the notes), and it is nonetheless still valuable because the CO2 it produces can be recycled into photosynthetic carbon fixation. Compared with plain glycolysis, fermentation starts where glycolysis leaves off -- with pyruvic acid -- and converts that pyruvic acid into alcohol or lactic acid; it occurs only in the absence of oxygen (unlike glycolysis, which proceeds regardless of oxygen availability); glycolysis nets 2 ATP while fermentation itself yields no additional net ATP; and where glycolysis produces $2\,NADH+H^+$, fermentation actually consumes (re-oxidises) that same $2\,NADH+H^+$ to regenerate the NAD+ that glycolysis needs to keep running. The net-products table shows the resulting bottom line: starting from one glucose molecule, glycolysis alone nets 8 ATP-equivalents (2 substrate-level ATP plus 2 NADH worth roughly 3 ATP each), but once the 2 NADH are spent re-oxidising in fermentation, the true net yield of the complete anaerobic pathway falls to just 2 ATP. A simple classroom demo …