Here's why:
* Aerobic respiration requires oxygen. It breaks down glucose completely, producing carbon dioxide and water as byproducts. This process yields a net gain of 36-38 ATP molecules per glucose molecule.
* Anaerobic respiration occurs in the absence of oxygen. It breaks down glucose only partially, producing lactic acid or ethanol as byproducts. This process yields a much smaller net gain of 2 ATP molecules per glucose molecule.
So, while both aerobic and anaerobic respiration produce ATP, aerobic respiration is significantly more efficient and produces a much greater amount of ATP.
