Final answer:
Glycolysis is an anaerobic process that breaks down glucose to produce ATP without the need for oxygen, allowing it to continue through fermentation under oxygen-limiting conditions.
Step-by-step explanation:
Glycolysis is an anaerobic process because it does not require oxygen to occur. This biochemical pathway takes place in the cytoplasm and can proceed under anaerobic conditions, which is why it is thought to be one of the oldest metabolic pathways, evolving before Earth's atmosphere contained oxygen. Glycolysis results in a net gain of 2 ATP and 2 NADH molecules per glucose molecule broken down. However, when oxygen is scarce, cells may undergo fermentation to regenerate NAD+, which allows glycolysis to continue. Though fermentation produces less ATP compared to aerobic processes like the Krebs cycle and electron transport chain, it enables organisms to survive in environments lacking oxygen.