Final answer:
Glycolysis occurs with or without oxygen, yielding two ATP molecules. In the presence of oxygen, it leads to aerobic respiration with a higher ATP yield. In its absence, fermentation occurs, resulting in less ATP production.
Step-by-step explanation:
Glycolysis is the first stage of cellular respiration in which glucose is split to form two molecules of pyruvate, yielding two net molecules of ATP regardless of the presence of oxygen. However, what follows glycolysis differs based on whether oxygen is available or not. In the presence of oxygen, aerobic respiration occurs, and pyruvate enters the mitochondria to be fully oxidized during the Krebs cycle and electron transport chain, producing a much higher yield of ATP. Conversely, in the absence of oxygen, anaerobic respiration or fermentation takes place, where pyruvate is metabolically converted into lactic acid or ethanol and carbon dioxide, resulting in a lower ATP yield compared to aerobic respiration.
During aerobic respiration, redox reactions are integral both in glycolysis and the Krebs cycle, aiding in the complete oxidation of glucose to CO2 and H2O, leaving no carbohydrates behind. Meanwhile, anaerobic respiration occurs more quickly but is less efficient in producing ATP, and fermentation is necessary as it regenerates NAD+ that allows glycolysis to continue in the absence of oxygen.
To sum up, glycolysis is a non-oxygen-dependent process that yields ATP and NADH, but the subsequent pathways it feeds into depend on oxygen availability, influencing the overall energy yield and by-products of cellular respiration.