What substance is produced from the breakdown of glucose in fermentation or during the initial stage of aerobic respiration?

During the breakdown of glucose in fermentation and at the initial stage of aerobic respiration, pyruvate is produced through a process called glycolysis. Glycolysis is the pathway that converts glucose into pyruvate, producing a small yield of ATP as well. This occurs in the cytoplasm of the cell and does not require oxygen, which is why it is a common first step in both fermentation (anaerobic) and aerobic (when oxygen is present) respiration.

Acetyl CoA is generated later in aerobic respiration from pyruvate before it enters the Krebs cycle, so it does not directly result from the breakdown of glucose in the initial stages. ATP is indeed produced during glycolysis, but it is a product rather than the direct result of glucose breakdown. Carbon dioxide is produced during the Krebs cycle in aerobic respiration and also during fermentation in some microbial processes, but this is not an output of the glycolysis phase itself. Thus, pyruvate is the key substance that directly results from the breakdown of glucose in these metabolic processes.