How is glucose changed during cellular respiration?

During cellular respiration, glucose undergoes a series of catabolic reactions designed to extract chemical energy and store it in adenosine triphosphate (ATP). This process involves the following stages:12

• Glycolysis: A single six-carbon glucose molecule is broken down into two molecules of pyruvate. During this sequence, energy is captured in ATP, and the coenzyme nicotinamide adenine dinucleotide (NAD+) is reduced to NADH.12
• TCA Cycle: Pyruvate is converted into acetyl coenzyme A and enters the tricarboxylic acid cycle. Here, the molecule is further degraded, releasing carbon dioxide as a waste product while transferring high-energy electrons to NAD+ and FAD, forming NADH and FADH2.2
• Oxidative Phosphorylation: In the final stage, electrons from NADH and FADH2 are transferred through the electron transport chain. This flow of electrons eventually reduces oxygen to form water, a process that facilitates the production of large amounts of ATP.2

In the absence of oxygen, organisms may instead utilize fermentation to regenerate NAD+ by transferring electrons to organic acceptors like pyruvate, producing substances such as lactate or ethanol.1