Cellular respiration consists of three primary stages: glycolysis, the Krebs cycle, and the electron transport chain.

This process encompasses a series of metabolic reactions that occur within the cells of organisms. Its purpose is to convert biochemical energy derived from nutrients into adenosine triphosphate (ATP) while simultaneously releasing waste products. The three main stages of cellular respiration are as follows: glycolysis, the Krebs cycle, and the electron transport chain.

The first stage, glycolysis, takes place in the cytoplasm of the cell. During this phase, one molecule of glucose, which is a six-carbon sugar, is broken down into two molecules of pyruvate, each a three-carbon compound. This conversion produces two molecules of ATP and two molecules of NADH, which serves as an electron carrier. Notably, glycolysis does not require oxygen, categorizing it as an anaerobic process.

The second stage, known as the Krebs cycle (or citric acid cycle, or tricarboxylic acid cycle), occurs in the mitochondria. In this stage, the pyruvate generated during glycolysis is further metabolized to yield ATP, NADH, and another electron carrier called FADH$_{2}$. Additionally, carbon dioxide is released as a waste product. Unlike glycolysis, the Krebs cycle is aerobic, meaning it necessitates the presence of oxygen.

The third and final stage is the electron transport chain, which also occurs in the mitochondria. This stage is responsible for the majority of ATP production. Here, the NADH and FADH$_{2}$ molecules produced in earlier stages transfer their electrons to a series of proteins located in the inner mitochondrial membrane. These proteins harness the energy from the electrons to pump hydrogen ions across the membrane, establishing a gradient. The subsequent flow of hydrogen ions back across the membrane drives the synthesis of ATP. Oxygen plays a crucial role in this stage as it acts as the final electron acceptor in the chain, combining with hydrogen ions to form water.

In summary, cellular respiration is a complex series of processes involving multiple stages, each characterized by distinct reactions and products. The three key stages—glycolysis, the Krebs cycle, and the electron transport chain—collaborate to convert the energy stored in nutrients into ATP, which serves as the primary energy source for the cell.