Which of the following are the two main stages of cellular respiration?

The two main stages of cellular respiration are glycolysis and oxidative phosphorylation. Glycolysis is the initial stage, occurring in the cytoplasm, where glucose is broken down into pyruvate, resulting in a small yield of ATP and the reduction of NAD+ to NADH. The pyruvate then enters the mitochondria, where oxidative phosphorylation takes place. This process includes the Krebs cycle (also known as the citric acid cycle) and the electron transport chain, which occur in the mitochondria. In oxidative phosphorylation, the energy from NADH and FADH2 (produced during glycolysis and the Krebs cycle) drives the synthesis of a large amount of ATP through the electron transport chain and chemiosmosis.

While fermentation does occur in cellular respiration, it is not one of the main stages; instead, it is an alternative pathway that occurs under anaerobic conditions. The option mentioning the electron transport chain and the Krebs cycle describes components within oxidative phosphorylation but does not encompass glycolysis, which is fundamental to initiating cellular respiration. Lastly, the mention of photosynthesis and cellular respiration refers to two different biological processes rather than stages within cellular respiration itself. Thus, glycolysis and oxidative phosphorylation comprehensively represent the two main stages of cellular respiration.