What are the two primary stages of cellular respiration?

The primary stages of cellular respiration consist of glycolysis and oxidative phosphorylation. Glycolysis is the first step in the process, occurring in the cytoplasm, where glucose is broken down into pyruvate, producing a small amount of ATP and NADH in the process. This stage does not require oxygen, meaning it is anaerobic.

Following glycolysis, if oxygen is present, the process continues with oxidative phosphorylation, which occurs in the mitochondria. This stage includes the Krebs cycle (also known as the citric acid cycle) and the electron transport chain. During oxidative phosphorylation, the NADH and FADH2 produced in previous steps are used to generate a significant amount of ATP via the electron transport chain and chemiosmosis. This stage is aerobic, meaning it requires oxygen.

The combination of these two stages—glycolysis, which can occur without oxygen, and oxidative phosphorylation, which requires oxygen—captures the entirety of cellular respiration effectively, providing the cell with a high yield of energy.