1. Glycolysis: This is the first phase and takes place in the cytoplasm. During glycolysis, a single molecule of glucose (a sugar) is broken down into two molecules of pyruvate. This process generates some ATP and also prepares the glucose for further breakdown.
2. The citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle): This phase takes place in the mitochondria. The pyruvate produced during glycolysis is further metabolized and converted into ATP, carbon dioxide, and water. The citric acid cycle also generates electrons that will be used to generate more ATP in the next phase.
3. Electron Transport Chain (ETC): This is the final phase and also takes place in the mitochondria. The electrons generated during the citric acid cycle are passed through a series of proteins in the inner mitochondrial membrane, generating a proton gradient that drives the production of ATP. This process is known as oxidative phosphorylation and generates the majority of the ATP produced during cellular respiration.