Answer: 3 stages- glycolysis, pyruvate oxidation, the citric acid or Krebs cycle, and oxidative phosphorylation. In glycolysis, the beginning process of all types of cellular respiration, two molecules of ATP are used to attach 2 phosphate groups to a glucose molecule, which is broken down into 2 separate 3-carbon PGAL molecules. PGAL releases electrons and hydrogen ions to the electron carrier molecule NADP+. A carboxyl group is removed from pyruvate and released as carbon dioxide. The two-carbon molecule from the first step is oxidized, and NAD+ accepts the electrons to form NADH. The oxidized two-carbon molecule, an acetyl group, is attached to Coenzyme A to form acetyl CoA. The citric acid cycle, where acetyl CoA is modified in the mitochondria to produce energy precursors in preparation for the next step. Oxidative phosphorylation, the process where electron transport from the energy precursors from the citric acid cycle (step 3) leads to the phosphorylation of ADP, producing ATP. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane.
