1.) The first stage is called glycolysis, it is a series of enzyme-controlled reactions that degrades glucose which is a 6-carbon molecule to pyruvate which is a 3-carbon molecule .pyruvate is further oxidized to acetylcoenzyme A (acetyl CoA). Amino acids and fatty acids may also be oxidized to acetyl CoA as well as glucose.
2.) In the second stage, acetyl CoA enters the citric acid (Krebs) cycle, where it is degraded to produce energy-rich hydrogen atoms which reduce the oxidized form of the coenzyme nicotinamide adenine dinucleotide (NAD) to NADH, and further reduces the coenzyme flavin adenine dinucleotide (FAD) to FADH. By reductioni mean the addition of electrons to a molecule, or the gain of hydrogen atoms, while oxidation is the loss of electrons or rather oxygen is added to a molecule.) In the second stage of cellular respiration, the carbon atoms of the intermediate metabolic products in the Krebs cycle are converted to carbon dioxide.
3.)The third stage of cellular respiration takes place when the energy-rich hydrogen atoms are separated to protons [H] and energy-rich electrons in the electron transport chain. At the start of the electron transport chain, the energy-rich hydrogen on NADH is removed from NADH, producing the oxidized coenzyme, NAD and a proton (H+) and two electrons (e-). The electrons are transferred along a chain of more than 15 different electron carrier molecules called the electron transport chain). These proteins are classified into three large respiratory enzyme complexes, each of which has proteins that span the mitochondrial membrane, securing the complexes into the inner membrane