In glycolysis, a six carbon sugar (glucose) undergoes several chemical transformations being converted into two pyruvate molecules in the end. ATP is made in the reaction and NAD+ converted to NADH.
In pyruvate oxidation(Acetyl CoA cycle), each pyruvate from glycolysis goes into the mitochondrial matrix,the innermost compartment of mitochondria hence converted into a two carbon molecule bound to a co enzyme A, called Acetyl CoA. Carbon (IV) oxide is released and NADH is generated in the process.
Krebs cycle or the cytric acid cycle involves the combination of Acetyl CoA made with a four carbon molecule, then goes through a series of reactions generating the four carbon starting molecules, ATP, NADH and FADH while Carbon (IV) oxide is released.
Oxidative phosphorylation( electron transport) involves deposit of NADH and FADH made in other steps into the electron transport chain, turning back into NAD+ and FAD. Energy released is used to pump protons out of the matrix forming gradient as electrons move down the chain. Protons flow back into the matrix through ATP synthase making ATP. Oxygen accepts electrons and takes up protons to form water at the end of the electron transport chain