Sugar molecules (i.e. glucose) are broken down into simpler molecules such as pyruvate. Pyruvate, in the presence of oxygen will be converted to Acetyl-CoA and it will enter the tricarboxylic acid cycle wherein it will produce series of reducing compounds such as NADH and FADH. These reducing compounds are then processed in the electron transport chain and oxidized, and the energy released in the oxidation of NADH and FADH is transferred to the high energy phosphate bond of adenosine triphosphate or ATP.