During rest and normal activities by the body fats contribute the biggest percentage as a source of energy followed by carbohydrates and the proteins contributing the least amount of energy. However, during strenuous exercises the body mostly uses energy from the stored carbohydrate stored in the muscles in the form of glycogen. Energy may be partially obtained from blood sugar, fatty acids in the blood and also triacylglycerols in the muscles.Therefore its not true that during intense exercise fat stores are used as a source of energy.
Pyruvic acid enters the mitochondria where in the presence of oxygen it is converted into acetyl coenzyme A. Acetyl coenzyme A joins the Krebs cycle in which it is broken down to carbon dioxide, water and energy which is used to produce ATP. ATP is a complex organic molecule.
