Respiratory Substrate The substrates, which are broken down in respiration for the release of energy, maybe carbohydrates, fats, or proteins. Proteins are used up in respiration only when carbohydrates and fats are not available. As regards carbohydrates, not only simple hexose sugars like glucose and fructose but complex disaccharides particularly sucrose and polysaccharides such as lignin, inulin, and hemicellulose are also used as respiratory substrates. Fats are used as respiratory substrates after their hydrolysis to fatty acids and glycerol by lipase and their subsequent conversion to hexose sugars. Proteins serve as respiratory substrates after their breakdown into aminoacids by proteolytic enzymes. During respiration, the complex substrates are broken down into simpler ones and finally, CO2 is liberated and water is formed. During the oxidation of the respiratory substrate, some energy is released. Part of this energy is trapped in the form of energy-rich compounds such as...