The food we take comprises complex carbohydrates, proteins, fats, etc. A substantial proportion is first broken down to simple oxo-acids by a series of enzyme reactions, collectively grouped in glycolysis and tricarboxylic acid pathways. These acids undergo another sequential oxidation breakdown to carbon dioxide and water taking the help of oxygen that we breathe. The entire process is known as catabolism. The last step of oxidation, called respiratory electron transport, is operative in the cellular powerhouse, mitochondria. Here, the highest amount of energy is released, which is immediately captured in the form of adenosine triphosphate (ATP) by a reaction known as phosphorylation (ADP + iP + Energy → ATP). Also known as cellular energy storehouse, ATP is a high-energy molecule, comprising a nitrogenous base – adenine, a five-carbon sugar – ribose, and three-terminal phosphates, of which the last phosphate bond is rich in captured energy. Enzymatic breakdown of this bond converts ATP back to ADP and iP by exactly opposite reaction and releases substantial energy (12 kcal mol-1), used for vital body functions.
To know the entire process, let us compare ATP with a spring-loaded dart gun and iP with a dart. To load the gun, we insert the dart until it clips. In energy terms we use energy (or do work) to contract the spring, thereby generating potential energy on the dart. This situation is analogous to phosphorylation in which energy, generated from various reactions of catabolism, is captured in the phosphate bond of ATP. As per the demand, we release the spring and fire. Consequently, the potential energy is converted to equal kinetic energy, clearly seen as the shot of the dart, which comes out with high energy. This is analogous to the release of iP from ATP, and the high chemical energy resting in the phosphate bond is released, to be used for different body functions.
Source
Plowman, S.A. &. Smith, D.L.(2007). Exercise physiology for health, fitness and performance. Philadelphia: Lippincott Williams & Wilkins.