Food molecules contain biochemical energy which is made available by a process called respiration.
Respiration is the process within cells by which living things break down food chemicals in their bodies and use them as a source of energy.
The proteins, lipids and polysaccharides that make up most of the food we eat must be broken down into smaller molecules before our cells can use them either as a source of energy or as building blocks for other molecules. This process is named catabolism and occurs in 3 stages.
Stage 1 is the enzymatic breakdown of food molecules in the digestion process into their monomer subunits- amino acids, glucose and glycerol.
Stage 2 is the process of glycolysis where each molecule of glucose is converted to pyruvate.
Stage 3 is production of ATP, the form of energy needed by the body to function. This stage takes place in the mitochondria of the cells. ATP is produced from conversion of pyruvate to acetylCoA in a process called the Citric Acid Cycle.
