Metabolism is the sum of all the reactions in a cell or in the body. The energy transferred by respiration is used for the continual, enzyme controlled processes of metabolism that synthesise new molecules. Sugars, amino acids, fatty acids and glycerol are the small building blocks that these reactions join together or release when large molecules are broken down.
Metabolic reactions include the conversion of glucose to starch, glycogen and cellulose. Lipids are formed from one molecule of glycerol and three molecules of fatty acids.
Glucose and nitrate ions are used to form amino acids, which are then joined to make proteins. Respiration is also part of metabolism. When there is excess protein, it is broken down to form urea, which is excreted.