Metabolism is the sum of all the reactions in a cell or the body. The energy transferred by respiration in cells is used for the continual enzyme controlled processes of metabolism that synthesise new molecules.
Metabolism includes making large molecules from small ones. Glucose is converted into starch, glycogen and cellulose. A lipid molecule is formed from one molecule of glycerol and three molecules of fatty acids. Glucose and nitrate ions are used to form amino acids, which in turn are used to synthesise proteins.
Metabolism also includes breakdown. Respiration breaks down glucose to transfer energy. Excess proteins are broken down to form urea, which is excreted. So sugars, amino acids, fatty acids and glycerol are the small molecules used to build, and released by the breakdown of, carbohydrates, proteins and lipids.