Monosaccharides are commonly used as a source of energy in respiration. This is due to the large number of carbon-hydrogen bonds which can be broken to release energy.
Monosaccharide also form the building blocks of larger molecules. Glucose molecules can be polymerised to make polysaccharides such as starch, glycogen and cellulose. Ribose (a pentose as it contains five carbon atoms) is used to make RNA (ribonucleic acid) and ATP.