Sugar is fundamental to the nutrition of plants and animals, and exists in one form or another in all living creatures. Sugars are carbohydrates, i.e. their molecules consist of carbon, hydrogen, and oxygen. (These molecules are relatively small. The more complex carbohydrates such as starch and cellulose are mainly composed of many sugar molecules joined together.) There are many different chemical forms of sugar.
The simplest sugars are termed monosaccharides, ‘single sugars’. Of these, the fundamental one is