Sugars are part of a class of biomolecules called carbohydrates that are made up of carbon, hydrogen, and oxygen atoms. The simplest sugars are called monosaccharides, but, like amino acids, they can polymerize to form disaccharides and oligosaccharides. Sugars can usually exist in either a ring form or an open chain form, as shown in the picture below. The sugar in this picture is called glucose, which is the sugar most commonly used for energy in the human body.