Carbohydrates are polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses) composed of C, H and O, or form polyhydroxy aldehydes or ketones upon hydrolysis. Carbohydrates come in the form of monosaccharides, disaccharides, oligosaccharides and polysaccharides. The simplest sugars, monosaccharides and disaccharides, consist of one or two monosaccharide units. Oligosaccharides are generally classified as carbohydrates containing 3-10 monosaccharide units. Polysaccharides contain more than 10 monosaccharide units and can be quite large. Glucose is an aldose, the most important monosaccharide in the body, used by cells as fuel. Other aldohexoses are galactose and mannose, which form part of complex molecules. Fructose is a ketohexose sugar, while ribose is the most important aldose pentose sugar and a component of RNA.