Sugar is a carbohydrate and is a rich source of energy. Its oxidation takes place in several small steps in the cell . Each step is controlled by its own specific enzyme.

Simple one molecule saccharides are known as monosaccharides. They are very soluble in water due to the presence of many hydroxy (OH) functional groups.

Monosaccharides include, glucose, fructose and galactose.

Glucose is the most abundant monosaccharide and has the molecular formula C6H12O6

Monosaccharides can link together via their hydroxy functional groups to form long carbohydrate polymers. Glucose molecules link together to form starch and cellulose. The reaction is a condensation reaction with water given off. An ether link is formed when the monosaccharide molecules link via their hydroxy functional groups. See the animation below.
A disaccharide is formed when two monosaccharides undergo a condensation reaction. As seen in the animation above, during such a condensation reaction a water molecule is formed. When two glucose molecules undergo a condensation reaction a disaccharide known as maltose is formed as well as water. Disaccharides, such as maltose, are very soluble in water. Other common disaccharides are lactose and fructose.