Simple carbohydrates are classified as either monosaccharides or disaccharides. Monosaccharides are the simplest, most basic units of carbohydrates and are made up of only one sugar unit. Glucose and fructose are monosaccharides and are the building blocks of sucrose, a disaccharide.Thus, disaccharides are just a pair of linked sugar molecules. They are formed when two monosaccharides are joined together and a molecule of water is removed -- a dehydration reaction.lucose and fructose are monosaccharides and are the building blocks of sucrose, a disaccharide. Thus, disaccharides are just a pair of linked sugar molecules.