A carbohydrate or saccharide is a group that includes sugars, starch, and cellulose. These perform numerous roles in living organisms, they play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development. There are various types of saccharides, including monosaccharides, disaccharides, and polysaccharides Monosaccharide is the smallest possible sugar unit. They are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. The 5-carbon monosaccharide ribose is an important component of coenzymes (e g., ATP.