Carbohydrates are the most abundant organic compounds in the plant world. They are natural products, produced by photosynthesis processes in plants. They act as storehouses of chemical energy (glucose, starch, and glycogen), they are components of supportive structures in plants (cellulose), crustacean shells (chitin), and connective tissues in animals (acidic polysaccharides); and they are essential components of nucleic acids (D-ribose and 2-deoxy-D-ribose (DNA and RNA)) and for energy transfer (in ATP). Carbohydrates are known as sugar and they composed of monosaccharaides, disaccharides and polysaccharides. The chemistry of carbohydrates most closely resembles that of alcohols, aldehydes, and ketones.