All carbohydrates are made from carbon, hydrogen and oxygen. They have twice as many hydrogen atoms as oxygen atoms. The word "carbohydrate" is a combination of the names of these elements and means "watered carbon." The presence of carbon makes carbohydrates organic, rather than inorganic, compounds.
Carbohydrate molecules vary in size. While monosaccharides and disaccharides are very small, polysaccharides are comparatively larger. Monosaccharides and disaccharides are considered simple sugars. This group includes the sugars found in candy and table sugar. The larger polysaccharides, which are formed when small units of carbohydrates combine, are categorized as starches, which are found in foods such as pasta and potatoes. Carbohydrates have many functions in various organisms. Glucose, a monosaccharide, provides fuel for cells that is vital for energy production in humans. Plants make their own glucose for energy during photosynthesis. Other monosaccharides are contained in the building blocks of DNA. Ribose is part of RNA, and DNA contains deoxyribose. Carbohydrates also combine to form structures in organisms. Polysaccharide cellulose is used to form plants' strong cell walls that help them hold their shapes. Cotton is made of cellulose. Polysaccharides are also found in the hard, protective shells of sea creatures like lobsters and crabs.