They connect the arterial and venous systems. Sources of CHO include vegetables, fruit, rice, bread, pasta and whole grains. 1 gm CHO yields 4 kilocalories of energy. They vary from simple sugars containing from three to seven carbon atoms to very complex polymers. Carbohydrates in food provide 4 calories per gram.
Plants manufacture and store carbohydrates as their chief source of energy.
The glucose synthesized in the leaves of plants is used as the basis for more complex forms of carbohydrates. Classification of carbohydrates relates to their structural core of simple sugars, saccharides. Principal monosaccharides that occur in food are glucose and fructose. Three common disaccharides are sucrose, maltose and lactose. Polysaccharides of interest in nutrition include starch, dextrin, glycogen and cellulose.