 Welcome to the carbohydrates. In this module, you'll explore what carbohydrates are. Carbohydrates are the sugars, starches, and fiber found in the foods we eat. They are one of the main nutrients in our diet. Carbohydrates are organic molecules with the general formula of C-H-O in a 1-2-2-2-1 ratio. In this formula, C stands for carbon, H for hydrogen, and O for oxygen. Although carbohydrates constitute only 1-2% of cell mass, they provide the raw fuel for cellular energy production. Carbohydrates are classified according to molecular size and solubility. In general, the smaller molecules are more soluble than the larger ones. Monosaccharides include glucose, fructose, galactose, deoxyribose, and ribose. Monosaccharides are a single unit of sugar and are the smallest carbohydrate. Glucose, a 6-carbon sugar, hexose, is the sugar in our blood. Fructose, the sugar that sweetens fruit, and galactose, the sugar found in milk, have the same chemical formula as glucose and are therefore isomers of glucose. Isomers have the same composition but have a different arrangement of their atoms and properties. Glucose can have a straight chain of carbon atoms. However, it's more common to find them in a ring structure. Two other 5-carbon sugars, or monosaccharides, called pentose, used in nucleic acid synthesis are deoxyribose and ribose. Disaccharides include sucrose, lactose, and maltose. Two monosaccharides join together by dehydration synthesis to form a disaccharide molecule. Sucrose, or table sugar, glucose plus fructose equals sucrose plus water. Lactose, or milk sugar, glucose plus galactose equals lactose plus water. Maltose, or malt sugar, glucose plus glucose equals maltose plus water. Polysaccharides include starch, cellulose, and glycogen. These long, chain-like polymers make ideal storage products due to their insoluibility. Starch is the storage molecule synthesized from glucose by plants. Cellulose, which is also synthesized by plants for cell wall construction, is indigestible because we lack enzymes for it. Cellulose provides fiber to promote peristalsis. Glycogen is the carbohydrate storage molecule found in muscle and liver cells. When blood sugar drops, liver cells hydrolyze glycogen and release glucose to the blood. Congratulations! You've completed biomolecules, the carbohydrates.