Glucose

Glucose is a monosaccharide, a single sugar unit, with the formula C6H12O6. It is a hexose, or six-carbon sugar, and because it carries an aldehyde group it is classed as an aldose. Fructose and galactose share its formula but arrange their atoms differently. In water, most glucose molecules close into a six-membered ring, which takes an alpha or a beta form depending on which way one hydroxyl group points.

Plants and other photosynthetic organisms make glucose from carbon dioxide and water using the energy of light, and nearly all living things break it down through glycolysis and cellular respiration to make ATP. Glucose is also the building block of larger carbohydrates: plants store it as starch, animals as glycogen, and plants join beta-glucose units into cellulose for their cell walls. Because the brain relies heavily on glucose, the body holds blood glucose within a narrow range using hormones from the pancreas. Insulin helps cells take glucose up after a meal, and glucagon raises blood levels between meals by releasing stored glucose and promoting gluconeogenesis.

In hospitals glucose is often called dextrose, especially in intravenous fluids. A persistently high blood glucose is the defining feature of diabetes mellitus, and glucose tests are among the most common blood tests.