<P> Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown, and interconversion of carbohydrates in living organisms . </P> <P> Carbohydrates are central to many essential metabolic pathways . Plants synthesize carbohydrates from carbon dioxide and water through photosynthesis, allowing them to store energy absorbed from sunlight internally . When animals and fungi consume plants, they use cellular respiration to break down these stored carbohydrates to make energy available to cells . Both animals and plants temporarily store the released energy in the form of high energy molecules, such as ATP, for use in various cellular processes . </P> <P> Although humans consume a variety of carbohydrates, digestion breaks down complex carbohydrates into a few simple monomers for metabolism: glucose, fructose, and galactose . Glucose constitutes about 80% of the products, and is the primary structure that is distributed to cells in the tissues, where it is broken down or stored as glycogen . In aerobic respiration, the main form of cellular respiration used by humans, glucose and oxygen are metabolized to release energy, with carbon dioxide and water as byproducts . Most of the fructose and galactose travel to the liver, where they can be converted to glucose . </P> <P> Some simple carbohydrates have their own enzymatic oxidation pathways, as do only a few of the more complex carbohydrates . The disaccharide lactose, for instance, requires the enzyme lactase to be broken into its monosaccharide components, glucose and galactose . </P>

The major carbohydrate that tissue cells use as fuel is