Glucose, a simple monosaccharide sugar, is one of the most important carbohydrates
and is used as a source of energy in animals and plants. Glucose is one of the main
products of photosynthesis and starts respiration. The natural form (D-glucose) is
also referred to as dextrose, especially in the food industry.
Carbohydrates are the human body's key source of energy, providing 4 calories (17
kilojoules) of food energy per gram. Breakdown of carbohydrates (e.g. starch) yields
mono- and disaccharides, most of which are glucose. Through glycolysis, glucose is
immediately involved in the production of ATP, the cell's energy carrier. In
addition, it is critical in the production of protein and in lipid metabolism. As the
central nervous system does not metabolise lipids, it is more dependent on glucose
than other tissues.
Glucose is absorbed into the bloodstream through the intestinal wall. Some of this
glucose goes directly to fuel brain cells, while the rest makes its way to the liver
and muscles, where it is stored as glycogen ("animal starch"), and to fat cells,
where it is stored as fat. Glycogen is the body's auxiliary energy source, tapped and
converted back into glucose when it needs more energy. Although stored fat can also
serve as a backup source of energy, it is never directly converted into glucose.
Fructose and galactose, other sugar products resulting from the breakdown of
carbohydrates, go straight to the liver, where they are converted into glucose.
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