Glucose (Glc), a monosaccharide (or simple sugar) also known as grape sugar, is an important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration in both prokaryotes and eukaryotes. The name comes from the Greek word glykys (lang: γλυκύς), meaning "sweet", plus the suffix "-ose" which denotes a sugar.

Two stereoisomers of the aldohexose sugars are known as glucose, only one of which (D-glucose) is biologically active. This form (D-glucose) is often referred to as dextrose monohydrate, or, especially in the food industry, simply dextrose (from dextrorotatory glucose). This article deals with the D-form of glucose. The mirror-image of the molecule, L-glucose, cannot be metabolized by cells in the biochemical process known as glycolysis.

Structure

Glucose (C₆H₁₂O₆) contains six carbon atoms, one of which is part of a sugar group, and is therefore referred to as an aldohexose. In solution, the glucose molecule can exist in an open-chain (acyclic) form and a ring (cyclic) form (in equilibrium). The cyclic form is the result of a covalent bond between the aldehyde C atom and the C-5 hydroxyl group to form a six-membered cyclic hemiacetal. At pH 7 the cyclic form is predominant. In the solid phase, glucose assumes the cyclic form. Because the ring contains five carbon atoms and one oxygen atom, which resembles the structure of pyran, the cyclic form of glucose is also referred to as glucopyranose. In this ring, each carbon is linked to a hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH₂OH group. Glucose is commonly available in the form of a white substance or as a solid crystal. It can also be dissolved in water as an aqueous solution.

Isomers

Aldohexose sugars have 4 chiral centers giving 2⁴ = 16 stereoisomers. These are split into two groups, L and D, with 8 sugars in each. Glucose is one of these sugars, and L-glucose and D-glucose are two of the stereoisomers. Only 7 of these are found in living organisms, of which D-glucose (Glu), D-galactose (Gal), and D-mannose (Man) are the most important. These eight isomers (including glucose itself) are related as diastereoisomers and belong to the D series.

An additional asymmetric center at C-1 (called the anomeric carbon atom) is created when glucose cyclizes and two ring structures called anomers are formed as α-glucose and β-glucose. These anomers differ structurally by the relative positioning of the hydroxyl group linked to C-1, and the group at C-6 which is termed the reference carbon. When D-glucose is drawn as a Haworth projection or in the standard chain conformation, the designation α means that the hydroxyl group attached to C-1 is positioned trans to the -CH₂OH group at C-5, while β means it is cis. Another popular method of distinguishing α from β is by observing whether the C-1 hydroxyl is below or above the plane of the ring; but this method is an inaccurate definition, and may fail if the glucose ring is drawn upside down or in an alternative chair conformation. The α and β forms interconvert over a timescale of hours in aqueous solution, to a final stable ratio of α:β 36:64, in a process called mutarotation.