Glucose is a monosaccharide molecule, meaning it is a sugar. Monosaccharides like glucose can be found in nature in two ways--they may be combined with other sugar units, or they may be found on their own. Humans most frequently consume glucose either combined with the monosaccharide fructose--this forms the compound sucrose, which is table sugar--or combined with many other glucose molecules to make cellulose, which is dietary fiber, or starch. Digestive processes free glucose from other sugar units, and the digestive tract absorbs glucose into the bloodstream for use by the cells. There are three forms of free glucose.
One form of glucose is simply called D-glucose. The "D" refers to the arrangement of alcohol, or OH, groups on the carbon skeleton of the sugar. D-glucose is an open-chain molecule consisting of six carbons arranged into a molecular backbone. There are five OH groups attached to this carbon backbone, forming "arms" or branches off the carbon skeleton. This form of glucose also has an aldehyde group at the top of the carbon skeleton, which consists of an atom of carbon with a double bond to an atom of oxygen, and a single bond to an atom of hydrogen.
In their 2007 book "Biochemistry," Drs. Reginald Garrett and Charles Grisham note that straight chain D-glucose exists only when glucose is free in solution, such as in the bloodstream--glucose bound to other sugar molecules is not found as straight chain D-glucose.
A second form of glucose is called beta-D-glucopyranose. As in straight chain D-glucose, the "D" refers to the arrangement of alcohol groups on the carbon skeleton. Unlike D-glucose, beta-D-glucopyranose has a cyclic carbon skeleton. The ring structure that forms the backbone of the molecule consists of five carbon atoms and one oxygen atom.
Garrett and Grisham explain that rings consisting of these six atoms are called pyranose rings, which explains the modification of the word "glucose" with the suffix "-pyranose." Beta-D-glucopyranose occurs both in solution, as in the bloodstream, and in glucose combined with other sugar units in some instances. Dietary fiber, or cellulose, consists of many units of chemically linked beta-D-glucopyranose.
The final form of glucose in the body and in nature is alpha-D-glucopyranose. Drs. Mary Campbell and Shawn Farrell, authors of the 2005 text “Biochemistry,” explain that the difference between alpha- and beta-D-glucopyranose lies in the orientation of a single alcohol group around the cyclic carbon backbone. This difference, while it seems minor, is actually very significant. Alpha-D-glucopyranose can be found in solution, or combined with other molecules of sugar. Digestible starch is made of many hundreds of linked alpha-D-glucopyranose units, for instance. Humans can split an alpha-linkage, but not an a beta-linkage, which explains why starch is digestible, but dietary fiber is not.