Glucose is sometimes called blood sugar, because it forms the bulk of the sugar taken up into the bloodstream by the digestive tract. Humans ingest glucose in many ways, but one of the most common is bound to other units of glucose, in the form of starch. Glucose is also commonly found bound to another sugar unit, called fructose, in the form of sucrose, or table sugar. The physical characteristics of the glucose molecule help researchers classify it and identify it in the lab.
Shape and Components
The glucose molecule exists in solution, as in the bloodstream, in any one of three different forms, but it's most stable as a ring. The ring-shaped backbone of a cyclic glucose molecule consists of five atoms of carbon and one atom of oxygen, producing a structure called a pyranose ring, explain biochemists Mary Campbell, Ph.D. and Shawn Farrell, Ph.D. in their book, "Biochemistry." There are a total of 6 atoms of carbon, 6 atoms of oxygen, and 12 atoms of hydrogen in glucose. Most of the oxygen atoms are found outside the main pyranose ring, as substituents or "arms" coming off the backbone. These substituent oxygen atoms have atoms of hydrogen bound to them, making them OH or "alcohol" groups.
Solubility
One of glucose's important physical properties is its ability to dissolve in aqueous, meaning water-based, solution. Since the blood, the extracellular fluid, and the fluid inside cells are all water-based, for glucose to dissolve in the bloodstream and be able to pass into cells, it must also dissolve well in water. Compounds consisting of only carbon and hydrogen show very poor water solubility. Elements such as oxygen and nitrogen, which are common in organic and bioorganic molecules, help increase water solubility. Glucose's six oxygen atoms increase its ability to dissolve in water. In particular, since five of those oxygen atoms are found in the form of alcohol groups, which are very water soluble, glucose dissolves quite readily in water, explain Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry."
Optical Activity
One of glucose's most interesting and least-observed physical properties is that it can rotate polarized light. A solution of pure naturally-occurring glucose rotates light to the right, notes the "CRC Handbook of Chemistry and Physics." While polarized light is rare in nature--in general, this property of glucose is observed only in the lab--the optical activity of glucose nevertheless is important to its ability to bind to receptors and enzymes in the body. A mirror-image of glucose, which doesn't occur naturally and is also called "glucose," rotates light to the left. The mirror image glucose can't bind to enzymes in the body--its shape means that it doesn't fit into receptor sites, the same way that a left hand can't fit into a right glove--meaning that if a human were to obtain a sample of mirror-image glucose, they wouldn't be able to use it for energy. The bioactivity of glucose in the body is a product of its optical activity.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "CRC Handbook of Chemistry and Physics"; David Lide; 1974
