21. Lipids and Carbohydrates   Previous PageNext Page

The most common sugar of all, glucose, is a hexose with n = 6, and with the molecular formula C6H12O6. Glucose also equilibrates between a closed-ring form and an open chain in aqueous solution. The ring form is shown on the facing page. Glucose has the chair structure of the cyclohexane ring seen in Chapter 19, with all of its -OH groups extending out from the perimeter of the ring (the "equatorial" positions) where they are farthest from one another, and hence most stable.

Each of two other common hexoses, mannose and galactose, has a single -OH group moved from the equatorial position to an axial position, extending up or down from the average plane of the puckered six membered ring.

The same atoms that are present in these hexoses also can combine into a five-membered ring, still with the molecular formula C6H12O6. This is fructose (fruit sugar), which has a ring structure similar to ribose (right) although it is a hexose and not a pentose.




  Page 15 of 25 HomeGlossary