21. Lipids and Carbohydrates   Previous PageNext Page

Carbohydrates are sugars and sugar derivatives. They have the overall formula Cx(H2O)y, in which x and y are integers. This composition led to the original erroneous impression that they in some sense were "hydrates of carbon," which gave them their name. Simple sugars, or monosaccharides, have the composition (CH2O)n, in which n can be three to six or more.

Ribose is a pentose, or a sugar with n = 5, and a derivative of ribose is an important part of the backbone of the DNA chain, which will be encountered in the next chapter. The ribose molecule is shown on the opposite page in both its open-chain and closed-ring forms. These forms are in equilibrium in aqueous solution, with the closed-ring form predominating.

The ring can close in one of two ways, leading to different positions of the -OH group on carbon atom one, to produce b-D-ribose or b-Dribose, as shown. If one were to begin with a pure sample of either type, equilibrium in solution quickly would lead to a 50-50 mixture of the a and b isomers.

  Page 14 of 25 HomeGlossary