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.