The Role of macromolecules
Carbohydrates are molecules containing carbon atoms flanked by hydrogen atoms and hydroxyl
groups (H—C—OH). They have two major biochemical roles:
•Carbohydrates are a source of energy that can be released in a form usable by body
•Carbohydrates serve as carbon skeletons that can be rearranged to form new molecules
that are essential for biological structures and functions.
Some carbohydrates are relatively small, with molecular weights of less than 100. Others are true
macromolecules, with molecular weights in the hundreds of thousands.
There are four categories of biologically important carbohydrates, which we will discuss in turn:
•Monosaccharide’s (mono, “one”; saccharide, “sugar”), such as glucose, ribose, and
fructose, are simple sugars. They are the monomers from which the larger carbohydrates are
•Disaccharides (di, “two”) consist of two monosaccharides linked together by covalent
•Oligosaccharides (oligo, “several”) are made up of several (3–20) monosaccharides.
•Polysaccharides (poly, “many”), such as starch, glycogen, and cellulose, are large
polymers composed of hundreds or thousands of monosaccharides.
The general formula for carbohydrates, CH2O, gives the relative proportions of carbon, hydrogen,
and oxygen in a monosaccharide (i.e., the proportions of these atoms are 1:2:1). In disaccharides,
oligosaccharides, and polysaccharides, these proportions differ slightly from the general formula
because two hydrogens and an oxygen are lost during each of the condensation reactions that
Different monosaccharides contain different numbers of carbons. Most of the monosaccharides found in
living systems belong to the d series of optical isomers. (Recall also that only l-amino acids occur in
proteins—there is amazing specificity in biology!) Some monosaccharides are structural isomers, with the
same kinds and numbers of atoms, but in different arrangements. For example, the hexoses (hex, “six”), a
group of structural isomers, all have the formula C6H12O6. Included among the hexoses are glucose,
fructose (so named because it was first found in fruits), mannose, and galactose
Pentoses (pente, “five”) are five-carbon sugars. Two pentoses are of particular biological importance:
ribose and deoxyribose form part of the backbones of the nucleic acids RNA and DNA, respectively .
These two pentoses are not isomers; rather, one oxygen atom is missing from carbon 2 in deoxyribose
(de-, “absent”). The absence of this oxygen atom is an important distinction between RNA and DNA.
Optical isomers: Two isomers that are mirror images of each other.