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2.3  Chemicals that Make Cells Work

bind together, they can do so through one of two possible places in the molecule. These are

described as either 1 → 4 or 1 → 6, referring to the numbering of the six carbon atoms in the

glucose molecule.

In addition, the chemical groups that link to the glycosidic bond itself are in general

different, and so it is again possible to have two possible stereoisomers (a chemistry term

simply describing something that has the same chemical formula but different potential spa­

tial arrangements of the constituent atoms), which are described as either α or β; cellulose is a

linear chain structure linked through β(1 → 4) glycosidic bonds containing as few as 100 and

as high as a few thousand glucose subunits; starch is actually a mixture of two types of poly­

saccharide called “amylose” linked through mainly α(1 → 4) glycosidic bonds, and amylo­

pectin that contains α(1 → 4) and as well as several α(1 → 6) links resulting in branching

of the structure; glycogen molecules are primarily linked through α(1 → 4), but roughly for

every 10 glucose subunits, there is an additional link of α(1 → 6), which results in significant

branching structure.

2.3.5  NUCLEIC ACIDS

Nucleic acids include molecules such as DNA and various forms of RNA. These are large

polymers composed of repeating subunits called “nucleotide bases” (Figure 2.3c) characterized

by having a nucleoside component, which is a cyclic molecule containing nitrogen as well

as carbon in a ringed structure, bound to a five-​carbon-​atom monosaccharide called either

“ribose,” in the case of RNA, or a modified form of ribose lacking a specific oxygen atom

called “deoxyribose,” in the case of DNA, in addition to bound phosphate groups. For DNA,

the nucleotide subunits consist of either adenine (A) or guanine (G), which are based on a

chemical structure known as purines, and cytosine (C) or thymine (T), which are based on a

smaller chemical structure known as pyrimidines, whereas for RNA, the thymine is replaced

by uracil (U).

FIGURE 2.6  Sugars. (a) Formation of larger sugars from monomer units of monosaccharide

molecules via loss of water molecule to form a disaccharide molecule. (b) Examples of polysac­

charide molecules.