42 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
Figure 2.13:(Molecular structure.) J. Watson and F. Crick demonstrate the complementarity of DNA base pairs.
The dotted lines denote hydrogen bonds (see Chapter 7). The shapes and chemical structure of the bases allow
hydrogen bonds to form optimally only between adenine (A) and thymine (T) and between guanine (G) and cytosine
(C); in these pairings, atoms that are able to form hydrogen bonds can be brought close together without distorting
the bases’ geometries. [Cartoon by Larry Gonick, from (Gonick & Wheelis, 1991).] [Copyrighted figure; permission
pending.]
Two more classes of small molecules will be of special interest to us. The first of these, the
“fatty acids,” have a simple structure: They consist of a chain of carbon atoms (for example, 15
for palmitic acid, derived from palm oil), with a carboxyl group (-COOH) at the end. Fatty acids
are partly important as building blocks of the phospholipids to be discussed in the next subsection.
Finally, theamino acidsare a group of about twenty building blocks from which all proteins are
constructed (Figure 2.15). As shown in the figure, each amino acid has a common central element,
with a “plug” at one end (the carboxyl group, –COOH) and a “socket” at the other (theamino
group,–NH 2 ). Attached to the side of the central carbon atom (the “α-carbon”) is one of about
twenty different side groups (or “residues,” generically denoted by R in Figure 2.15a) determining
the identity of the amino acid; for example, alanine is the amino acid whose side group is –CH 3.
Protein synthesis consists of successively attaching the socket of the next amino acid to the plug of
the previous one by the “condensation reaction” shown, creating a polymer called apolypeptide.
The C-N bond formed in this process is called thepeptide bond.Section 2.2.3 and Chapter 9 will
sketch how polypeptides turn into functioning proteins.
2.2.2 Medium-size molecules
Ahuge number of medium-sized molecules can be formed from the handful of atoms used by living
organisms. Remarkably, only a tiny subset of these are actually used by living organisms. Indeed,
the list of possible compounds with mass under 25 000 times that of water probably runs into the
billions, and yet fewer than a hundred of these (and their polymers) account for most of the weight
of any given cell (see Table 2.1).