Essential Cell Biology 5th edition

Small Molecules in Cells
51
with other atoms (see Figure 2–9). Most importantly, one carbon atom
can link to other carbon atoms through highly stable covalent C–C bonds,
producing rings and chains that can form the backbone of complex molecules
with no obvious upper limit to their size. These carbon-containing
compounds are called organic molecules. By contrast, all other molecules,
including water, are said to be inorganic.
In addition to containing carbon, the organic molecules produced by cells
frequently contain specific combinations of atoms, such as the methyl
(–CH 3 ), hydroxyl (–OH), carboxyl (–COOH), carbonyl (–C=O), phosphoryl
(–PO 3 2– ), and amino (–NH 2 ) groups. Each of these chemical groups has
distinct chemical and physical properties that influence the behavior of
the molecule in which the group occurs, including whether the molecule
tends to gain or lose protons when dissolved in water and with which
other molecules it will interact. Knowing these groups and their chemical
properties greatly simplifies understanding the chemistry of life. The most
common chemical groups and some of their properties are summarized
in Panel 2–1 (pp. 66–67).
Cells Contain Four Major Families of Small Organic
Molecules
The small organic molecules of the cell are carbon compounds with
molecular weights in the range 100–1000 that contain up to 30 or so
carbon atoms. They are usually found free in solution in the cytosol and
have many different roles. Some are used as monomer subunits to construct
the cell’s polymeric macromolecules—its proteins, nucleic acids,
and large polysaccharides. Others serve as energy sources, being broken
down and transformed into other small molecules in a maze of
intracellular metabolic pathways. Many have more than one role in the
cell—acting, for example, as both a potential subunit for a macromolecule
and as an energy source. The small organic molecules are much
less abundant than the organic macromolecules, accounting for only
about one-tenth of the total mass of organic matter in a cell. But small
organic molecules adopt a huge variety of chemical forms. Nearly 4000
different kinds of small organic molecules have been detected in the
well-studied bacterium Escherichia coli.
All organic molecules are synthesized from—and are broken down
into—the same set of simple compounds. Both their synthesis and their
breakdown occur through sequences of simple chemical changes that
are limited in variety and follow step-by-step rules. As a consequence,
the compounds in a cell are chemically related, and most can be classified
into a small number of distinct families. Broadly speaking, cells
contain four major families of small organic molecules: the sugars, the
fatty acids, the amino acids, and the nucleotides (Figure 2–17). Although
many compounds present in cells do not fit into these categories, these
four families of small organic molecules—together with the macromolecules
made by linking them into long chains—account for a large fraction
of a cell’s mass (Table 2–2).
small organic building blocks
of the cell
SUGARS
FATTY ACIDS
AMINO ACIDS
NUCLEOTIDES
larger organic molecules
of the cell
POLYSACCHARIDES, GLYCOGEN,
AND STARCH (IN PLANTS)
FATS AND MEMBRANE LIPIDS
PROTEINS
NUCLEIC ACIDS
Figure 2–17 Sugars, fatty acids, amino
acids, and nucleotides are the four main
families of small organic molecules
in cells. They form the monomeric
building blocks, or subunits, for larger
organic molecules, including most of the
macromolecules and other molecular
assemblies of the cell. Some, like the sugars
and the fatty acids, are also energy sources.