Having reviewed the ways atoms combine into molecules and how these molecules behave in an aqueous environment, we now examine the main classes of small molecules found in cells. We shall see that a few categories of molecules, formed from a handful of different elements, give rise to all the extraordinary richness of form and behavior shown by living things.
If we disregard water and inorganic ions such as potassium, nearly all the molecules in a cell are based on carbon. Carbon is outstanding among all the elements in its ability to form large molecules; silicon is a poor second. Because carbon is small and has four electrons and four vacancies in its outermost shell, a carbon atom can form four covalent bonds with other atoms. Most important, one carbon atom can join to other carbon atoms through highly stable covalent C–C bonds to form chains and rings and hence generate large and complex molecules with no obvious upper limit to their size. The carbon compounds made by cells are called organic molecules. In contrast, all other molecules, including water, are said to be inorganic.
Certain combinations of atoms, such as the methyl (–CH3), hydroxyl (–OH), carboxyl (–COOH), carbonyl (–C=O), phosphate (–PO32–), sulfhydryl (–SH), and amino (–NH2) groups, occur repeatedly in the molecules made by cells. Each such chemical group has distinct chemical and physical properties that influence the behavior of the molecule in which the group occurs. The most common chemical groups and some of their properties are summarized in Panel 2–1, pp. 90–91.
Cells Contain four major families of small organic molecules
The small organic molecules of the cell are carbon-based compounds that have molecular weights in the range of 100–1000 and contain up to 30 or so carbon atoms. They are usually found free in solution and have many different fates. Some are used as monomer subunits to construct giant polymeric macromolecules— proteins, nucleic acids, and large polysaccharides. Others act as energy sources and are broken down and transformed into other small molecules in a maze of intracellular metabolic pathways. Many small molecules have more than one role in the cell—for example, acting both as a potential subunit for a macromolecule and as an energy source. 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. As a rough guess, there may be a thousand different kinds of these small molecules in a typical cell.
All organic molecules are synthesized from and are broken down into the same set of simple compounds. As a consequence, the compounds in a cell are chemically related and most can be classified into a few distinct families. Broadly speaking, cells contain four major families of small organic molecules: the sugars, the fatty acids, the nucleotides, and the amino acids (Figure 2–6). 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 the cell mass.
Amino acids and the proteins that they form will be the subject of Chapter 3. A summary of the structures and properties of the remaining three families— sugars,  fatty acids, and nucleotides—is presented in Panels 2–4, 2–5, and 2–6, respectively (see pages 96–101).
The Chemistry of Cells is Dominated by macromolecules with Remarkable properties
By weight, macromolecules are the most abundant carbon-containing molecules in a living cell (Figure 2–7). They are the principal building blocks from which a cell is constructed and also the components that confer the most distinctive properties of living things. The macromolecules in cells are polymers that are constructed by covalently linking small organic molecules (called monomers) into of the cell of the cell
SUGARS FATTY ACIDS POLYSACCHARIDES AMINO ACIDS NUCLEOTIDES PROTEINS FATS, LIPIDS, MEMBRANES NUCLEIC ACIDS