Figure 2–6 The four main families of small organic molecules in cells. These small molecules form the monomeric building blocks, or subunits, for most of the macromolecules and other assemblies of the cell. some, such as the sugars and the fatty acids, are also energy sources. Their structures are outlined here and shown in more detail in the panels at the end of this chapter and in Chapter 3.
long chains (Figure 2–8). They have remarkable properties that could not have been predicted from their simple constituents.
Proteins are abundant and spectacularly versatile, performing thousands of distinct functions in cells. Many proteins serve as enzymes, the catalysts that facilitate the many covalent bond-making and bond-breaking reactions that the cell needs. Enzymes catalyze all of the reactions whereby cells extract energy from food molecules, for example, and an enzyme called ribulose bisphosphate carboxylase helps to convert CO2 to sugars in photosynthetic organisms, producing most of the organic matter needed for life on Earth. Other proteins are used to build structural components, such as tubulin, a protein that self-assembles to make the cell’s long microtubules, or histones, proteins that compact the DNA in chromosomes. Yet other proteins act as molecular motors to produce force and
Figure 2–7 The distribution of molecules in cells. The approximate composition of a bacterial cell is shown by weight. The composition of an animal cell is similar, even though its volume is roughly 1000 times greater. note that macromolecules dominate. The major inorganic ions include na+, K+, mg2+, Ca2+, and Cl–.
movement, as for myosin in muscle. Proteins perform many other functions, and we shall examine the molecular basis for many of them later in this book.
Although the chemical reactions for adding subunits to each polymer are different in detail for proteins, nucleic acids, and polysaccharides, they share important features. Each polymer grows by the addition of a monomer onto the end of a growing chain in a condensation reaction, in which one molecule of water is lost with each subunit added (Figure 2–9). The stepwise polymerization of monomers into a long chain is a simple way to manufacture a large, complex molecule, since the subunits are added by the same reaction performed over and over again by the same set of enzymes. Apart from some of the polysaccharides, most macromolecules are made from a limited set of monomers that are slightly different from one another—for example, the 20 different amino acids from which proteins are made. It is critical to life that the polymer chain is not assembled at random from these subunits; instead the subunits are added in a precise order, or sequence. The elaborate mechanisms that allow enzymes to accomplish this task are described in detail in Chapters 5 and 6.
noncovalent Bonds specify Both the precise shape of a macromolecule and its Binding to other molecules
Most of the covalent bonds in a macromolecule allow rotation of the atoms they join, giving the polymer chain great flexibility. In principle, this allows a macromolecule to adopt an almost unlimited number of shapes, or conformations, as random thermal energy causes the polymer chain to writhe and rotate. However, the shapes of most biological macromolecules are highly constrained because of the many weak noncovalent bonds that form between different parts of the same molecule. If these noncovalent bonds are formed in sufficient numbers, the polymer chain can strongly prefer one particular conformation, determined by the linear sequence of monomers in its chain. Most protein molecules and many of the small RNA molecules found in cells fold tightly into a highly preferred conformation in this way (Figure 2–10).