2.2 Chemical Building Blocks of Cells
A common theme in biology is the construction of large macromolecules and macromolecular structures out of smaller molecular subunits, which can be thought of as building blocks. Often these subunits are similar or identical. The three main types of biological macromolecules—proteins, nucleic acids, and polysaccharides—are all polymers composed of multiple covalently linked small molecules, or monomers (Figure 2-13). Proteins are linear polymers containing up to several thousand amino acids linked by peptide bonds. Nucleic acids are linear polymers containing hundreds to millions of nucleotides linked by phosphodiester bonds. Polysaccharides are linear or branched polymers of monosaccharides (sugars) such as glucose linked by glycosidic bonds.
FIGURE 2-13 Overview of the cell’s principal chemical building blocks. (Top) The three major types of biological macromolecules are each assembled by the polymerization of multiple small molecules (monomers) of a particular type: proteins from amino acids (see Chapter 3), nucleic acids from nucleotides (see Chapter 5), and polysaccharides from monosaccharides (sugars). Each monomer is covalently linked into the polymer by a reaction whose net result is loss of a water molecule (dehydration). (Bottom) In contrast, phospholipid monomers noncovalently assemble into a bilayer structure, which forms the basis of all cellular membranes (see Chapter 7).
Although the actual mechanisms of covalent bond formation between monomers are complex, as we will see, the formation of a covalent bond between two monomers usually involves the net loss of a hydrogen (H) from one monomer and a hydroxyl (OH) from the other monomer—or the net loss of one water molecule—and can therefore be thought of as a dehydration reaction. The breakdown, or cleavage, of a covalent bond in a polymer that releases a monomeric subunit involves the reverse reaction, or the addition of water, called hydrolysis. The covalent bonds that link monomers together are normally stable under normal biological conditions (e.g., 37 °C, neutral pH), so these biopolymers are stable and can perform a wide variety of jobs in cells, such as storing information, catalyzing chemical reactions, serving as structural elements that define cell shape and movement, and many others. Macromolecular structures can also be assembled using noncovalent interactions. The two-ply, or “bilayer,” structure of cellular membranes is built up by the noncovalent assembly of many thousands of small molecules called phospholipids (see Figure 2-13).
In this chapter, we focus on the chemical building blocks making up cells—amino acids, nucleotides, sugars, and phospholipids. The structure, function, and assembly of proteins, nucleic acids, polysaccharides, and biomembranes are discussed in subsequent chapters.