phosphoryl of the cell or between compartments in eukaryotic cells. The universal occurrence of the same set of compounds in living cells is a manifestation of the universality of metabolic design, reflecting the evolutionary conservation of metabolic pathways that developed in the earliest cells.

There are other small biomolecules, specific to certain types of cells or organisms. For example, vascular plants contain, in addition to the universal set, small molecules called secondary metabolites, which play a role specific to plant life. These metabolites include compounds that give plants their characteristic scents, and compounds such as morphine, quinine, nicotine, and caffeine that are valued for their physiological effects on humans but used for other purposes by plants. The entire collection of small molecules in a given cell has been called that cell's metabolome, in parallel with the term "genome" (defined earlier and expanded on in

Section 1.4). If we knew the composition of a cell's metabolome, we could predict which enzymes and metabolic pathways were active in that cell.

Macromolecules Are the Major Constituents of Cells

Many biological molecules are macromolecules, polymers of high molecular weight assembled from relatively simple precursors. Proteins, nucleic acids, and polysaccharides are produced by the polymerization of relatively small compounds with molecular weights of 500 or less. The number of polymerized units can range from tens to millions. Synthesis of macromolecules is a major energy-consuming activity of cells. Macromol-ecules themselves may be further assembled into supramolecular complexes, forming functional units such as ribosomes. Table 1-2 shows the major classes of biomolecules in the bacterium E. coli.

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