Cysteine reversible, and subject to regulation in response to various intracellular and extracellular signals.

Cells Build Supramolecular Structures

Macromolecules and their monomeric subunits differ greatly in size (Fig. 1-10). A molecule of alanine is less than 0.5 nm long. Hemoglobin, the oxygen-carrying protein of erythrocytes (red blood cells), consists of nearly 600 amino acid subunits in four long chains, folded into globular shapes and associated in a structure 5.5 nm in diameter. In turn, proteins are much smaller than ribo-somes (about 20 nm in diameter), which are in turn much smaller than organelles such as mitochondria, typically 1,000 nm in diameter. It is a long jump from simple biomolecules to cellular structures that can be seen

FIGURE 1-10 The organic compounds from which most cellular materials are constructed: the ABCs of biochemistry. Shown here are (a) six of the 20 amino acids from which all proteins are built (the side chains are shaded pink); (b) the five nitrogenous bases, two five-carbon sugars, and phosphoric acid from which all nucleic acids are built; (c) five components of membrane lipids; and (d) d-glucose, the parent sugar from which most carbohydrates are derived. Note that phosphoric acid is a component of both nucleic acids and membrane lipids.

(b) The components of nucleic acids

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