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Many bacteria are able to build all of their own molecules from a few simple raw materials like carbon dioxide, oxygen, and ammonia. A single bacterial cell knows how to build several thousand types of proteins, including motors, girders, toxins, catalysts, and construction machinery. This cell also builds hundreds of RNA molecules with different orderings of nucleotides, as well as a diverse collection of lipids, sugar polymers, and a bewildering collection of exotic small molecules. All of these different molecules must be created from scratch, using only the molecules that the cell eats, drinks, and breathes.

To make proteins, we require a few amino acids in our diet that we cannot make from scratch ourselves. We also require vitamins—small molecules that we cannot make, but that are absolutely required by some of our enzymes (vitamins are discussed in more detail in the last chapter). We do, however, make hundreds of useful molecules, including lipids, most amino acids, nucleotides, and sugars. Our cells also make many unusual molecules for specialized tasks, such as thyroid hormones that include iodine atoms, tiny nerve signal transmitters, natural painkillers, and bright red molecules that use iron ions to capture oxygen.

Even rocks, mountains, and entire planets are subject to this unstoppable force of decay and dilution—over millennia they weather and crumble, and ultimately they will be scattered as dust. Living things resist these forces of decay. They strive to stay the same in spite of whatever challenges they face. Our bodies create heat when it is cold outside, resisting the natural tendency to cool down. Fig. 1 Information Transfer The binding of transfer RNA to messenger RNA is the step where genetic information is translated into a sequence of amino acids in a protein.