Because all cells make DNA, RNA, and protein, and these macromolecules are
composed of the same set of subunits in every case, all cells have to contain
and manipulate a similar collection of small molecules, including simple
sugars, nucleotides, and amino acids, as well as other substances that are
universally required for their synthesis. All cells, for example, require the
phosphorylated nucleotide
for the synthesis of DNA and RNA; and all cells also make and consume this
molecule as a carrier of free energy and phosphate groups to drive many other
chemical reactions.
Although all cells function as biochemical factories of a broadly similar type,
many of the details of their small-molecule transactions differ, and it is not as
easy as it is for the informational macromolecules to point out the features that
are strictly universal. Some organisms, such as plants, require only the
simplest of nutrients and harness the energy of sunlight to make from these
almost all their own small organic molecules; other organisms, such as
animals, feed on living things and obtain many of their organic molecules
ready-made. We return to this point below.ATP (adenosine triphosphate) as a building block
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