Phosphorus (P) has a larger atomic mass than C or N, and it is less abundant in the universe than those two elements. However, it is more abundant on the whole of the Earth than is either C or N. P lacks a significant gaseous phase, imparting very different biogeochemical behaviors to this element compared to C or N. Its abundance in the aqueous phase is strongly controlled by the presence of other elements, most notably oxygen (O) and iron (Fe). Most of the P at Earth's surface is found in the oxidized form of PO4~ (phosphate). Phosphates can polymerize, and the reaction
has a high free energy change but occurs slowly without a catalyst; these characteristics make P a highly suitable element to be involved in transfer of chemical energy around a cell. The soft tissue of living organisms is generally 0.3-2% P by dry mass.
Phosphorus is widespread in biochemistry, being found in relatively high abundance in membranes and being involved in many biochemical reactions throughout a cell. However, P is thought to have two focal roles in whole-organism stoichiometry. First, P alternates with sugars in forming the backbone of nucleic acids. As rRNA, nucleic acid serves a key role in allowing a cell to make proteins. Eukaryotic ribosomes are ^50% rRNA while prokaryotic ribosomes are ^65% rRNA. In this way, we can say that phosphorus is the elemental engine for protein manufacture. Second, P in the form of the mineral apatite is a major component of bone and thus has a major structural role in vertebrates. In small, unicellular heterotrophs such as bacteria and in rapidly growing larvae of insects, generally around 50% and sometimes as much as 90% of total cellular phosphorus may be contained in nucleic acids, especially rRNA. In larger heterotrophs, structural support tissues such as bones take on greater importance in body P-pools and in accounting for differences between species. These interspecific differences can be ancient in evolutionary origin and represent major differences in body plans comparable to taxonomic levels such as family or above.
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