When one considers the chemistry of any element in the context of biological organisms, it is exceedingly important to observe it in relationship to the chemistries of all the other elements in the environment that are used, or even not used, by cells. One major concern is the limitation of the availability of the element concerned, in part because of the presence of other elements, which may have changed during the evolution of Earth and its organisms. This is a geochemical problem, but even when the element has an easily available form in a compound, the accessibility to biology may be restricted through this very combination. One or two simple points then need stressing. Elements such as H, C, O, and, to lesser extent, N, S, and P, the major elements of bioorganic chemistry, are all abundant in the universe and are geochemically available. However, unfortunately, several of these elements are locked up in compounds so their accessibility for transformation into manipulatable atomic elements in cells is very restricted - consider H in H₂O, C in CO₂, N in N₂, and S in SO₄ ^2-, where in each case the respective elements, H, C, N, and S are difficult for organisms to obtain. Only O and P as O₂ today and HPO₄ ^2- are genuinely available and in a suitable form for immediate use in a cell. By way of comparison, metal ions such as calcium and a few nonmetals such as chlorine (as chloride) are quite abundant and are relatively freely available as ions in the forms in which organisms use them.