Each of the 100+ elements in the periodic table has characteristic chemical and physical features governing properties like the elements it will bond with, how easily it will ionize, etc. These properties arise mostly from the atomic size of each element and the arrangement of electrons within shells surrounding the nucleus. Metals such as Fe or Mo for instance can exist in different oxidation-reduction states, and thus these elements are involved in electron transport in membranes and in other biological locations. Other elements such as C or N are very useful for making highly complex three-dimensional (3-D) shapes. Thirty or more elements are thought to be essential to the growth of at least some organisms. This number and range of reactivities give biological systems a great range of chemical behaviors to choose from in order to organize their activities. The physical and chemical properties of elements have a great bearing on ecological stoichiometry.
As discussed above, differences in organism elemental content between species are very relevant in ecological stoichiometry and ecology in general. Nevertheless, some elements are consistently high in abundance in biological systems while others are consistently rare. For example, the chemical composition for a living human can be written:
This formula combines all the countless different compounds in a human being into a single abstract 'molecule'. Ecological stoichiometry asks how far this analogy to a single complex molecule will take us.
Ecological stoichiometric principles should apply to any element, and possibly to some of the less-reactive biochemicals as well. However, stoichiometric analysis has focused most on several elements that make up moderate to large proportions of living biomass and that also may become limiting to organism growth. The four elements considered below relate strongly to ecological dynamics and evolutionary fitness.
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