A biophysical basis for modeling element cycling is described. The scheme consists of element cycles, organisms necessary to completely catalyze all the component reactions, and higher organisms as structurally complex systems and as subsystems of more complex ecosystems, all to the degree of complexity supportable by the system power input and rate that elements are made available by the biotic system itself. In the limit, organisms that persist are those that achieve a balance between their power supply and demand and that are able to obtain elements for biosynthesis at rates that match their rates of loss via deaths, excretion, etc. Comparison of power demand, power supply, and rates of obtaining elements as substrate for biosynthesis for the various modes of metabolism permit the calculation of population growth rates. Thus, the foundation is laid for further consideration of element cycling rates via a system of coupled equations in which the equations represent populations of organisms carrying out the element cycle transformations. (Copyright (c) Springer-Verlag Berlin Heidelberg, 1986.)