A system of differential equations was presented which describes the rate of linear and cyclic electron flow through photosystems II and I. The system describes the rate of photochemistry in terms of electrons generated that are available for cellular metabolism, and results in a realistic description of photosynthesis as a function of irradiance without implicit assumptions for the relationship. The system allows a concise and detailed simulation of fluorescence kinetics. The derivation of the general degree of reduction (γx) and its application to translate the rates of photochemistry (or measured fluorescence yields) to steady-state rates of carbon fixation and growth was shown. The efficiency of light-limited photosynthesis (α) was shown to depend on the cellular ratio of carbon to nitrogen. For any given antenna size, α increases with nitrate as N-source, and decreases with ammonium as N-source, if the cellular carbon to nitrogen ratio of the phytoplankton increases. Cyclic electron transport around photosystem I increases the ratio of ATP generated relative to linear electron (e-) flow. The increase of ATP/e- is larger under extreme light-limiting conditions. The long-known fact that protein synthesis saturates at lower light intensities than carbonate synthesis was explained in terms of the decrease of ATP/e- with increasing irradiance and the higher ATP demand of protein synthesis. © 2006 by the Phycological Society of America.