• Light reactions increase the permeability of the stromal membrane to cofactors such as Mg++ that are necessary for the Calvin cycle. An important aspect of the problem lies in the hypothetical "radiation temperature" attributed to the light beam. This concept has its origins in Planck's vision of assigning entropy, and therefore temperature, to radiation. However, Planck was very clear that there is only one thermodynamic radiation temperature: that of the equilibrium black body (Planck 1912). In fact, he asserts that since the rays of radiation, used to define a temperature, passing through a point can be arbitrary, there are an infinite number of such "temperatures". Almost all previous discussions have used these arbitrary "temperatures" in thermodynamic equations that require equilibrium to be achieved.