The individual flux contributions are both proportional to the concentration of S in the originating compartment, the compartment from which the flux flows. A final point, perhaps obvious by now, is that the concentrations of S are absolutely required on the right hand side of the equation. The mass of S cannot be substituted for the concentration because you could then cause transport up a concentration gradient just by giving the low concentration compartment a much larger volume than the high concentration compartment. In order to develop some intuition about diffusion, you should learn the absolute values of some typical diffusion coefficients. These are reported values for diffusion in water at 20 oC. Notice that the bigger the molecule, the smaller the diffusion coefficient.
P is referred to as a permeability.
The temperature appears linearly in this equation, but it is important to remember that this is absolute temperature. Perhaps you anticipated the increase in diffusion coefficient with temperature since you know that increasing temperature causes increased molecular velocity. A quantity frequently used to characterize the temperature sensitivity of a physical or chemical process is the Q10. Such arguments are not supported by the facts. Activation energies for active and passive processes overlap extensively.