Photosynthesis conductance/resistance

For our applications to transpiration and photosynthesis, we will define a stomatal conductance, and a stomatal resistance, r for the diffusion of species j using Equation 8.4 ... 

We next consider the main function of a leaf, photosynthesis, in terms of the conductances and the resistances encountered by CO2 as it diffuses from the turbulent air, across the boundary layers next to the leaf surface, through the stomata, across the intercellular air spaces, into the mesophyll cells, and eventually into the chloroplasts. The situation is obviously more complex than the movement of water vapor during transpiration. Indeed, CO2 not only must diffuse across the same components encountered by water vapor moving in the opposite direction5 but also must cross the cell wall of a mesophyll cell, the plasma membrane, part of the cytosol, the membranes surrounding a chloroplast, and some of the chloroplast stroma. Resistances are easier to deal with than are conductances for the series of components involved in the pathway for CO2 movement, so we will specifically indicate the resistance of each component. 

Now that we have discussed the CO2 resistances and conductances, we are ready to examine CO2 fluxes. We will do this for photosynthesis, a process that consumes CO2, as well as for respiration and photorespiration, processes that evolve CO2. Our analysis will use an electrical circuit so that we can represent the interrelationships among the various factors influencing net CO2 uptake by a leaf... 

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