CO2 travels from the atmosphere through stomata to the carboxylation sites where CO2 is fixed (Figure 4.1). During this diffusional process, CO2 travels through the leaf boundary layer, stomata, mesophyll cell wall, mesophyll plasmalemma, and cytosol and chloroplast membranes. Carbonic anhydrase catalyzes the reversible hydration of CO2 to HCO3 − in the aqueous phase (i.e., chloroplast, cytosol, plasma membrane) and is thought to maintain the supply of CO2 to ribulose 1,5-bisphosphate carboxynase/oxynase (RuBisCO) by speeding up the dehydration of HCO3 −. In the chloroplast, RuBisCO catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) by CO2 and produces 3-phosphoglyceric acid (PGA). ATP and NADPH produced by chloroplast electron transport are used to produce sugars and starch, as well as for the regeneration of RuBP from PGA in the Calvin–Benson cycle. During CO2 diffusion from the atmosphere into mesophyll, each step is influenced by multiple factors, and depending on the environmental variables, the diffusional conductance at each step could change independently or simultaneously [1–4]. It is therefore important to investigate the diffusional conductance steps separately.