Comparing cell wall and membrane contributions to mesophyll conductance in plants

Photosynthesis in plants sequesters carbon from carbon dioxide into biomass, while releasing oxygen back into the atmosphere. Gases such as carbon dioxide and oxygen must thus pass through spongy mesophyll cells, which feature both a rigid cell wall and a plasma membrane that may create barriers to gas transfer. Experimentally, this is quantified through measuring mesophyll conductance. Mesophyll conductance is a composite metric, which is controlled by the barriers between intracellular airspaces and photosynthetic membranes within plant cells, including plant cell walls and membranes. Using molecular simulation techniques, we track gas permeation events across both plant plasma membrane models as well as models for a chunk of secondary cell wall. For the membrane, we find gases accumulate within the membrane interior, consistent with prior partitioning measurements. Carbon dioxide permeability measured from observed transition events increases approximately threefold over temperature ranges experienced in the environment, similar to experimental measures for carbon dioxide assimilation. Under optimistic transport assumptions, we find that carbon dioxide membrane permeation is only ten times faster than measured carbon dioxide assimilation rates, suggesting that membrane transport may be limiting under some growth conditions where carbon dioxide availability is low. By contrast, we find that gases permeate readily through plant cell walls, with diffusion coefficients that are only approximately ten times slower than water. As a consequence, diffusion estimates reveal that a gas molecule would spend only 100 miliseconds transiting across a cell wall that is a micron thick. Thus, we find that biological membranes are likely the limiting factor in gas permeation through the plant mesophyll.