A Revised View on Cellular CO2 Transport Mechanisms: Biophysics, Physiology and Genomics of Aquaporin-Facilitated CO2 Transport in Plants

Abstract

It is a general opinion that the Meyer-Overton correlation determines bio-membrane transport of hydrophobic molecules. This so called “Overton rule” says that the easier it is for a chemical to dissolve in a lipid the easier and faster it will be transported into a cell. In medical science for example, this passive transport is crucial for the effective delivery of many pharmaceutical agents to intracellular targets. The prediction also concerns CO2 as a hydrophobic molecule. Certainly, the membrane diffusion of CO2 is of critical importance for bacteria, animal- and plant cells. In contrast to many other organisms, plants require CO2 and its availability at the site of CO2 fixation limits the rate of net photosynthesis. In this regard, plants provide an excellent system to study CO2 transport mechanisms. Findings will be presented, which question the applicability of Overton's rule to specific plant CO2 transport processes. It could be demonstrated that the function of specific membrane proteins, i.e. distinct aquaporins, increased CO2 transport rates. The experiments were performed on synthetic membranes and cell based systems as well as plant tissues and complete plants. Techniques from biophysics, cell biology, molecular biology and plant physiology were employed and it was found that in the analyzed systems CO2 transport rates were limited by the function of these aquaporins. The results could be interpreted in a way that supports alternative cellular CO2 transport mechanisms and a modified model of cellular surfaces. If the findings were of general validity and not specific for plants, our view on diverse transport processes in many living organisms from all kingdoms could be modified.