title: Aquaporins and membrane diffusion of CO2 in living organisms.
abstract: BACKGROUND
  Determination of CO2 diffusion rates in living cells revealed inconsistencies with existing models about the mechanisms of membrane gas transport. Mainly, these discrepancies exist in the determined CO2 diffusion rates of bio-membranes, which were orders of magnitudes below those for pure lipid bilayers or theoretical considerations as well as in the observation that membrane insertion of specific aquaporins was rescuing high CO2 transport rates. This effect was confirmed by functional aquaporin protein analysis in heterologous expression systems as well as in bacteria, plants and partly in mammals.
  
  
  METHODS
  This review summarizes the arguments in favor of and against aquaporin facilitated membrane diffusion of CO2 and reports about its importance for the physiology of living organisms.
  
  
  CONCLUSIONS
  Most likely, the aquaporin tetramer forming an additional fifth pore is required for CO2 diffusion facilitation. Aquaporin tetramer formation, membrane integration and disintegration could provide a mechanism for regulation of cellular CO2 exchange. The physiological importance of aquaporin mediated CO2 membrane diffusion could be shown for plants and cyanobacteria and partly for mammals.
  
  
  CONCLUSIONS
  Taking the mentioned results into account, consequences for our current picture of cell membrane transport emerge. It appears that in some or many instances, membranes might not be as permeable as it was suggested by current bio-membrane models, opening an additional way of controlling the cellular influx or efflux of volatile substances like CO2. This article is part of a Special Issue entitled Aquaporins.
authors.affiliation: Department of Biology, Applied Plant Sciences, Technische Universität Darmstadt, Schnittspahnstrasse 10, D-64287 Darmstadt, Germany. Electronic address: kaldenhoff@bio.tu-darmstadt.de.
authors.forename: Ralf
authors.initials: R
authors.lastname: Kaldenhoff
authors.affiliation: Department of Biology, Applied Plant Sciences, Technische Universität Darmstadt, Schnittspahnstrasse 10, D-64287 Darmstadt, Germany.
authors.forename: Lei
authors.initials: L
authors.lastname: Kai
authors.affiliation: Department of Biology, Applied Plant Sciences, Technische Universität Darmstadt, Schnittspahnstrasse 10, D-64287 Darmstadt, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Uehlein
mesh_terms: D000818:Animals; D020346:Aquaporins / Q000378:metabolism*; D001419:Bacteria / Q000378:metabolism; D001692:Biological Transport; D002245:Carbon Dioxide / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D008322:Mammals / Q000378:metabolism; D010944:Plants / Q000378:metabolism
chemical_list: D020346:Aquaporins; D002245:Carbon Dioxide
keywords: Aquaporin; Aquaporin structure-function; CO(2) conductance of aquaporins; Gas exchange; Membrane permeability for CO(2)