The ins and outs of CO2

John A. Raven (Lead / Corresponding author), John Beardall

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Abstract

It is difficult to distinguish influx and efflux of inorganic C in photosynthesizing tissues; this article examines what is known and where there are gaps in knowledge. Irreversible decarboxylases produce CO2, and CO2 is the substrate/product of enzymes that act as carboxylases and decarboxylases. Some irreversible carboxylases use CO2; others use HCO3 -. The relative role of permeation through the lipid bilayer versus movement through CO2-selective membrane proteins in the downhill, non-energized, movement of CO2 is not clear. Passive permeation explains most CO2 entry, including terrestrial and aquatic organisms with C3 physiology and biochemistry, terrestrial C4 plants and all crassulacean acid metabolism (CAM) plants, as well as being part of some mechanisms of HCO3 - use in CO2 concentrating mechanism (CCM) function, although further work is needed to test the mechanism in some cases. However, there is some evidence of active CO2 influx at the plasmalemma of algae. HCO3 - active influx at the plasmalemma underlies all cyanobacterial and some algal CCMs. HCO3 - can also enter some algal chloroplasts, probably as part of a CCM. The high intracellular CO2 and HCO3 - pools consequent upon CCMs result in leakage involving CO2, and occasionally HCO3 -. Leakage from cyanobacterial and microalgal CCMs involves up to half, but sometimes more, of the gross inorganic C entering in the CCM; leakage from terrestrial C4 plants is lower in most environments. Little is known of leakage from other organisms with CCMs, though given the leakage better-examined organisms, leakage occurs and increases the energetic cost of net carbon assimilation.

Original languageEnglish
Pages (from-to)1-13
Number of pages13
JournalJournal of Experimental Botany
Volume67
Issue number1
Early online date14 Oct 2015
DOIs
Publication statusPublished - Jan 2016

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Keywords

  • Aquaporins
  • Bicarbonate
  • C
  • Carbon concentrating mechanisms
  • Carbon dioxide
  • Crassulacean acid metabolism
  • Leakage
  • Lipid bilayer
  • Permeability

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