Alternative Carbon Fixation

Principles Module 31


  • Distinguish between the two alternative pathways of carbon fixation
  • Explain why these alternative pathways evolved (identify the selective pressures involved)

Key points

  • RuBisCO is an ancient enzyme, having evolved under an atmosphere without O2
    • it can accidentally bind to O2 rather than CO2 in the carboxylation reaction
    • incorporation of an O2 results in a waste product that is energetically expensive to recover
  • Some plants have evolved a carbon fixing step prior to the RuBisCO step that concentrates CO2for RuBisCO
    • C4 pathway plants have a carbon fixing step that is spatially separated from the pathway with RuBisCO
    • CAM pathway plants have a carbon fixing step that is temporally separated from the pathway with RuBisCO
    • both of these pathways incur greater ‘costs’ because they concentrate CO2 for RuBisCO, but can also achieve greater rates of CO2 fixation under certain conditions

Questions for Practice

  • Under what conditions does RuBisCO ‘misfire’ and incorporate an O2 instead of a CO2? What is the result of this misfire?
  • What benefit does oxygenation provide to the plant under these conditions?
  • Compare and contrast C4 and CAM pathways.
  • Why are C4 and CAM plants found in high-light environments? Why can’t they grow well in shade?