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Principles Module 31

Objectives

• 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?