C3, C4 and CAM Plants


The Calvin cycle uses the enzyme Rubisco (RuBP carboxylase) to fix CO2 to RuBP and make a 3C compound (GP)

  • Plants that fix carbon dioxide directly from the air are called C3 plants (as the initial product is a 3C compound)


Rubisco can also use oxygen (O2) as an alternative substrate to undergo a series of reactions known as photorespiration

  • Photorespiration creates a product that cannot be used to make sugars and hence reduces the efficiency of the Calvin cycle
  • Photorespiration reduces levels of photosynthesis by up to ~25% in C3 plants, reducing energy yield in these plants


Photorespiration vs Photosynthesis

photorespiration


C4 and CAM Plants

Because oxygen acts as a competitive inhibitor for Rubisco, photosynthesis in C3 plants is reduced in the presence of oxygen

  • C3 plants are less efficient in hot and dry regions, as stomata must remain closed to prevent excessive water loss
  • When stomata are closed, O2 cannot diffuse out of the leaf, increasing O2 concentration relative to CO2


In these hot and arid conditions, other types of plants have evolved to limit the exposure of Rubisco to oxygen

  • C4 and CAM plants uses the enzyme PEP carboxylase to combine CO2 to a 3C compound (PEP) and make a 4C compound
  • PEP carboxylase has a higher affinity for CO2 than Rubisco and doesn’t bind to oxygen at all
  • These plants can then transfer the CO2 (stored in the 4C compound) to regions with low oxygen concentrations


In the C4 pathway, carbon dioxide is physically separated from oxygen in order to improve CO2 binding to Rubisco 

  • The CO2 is converted to a 4C compound and then sequestered to a deeper tissue layer where less oxygen is present
  • In this deeper tissue layer, the CO2 is released and can enter the Calvin cycle without competition from oxygen


In the CAM pathway, carbon dioxide reserves are created (temporal isolation) in order to improve CO2 binding to Rubisco

  • CAM plants are adapted to arid environments where water loss is high and stomata must remain closed during the day
  • The CO2 is converted into a 4C compound during the night, when stomata are open and CO2 is able to diffuse into the leaf
  • This allows reserves of CO2 to be created for use during the day, when stomata are closed and O2 cannot be released


Comparison of Carbon Fixation Pathways

C3, C4 and CAM