Plant Adaptations

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Key Knowledge:
  • The role of Rubisco in photosynthesis, including adaptations of C3, C4 and CAM plants to maximise the efficiency of photosynthesis


The light independent reactions use the enzyme Rubisco to fix carbon (from CO2) in order to make carbohydrates (such as glucose)

  • This process (called the Calvin cycle) involves the production of a 3C intermediate called GP (glycerate-3-phosphate)
  • As each cycle produces one GP molecule, it takes two cycles to synthesise glucose and more to produce polysaccharides (like starch)


Plants that exclusively fix carbon dioxide this way are called C3 plants – as the initial product (GP) is a 3C compound


Photorespiration

Rubisco can alternatively use oxygen (O2) as a substrate to undergo a different 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


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 the stomata must remain closed in order to prevent excessive water loss
  • When the stomata are closed, oxygen cannot diffuse out of the leaf, increasing oxygen concentration relative to CO2 levels


In an effort to maximise photosynthesis, some plants have evolved mechanisms to limit the exposure of Rubisco to oxygen

  • C4 and CAM plants use an alternate enzyme called PEP carboxylase to initially fix the carbon and make a 4C compound
  • PEP carboxylase has a higher affinity for carbon dioxide than Rubisco and doesn’t bind to oxygen at all
  • These plants can then store and transfer the 4C compounds to regions with lower oxygen concentrations


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

  • The CO2 is converted to the 4C compound in the mesophyll and then sequestered to a deeper tissue layer where less O2 is present
  • In this deeper tissue layer (the bundle sheath), the CO2 is released and can enter the Calvin cycle without competition from oxygen


CAM Pathway

In the CAM pathway, carbon reserves are created at night and then released for use during the day (temporal isolation)

  • CAM plants are suited to hot and arid environments where water loss is high and stomata must therefore remain closed during the day
  • The CO2 is converted into the 4C compound during the night, when stomata are open and the CO2 is able to diffuse into the leaf
  • The stored CO2 is then released for use during the day, when closed stomata would otherwise prevent photosynthesis from proceeding


Comparison of the Carbon Fixation Pathways

C3, C4 and CAM