Gr 12 C3 C4 CAM plants

Comparison of C3, C4 and CAM plants

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

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

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 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

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