Question

Rubisco is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C₄ plants?

Answer 1

RuBisCO is an enzyme which acts both as carboxylase (carboxylationduring photosynthesis) and oxygenase(during photorespiration). But RuBisCO carries out more carboxylation in C4 plants. In C4 plants, initial fixation of carbon dioxide occurs in mesophyll cells.

Answer 2

Answer:

In C4 plants, Rubisco is located only in the bundle sheath cells where photosynthetic release of oxygen does not occur. Hence, Rubisco carries out carboxylation as more carbon dioxide is available.  

Explanation:

The enzyme RubisCo ( Ribulose 1,5 Bisphosphate Carboxylase-Oxygenase ) acts both as a carboxylase and oxygenase. The active site of RuBisCo for both carboxylation and oxygenation is the same. The enzyme has more affinity to Carbon dioxide than oxygen. In C3 plants, some O2 will always bind to Rubisco. However, active oxygenation only when CO2:O2 ratio favors it (Figure 1).  

In C4 plants, which are adapted to regions with high temperature, the enzyme Rubisco is present only in special cells called bundle sheath cells, which are absent in C3 plants. Bundle sheath cells have a high intracellular CO2 concentration due to flow of C4 acids like OAA (Oxalo-acetic acid) and Malic Acid and their decarboxylation to release CO2. Therefore, Rubisco functions purely as Carboxylase in C4 plants, because of the presence of special anatomical characters which are bundle sheath cells.