Question -
Answer -
The affinity ofRUBISCO is much higher than its affinity for Oxygen. It is the concentration ofOxygen and CO2
that determines thebinding of the enzyme. Mesophyll cells of C4 plants lack thisenzyme but are found in the bundle sheath cells that girdle the vascularbundles where the Calvin cycle occurs.
RuBisco functions asoxygenase when the concentration of Oxygen is higher and it acts as carboxylasewhen the concentration of CO2 is high. In the mesophyll cells,the primary carbon dioxide acceptor is a three carbon compound – phosphoenolpyruvate which is converted into a four-carbon compound, oxaloacetic acid orOAA. This is converted further into malic acid which is transported to thebundle-sheath cells where it undergoes decarboxylation and carbon fixationtakes place through the Calvin cycle which prevents RuBisCo to serve as anoxygenase.