Solution:

RUBISCO has a substantially stronger affinity for oxygen than Oxygen. The enzyme’s binding is determined by the concentrations of oxygen and carbon dioxide. This enzyme is not found in the mesophyll cells of C4 plants, but it is abundant in the bundle sheath cells that girdle the vascular bundles where the Calvin cycle occurs.

When the concentration of oxygen is high, RuBisco operates as an oxygenase, and when the concentration of CO2 is high, it acts as a carboxylase. The major carbon dioxide acceptor in mesophyll cells is phosphoenol pyruvate, a three-carbon chemical that is transformed to oxaloacetic acid, or OAA, a four-carbon product. This is then converted to malic acid, which is delivered to the bundle-sheath cells, where it is decarboxylated and carbon fixed via the Calvin cycle, preventing RuBisCo from acting as an oxygenase.