Describe why Rubisco is vulnerable to photorespiration in C3 leaves and protected from photorespiration in C4 leaves.

a. Rubisco is less abundant in C4 leaves.
b. C3 leaves lack the enzyme necessary to convert RuBP to PGA.
c. In C4 leaves, Rubisco is localized in the bundle sheath cells where CO₂ levels are higher.
d. C4 leaves have a lower oxygen concentration, preventing Rubisco from reacting with oxygen.

The correct answer is c. In C4 leaves, Rubisco is localized in the bundle sheath cells where CO₂ levels are higher. In C3 leaves, high oxygen concentration leads Rubisco to favor photorespiration, wasting resources. By contrast, C4 leaves protect Rubisco from photorespiration by concentrating CO2 in the bundle sheath cells through the C4 pathway, thus enhancing carbon fixation efficiency.

Explanation:

The correct answer is c. In C4 leaves, Rubisco is localized in the bundle sheath cells where CO₂ levels are higher. Rubisco is vulnerable to photorespiration in C3 leaves because, under conditions of high oxygen concentration typically resulting from intense light, Rubisco oxygenates RuBP leading to the formation of PGA and PGAL, and subsequently glycolate. This process consumes ATP and does not contribute to carbon fixation, essentially wasting resources.

In contrast, in C4 leaves, Rubisco is protected from photorespiration. This protection is achieved by spatially concentrating CO2 within the bundle sheath cells, where Rubisco resides. In these cells, CO2 levels are enhanced due to the activity of PEP Carboxylase in the mesophyll cells, which initially captures CO2 to form a four-carbon sugar. This CO2 is then delivered to the Rubisco enzyme in the bundle sheath cells, significantly increasing the local CO2 concentration and minimizing Rubisco's interaction with O2, therefore reducing photorespiration.