AP BIOLOGY

Detailed explanation-1: -C3 plants do not have the anatomic structure (no bundle sheath cells) nor the abundance of PEP carboxylase to avoid photorespiration like C4 plants. One focus of the RIPE project is to create a more efficient pathway for photorespiration to improve the productivity of C3 crops.

Detailed explanation-2: -Photorespiration happens in C3 plants when the CO2 concentration drops to about 50 ppm. The key enzyme that accomplishes the fixing of carbon is rubisco, and at low concentrations of CO2 it begins to fix oxygen instead.

Detailed explanation-3: -Photorespiration does not occur in C4 plants because they have a mechanism that increases the concentration of CO2 at the enzyme site. In the C4 pathway, carbon fixation ultimately results in the production of malic acid in the mesophyll cells. The malic acid is broken down in the bundle sheath to release CO2.