Question

Most plants are what are called C3 plants. They have not evolved mechanisms to avoid photorespiration. C4 and CAM plants have mechanisms to decrease the likelihood of photorespiration occuring. Briefly describe how each type of plants avoids photorespiration.

Answer 1

C4 plants capture CO2 in cells of their mesophyll, and oxaloacetate is formed. oxaloacetate is then converted to malate and is released into the bundle sheath cells where O2 concentration is low to avoid photo respiration. Here, CO2 is removed from the malate and combined with RuBP in the usual way, and the Calvin cycle proceeds as normal.

instead of separating the light dependent reactions and the use of CO2 in the Calvin cycle in space, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves. This CO2 is fixed into the oxaloacetate, by PEP carboxylase, then converted to malate.
It is stored inside the vacoules until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesize. That's because the malate is transported out of the vacoule and broken down to release CO2, which enters the Calvin cycle.This controlled release maintains a high concentration of CO2 around RuBIsCO.