what is photorespiration? and why it take place c3 but not in c4
It is a process in which there is no formation of ATP or NADPH, but there is utilization of ATP with release of CO2. It is also considered a wasteful process.
Photorespiration is responsible for the difference between C3 and C4 plants
Photorespiration does not occur in C4 plants. These plants have a special type of protective anatomical arrangement called as Kranz anatomy in which two types of photosynthetic cells are present in chloroplast viz. mesophyll cells and Bundle sheath cells unlike C3 plants which have one type of cell like mesophyll cells (lacking bundle sheath cells). These two types of cells present in C4 plants allows two different compartment inside chloroplast having two different reaction occuring separately (light reaction and dark reaction). Light reaction occur in mesophyll cells and dark reaction occur in bundle sheath cells. So there is release of oxygen during light reaction in mesophyll cells and fixation of CO2 during dark reaction in bundle sheath cells in C4 plants. Carbon dioxide fixation is catalised by RuBisCO and the active site of RuBisCO can bind to both O2 and CO2, though it has a greater affinity for CO2.
In C3 plants, some of the O2 binds to RuBisCO to form phosphoglycerate and phosphoglycolate (instead of PGA). There is no production of sugar or ATP. This mechanism in C3 plants results in utilization of ATP with the release of O2.
In C4 plants, photorespiration does not occur since they have mechanism to increase the CO2 concentration at enzyme site. C4 acids are broken in the bundle sheath cells to release CO2, thus increasing the intracellular concentration of CO2.
This is the reason why C4 plants are better yielding as compared to C3 plants.