What is the photorespiratory pump and what does it do in C3C

What is the photorespiratory pump and what does it do in C3-C4 intermediates?

Solution

C3-C4 intermediate plants are able to reduce photorespiration (and hence CO2 compensation point) with the help of photorespiratory pump.

This is due to modified localisation of mitochondrial photorespiratory enzyme glycine decarboxylase. In these plants the enzyme is present in bundle sheath cells and not in mesoohyll cells. As a result of this photorespired CO2 is released in bundle sheath cells that is fixed by Rubisco and photosynthetic reductive cycle. Thus CO2 may mildly elevate in bundle sheath cells and therefore carboxylation activity of Rubisco increases while oxygenase activity outcompeted.

Thus, photorespiratory pump is the pump present in C3-C4 intermediate species that pumps CO2 into the bundle sheath cells. All photorespired CO2 is set free inside the leaf and additionally Gly shuttle increases CO2 concentration in bundle sheath cells.