Photorespiration - Formation of Phosphoglycerate and Phosphoglycolate

PHOTORESPIRATION

This is the process that creates a difference between C3 plants and  C4 plants. Photorespiration is a wasteful pathway that begins when RuBisCO acts on oxygen instead of carbon dioxide.

During the first step of the Calvin  cycle or  the fixation of carbon dioxide,  the two molecules of Phosphoglyceric acid are formed in presence of  enzyme RuBisCO when ribulose bi phosphate is combined with the carbon dioxide.

👌👌Remember RuBisCO is the most abundant enzyme in the world. 

This RuBisCO has a unique feature. Its active site can bind both carbon dioxide and oxygen. But the  RuBisCO has  greater affinity for carbon dioxide rather than oxygen and if the ratio of carbon dioxide and oxygen  is nearly equal for RuBisCO  then the binding process is competitive. The relative concentration of oxygen and carbon dioxide determines which one  of the two will bind to the enzyme. 

In C3 plants, When  oxygen  binds to RuBisCO and CO2 fixation is decreased. As a result the RuBP is  not converted into two  molecules of phospho glyceric acid and forms one molecule of phosphoglycerate and phosphoglycolate  when it binds with oxygen. This pathway is called photorespiration. 

The photorespiration is regarded as the waste process because  In the photorespiratory pathway, there is neither synthesis of sugars nor ATP. In addition to this this process releases  Carbon di oxide with the utilisation of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH. The biological function of photorespiration is not known yet. 

In C4 plants photorespiration does not occur therefore these plants have a mechanism by which they  increase the concentration of Carbon dioxide  at the enzyme site. It happens when the  oxalo acetic acid is transported from the mesophyll and   broken down in the bundle sheath cells to release  carbon dioxide . As a result  the concentration of carbon dioxide is increased. In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity.  With the increased concentration of carbon dioxide these plants have better productivity and yields. In addition these plants show tolerance to higher temperatures.

The protective role of photorespiration may be especially important under stress conditions such as drought when leaf stomata  are closed in the light in order to prevent excess water loss.