12 photosynthesis - part 07 - Photorespiration

 

12 photosynthesis - part 07 - Photorespiration


Photorespiration / PCO cycle.  :


  • Photorespiration occurs under the conditions like 
  1. high temperature
  2. bright light 
  3. high oxygen and 
  4. low CO2 concentration.
     
  • It is a wasteful process linked with C3-Cycle, where instead of fixation of CO2 it is given out.
  • It involves three organelles chloroplast peroxisomes and mitochondria and occurs in a series of cyclic reactions which is also called PCO cycle. 
  • Enzyme Rubisco acts as oxygenase at higher concentration of O2 and photorespiration begins. 
  • When RuBP reacts with O2 rather than CO2 to form a 3-carbon compound (PGA) and 2-carbon compound phosphologycolate. 
  • Later is converted to glycolate which is shuttled out of the chloroplast into the peroxisomes.
  • In peroxisomes, enzyme glycolate oxidase converts glycolate into glyoxylate, which is converted into amino acid glycine by transamination.
  •  In mitochondria, two molecules of glycine are converted into serine (amino acid) and CO2 is given out. 
  • Thus, it looses 25% of photosynthetically fixed carbon.
  • Serine is transported back to peroxisomes and converted into glycerate. 
  • It is shuttled back to chloroplast to undergo phosphorylation and utilized in formation of 3-PGA, which get utilized in C3 pathway.


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