Photorespiration

Photorespiration also referred as oxidative photosynthetic carbon cycle or C2 photosynthesis is a light-dependent release of carbondioxide from photosynthetic organisms, which is caused by substituting oxygen in the first step of photosynthetic fixation by carbondioxide.


During the peak sunny hours, when the temperature gets high, the plant has a character to close its stomata to prevent excess water loss by transpiration. This process occurs due to higher ration of O2 to CO2 in the plant, that is, when the concentration of CO2 gets below or around 50 ppm.

The photorespiration is initiated in Chloroplast. In this oxygenase reaction, the RuBP oxygenase-carboxylase (Rubisco) starts by combining O2 with RuBP (ribulose-1,5-bisphosphate) in place of CO2, which in result produces two products, of which one molecule is a three carbon compound 3-PGA and the other is a two carbon compound Phosphoglycolate.

Now, the 3-Phosphoglycerate (3-PGA) formed by the above reaction can enter the Calvin-Cycle but the toxic two carbon compound Phosphoglycolate must be converted into a useful product and this process takes place in a specialized organelle called Peroxisome, where the two carbon compound is further metabolized. In the chloroplast the dephosphorylation of phosphoglycolate takes place by the action of phosphatase to form a compound called Glycolate, which is transported to peroxisome where it is oxidized to form glyoxylate, which form Glycine by reacting with hydrogen peroxide.
The Glycine is formed in the peroxisome is the transported to the mitochondrial matrix, where two glycine molecule is combined to form an amino acid called serine by the removal of CO2 and NH3 with the action of an engyme called methyltransferase in the process. The serine is then dominated to form glycerate by the action of hydroxypyruvate and thus phosphorylated to form 3-phosphoglycerate with an expenditure of ATP by the action of kinase, which reenter the Calvin-Cycle.


External Links:

https://goo.gl/cgWpRU

https://goo.gl/4Jsnqs 

https://goo.gl/JP7aMB 

https://goo.gl/XsVFgh 


Questions:

Photorespiration

  1. Where does photorespiration occur?
  2. Name the enzyme that catalyses the reaction between ribulose-1,5-bisphosphate and oxygen.
  3. Define photorespiration or C2 photosynthesis.
  4. Photorespiration is a metabolic pathway, that occurs in plant –
    a) In presence of light.
    b) In absence of light.
  5. What are the final products of the reaction catalysed by Rubisco-
    a) 3-phosphoglycerate
    b) 2-phosphoglycolate
    c) Glyceraldehyde-3-phosphate
    d) Both (a) and (b)
Query Box