PHOTORESPIRATION (C2 Cycle or Glycolate Metabolism) 

Term given by (Decker and Tio). Photorespiration is a process which involves oxidation of organic compounds in plants by oxygen in the presence of light.

Like ordinary respiration, this process also releases carbon from organic compounds in the form of carbon dioxide but does not produce ATP.

Thus, apparently it seems to be a wasteful process, but it must have some functions which are still unknown.

The process occurs in C3 plants only.

There are two oxygen consuming reactions in this process.

First oxidation occur in chloroplast and second oxidation occur in peroxisome.

Photorespiration occurs usually when there is high concentration of oxygen.

Under such circumstances RuBP carboxylase (or Rubisco), the enzyme that joins carbon dioxide to RuBP, functions as oxygenase.

As a result, oxygen, instead of carbon dioxide, gets attached to the binding site of the enzyme (i.e. RuBP).

This reaction is competitive.

It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.

On oxidation, RuBP releases one molecule of C3 compound.

 PGA (which enters C3 cycle) and one molecule of a C2 compound phosphoglycolate.

The latter almost immediately changes into glycolate.

The glycolate leaves the chloroplast and enters a membrane bound sac of enzymes called peroxisome.

Here the glycolate is oxidised into glyoxylate which is aminated into glycine.

Further condensation of glycine takes place inside the mitochondria where two molecules of glycine interact and give rise to a molecule of serine and carbon dioxide each.

The CO2 is then released from mitochondria.

Serine regenerates PGA. With increase in temperature, high light intensity and oxygen concentration, affinity of Rubisco for CO2 decreases and its affinity for oxygen increases.

Thus, a rise in temperature means more loss of photosynthetically fixed carbon by photorespiration.

It reduces potential yield of plants growing in the tropics by 30-40%. Upto 1/4th of the photosynthetically fixed CO2 may be lost by this pathway.