C3 Plants: Photorespiration
Most of the plants fix CO2 through Calvin cycle and are called C3 plants.
However, Rubisco is thermolabile and requires higher concentration of CO2 for its activity.
- At high temperature the stomata close partially so that availability of CO2 falls.
- At high temperature and at low CO2 concentration, RUBPcarboxylase, (i.e. RUBISCO-Ribulose Bis phosphate carboxylase oxygenase), functions as oxygenase.
- It brings about oxidation of RUBP instead of carboxylation
- Due to this a considerable (approximately 25%) part of photosynthetically fixed CO2 goes back to atmosphere.
- This is called Photorespiration.
- The process of respiration (oxidation) that is initiated in the chloroplast and takes place only during day is called photorespiration.
- RUBISCO is the most abundant enzyme in the world and its active site can bind to both CO2 and O2. This binding is competitive.
Process of Photorespiration
- At high temperature, high light intensity and low CO2 concentration, oxidation of RUBP by O2 takes place.
- It results in the formation of one molecule of 2-C compound, phosphoglycolate and one molecule of PGA.
- PGA gets incorporated in Calvin cycle while the phosphoglycolate gets dephosphorylated in chloroplast to form glycolate.
- The glycolate then gets diffused into peroxisomes where it is oxidised to glyoxlate and then gets conveted into an amino acid glycine (2C).
- Glycine Enters mitochondria and gives rise amolecule of serine (3C) and one CO2.
- The serine is taken up by peroxisome and gets converted into glycerate.
- The glycerate enters the chloroplast gets phosphorylated to form PGA and enters the Calvin cycle.
- Thus, 75% of the carbon lost by oxygenation of RUBP is recovered but 25% is lost as release of one molecule of CO2.
Photorespiration is also called photosyntlietic carbon oxidation cycle. It protects the plants from photooxidative damage.
In C4 plants, photorespiration is avoided.