Photorespiration is defined as wasteful light-dependent process where there is uptake of oxygen and output of carbondioxide molecules. It is also called as C2 cycle as 2-carbon containing compound, glycolate is produced.
RuBP + O2 → Phosphoglycolate + PGA
[Fig. Pathway of photorespiration]
|1. It takes place only in presence of light.||1.It is a light independent process.|
|2.It occurs in chloroplast, peroxisomes and mitochondria.||2.It occurs in mitochondria and cytoplasm.|
|3.It occurs in green plants only.||3.It occurs in all living cells.|
|4.It is a wasteuful process as ATP and NADPH2 are used.||4.It release energy in the form of ATP and NADPH2.|
|5.It is not essential process.||5.It is an essential process.|
|6.It reduces the yield of C3 plants growing in tropics.||6.It is an usual process where stored food is broken down to release energy.|
What are the main features of photorespiration ?
- It takes place only in presece of light so, light dependent reaction.
- It's function is to recover some carbon atoms from excess glycolate.
- It is the wastefull process because ATP and NADPH2 are used but not produced. Photorespiration is a part of photosynthesis and we know photosynthesis is energy releasing process but photorespiration does not produce energy.
- It takes place in chloroplast, peroxisomes & mitochondria organells.
- With increased temperature and oxygen concentration, affinity of RuBP carboxylase(RuBisCo) for O2 increases and decreases for CO2. Hence, RuBP function as oxygenase rather than carboxylase.
- Increasing temperature increases photorespiration which results in loss of more fixed carbon.
- It reduces the yields of C3 plants(wheat, rice, bamboo, oat) by 30-45%.
How photorespiration decreases yields and productivity ?
This process reduces the rate and yield of phototosynthesis. 1 mole of ATP and 1 mole of NADPH2 are used up during this process without gainig any ATP or NADPH2. Two mole of O2 is used, one during conversion of glycolate into glyoxylate inside peroxisome and other during conversion of glycine to serine inside mitochondria with release of only one mole of CO2. It also decreases nos. of photosynthetically fixed carbon.
Why does a green plant start evolving CO2 instead of O2, in hot summer days?
Due to photorespiration, green plants start evolving CO2 instead of O2 in hot summer days. In hot days, temperature increases drastically and affinity of RuBisCo increases for CO2 and decreases for O2 which results in evolution of carbondioxide.