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by durai raja on April 13th, 2005

durai raja

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What is "photorespiration" and why it is needed by plants?

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  • by Erin Albrecht on April 14th, 2005

    Erin Albrecht

    Photorespiration occurs when carbon dioxide levels within the leaf tissue dwindle to about 50 parts per million, usually on a hot, dry day where a plant has closed its stomates to prevent water loss. At this concentration of CO2, oxygen reacts with ribulose bisphosphate (RuBP) in the presence of RubisCO, an enzyme used in the Calvin cycle of photosynthesis, and creates 2 molecules of 3-phosphoglycerate and 1 molecule of 2-phosphoglycolate. Under normal oxygen and carbon dioxide concentrations, the reaction is between CO2, RuBP, and RubisCO to produce 12 molecules of 3-phosphoglycerate. To top it all off, 2-phosphoglycolate is toxic, so the plant has to convert it into a non-detrimental compound through a number of steps, each of which costs energy and CO2. Photorespiration is wasteful, as well, in that it does not produce any 3-phosphoglyceraldehyde (PGAL), which is used by the plant for a large number of functions. If you take a look at the Calvin cycle (http://en.wikipedia.org/wiki/Calvin_cycle) you can see where things go wrong with photorespiration. It is interesting to note that photorespiration only occurs in C3 plants, which thrive where temperature and light intensity are moderate, water is abundant, and carbon dioxide concentrations are 200 ppm or higher. Something like 95% of all plants on Earth use this form of metabolism. Plants that have developed a way to circumvent the problems of photorespiration are called C4 and CAM (crassulacean acid metabolism) plants. The website http://www.park.edu/bhoffman/courses/bi225/recaps/C3C4.htm has an excellent, brief overview of the differences between C3 and C4 plants.

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