9. What happens during the process called "photorespiration"?

It has long intrigued biologists that O2 has an inhibitory effect upon photosynthesis. As shown in the following hypothetical experiment, in the presence of elevated O2 levels, photosynthesis rates are lower.

It is now known that this response to oxygen is due to competition between O2 and CO2 on the Rubisco enzyme of the Calvin-Benson cycle. You will recall that in the "normal" reaction, CO2 is joined with RUBP to form 2 molecules of 3PGA.


In the process called photorespiration, O2 replaces CO2 in a non-productive, wasteful reaction.

It is believed that photorespiration in plants has increased over geologic time and is the result of increasing levels of O2 in the atmosphere--the byproduct of photosynthetic organisms themselves. The appearance of C4-type plants appears to be an evolutionary mechanism by which photorespiration is suppressed. It has long been the dream of biologists to increase the production of certain crop plants, such as wheat, that carry on C3-type photosynthesis by genetically re-engineer them to perform C4-type photosynthesis. It seems unlikely that this goal will be accomplished in the near future due to the complex anatomical and metabolic differences that exist between C3- and C4-type plants.


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