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Wize University Biology Textbook > Photosynthesis

Photorespiration

Calvin Cycle [Carbon Fixation] [detailed]Previous Section
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Photorespiration

Before we talk about photorespiration, let's review where photosynthesis occurs in plants:
  • Guard cells open and close pores called stomata depending on the environmental conditions including light intensity, humidity, and CO2 concentration.
  • Stomata let CO2 enter and and O2 to leave and also lead to water loss through transpiration.
  • Stomata close during hot, dry conditions to prevent water loss, but this leads to a build up of O2.

Photo by Zephyris / CC BY Photo by Ali Zifan / CC BY


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Why is build up of O2 a problem?

  • Rubisco, the enzyme that usually fixes CO2 can also use O2 as a substrate.
  • Photorespiration, also known as oxidative photosynthetic carbon cycle or C2 photosynthesis, is the process in which Rubisco adds an O2 to RuBP, rather than a CO2 as in the Calvin Cycle.
  • This wastes energy and decreases sugar production.
  • Under hot, dry conditions, photorespiration increases.



Watch Out!
Don't be alarmed that this cycle is showing the required amount of CO2 (6) to make a full molecule of glucose instead of 3 which is required amount to allow one molecule of G3P to leave.

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  • Some plants - C4 plants and CAM plants - have strategies to minimize this problem
  • In C3 plants, 6 molecules of phosphoglycolate are produced in photorespiration.
  • Three of these go through a salvage pathway, recovering half of the 3-PGA. The other 3 are lost to CO2.


Practice: Problem of Photorespiration

Why is photorespiration a problem for plants?