Wize University Biology Textbook > Photosynthesis
Photorespiration
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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.
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.
- 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?