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Understanding Photosynthesis: Alternative Mechanisms and the Role of RuBisCO

This article explores the critical processes of photosynthesis, focusing on the enzyme RuBisCO, which is the most abundant protein on Earth and catalyzes the first reaction of the Calvin cycle. It discusses photorespiration and its implications, particularly in warmer climates where O2 competes with CO2 for RuBisCO's active site, resulting in less efficient carbon fixation. Additionally, the article covers alternative carbon fixation mechanisms such as C4 photosynthesis and CAM (Crassulacean Acid Metabolism), highlighting how plants have evolved to adapt to varying environmental conditions.

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Understanding Photosynthesis: Alternative Mechanisms and the Role of RuBisCO

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  1. Photosynthesis Alternative Mechanisms of Carbon Fixation

  2. RuBisCOImage from: http://www.ncbi.nlm.nih.gov/books/NBK22344/ • RuBisCO is the most abundant protein on Earth. • It catalyzes the first reaction of the Calvin cycle.

  3. PhotorespirationImage from: http://access.mmhs.ca/docs/Science/MMHS%20Web%20folder/Kamla/20102008.htm • In the first reaction of the Calvin cycle, CO2 is “fixed”, forming 2 molecules of 3-PGA. • O2 competes with CO2 for the active site of RuBisCO.

  4. PhotorespirationImage from: http://access.mmhs.ca/docs/Science/MMHS%20Web%20folder/Kamla/20102008.htm • When RuBP is oxidized, it produces only 1 molecule of 3-PGA. • This process is called photorespiration.

  5. Photorespiration • In warmer temperatures, RuBisCO’s affinity for O2 increases, resulting in less CO2 being fixed into G3P. • How have plants in warmer climates evolved to cope with RuBisCO’s affinity for O2?

  6. C4 PhotosynthesisImage from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html • CO2 is combined with PEP to form 4-carbon oxaloacetate (enzyme PEP carboxylase) in mesophyll cells • oxaloacetate is converted to malate, then transported into bundle-sheath cells • malate is decarboxylated forming pyruvate, and CO2 enters the Calvin cycle

  7. C4 PhotosynthesisImage from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html

  8. C4 PhotosynthesisImage from: http://www.botany.hawaii.edu/faculty/webb/bot311/bot311-00/psyn/PsynDark2.htm

  9. C4 Plants

  10. CAM Plants • crassulacean acid metabolism (CAM), first discovered in Crassulacea family of plants • stomata are closed during the day, so CO2 is only absorbed at night • C4 pathway at night, C3 pathway during the day

  11. CAM PlantsImage from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html

  12. CAM plants

  13. Animation Narrated animation of C4 photosynthesis and CAM plants: http://www.uic.edu/classes/bios/bios100/lectures/c4.htm

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