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The Inefficiencies of Photosynthesis
Photosynthesis versus Photorespiration:
In photosynthesis, plants use sunlight and water for carbon fixation, the conversion of inorganic CO2 to organic glucose, resulting in energy for plant growth and oxygen (O2).
However, in photorespiration, O2 is taken in (instead of CO2), decreasing the efficiency of carbon fixation by 25-50%.
Rubisco versus Rubisco Activase:
Rubisco (Ribulose bisphosphate carboxylase/oxygenase) is the most abundant protein on Earth and is essential for photosynthesis. See animation below.
1. Rubisco binds CO2 and ribulose bis-phosphate (RuBP) for photosynthesis (carbon fixation).
2. However, Rubisco can also bind O2 and RuBP for photorespiration, a wasteful side-reaction which not only prevents Rubisco from binding CO2, but also releases more CO2.
3. Rubisco Activase (RCA) is needed to reactivate Rubisco, allowing Rubisco to bind CO2 again for photosynthesis.
4. But RCA is itself temperature sensitive (thermolabile) and works poorly at higher temperatures (especially in those climates most impacted by global warming).
This perpetuates a vicious circle with the higher temperatures from global warming decreasing RCA activity and reducing photosynthesis.
Previous efforts have been made to improve photosynthesis by genetic engineering Rubisco in some crops to increase agricultural yield. However, this has proven to be technically very challenging as Rubisco is a complex enzyme with 16 subunits, half coded by the nuclear genome and half coded by the chloroplast genome. Genetic engineering of chloroplast DNA is not yet as facile as that of nuclear DNA.
Previous efforts to genetically engineer trees have been successful. In 2020, the phytopathogenic bacterium, Agrobacterium tumefaciens, was used to transform blight-resistant genes into the American Chestnut Tree.
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