Question

How does photorespiration reduce the photosynthetic yield of C3 plants?

Answer 1

Photorespiration:

1. Photorespiration is a metabolic mechanism that happens in photosynthetic organisms.
2. This process produces no chemical energy or food but results in the release of carbon dioxide and consumption of oxygen. Plant growth rates are slowed by this procedure.

Photorespiration and photosynthetic efficiency:

1. For a number of reasons, photorespiration reduces photosynthetic efficiency. Carbon is first given an oxygen boost. In other words, the carbon is oxidised, as opposed to photosynthesis, in which carbon is reduced to carbohydrate. Second, the ribulose bisphosphate must be resynthesized, and phosphoglycolate must be reduced.
2. RuBisCo creates a 3-carbon chemical, 3-phosphoglycerate, and a 2-carbon compound, phosphoglycolate, by oxidising ribulose-1,5-bisphosphate. Photorespiration is the name given to this process, because carbon is oxidised.
3. The Calvin-Benson process can re-use 3-phosphoglycerate from photorespiration, but the phosphoglycolate must be recycled to generate a useful molecule. The peroxisome is a specialised organelle that performs this recycling.
4. The phosphoglycolate is dephosphorylated in the chloroplast. Glycolate is carried to the peroxisome, where it is further oxidised to glyoxylate by molecular oxygen. The product is hydrogen peroxide, H₂O₂, which is promptly broken down by catalase into water and oxygen.
5. In the peroxisome, the glyoxylate is amidated to the amino acid glycine.
6. Glycine is then transported to the mitochondrial matrix, where two glycine molecules are converted to one serine with the loss of CO₂ and NH₃ from the fixed molecule pool. The serine is carried to the peroxisome, where it is deaminated and converted to glycerate. For the Calvin-Benson cycle, the glycerate is transported back to the chloroplast and phosphorylated to 3-phosphoglycerate.
7. The efficiency of photosynthesis is severely harmed by this group of processes. CO₂ is lost, energy is expended, and ribulose bisphosphate is destroyed when oxygen is added to carbon.
8. A bypass system for the transport of CO₂ to Rubisco has evolved in a variety of C₄ plants, including grasses like maize (corn) and bamboo.