What is Photorespiration?
Respiration leads to oxygen metabolism, and carbon dioxide production. In cellular respiration it is a positive term, a critical process for survival. Yet Photorespiration is a completely negative term because it indicates a serious loss to the method of using light energy in photosynthetic organisms to fix carbon for subsequent carbohydrates.
By causing the loss of up to half the carbon fixed at the cost of light energy, photospiration undoes the photosynthesis work.
RuBisCO is the globally most abundant enzyme. Its active location can bind both to CO2 and to O2. But RuBisCO 's affinity to CO2 is far greater than O2. The relative concentration of O2 and CO2 determines which enzyme will bind.
Definition: It is a trend seen in almost all C3 plants where an increase in carbon dioxide concentration results in a decrease in photosynthesis rate.
Photorespiration in C3 Plants
Any O2 binds to RuBisCO in C3 plants and hence CO2 fixation is reduced.
Here the RuBP binds with O2 instead of being converted into 2 PGA molecules to form one phosphoglycerate and phosphoglycolate molecule in a pathway called Photorespiration.
There is no synthesis of sugars or ATP in the photorespiratory pathway. Instead it helps in CO2 release with the use of ATP.
There is no synthesis of either ATP or NADPH in the photorespiratory pathway. Photo-Respiration is therefore a costly operation.
Photorespiration cycle is explained in detail through the Photorespiration diagram below.
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Photorespiration in C4 plants
Photorespiration does not occur in C4 plants. This is because they have a mechanism which increases the CO2 concentration at the site of the enzyme.
This happens when the mesophyll C4 acid is broken down in the bundle sheath cells to release CO2 this results in an increase in the intercellular CO2 concentration.
This in turn ensures that the RuBisCO acts as a carboxylase which minimizes oxygenase activity.
Now it’s understandable that C4 plants lack Photorespiration. Additionally, these plants show higher temperature tolerance.
Detailed Difference between C3 and C4 Plants
Around 95 per cent of shrubs, trees, and plants are C3 species. C4 plants, on the other hand, are those that employ the C4 pathway during the dark response. These plants' chloroplasts are dimorphic, and unlike C3 plants, C4 plants' leaves have kranz anatomy. C4 plants make up around 5% of all plants on the planet. Here's how to tell the difference between C3 and C4 plants.
Conclusion
RuBisCO is the globally most abundant enzyme. Its active location can bind both to CO2 and to O2. The relative concentration of O2 and CO2 determines which enzyme will bind to the enzyme. Photorespiration is a trend where an increase in carbon dioxide concentration results in a decrease in photosynthesis rate.
FAQs on Photorespiration in C3 and C4 Plants
1. Does Photorespiration occur in Cs plants?
The oxygenation of ribulose-5-bisphosphate (rubp) by oxygen is the first step in Photorespiration. Rubisco, the same enzyme that catalyses rubp carboxylation, which leads to photosynthetic products, also catalyses wasteful oxygenation. To "pump" carbon dioxide to the rubp carboxylation site, C4 plants need metabolic energy (ATP). C4 plants reduce the oxygen level of the carboxylation site by limiting gas transport to the cells where carboxylation takes place. High carbon dioxide concentrations and low oxygen concentrations reduce the likelihood of rubp oxygenation and increase the likelihood of carboxylation. Due to which C4 plants do not have Photorespiration.
2. What happens during Photorespiration?
O2 absorbs CO2 in a non - productive, inefficient reaction, in a process called Photorespiration. Photorespiration in plants is thought to have risen over time and is the result of increasing levels of O2 in the atmosphere-the by-product of photosynthetic organisms themselves. The intended reaction is the addition of carbon dioxide to RuBP (carboxylation), which is a crucial step in the Calvin–Benson cycle. However, around 25% of RuBisCO reactions add oxygen to RuBP (oxygenation), resulting in a product that cannot be utilised in the Calvin–Benson cycle. This mechanism affects photosynthesis efficiency, potentially lowering photosynthetic production by 25% in C3 plants.
3. Where does Photorespiration occur?
Photorespiration A light - activated form of respiration occurring in many plant chloroplasts. Biochemically it differs from normal (dark) breathing in that it requires glycolate metabolism.
4. What are the disadvantages of Photorespiration?
This cycle does not yield either ATP or NADPH, and is inefficient. This helps instead in the release of CO2 with the use of ATP. It results in a loss of 25 percent of the fixed CO2. Sugarcane has developed a mechanism for surmounting photorespiratory failure.
5. How does Photorespiration affect Photosynthesis?
The oxygenation of RuBP by RUBISCO, followed by photorespiratory glycolate metabolism, is known as Photorespiration. The rate of carbon absorption, the energy efficiency of photosynthesis, and the photosynthetic quotient (PQ = O2 produced/CO2 absorbed) are all affected by the competition between O2 and CO2. Protein and lipid synthesis are represented by PQ values of 1.2–1.8. At the CO2 compensation point, where CO2 absorption equals CO2 evolution, Photorespiration and glycolate excretion can provide PQ values as low as 0.75. Only imbalanced growth circumstances, in which respiration contributes to net gas exchange, may explain PQ values below 0.75.
