Photorespiration in C3 and C4 Plants

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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 carbohydrate.

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 will bind to enzyme.

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.

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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 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 intracellular 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.

FAQ (Frequently Asked Questions)

Q1. Does Photorespiration occur in Cplants?

Ans. Photorespiration in plants is an inefficient pathway that happens when the Calvin cycle Enzyme rubisco acts on Oxygen instead of carbon dioxide. C4 plants minimize for respiration by splitting original CO2 fixation and the calvin cycle.

Q2. What happens during Photorespiration?

Ans. 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.

Q3. Where does Photorespiration occur?

Ans. 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.

Q4. What are the disadvantages of Photorespiration?

Ans. 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.