What is Photorespiration?

Photorespiration is also called photorespiration or C2 cycle or photosynthetic carbon oxidative cycle. Photorespiration or C2 cycle was discovered in 1959 by two Biologist namely Dicker and Tio.

The whole process of photorespiration is completed in three cell organelles of the cell. Their names are as follows. Chloroplast, mitochondria and peroxisomes

The first intermediate in this process is the organic chemical phosphoglycolate, which has 2 carbons, so it is also called the C2 cycle.

The process of photorespiration is found in C2 plants, when some of the oxygen released during photosynthesis and sometimes reacts with the RuBisCO enzyme (ribulose 1,5 bisphosphate carboxylase-oxygenase).

Due to which RuBP, instead of forming two molecules of Phosphoglycerate, forms one molecule of Phosphoglycerate (3-phosphoglycerate) and one molecule of phosphoglycolate.

Therefore, there is no fixation of CO2 gas and there is no formation of organic compounds like glucose.

What is the reason for Photorespiration?

We know that the full name of RuBisCO enzyme-RuBisCO- ribulose 1,5 bisphosphate carboxylase-oxygenase.

It has two active sites, (one for Oxygenation & other for Carboxylation) Carboxylase part does carboxylation, the same oxygenase part does oxygenation.

RuBisCO will bind to carbon dioxide gas or oxygen at the same time, depending on which of the two gases is greater in concentrations (relative concentrations of CO2 and O2 decides the RuBisCO activity).

If the relative concentrations of oxygen in the chloroplast becomes high during photosynthesis, then RuBisCO will binds with oxygen.

Thus, instead of forming two molecules of PGA, RuBP forms one molecule of PGA and one molecule of phosphoglycolate.

Apart from this, it also depends on the fact that the temperature of the environment is also high (and also high light intensity), so RuBisCO performs oxygenase activity at high temperature and relative high concentrations of oxygen as compare to CO2.

Where does Photorespiration Occurs?

1-Three cell organelles participate in the process of photorespiration-

First Chloroplast
Second Mitochondria
Third Peroxisome

2- When RuBisCO is oxidized, one molecule of PGA and one molecule of phosphoglycolate are formed.

3-PGA-Phosphoglycerate gets used in the Calvin cycle.

4-While phosphoglycolate is dephosphorylated into glycolate.

5-Glycolate present in the mesophyll cell in leaves is oxidized into glyoxylate.

6-Glyoxylate then undergoes amination (addition of NH2-Amino group), glycine amino acid is formed.

7-Glycine (Glycine amino acid) now enters the mitochondria, where two molecules of glycine condense to form another amino acid called serine amino acid.

Ammonia and carbon dioxide are released in this reaction.

8-serine enters the peroxisome and is deaminated into glycerate (serine delaminated into glycerate).

The 9-glycerate then moves into the chloroplast, and is used to make photosynthetic products.

10-In the process of complete photorespiration, two molecules are formed by the oxidation of phosphoglycolate.

And these two molecules further form one molecule of phosphoglycerate and one molecule of CO2.

Factors Affecting On Photorespiration

1. Photorespiration increases with increase in temperature.

2. It will increase with the increase of high intensity of light.

3. Similarly it also increases with increase in oxygen concentrations.

4. Even when the leaves are old, the process of photorespiration will increase.

5. However, the process of photorespiration is not found in C4 plant, because it avoids photorespiration by C2 cycle.

Significance Of Photorespiration

1. In this no type of ATP NADH is formed.

2. ATP is used in this, and CO2 is released, so it is a wasteful process.

3. There is loss of 25% fixed CO2 gas in photorespiration.

4. It is found only in C3 plants.

5. However, according to Kozaki & Takeba (1996), this process is applicable to C3 plants.

6. Photorespiration also Prevent to C3 from photo-oxidative damage.

Productivity Comparison between C4 & C3 Plants

C4 plants are generally more productive than C3 plants due to their unique photosynthetic pathway, which allows them to efficiently fix carbon dioxide (CO2) and minimize water loss. Here are some key reasons why C4 plants exhibit higher productivity:

1. Carbon fixation efficiency: C4 plants have an additional step in their photosynthetic process compared to C3 plants.

They possess specialized cells called bundle sheath cells that concentrate CO2 using an enzyme called PEP carboxylase.

This process allows them to efficiently fix CO2, even at lower atmospheric concentrations.

In contrast, C3 plants directly fix CO2 using an enzyme called RuBisCO, which is less efficient, especially under conditions of high temperature and water stress.

2. Reduced photorespiration: C4 plants have a biochemical mechanism that minimizes photorespiration by spatially separating the initial CO2 fixation (in mesophyll cells) from the CO2 release and Calvin cycle (in bundle sheath cells). This separation reduces the chances of oxygen interfering with the CO2 fixation process, resulting in increased efficiency and productivity.

3. Enhanced water-use efficiency: C4 plants have an advantage in arid or hot environments due to their ability to partially close their stomata during hot daytime conditions.

This reduces water loss through transpiration while still allowing CO2 uptake. In contrast, C3 plants need to keep their stomata open to ensure sufficient CO2 entry, leading to higher water loss.

The improved water-use efficiency of C4 plants allows them to thrive in water-limited conditions and maintain productivity even under drought stress.

4. Higher light saturation point: C4 plants have a higher light saturation point, which means they can utilize higher light intensities for photosynthesis without being limited by light availability.

This characteristic enables C4 plants to capture more light energy and convert it into biomass, resulting in increased productivity.

However, it is important to note that the productivity advantage of C4 plants is context-specific and varies depending on environmental conditions.

While C4 plants excel in high-temperature and water-limited environments, C3 plants may have an advantage in cooler and more temperate regions.

Additionally, there are exceptions and variations within C3 and C4 plants, and some C3 plants can also exhibit high productivity under certain conditions.

Common Examples of C4 Plants

There are several common examples of C4 plants found in various plant families. Here are some well-known examples:

1. Maize (corn): Maize is one of the most widely cultivated C4 plants.

2. Sugarcane: Sugarcane is another important C4 plant. It is known for its rapid growth and high biomass accumulation.

3. Sorghum: Sorghum is a versatile C4 plant. It is known for its ability to thrive in hot and arid conditions.

4. Millet: Millet is a group of C4 plants that includes various types such as pearl millet, finger millet, and foxtail millet.

5. Switchgrass: Switchgrass is a C4 grass commonly used as a bioenergy crop. It has the potential for high biomass production and is being explored as a sustainable alternative to fossil fuels.

6. Bermuda grass: Bermuda grass is a widely distributed C4 grass.

7. Amaranth: Amaranth species, such as grain amaranth and vegetable amaranth, are C4 plants.

8. Panicum grasses: Various species of Panicum grasses, including switchgrass mentioned earlier, are C4 plants commonly found in grasslands.

These are some examples of C4 plants, and there are many more species across different plant families that exhibit C4 photosynthesis on the Earth.

FAQ

1-What is C2 cycle?

Photorespiration is also called C2 cycle. Because 2 carbon phosphoglycolate is formed in it.

2-In which type of plants does the C2 cycle take place?

The process of photorespiration is found in all C3 plants.

3-Is the process of photorespiration found in C4 plants?

No, it is not found in C4 plants.

4-Glucose is formed in the C2 cycle?

CO2 is lost in the C2 cycle. ATP is spent, but no glucose is formed.

5-Where does the C2 cycle take place?

The C2 cycle occurs in chloroplasts, mitochondria, and oxysomes.

6-Who discovered the photorespiration?

Photorespiration was discovered by Dicker and Tio.

Conclusion

We have tried to cover all the points related to C2 cycle or photorespiration, and also shared important questions at the end.

But still any kind of suggestion or update and if you see any mistake, then you must tell us.

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