4.3 Photorespiration | Bioenergetics | Biology 11

Photorespiration is a metabolic pathway that occurs in photosynthetic organisms. It is a respiratory process in green cells that is dependent on light, consumes oxygen, and releases carbon dioxide. Unlike cellular respiration, it does not produce any ATP.

Mechanism of Photorespiration

Photorespiration is initiated when the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) acts as an oxygenase instead of a carboxylase. This typically happens under conditions of low internal $C O_{2}$ concentration (around 50 ppm) and high $O_{2}$ concentration.

Initiation: RuBisCO combines $O_{2}$ with Ribulose-1,5-bisphosphate (RuBP).
Products: This reaction produces one molecule of 3-phosphoglycerate (PGA), which can enter the Calvin cycle, and one molecule of a toxic 2-carbon compound called phosphoglycolate.
Detoxification Pathway: The plant must metabolize the toxic phosphoglycolate in a complex pathway involving three different organelles: the chloroplast, the peroxisome, and the mitochondrion.

Chloroplast: Phosphoglycolate is converted to glycolate. $RuBP + O_{2} \to Phosphoglycolate + PGA$ $Phosphoglycolate \to Glycolate$
Peroxisome: Glycolate is transported to the peroxisome and converted first to glyoxylate and then to the amino acid glycine.
Mitochondrion: Two molecules of glycine are transported into the mitochondrion, where they are converted into one molecule of the amino acid serine, releasing a molecule of $C O_{2}$ in the process.

Photorespiration pathway — Figure 4.20: Schematic representation of photorespiration pathway in chloroplast, peroxisomes and mitochondria

Disadvantages and Evolution

Disadvantages:
- It is an energy-costly process that consumes ATP and reducing power.
- It results in the net loss of previously fixed carbon as $C O_{2}$ , reducing the efficiency of photosynthesis by as much as 25%.
- Photorespiration is not essential for plant growth; inhibiting it does not harm the plant.
Evolutionary Context:
- RuBisCO evolved in an ancient atmosphere with very little $O_{2}$ and high levels of $C O_{2}$ . Its active site can bind to both molecules.
- As photosynthesis released $O_{2}$ into the atmosphere over geological time, the oxygenase activity of RuBisCO became a significant problem, leading to the wasteful process of photorespiration.

Factors Triggering Photorespiration

RuBisCO's Dual Activity: The enzyme's activity (carboxylase vs. oxygenase) depends on the relative concentrations of $C O_{2}$ and $O_{2}$ .
High Temperature and Dry Conditions: On hot, dry days, plants close their stomata to conserve water. This prevents $C O_{2}$ from entering the leaf and $O_{2}$ from exiting.
Shift in Gas Ratio: As photosynthesis continues, the internal $C O_{2}$ level drops while the $O_{2}$ level rises, increasing the $O_{2}$ to $C O_{2}$ ratio and favoring the oxygenase activity of RuBisCO.

C4 Photosynthesis: An Adaptation

Some plants, especially those in tropical climates (e.g., maize, sugarcane), have evolved a mechanism to minimize photorespiration. This is known as C4 photosynthesis or the Hatch-Slack pathway.

Spatial Separation: C4 plants separate initial $C O_{2}$ fixation from the Calvin cycle into two different cell types: mesophyll cells and bundle sheath cells.
CO₂ Pumping Mechanism:
1. Initial Fixation (Mesophyll Cells): $C O_{2}$ is first fixed by the enzyme PEP carboxylase, which has a high affinity for $C O_{2}$ and no affinity for $O_{2}$ . It combines $C O_{2}$ with a 3-carbon molecule, phosphoenolpyruvate (PEP), to form a 4-carbon molecule, oxaloacetate.
2. Transport: Oxaloacetate is converted to malate (another 4-carbon acid) and transported to the adjacent bundle sheath cells.
3. CO₂ Release (Bundle Sheath Cells): Inside the bundle sheath cells, malate releases the $C O_{2}$ , creating a very high concentration of $C O_{2}$ around the RuBisCO enzyme.
4. Calvin Cycle: With high $C O_{2}$ levels, RuBisCO functions efficiently as a carboxylase, running the Calvin cycle and minimizing photorespiration.
5. Regeneration: The remaining 3-carbon molecule (pyruvate) is transported back to the mesophyll cell to regenerate PEP, a process that requires ATP.

Feature	C3 Photosynthesis	C4 Photosynthesis
Initial $C O_{2}$ Acceptor	RuBP (5-carbon)	PEP (3-carbon)
Initial $C O_{2}$ Fixing Enzyme	RuBisCO	PEP Carboxylase
First Stable Product	3-PGA (3-carbon)	Oxaloacetate (4-carbon)
Location of Calvin Cycle	Mesophyll cells	Bundle sheath cells
Photorespiration Rate	High, especially in hot/dry conditions	Negligible or very low
Optimal Temperature	Lower (20-25°C)	Higher (30-40°C)
Example Plants	Rice, Wheat, Soybeans	Maize, Sugarcane, Sorghum

Possible Questions and Answers

Q: What are the disadvantages of photorespiration? A: Photorespiration is a wasteful process that costs the plant energy (ATP) and results in the net loss of fixed carbon dioxide. It can reduce the efficiency of photosynthesis by up to 25%.
Q: How did photorespiration evolve? A: The enzyme RuBisCO evolved when Earth's atmosphere had very little oxygen. Its active site can bind both $C O_{2}$ and $O_{2}$ . As oxygen levels in the atmosphere increased due to photosynthesis, the enzyme's ability to bind with $O_{2}$ became a liability, leading to the process of photorespiration.
Q: What is the effect of temperature on the activities of RuBisCO? A: High temperatures cause plants to close their stomata to conserve water. This leads to a decrease in the internal $C O_{2}$ concentration and an increase in the $O_{2}$ concentration inside the leaf. This high $O_{2}$ : $C O_{2}$ ratio favors the oxygenase activity of RuBisCO, thus increasing the rate of photorespiration.

References

(Derived from FBISE textbook)

Mechanism of Photorespiration

Initiation: RuBisCO combines $O_{2}$ with Ribulose-1,5-bisphosphate (RuBP).
Products: This reaction produces one molecule of 3-phosphoglycerate (PGA), which can enter the Calvin cycle, and one molecule of a toxic 2-carbon compound called phosphoglycolate.
Detoxification Pathway: The plant must metabolize the toxic phosphoglycolate in a complex pathway involving three different organelles: the chloroplast, the peroxisome, and the mitochondrion.

Chloroplast: Phosphoglycolate is converted to glycolate. $RuBP + O_{2} \to Phosphoglycolate + PGA$ $Phosphoglycolate \to Glycolate$
Peroxisome: Glycolate is transported to the peroxisome and converted first to glyoxylate and then to the amino acid glycine.
Mitochondrion: Two molecules of glycine are transported into the mitochondrion, where they are converted into one molecule of the amino acid serine, releasing a molecule of $C O_{2}$ in the process.

Disadvantages and Evolution

Disadvantages:
- It is an energy-costly process that consumes ATP and reducing power.
- It results in the net loss of previously fixed carbon as $C O_{2}$ , reducing the efficiency of photosynthesis by as much as 25%.
- Photorespiration is not essential for plant growth; inhibiting it does not harm the plant.
Evolutionary Context:
- RuBisCO evolved in an ancient atmosphere with very little $O_{2}$ and high levels of $C O_{2}$ . Its active site can bind to both molecules.
- As photosynthesis released $O_{2}$ into the atmosphere over geological time, the oxygenase activity of RuBisCO became a significant problem, leading to the wasteful process of photorespiration.

Factors Triggering Photorespiration

RuBisCO's Dual Activity: The enzyme's activity (carboxylase vs. oxygenase) depends on the relative concentrations of $C O_{2}$ and $O_{2}$ .
High Temperature and Dry Conditions: On hot, dry days, plants close their stomata to conserve water. This prevents $C O_{2}$ from entering the leaf and $O_{2}$ from exiting.
Shift in Gas Ratio: As photosynthesis continues, the internal $C O_{2}$ level drops while the $O_{2}$ level rises, increasing the $O_{2}$ to $C O_{2}$ ratio and favoring the oxygenase activity of RuBisCO.

C4 Photosynthesis: An Adaptation

Spatial Separation: C4 plants separate initial $C O_{2}$ fixation from the Calvin cycle into two different cell types: mesophyll cells and bundle sheath cells.
CO₂ Pumping Mechanism:
1. Initial Fixation (Mesophyll Cells): $C O_{2}$ is first fixed by the enzyme PEP carboxylase, which has a high affinity for $C O_{2}$ and no affinity for $O_{2}$ . It combines $C O_{2}$ with a 3-carbon molecule, phosphoenolpyruvate (PEP), to form a 4-carbon molecule, oxaloacetate.
2. Transport: Oxaloacetate is converted to malate (another 4-carbon acid) and transported to the adjacent bundle sheath cells.
3. CO₂ Release (Bundle Sheath Cells): Inside the bundle sheath cells, malate releases the $C O_{2}$ , creating a very high concentration of $C O_{2}$ around the RuBisCO enzyme.
4. Calvin Cycle: With high $C O_{2}$ levels, RuBisCO functions efficiently as a carboxylase, running the Calvin cycle and minimizing photorespiration.
5. Regeneration: The remaining 3-carbon molecule (pyruvate) is transported back to the mesophyll cell to regenerate PEP, a process that requires ATP.

Feature	C3 Photosynthesis	C4 Photosynthesis
Initial $C O_{2}$ Acceptor	RuBP (5-carbon)	PEP (3-carbon)
Initial $C O_{2}$ Fixing Enzyme	RuBisCO	PEP Carboxylase
First Stable Product	3-PGA (3-carbon)	Oxaloacetate (4-carbon)
Location of Calvin Cycle	Mesophyll cells	Bundle sheath cells
Photorespiration Rate	High, especially in hot/dry conditions	Negligible or very low
Optimal Temperature	Lower (20-25°C)	Higher (30-40°C)
Example Plants	Rice, Wheat, Soybeans	Maize, Sugarcane, Sorghum

Possible Questions and Answers

Q: What are the disadvantages of photorespiration? A: Photorespiration is a wasteful process that costs the plant energy (ATP) and results in the net loss of fixed carbon dioxide. It can reduce the efficiency of photosynthesis by up to 25%.
Q: How did photorespiration evolve? A: The enzyme RuBisCO evolved when Earth's atmosphere had very little oxygen. Its active site can bind both $C O_{2}$ and $O_{2}$ . As oxygen levels in the atmosphere increased due to photosynthesis, the enzyme's ability to bind with $O_{2}$ became a liability, leading to the process of photorespiration.
Q: What is the effect of temperature on the activities of RuBisCO? A: High temperatures cause plants to close their stomata to conserve water. This leads to a decrease in the internal $C O_{2}$ concentration and an increase in the $O_{2}$ concentration inside the leaf. This high $O_{2}$ : $C O_{2}$ ratio favors the oxygenase activity of RuBisCO, thus increasing the rate of photorespiration.

References

(Derived from FBISE textbook)