4.3.1 Mechanism of Photorespiration

Introduction

Photorespiration is a process that occurs in plants in the presence of light, where oxygen is consumed and carbon dioxide is released. It is a wasteful process because it consumes energy and reduces the efficiency of photosynthesis. It occurs because the enzyme RuBisCO can bind to oxygen instead of carbon dioxide when CO₂ levels are low.

Definition

Photorespiration: The respiratory activity that occurs in green cells in the presence of light resulting in release of carbon dioxide is termed as photorespiration.

In simple words: Photorespiration is a process in plants where oxygen is used and carbon dioxide is released in the presence of light, wasting energy.

Core Concept

• Photorespiration needs oxygen and produces CO₂ and H₂O like aerobic respiration.
• However, ATP is not produced during photorespiration.
• It occurs when the CO₂ levels inside the leaf drop to around 50 ppm (parts per million).
• At this low CO₂ concentration, RuBisCO (Ribulose bisphosphate carboxylase/oxygenase) starts to combine O₂ with RuBP instead of CO₂.
• The net result is that instead of producing two 3C molecules of phosphoglycerate (PGA), only one molecule of PGA and a toxic 2C molecule called phosphoglycolate are produced.
• The plant must get rid of the phosphoglycolate.
• First, it immediately gets rid of the phosphate group, converting the molecule to glycolate.
• The glycolate is then transported to the peroxisome and there converted to glyoxalate and then glycine.
• The glycine is then transported into the mitochondria where it is converted into serine.
• The serine is then used to make other organic molecules.

Detailed Explanation

Conditions for Photorespiration

• CO₂ levels are low (around 50 ppm).
• O₂ levels are high.
• Light is present.
• Stomata are closed (due to water stress or high temperature).

• To conserve water.
• This reduces CO₂ intake.
• O₂ produced by photosynthesis accumulates.
• RuBisCO binds O₂ instead of CO₂.

Mechanism of Photorespiration

Step 1: Oxygenation of RuBP

• RuBisCO normally fixes CO₂ to RuBP (5C) to form two molecules of 3-PGA (3C).
• When CO₂ is low and O₂ is high, RuBisCO fixes O₂ to RuBP.
• This produces one molecule of 3-PGA (3C) and one molecule of phosphoglycolate (2C).

Equation: RuBP + O₂ RuBisCO→ Phosphoglycolate + Phosphoglycerate

Step 2: Removal of Phosphate Group

• The plant immediately gets rid of the phosphate group from phosphoglycolate.
• This converts it to glycolate.

Equation: Phosphoglycolate → Glycolate + Pi

Step 3: Transport to Peroxisome

• Glycolate is transported to the peroxisome.
• In the peroxisome, glycolate is converted to glyoxalate.
• Glyoxalate is then converted to glycine.

Equation: Glycolate → Glyoxalate → Glycine

Step 4: Transport to Mitochondria

• Glycine is transported to the mitochondria.
• In the mitochondria, two molecules of glycine are converted to one molecule of serine and one molecule of CO₂.

Equation: 2 Glycine → Serine + CO₂

Step 5: Utilization of Serine

• Serine is then used to make other organic molecules.
• The CO₂ released is lost to the atmosphere.
• This reduces the efficiency of photosynthesis.

Summary of Photorespiration

Equation Summary

RuBP + O₂ → Phosphoglycolate + Phosphoglycerate \] \[ Phosphoglycolate → Glycolate + Pi \] \[ Glycolate → Glyoxalate → Glycine \] \[ 2 Glycine → Serine + CO₂

Why is Photorespiration a Wasteful Process?

• No ATP is produced: Unlike aerobic respiration, photorespiration does not produce ATP.
• No sugar is produced: The process does not produce glucose or other sugars.
• CO₂ is released: The plant loses carbon that was already fixed.
• Energy is consumed: The plant uses energy to carry out photorespiration.
• Reduces photosynthetic efficiency: It reduces the rate of photosynthesis.
• Net loss of CO₂ fixation: In most plants, photorespiration reduces the amount of carbon fixed into carbohydrate during photosynthesis by 25 percent.

Science, Technology and Society Connections (from textbook)

• Photorespiration decreases net photosynthesis because a portion of the CO₂ fixed in photosynthesis escapes from the leaf after it is fixed.
• Under certain conditions, up to 5% of the photosynthetic potential is lost in photorespiratory metabolism.
• Thus photorespiration reduces dry matter production and agricultural yield in tropical climates.

Comparison of Photorespiration and Aerobic Respiration

Why Does Photorespiration Exist?

• Photorespiration is not essential for plants.
• It is also observed that if photorespiration is inhibited chemically, the plant can still grow.
• Furthermore, some plants are naturally resistant to photorespiration.
• Then why does photorespiration exist?
• The common simple answer to this question is that the active site of RuBisCO is evolved to bind both carbon dioxide and oxygen.
• Originally it was not a problem as there was no oxygen in the atmosphere at the time of establishment of earth, so the carbon dioxide binding activity was the only one used.
• The photorespiration started when the oxygen began to accumulate in the atmosphere.

Historical Background

• Photorespiration evolved because RuBisCO evolved when there was no oxygen in the atmosphere.
• RuBisCO's active site can bind both CO₂ and O₂.
• When oxygen accumulated in the atmosphere (due to photosynthesis), RuBisCO started binding O₂.
• This led to photorespiration.
• Photorespiration is a wasteful process, but it is an unavoidable consequence of RuBisCO's dual affinity.

Intermediate Level

Mechanism of Photorespiration in Detail

Comparison of Photorespiration and Photosynthesis

Comparison of Photorespiration and Aerobic Respiration

Impact on Agriculture

• Photorespiration reduces the amount of carbon fixed into carbohydrate during photosynthesis by 25 percent.
• Up to 5% of the photosynthetic potential is lost in photorespiratory metabolism.
• This reduces dry matter production and agricultural yield in tropical climates.
• C4 plants have evolved mechanisms to reduce photorespiration.

Important Points

• 1. The respiratory activity that occurs in green cells in the presence of light resulting in release of carbon dioxide is termed as photorespiration.
• 2. It needs oxygen and produce CO₂ and H₂O like aerobic respiration.
• 3. However ATP is not produced during photorespiration.
• 4. When the CO₂ levels inside the leaf drop to around 50 ppm (part per million), ribulose bisphosphate carboxylase/oxygenase (RuBisCO) starts to combine O₂ with RuBP instead of CO₂.
• 5. The net result of this is that instead of producing two 3C molecules of phosphoglycerate (PGA), only one molecule of PGA and a toxic 2C molecule called phosphoglycolate are produced.
• 6. The plant must get rid of the phosphoglycolate.
• 7. First it immediately gets rid of the phosphate group, converting the molecule to glycolate.
• 8. The glycolate is then transported to the peroxisome and there converted to glyoxalate and then glycine.
• 9. The glycine is then transported into the mitochondria where it is converted into serine.
• 10. The serine is then used to make other organic molecules.
• 11. Disadvantages and Evolution of Photorespiration:
• - Photorespiration costs the plant energy and results in the net loss of CO₂ fixation from the plant.
• - Thus, it reduces the rate of photosynthetic process.
• - In most plants, photorespiration reduces the amount of carbon fixed into carbohydrate during photosynthesis by 25 percent.
• - Photorespiration is not essential for plant.
• - It is also observed that if photorespiration is inhibited chemically, the plant can still grow.
• - Furthermore, some plants are naturally resistant to photorespiration.
• - Then why photorespiration exists?
• - The common simple answer to this question is that the active site of RuBisCO is evolved to bind both carbon dioxide and oxygen.
• - Originally it was not a problem as there was no oxygen in the atmosphere at the time of establishment of earth so the carbon dioxide binding activity was the only one used.
• - The photorespiration started when the oxygen began to accumulate in the atmosphere.
• 12. Science, Technology and Society Connections:
• - The impact of photorespiration on the agriculture yield in the tropic climates.
• - Photorespiration decreases net photosynthesis because a portion of the CO₂ fixed in photosynthesis escapes from the leave after it is fixed.
• - Under certain conditions, up to 5% of the photosynthetic potential is lost in photorespiratory metabolism.
• - Thus photorespiration reduces dry matter production and agricultural yield in tropical climate.

Remember

• - Photorespiration = wasteful process in plants
• - **Occurs when** = CO₂ is low, O₂ is high
• - **Enzyme** = RuBisCO
• - RuBisCO binds = O₂ instead of CO₂
• - Produces = phosphoglycolate (toxic)
• - Cleanup = peroxisome, mitochondria
• - **Releases** = CO₂
• - No ATP produced
• - Reduces photosynthesis
• - Reduces crop yield

Common Mistakes

• 1. Confusing photorespiration with aerobic respiration – Photorespiration occurs in green cells in the presence of light; aerobic respiration occurs in all living cells.
• 2. Thinking photorespiration produces ATP – Photorespiration does not produce ATP.
• 3. Confusing photorespiration with photosynthesis – Photorespiration releases CO₂; photosynthesis fixes CO₂.
• 4. Thinking photorespiration is essential – Photorespiration is not essential for plants.
• 5. Forgetting the role of RuBisCO – RuBisCO binds both CO₂ and O₂.
• 6. Confusing the location of the steps – Steps occur in the chloroplast, peroxisome, and mitochondria.
• 7. Thinking photorespiration occurs only in C3 plants – Photorespiration occurs in C3 plants; C4 plants have mechanisms to reduce it.
• 8. Forgetting the energy cost – Photorespiration costs the plant energy.
• 9. Thinking photorespiration is a recent phenomenon – Photorespiration started when oxygen accumulated in the atmosphere.
• 10. Confusing the products – Photorespiration produces phosphoglycolate, glycolate, glycine, serine, and CO₂.
• 11. Forgetting the impact on agriculture – Photorespiration reduces crop yield by 20–30%.
• 12. Thinking photorespiration can be easily stopped – Photorespiration is an unavoidable consequence of RuBisCO's dual affinity.

Quick Revision

• The respiratory activity that occurs in green cells in the presence of light resulting in release of carbon dioxide is termed as photorespiration.
• It needs oxygen and produce CO₂ and H₂O like aerobic respiration.
• However ATP is not produced during photorespiration.
• When the CO₂ levels inside the leaf drop to around 50 ppm (part per million), ribulose bisphosphate carboxylase/oxygenase (RuBisCO) starts to combine O₂ with RuBP instead of CO₂.
• The net result of this is that instead of producing two 3C molecules of phosphoglycerate (PGA), only one molecule of PGA and a toxic 2C molecule called phosphoglycolate are produced.
• The plant must get rid of the phosphoglycolate.
• First it immediately gets rid of the phosphate group, converting the molecule to glycolate.
• The glycolate is then transported to the peroxisome and there converted to glyoxalate and then glycine.
• The glycine is then transported into the mitochondria where it is converted into serine.
• The serine is then used to make other organic molecules.
• RuBP + O₂ → Phosphoglycolate + Phosphoglycerate
• Phosphoglycolate → Glycolate + Pi
• Glycolate → Glyoxalate → Glycine
• 2 Glycine → Serine + CO₂

60-Second Revision

• Photorespiration = wasteful process in plants
• **Occurs when** = CO₂ is low, O₂ is high
• **Enzyme** = RuBisCO
• RuBisCO binds = O₂ instead of CO₂
• Produces = phosphoglycolate (toxic)
• Cleanup = peroxisome, mitochondria
• **Releases** = CO₂
• No ATP produced
• Reduces photosynthesis
• Reduces crop yield
• Warning: Photorespiration is not the same as aerobic respiration. It does not produce ATP. It occurs in green cells in the presence of light. RuBisCO binds O₂ instead of CO₂ when CO₂ is low. Phosphoglycolate is toxic and must be cleaned up. The cleanup occurs in the peroxisome and mitochondria. CO₂ is released. Photorespiration reduces crop yield. It evolved because RuBisCO can bind both CO₂ and O₂. It is not essential for plants. C4 plants have mechanisms to reduce photorespiration.