4.3.2 Disadvantages and Evolution of Photorespiration

Introduction

Photorespiration is a wasteful process that occurs in plants when RuBisCO binds oxygen instead of carbon dioxide. It reduces the efficiency of photosynthesis, costs the plant energy, and results in the net loss of carbon fixation. Despite being disadvantageous, photorespiration exists because RuBisCO evolved when there was no oxygen in the atmosphere, and it can bind both CO₂ and O₂.

Definition

Disadvantages 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.

Evolution of Photorespiration: 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.

In simple words: Photorespiration is a wasteful process that reduces plant productivity, but it exists because RuBisCO can bind both CO₂ and O₂, and oxygen appeared in the atmosphere after RuBisCO evolved.

Core Concept

• Photorespiration costs the plant energy.
• It results in the net loss of CO₂ fixation from the plant.
• Thus, it reduces the rate of the photosynthetic process.
• In most plants, photorespiration reduces the amount of carbon fixed into carbohydrate during photosynthesis by 25 percent.
• 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 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.

Detailed Explanation

Disadvantages of Photorespiration

1. Energy Cost

• Photorespiration consumes ATP.
• ATP is used to convert phosphoglycolate to glycolate.
• ATP is also used in the peroxisome and mitochondria.
• This energy could have been used for growth and other processes.

Key Point: Photorespiration wastes energy that could be used for plant growth and development.

2. Net Loss of CO₂ Fixation

• Photorespiration releases CO₂.
• This CO₂ was already fixed by photosynthesis.
• The released CO₂ is lost to the atmosphere.
• This reduces the net carbon gain.

Key Point: Photorespiration results in a net loss of carbon that could have been used to make sugars.

3. Reduction in Photosynthetic Rate

• Photorespiration reduces the rate of photosynthesis.
• In most plants, photorespiration reduces the amount of carbon fixed into carbohydrate during photosynthesis by 25 percent.
• This means that 25% of the carbon that could have been fixed is lost.

Key Point: Photorespiration reduces the efficiency of photosynthesis by 25%.

4. Reduction in Crop Yield

• Photorespiration reduces dry matter production.
• It reduces agricultural yield in tropical climates.
• Under certain conditions, up to 5% of the photosynthetic potential is lost in photorespiratory metabolism.

Key Point: Photorespiration reduces crop yield in tropical climates.

5. Not Essential for Plants

• Photorespiration is not essential for plants.
• If photorespiration is inhibited chemically, the plant can still grow.
• Some plants are naturally resistant to photorespiration.

Key Point: Plants can survive without photorespiration.

Summary of Disadvantages

Science, Technology and Society Connections (from textbook)

The impact of photorespiration on agriculture yield in tropical climates:

• 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.

Evolution of Photorespiration

1. RuBisCO Evolved in an Oxygen-Free Atmosphere

• RuBisCO evolved about 3.5 billion years ago.
• At that time, there was no oxygen in the atmosphere.
• The active site of RuBisCO was evolved to bind CO₂.
• The oxygen-binding activity was not a problem because there was no oxygen.

Key Point: RuBisCO evolved when there was no oxygen in the atmosphere.

2. Oxygen Accumulated in the Atmosphere

• About 2.5 billion years ago, oxygen began to accumulate in the atmosphere.
• This was due to the evolution of oxygenic photosynthesis in cyanobacteria.
• Oxygen levels increased dramatically.

Key Point: Oxygen accumulated in the atmosphere due to photosynthesis.

3. RuBisCO Started Binding Oxygen

• The active site of RuBisCO can bind both CO₂ and O₂.
• With oxygen present, RuBisCO started binding O₂ instead of CO₂.
• This led to photorespiration.

Key Point: RuBisCO's dual affinity for CO₂ and O₂ led to photorespiration.

4. Photorespiration Is an Unavoidable Consequence

• Photorespiration is a wasteful process.
• However, it is an unavoidable consequence of RuBisCO's dual affinity.
• Plants cannot easily evolve a new enzyme because RuBisCO is essential for photosynthesis.

Key Point: Photorespiration is an unavoidable consequence of RuBisCO's dual affinity.

5. Plants Have Evolved Mechanisms to Reduce Photorespiration

• C4 pathway: Concentrates CO₂ in bundle sheath cells.
• CAM pathway: Separates CO₂ fixation and Calvin cycle in time.
• These mechanisms reduce photorespiration and increase photosynthetic efficiency.

Key Point: Some plants have evolved mechanisms to reduce photorespiration.

Summary of Evolution

Comparison of C3, C4, and CAM Plants

Why Doesn't Photorespiration Disappear?

• Photorespiration is a wasteful process.
• However, it is an unavoidable consequence of RuBisCO's dual affinity.
• Plants cannot easily evolve a new enzyme because RuBisCO is essential for photosynthesis.
• RuBisCO is the most abundant protein on Earth.
• It is encoded by a single gene in chloroplasts.
• Mutations in RuBisCO that reduce oxygen binding also reduce CO₂ binding.
• Therefore, photorespiration persists.

Key Point: Photorespiration persists because RuBisCO is essential and cannot easily be replaced.

Intermediate Level

Disadvantages of Photorespiration in Detail

Evolution of Photorespiration in Detail

Comparison of C3, C4, and CAM Plants

Impact on Agriculture

Key Point: Photorespiration reduces crop yield in tropical climates.

Important Points

• 1. Photorespiration costs the plant energy and results in the net loss of CO₂ fixation from the plant.
• 2. Thus, it reduces the rate of photosynthetic process.
• 3. In most plants, photorespiration reduces the amount of carbon fixed into carbohydrate during photosynthesis by 25 percent.
• 4. Photorespiration is not essential for plant.
• 5. It is also observed that if photorespiration is inhibited chemically, the plant can still grow.
• 6. Furthermore, some plants are naturally resistant to photorespiration.
• 7. Then why photorespiration exists?
• 8. The common simple answer to this question is that the active site of RuBisCO is evolved to bind both carbon dioxide and oxygen.
• 9. 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.
• 10. The photorespiration started when the oxygen began to accumulate in the atmosphere.
• 11. 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
• - Disadvantages = energy cost, CO₂ loss, reduced photosynthesis, reduced crop yield
• - Evolution = RuBisCO evolved without oxygen; oxygen appeared; photorespiration started
• - Not essential = plants can grow without it
• - C4 and CAM plants = evolved to reduce photorespiration
• - RuBisCO = can bind both CO₂ and O₂
• - Oxygen accumulated = ~2.5 billion years ago
• - RuBisCO evolved = ~3.5 billion years ago

Common Mistakes

• 1. Thinking photorespiration is beneficial – Photorespiration is a wasteful process.
• 2. Thinking photorespiration is essential – Photorespiration is not essential for plants.
• 3. Thinking photorespiration can be easily stopped – Photorespiration is an unavoidable consequence of RuBisCO's dual affinity.
• 4. Confusing photorespiration with aerobic respiration – Photorespiration occurs in green cells in the presence of light; aerobic respiration occurs in all living cells.
• 5. Thinking RuBisCO only binds CO₂ – RuBisCO binds both CO₂ and O₂.
• 6. Forgetting the energy cost – Photorespiration consumes ATP.
• 7. Thinking photorespiration only occurs in C3 plants – Photorespiration occurs in C3 plants; C4 and CAM plants have mechanisms to reduce it.
• 8. Confusing the evolution timeline – RuBisCO evolved ~3.5 billion years ago; oxygen accumulated ~2.5 billion years ago.
• 9. Thinking photorespiration can be easily engineered away – Engineering RuBisCO is challenging.
• 10. Forgetting the impact on agriculture – Photorespiration reduces crop yield by 20–30%.
• 11. Thinking photorespiration is a recent phenomenon – Photorespiration started when oxygen accumulated in the atmosphere.
• 12. Confusing the products of photorespiration – Photorespiration produces phosphoglycolate, glycolate, glycine, serine, and CO₂.

Quick Revision

• 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.
• 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.

60-Second Revision

• Photorespiration = wasteful process
• Disadvantages = energy cost, CO₂ loss, reduced photosynthesis, reduced crop yield
• Evolution = RuBisCO evolved without oxygen; oxygen appeared; photorespiration started
• Not essential = plants can grow without it
• C4 and CAM plants = evolved to reduce photorespiration
• RuBisCO = can bind both CO₂ and O₂
• Oxygen accumulated = ~2.5 billion years ago
• RuBisCO evolved = ~3.5 billion years ago
• Warning: Photorespiration is not beneficial. It is not essential. It cannot be easily stopped. RuBisCO binds both CO₂ and O₂. Photorespiration reduces crop yield by 20–30%. C4 and CAM plants have evolved mechanisms to reduce photorespiration. Engineering RuBisCO is challenging. Photorespiration persists because RuBisCO is essential.