4.3.3 EFFECT OF TEMPERATURE ON THE ACTIVITIES OF RuBisCO

Introduction

RuBisCO (Ribulose bisphosphate carboxylase/oxygenase) is the enzyme that catalyzes the first major step of carbon fixation in the Calvin cycle. It is also the enzyme responsible for photorespiration. The activity of RuBisCO is highly dependent on temperature. Understanding how temperature affects RuBisCO is essential for understanding photosynthesis and photorespiration, especially in the context of climate change and agriculture.

Definition

RuBisCO: RuBisCO (Ribulose bisphosphate carboxylase/oxygenase) is the enzyme that catalyzes the fixation of carbon dioxide into an organic molecule during the Calvin cycle. It can also catalyze the oxygenation of RuBP, leading to photorespiration.

In simple words: RuBisCO is the enzyme that fixes CO₂ in photosynthesis, but it can also cause photorespiration by binding O₂.

Core Concept

• Photorespiration is related to the functioning of the enzyme ribulose bisphosphate (RuBP) carboxylase/oxygenase.
• It is often called RuBisCO because it can have an oxygenase activity in addition to carboxylase activity.
• Its activity depends upon the relative concentration of O₂ and CO₂ in leaves.
• Photorespiration starts when the CO₂ levels inside a leaf become low.
• If the plant continues CO₂ fixation in photosynthesis when its stomata are closed, the CO₂ will be used up and the O₂ released from photosynthesis will be prevented to release out.
• Temperature plays a crucial role in determining the relative concentrations of O₂ and CO₂, and thus the activity of RuBisCO.
• Higher temperatures favor photorespiration over photosynthesis.
• Lower temperatures favor photosynthesis over photorespiration.

Detailed Explanation

RuBisCO: A Dual-Function Enzyme

RuBisCO has two activities:

• Fixes CO₂ to RuBP.
• Produces two molecules of 3-PGA (3C).
• This is the first step of the Calvin cycle.
• Leads to sugar production.

• Fixes O₂ to RuBP.
• Produces one molecule of 3-PGA (3C) and one molecule of phosphoglycolate (2C).
• This is the first step of photorespiration.
• Leads to energy loss and CO₂ release.

Equation for Carboxylase Activity: RuBP + CO₂ → 2 × 3-PGA

Equation for Oxygenase Activity: RuBP + O₂ → 3-PGA + Phosphoglycolate

Factors Affecting RuBisCO Activity

The activity of RuBisCO depends on:

• Relative concentration of CO₂ and O₂
• Temperature
• Light intensity
• Water availability
• Stomatal opening

Effect of Temperature on RuBisCO Activity

Key Points:

• RuBisCO has higher affinity for CO₂.
• Carboxylase activity is favored.
• Photosynthesis is favored.
• Photorespiration is reduced.
• More sugar is produced.

• RuBisCO has higher affinity for O₂.
• Oxygenase activity is favored.
• Photorespiration is favored.
• Photosynthesis is reduced.
• Less sugar is produced.
• More CO₂ is released.

Why does temperature affect RuBisCO activity?

• Solubility of gases: CO₂ is more soluble in water than O₂ at low temperatures. At high temperatures, the solubility of CO₂ decreases more than O₂, making the CO₂/O₂ ratio lower.
• Stomatal closure: At high temperatures, plants close their stomata to conserve water. This reduces CO₂ intake and increases O₂ accumulation.
• Enzyme kinetics: The affinity of RuBisCO for CO₂ and O₂ changes with temperature.
• Photorespiration rate: Photorespiration increases more rapidly with temperature than photosynthesis.

Temperature and the CO₂/O₂ Ratio

Key Point: At high temperatures, the CO₂/O₂ ratio decreases, favoring photorespiration.

Temperature and Stomatal Closure

Key Point: At high temperatures, stomata close to conserve water, reducing CO₂ intake and increasing O₂ accumulation, which favors photorespiration.

Temperature and Photorespiration Rate

• Photorespiration increases more rapidly with temperature than photosynthesis.
• At 25°C, photorespiration is about 20% of photosynthesis.
• At 35°C, photorespiration is about 50% of photosynthesis.
• At 40°C, photorespiration can exceed photosynthesis.

Key Point: Higher temperatures increase photorespiration more than photosynthesis.

Why Is This Important?

• High temperatures reduce crop yield due to increased photorespiration.
• This is especially important in tropical climates.

• Global warming is increasing temperatures.
• This may increase photorespiration and reduce crop yields.

• These plants have evolved mechanisms to concentrate CO₂ and reduce photorespiration.
• They are more efficient at high temperatures.

Comparison of C3 and C4 Plants at Different Temperatures

Key Point: C4 plants are more efficient at high temperatures because they reduce photorespiration.

Intermediate Level

Effect of Temperature on RuBisCO Activity in Detail

Factors Affecting the CO₂/O₂ Ratio

Temperature and Photorespiration Rate

Comparison of C3 and C4 Plants

Impact on Agriculture

Key Point: C4 crops (maize, sugarcane) are more tolerant to high temperatures than C3 crops (wheat, rice).

Important Points

• 1. Photorespiration is related to the functioning of the enzyme ribulose bisphosphate (RuBP) carboxylase/oxygenase.
• 2. It is often called RuBisCO because it can have an oxygenase activity in addition to carboxylase activity.
• 3. Its activity depends upon the relative concentration of O₂ and CO₂ in leaves.
• 4. Photorespiration starts when the CO₂ levels inside a leaf become low.
• 5. If the plant continues CO₂ fixation in photosynthesis when its stomata are closed, the CO₂ will be used up and the O₂ released from photosynthesis will be prevented to release out.
• 6. Temperature plays a crucial role in determining the relative concentrations of O₂ and CO₂, and thus the activity of RuBisCO.
• 7. Higher temperatures favor photorespiration over photosynthesis.
• 8. Lower temperatures favor photosynthesis over photorespiration.

Remember

• - RuBisCO = dual-function enzyme (carboxylase and oxygenase)
• - Low temperature = photosynthesis favored
• - High temperature = photorespiration favored
• - High temperature = stomata close = low CO₂ = high O₂ = photorespiration
• - Photorespiration = wasteful process
• - C4 plants = better at high temperatures
• - Climate change = may increase photorespiration
• - Engineering RuBisCO = challenging

Common Mistakes

• 1. Thinking RuBisCO only fixes CO₂ – RuBisCO can also fix O₂.
• 2. Thinking temperature only affects enzyme kinetics – Temperature also affects gas solubility and stomatal opening.
• 3. Confusing the effect of temperature on photosynthesis and photorespiration – High temperature favors photorespiration; low temperature favors photosynthesis.
• 4. Thinking photorespiration is not affected by temperature – Photorespiration is strongly affected by temperature.
• 5. Forgetting the role of stomata – Stomatal closure at high temperatures reduces CO₂ intake and increases O₂ accumulation.
• 6. Thinking C3 and C4 plants are equally affected by temperature – C4 plants are more tolerant to high temperatures.
• 7. Confusing the CO₂/O₂ ratio – High CO₂/O₂ ratio favors photosynthesis; low CO₂/O₂ ratio favors photorespiration.
• 8. Thinking RuBisCO can be easily engineered – Engineering RuBisCO is challenging.
• 9. Forgetting the impact on agriculture – High temperatures reduce crop yield due to photorespiration.
• 10. Thinking climate change has no effect on photorespiration – Climate change may increase photorespiration and reduce crop yields.
• 11. Confusing the specificity factor – The specificity factor decreases with increasing temperature.
• 12. Thinking photorespiration only occurs in C3 plants – Photorespiration occurs in C3 plants; C4 and CAM plants have mechanisms to reduce it.

Quick Revision

• Photorespiration is related to the functioning of the enzyme ribulose bisphosphate (RuBP) carboxylase/oxygenase.
• It is often called RuBisCO because it can have an oxygenase activity in addition to carboxylase activity.
• Its activity depends upon the relative concentration of O₂ and CO₂ in leaves.
• Photorespiration starts when the CO₂ levels inside a leaf become low.
• If the plant continues CO₂ fixation in photosynthesis when its stomata are closed, the CO₂ will be used up and the O₂ released from photosynthesis will be prevented to release out.
• Temperature plays a crucial role in determining the relative concentrations of O₂ and CO₂, and thus the activity of RuBisCO.
• Higher temperatures favor photorespiration over photosynthesis.
• Lower temperatures favor photosynthesis over photorespiration.
• At high temperatures, stomata close to conserve water, reducing CO₂ intake and increasing O₂ accumulation.
• Photorespiration increases more rapidly with temperature than photosynthesis.
• C4 plants are more efficient at high temperatures because they reduce photorespiration.
• Climate change may increase photorespiration and reduce crop yields.
• Engineering RuBisCO to reduce photorespiration is a promising but challenging approach.

60-Second Revision

• RuBisCO = dual-function enzyme (carboxylase and oxygenase)
• Low temperature = photosynthesis favored
• High temperature = photorespiration favored
• High temperature = stomata close = low CO₂ = high O₂ = photorespiration
• Photorespiration = wasteful process
• C4 plants = better at high temperatures
• Climate change = may increase photorespiration
• Engineering RuBisCO = challenging
• Warning: RuBisCO can bind both CO₂ and O₂. High temperature favors photorespiration. Low temperature favors photosynthesis. Stomatal closure at high temperatures reduces CO₂ intake and increases O₂ accumulation. Photorespiration increases more rapidly with temperature than photosynthesis. C4 plants are more tolerant to high temperatures. Climate change may increase photorespiration and reduce crop yields. Engineering RuBisCO is challenging.