Photorespiration, often described as a wasteful process, is a metabolic pathway that occurs in plants under certain conditions, particularly when carbon dioxide (CO2) levels are low and oxygen (O2) levels are high. It’s a consequence of the oxygenase activity of the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). While seemingly counterproductive, photorespiration is believed to be a relic of early Earth’s atmosphere, which had significantly lower CO2 and higher O2 concentrations. Understanding this process is crucial for comprehending plant physiology and exploring strategies to enhance photosynthetic efficiency, especially in the context of climate change. Understanding Photorespiration: A Detailed Overview Photorespiration is a complex metabolic pathway involving chloroplasts, peroxisomes, and mitochondria. It begins when RuBisCO, instead of binding to CO2, binds to O2. This oxygenation reaction initiates a series of reactions that ultimately lead to the release of CO2, consuming ATP and NADPH in the process – effectively reversing some of the gains made during photosynthesis. The Biochemical Pathway The photorespiratory pathway can be summarized in the following steps: Oxygenation by RuBisCO: RuBisCO catalyzes the reaction between RuBP (Ribulose-1,5-bisphosphate) and O2, forming one molecule of 3-PGA (3-phosphoglycerate) and one molecule of 2-phosphoglycolate. Processing of 2-Phosphoglycolate: 2-phosphoglycolate is toxic and must be detoxified. It is converted to glycolate in the chloroplast. Glycolate Transport & Conversion: Glycolate is transported to the peroxisome, where it is converted to glyoxylate and then to glycine. Glycine Transport & Conversion: Glycine is transported to the mitochondria, where two molecules of glycine are converted to serine, CO2, and ammonia (NH3). This is the step where CO2 is released, diminishing the carbon fixed during photosynthesis. Serine Transport & Conversion: Serine is transported back to the peroxisome and then to the chloroplast, where it is converted back to glycerate. Glycerate Conversion: Glycerate is phosphorylated to 3-PGA, which can re-enter the Calvin cycle. Differences between Photosynthesis and Photorespiration Feature Photosynthesis Photorespiration Primary Substrate CO2 O2 Enzyme RuBisCO (carboxylase activity) RuBisCO (oxygenase activity) Products Glucose, O2 CO2, NH3 Energy Requirement Requires light energy Consumes ATP & NADPH Net Carbon Gain Positive Negative Factors Affecting Photorespiration Temperature: Higher temperatures increase the rate of photorespiration because RuBisCO’s affinity for O2 increases with temperature. CO2 Concentration: Low CO2 concentrations favor oxygenation by RuBisCO, increasing photorespiration. O2 Concentration: High O2 concentrations promote oxygenation. Light Intensity: High light intensity can lead to stomatal closure, reducing CO2 uptake and increasing photorespiration. Evolutionary Significance and Mitigation Strategies Photorespiration is thought to be a relic of an ancient atmosphere with low CO2 and high O2. Plants like C4 and CAM plants have evolved mechanisms to minimize photorespiration. C4 plants spatially separate initial CO2 fixation from the Calvin cycle, concentrating CO2 around RuBisCO. CAM plants temporally separate these processes. Genetic engineering efforts are also underway to improve RuBisCO’s specificity for CO2, reducing photorespiration and enhancing photosynthetic efficiency. Photorespiration, while seemingly a wasteful process, is an integral part of plant metabolism with roots in Earth’s evolutionary history. Its occurrence is significantly influenced by environmental factors, and its impact on plant productivity is substantial. Understanding the intricacies of this pathway is crucial for developing strategies to enhance photosynthetic efficiency, particularly in the face of changing climate conditions. Future research focusing on improving RuBisCO and implementing C4-like mechanisms in C3 plants holds promise for increasing crop yields and ensuring food security.

Photorespiration: Process & Significance | UPSC Mains BOTANY-PAPER-II 2019

Photorespiration

How to Approach

This question requires a detailed understanding of photorespiration, its biochemical pathway, significance, and factors affecting it. The answer should cover the process itself, its evolutionary context, differences from photosynthesis, and its implications for plant productivity. A structured approach involving defining photorespiration, explaining the pathway, discussing its consequences, and highlighting potential mitigation strategies is recommended. Focus on clarity and conciseness, using biological terminology accurately.

Understanding Photorespiration: A Detailed Overview

Photorespiration is a complex metabolic pathway involving chloroplasts, peroxisomes, and mitochondria. It begins when RuBisCO, instead of binding to CO2, binds to O2. This oxygenation reaction initiates a series of reactions that ultimately lead to the release of CO2, consuming ATP and NADPH in the process – effectively reversing some of the gains made during photosynthesis.

The Biochemical Pathway

The photorespiratory pathway can be summarized in the following steps:

Oxygenation by RuBisCO: RuBisCO catalyzes the reaction between RuBP (Ribulose-1,5-bisphosphate) and O2, forming one molecule of 3-PGA (3-phosphoglycerate) and one molecule of 2-phosphoglycolate.
Processing of 2-Phosphoglycolate: 2-phosphoglycolate is toxic and must be detoxified. It is converted to glycolate in the chloroplast.
Glycolate Transport & Conversion: Glycolate is transported to the peroxisome, where it is converted to glyoxylate and then to glycine.
Glycine Transport & Conversion: Glycine is transported to the mitochondria, where two molecules of glycine are converted to serine, CO2, and ammonia (NH3). This is the step where CO2 is released, diminishing the carbon fixed during photosynthesis.
Serine Transport & Conversion: Serine is transported back to the peroxisome and then to the chloroplast, where it is converted back to glycerate.
Glycerate Conversion: Glycerate is phosphorylated to 3-PGA, which can re-enter the Calvin cycle.

Differences between Photosynthesis and Photorespiration

Feature	Photosynthesis	Photorespiration
Primary Substrate	CO2	O2
Enzyme	RuBisCO (carboxylase activity)	RuBisCO (oxygenase activity)
Products	Glucose, O2	CO2, NH3
Energy Requirement	Requires light energy	Consumes ATP & NADPH
Net Carbon Gain	Positive	Negative

Factors Affecting Photorespiration

Temperature: Higher temperatures increase the rate of photorespiration because RuBisCO’s affinity for O2 increases with temperature.
CO2 Concentration: Low CO2 concentrations favor oxygenation by RuBisCO, increasing photorespiration.
O2 Concentration: High O2 concentrations promote oxygenation.
Light Intensity: High light intensity can lead to stomatal closure, reducing CO2 uptake and increasing photorespiration.

Evolutionary Significance and Mitigation Strategies

Photorespiration is thought to be a relic of an ancient atmosphere with low CO2 and high O2. Plants like C4 and CAM plants have evolved mechanisms to minimize photorespiration. C4 plants spatially separate initial CO2 fixation from the Calvin cycle, concentrating CO2 around RuBisCO. CAM plants temporally separate these processes. Genetic engineering efforts are also underway to improve RuBisCO’s specificity for CO2, reducing photorespiration and enhancing photosynthetic efficiency.

Additional Resources

Key Definitions

RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase, an enzyme responsible for the initial carbon fixation step in photosynthesis, but also catalyzes the oxygenation of RuBP, initiating photorespiration.

Glycolate Pathway

The metabolic pathway that detoxifies 2-phosphoglycolate, a byproduct of the oxygenase activity of RuBisCO, involving chloroplasts, peroxisomes, and mitochondria.

Key Statistics

Photorespiration can reduce photosynthetic efficiency by up to 25-50% in C3 plants under hot, dry conditions.

Source: Taiz & Zeiger, Plant Physiology and Development (2010)

Globally, approximately 30% of potential photosynthetic productivity is lost due to photorespiration in C3 crops.

Source: Long, S. P. (2013). Intercropping and the synergy of ecological processes. Frontiers in Ecology and the Environment, 11(1), 45-54.

Examples

C4 Plants: Maize and Sugarcane

Maize and sugarcane are C4 plants that have evolved a mechanism to concentrate CO2 around RuBisCO, minimizing photorespiration and allowing them to thrive in hot, dry environments. They utilize a different enzyme, PEP carboxylase, to initially fix CO2, which has a higher affinity for CO2 than RuBisCO.

Frequently Asked Questions

▶Why is photorespiration considered wasteful?

Photorespiration is considered wasteful because it consumes energy (ATP and NADPH) and releases CO2, effectively reversing some of the carbon fixation achieved during photosynthesis, leading to a net loss of carbon.