What Molecule Is Released During Photorespiration

Photorespiration, a seemingly wasteful process in plants, has intrigued scientists for decades. Understanding what molecule is released during photorespiration is key to grasping the process's impact on plant productivity and its potential for manipulation. This article delves into the intricate details of photorespiration, focusing on the specific molecule released and the implications of this process.

The Basics of Photorespiration

Photorespiration, also known as the oxidative photosynthetic carbon cycle, is a metabolic pathway that occurs in plants when the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) oxygenates ribulose-1,5-bisphosphate (RuBP) instead of carboxylating it. This happens primarily when carbon dioxide (CO2) levels are low and oxygen (O2) levels are high, conditions often found in the hot, dry environments where C3 plants thrive.

RuBisCO, the most abundant enzyme on Earth, is responsible for the first major step in carbon fixation during photosynthesis. Ideally, RuBisCO binds with CO2, initiating the Calvin cycle and leading to the production of sugars. However, RuBisCO is not perfectly specific to CO2; it can also bind with O2. When O2 binds to RuBP, it initiates the photorespiratory pathway.

The Molecule Released: Carbon Dioxide (CO2)

The primary molecule released during photorespiration is carbon dioxide (CO2). This is a crucial point because the release of CO2 effectively reverses some of the carbon fixation achieved during photosynthesis. Instead of gaining carbon to produce sugars, the plant loses carbon back into the atmosphere. This loss of fixed carbon makes photorespiration appear to be a wasteful process.

Here's a more detailed breakdown of why and how CO2 is released:

1. Oxygenation of RuBP: When RuBisCO binds with O2, RuBP is oxygenated, resulting in one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate.
2. Conversion to Glycolate: The 2-phosphoglycolate is then converted to glycolate in the chloroplast.
3. Transport to Peroxisome: Glycolate is transported to the peroxisome.
4. Conversion to Glyoxylate and Hydrogen Peroxide: In the peroxisome, glycolate is converted to glyoxylate, and hydrogen peroxide (H2O2) is produced as a byproduct.
5. Conversion to Glycine: Glyoxylate is converted to glycine.
6. Transport to Mitochondria: Glycine is transported to the mitochondria.
7. Conversion to Serine and CO2 Release: In the mitochondria, two molecules of glycine are converted into one molecule of serine, with the release of one molecule of CO2 and one molecule of ammonia (NH3).

The release of CO2 in the mitochondria is the critical step that characterizes photorespiration as a carbon-releasing process. The serine produced is then transported back to the peroxisome and eventually re-enters the Calvin cycle as 3-PGA, but not without significant energy expenditure and carbon loss.

The Photorespiratory Pathway: A Step-by-Step Explanation

To fully understand the release of CO2 during photorespiration, it's essential to trace the pathway through the various cellular compartments:

1. Chloroplast

• Oxygenation of RuBP: The process begins in the chloroplast when RuBisCO catalyzes the reaction between RuBP and O2, forming 3-PGA and 2-phosphoglycolate.
• Conversion to Glycolate: 2-phosphoglycolate is rapidly converted into glycolate by a phosphatase enzyme.
• Export of Glycolate: Glycolate is then transported out of the chloroplast and into the peroxisome.

2. Peroxisome

• Conversion to Glyoxylate: In the peroxisome, glycolate oxidase converts glycolate into glyoxylate, producing hydrogen peroxide (H2O2) as a byproduct. The enzyme catalase then breaks down the toxic H2O2 into water and oxygen.
• Transamination to Glycine: Glyoxylate is converted into glycine through a transamination reaction, where an amino group is transferred from another molecule to glyoxylate.
• Export of Glycine: Glycine is then transported out of the peroxisome and into the mitochondria.

3. Mitochondria

• Conversion to Serine, CO2, and NH3: Inside the mitochondria, two molecules of glycine are converted into one molecule of serine, one molecule of CO2, and one molecule of ammonia (NH3). This reaction is catalyzed by the enzyme glycine decarboxylase complex (GDC).
• Release of CO2: The CO2 released in this step is the key carbon loss associated with photorespiration.
• Ammonia Recycling: The released ammonia is toxic to the plant and must be reassimilated. This occurs through the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway in the chloroplast, which requires additional energy.
• Export of Serine: Serine is then transported out of the mitochondria and back to the peroxisome.

4. Return to Peroxisome and Chloroplast

• Conversion to Hydroxypyruvate: In the peroxisome, serine is converted to hydroxypyruvate.
• Conversion to Glycerate: Hydroxypyruvate is reduced to glycerate.
• Transport to Chloroplast: Glycerate is transported back to the chloroplast.
• Conversion to 3-PGA: In the chloroplast, glycerate is phosphorylated to form 3-phosphoglycerate (3-PGA), which can then re-enter the Calvin cycle.

The Energetic Cost of Photorespiration

Photorespiration is not only a carbon-releasing process but also an energy-intensive one. The pathway involves multiple organelles and requires significant energy input to recycle the intermediates and reassimilate the released ammonia. The energetic costs include:

• ATP Consumption: ATP is required for the phosphorylation of glycerate to 3-PGA in the chloroplast and for the reassimilation of ammonia via the GS-GOGAT pathway.
• NAD(P)H Consumption: NAD(P)H is used in the reduction of hydroxypyruvate to glycerate in the peroxisome.

Estimates suggest that photorespiration can reduce photosynthetic efficiency by as much as 25-50% in C3 plants under hot and dry conditions. This reduction in efficiency has significant implications for crop yields and overall plant productivity.

Why Does Photorespiration Occur?

The existence of photorespiration raises the question of why such a seemingly wasteful process evolved in plants. Several hypotheses attempt to explain the role and evolutionary significance of photorespiration:

1. Evolutionary Relic: One hypothesis suggests that photorespiration is an evolutionary relic from a time when atmospheric CO2 levels were much lower, and O2 levels were higher. In these conditions, RuBisCO's oxygenase activity would have been more prevalent, and photorespiration may have served a protective role.
2. Photoprotection: Photorespiration may play a role in photoprotection by dissipating excess light energy when CO2 is limiting. By consuming ATP and NAD(P)H, photorespiration can help prevent the over-reduction of the electron transport chain and the formation of damaging reactive oxygen species (ROS).
3. Nitrogen Metabolism: Photorespiration is closely linked to nitrogen metabolism through the reassimilation of ammonia released in the mitochondria. This connection suggests that photorespiration may play a role in regulating nitrogen balance within the plant.
4. Metabolic Regulation: Photorespiration may also have a role in regulating carbon metabolism by preventing the accumulation of toxic intermediates. By converting 2-phosphoglycolate into less harmful compounds, photorespiration may help maintain metabolic homeostasis.

Strategies to Mitigate Photorespiration

Given the significant impact of photorespiration on plant productivity, researchers have explored various strategies to mitigate its effects:

1. Genetic Engineering of RuBisCO: One approach involves engineering RuBisCO to increase its specificity for CO2 over O2. While this has proven challenging due to the enzyme's complex structure, recent advances in protein engineering offer some promise.
2. Introduction of CO2 Concentrating Mechanisms: Another strategy involves introducing CO2 concentrating mechanisms, such as those found in C4 and CAM plants, into C3 plants. These mechanisms increase the CO2 concentration around RuBisCO, reducing its oxygenase activity.
3. Engineering Alternative Photorespiratory Pathways: Researchers are also exploring the possibility of engineering alternative photorespiratory pathways that bypass the CO2-releasing step in the mitochondria. This could potentially reduce carbon loss and increase photosynthetic efficiency.
4. Improving Glycolate Metabolism: Enhancing the efficiency of glycolate metabolism in the peroxisome could reduce the flux of carbon through the photorespiratory pathway, minimizing carbon loss.
5. Optimizing Environmental Conditions: Modifying environmental conditions, such as increasing CO2 levels in greenhouses, can also help reduce photorespiration and improve plant growth.

Photorespiration in Different Plant Types

Photorespiration is most prominent in C3 plants, which rely solely on the Calvin cycle for carbon fixation. However, C4 and CAM plants have evolved mechanisms to minimize photorespiration:

C3 Plants

• C3 plants are the most common type of plant and include many important crops such as rice, wheat, and soybeans.
• In C3 plants, photorespiration occurs at a significant rate, especially under hot, dry conditions when stomata close to conserve water, leading to low CO2 and high O2 concentrations in the leaves.

C4 Plants

• C4 plants, such as corn and sugarcane, have evolved a spatial separation of carbon fixation and the Calvin cycle.
• In C4 plants, CO2 is initially fixed in mesophyll cells by the enzyme PEP carboxylase, which has a high affinity for CO2 and does not bind to O2. The resulting four-carbon compound is then transported to bundle sheath cells, where it is decarboxylated, releasing CO2 and increasing the CO2 concentration around RuBisCO. This reduces the rate of photorespiration.

CAM Plants

• CAM (Crassulacean Acid Metabolism) plants, such as cacti and succulents, have evolved a temporal separation of carbon fixation and the Calvin cycle.
• CAM plants open their stomata at night, when temperatures are cooler and water loss is minimized, and fix CO2 using PEP carboxylase. The resulting organic acids are stored in vacuoles until daytime, when they are decarboxylated, releasing CO2 for the Calvin cycle. This temporal separation also helps to minimize photorespiration.

The Role of Photorespiration in a Changing Climate

As global climate change leads to higher temperatures and more frequent droughts, understanding and mitigating photorespiration becomes increasingly important. Higher temperatures increase the rate of photorespiration, while drought conditions exacerbate the problem by limiting CO2 uptake.

Strategies to reduce photorespiration, such as engineering more efficient RuBisCO enzymes or introducing CO2 concentrating mechanisms into C3 crops, could help improve plant productivity and resilience in a changing climate. Furthermore, understanding the genetic and physiological factors that regulate photorespiration could lead to the development of more stress-tolerant crops.

Recent Advances in Photorespiration Research

Recent advances in molecular biology, genetics, and systems biology have provided new insights into the mechanisms and regulation of photorespiration:

1. Identification of New Photorespiratory Mutants: Researchers have identified new mutants with altered photorespiratory phenotypes, providing valuable tools for studying the pathway and its regulation.
2. Elucidation of Regulatory Networks: Systems biology approaches have been used to map the regulatory networks that control photorespiration, revealing new targets for genetic engineering.
3. Development of Synthetic Biology Tools: Synthetic biology tools are being used to engineer alternative photorespiratory pathways and improve carbon fixation efficiency.
4. Advancements in RuBisCO Engineering: New techniques, such as directed evolution and computational design, are being used to engineer RuBisCO enzymes with improved catalytic properties.
5. Understanding the Role of Photorespiration in Stress Response: Research has shown that photorespiration plays a complex role in plant stress responses, with both beneficial and detrimental effects depending on the specific stress and plant species.

Conclusion

In summary, the molecule released during photorespiration is carbon dioxide (CO2), which is a critical factor that defines the process as one that reduces photosynthetic efficiency. Photorespiration is a complex metabolic pathway that involves multiple organelles and has significant implications for plant productivity, especially in C3 plants under hot and dry conditions. Understanding the intricacies of photorespiration, its regulation, and its role in plant stress responses is crucial for developing strategies to improve crop yields and enhance plant resilience in a changing climate. By mitigating the negative effects of photorespiration and harnessing its potential benefits, we can pave the way for more sustainable and productive agriculture in the future. The ongoing research in this field promises to unlock new approaches for engineering plants with improved photosynthetic efficiency and enhanced adaptation to environmental stresses.