Model Answer
0 min readIntroduction
Photorespiration is a metabolic pathway that occurs in plants, particularly C3 plants, under conditions of high oxygen and low carbon dioxide concentrations. It is initiated by the oxygenase activity of the enzyme RuBisCO, which mistakenly fixes oxygen instead of carbon dioxide during the Calvin cycle. This process is energetically costly and reduces photosynthetic efficiency. However, it is not merely a wasteful process; it plays a role in protecting the plant from photooxidative damage and recycling some of the products of photosynthesis. The compartmentalization of photorespiration across three organelles – chloroplasts, peroxisomes, and mitochondria – is crucial for its efficient functioning and regulation.
Compartmentation of Biochemical Events in Photorespiration
Photorespiration is not a single reaction but a series of enzymatic reactions distributed across three organelles. This compartmentalization ensures efficient substrate channeling and prevents the accumulation of toxic intermediates.
1. Chloroplast: The Initiation Phase
- The process begins in the chloroplast when RuBisCO oxygenates RuBP, forming one molecule of 3-phosphoglycerate (3-PGA) – which enters the Calvin cycle – and one molecule of 2-phosphoglycolate.
- 2-phosphoglycolate is rapidly converted to glycolate.
- Glycolate is then transported to the peroxisome.
2. Peroxisome: Glycolate Metabolism
- In the peroxisome, glycolate is oxidized to glyoxylate by glycolate oxidase, producing hydrogen peroxide (H2O2).
- H2O2 is detoxified by catalase.
- Glyoxylate is transaminated to glycine.
- Glycine is then transported to the mitochondria.
3. Mitochondria: Serine Production and CO2 Release
- Within the mitochondria, two molecules of glycine are converted into one molecule of serine, releasing CO2 and ammonia (NH3). This is the step where carbon is lost during photorespiration.
- Serine is transported back to the peroxisome.
4. Peroxisome (Continued) & Chloroplast: Regeneration of Glycerate
- In the peroxisome, serine is converted to hydroxypyruvate.
- Hydroxypyruvate is reduced to glycerate.
- Glycerate is transported back to the chloroplast.
- In the chloroplast, glycerate is phosphorylated to 3-PGA, re-entering the Calvin cycle.
The following table summarizes the key events in each organelle:
| Organelle | Key Reactions | Inputs | Outputs |
|---|---|---|---|
| Chloroplast | Oxygenation of RuBP, conversion of 2-phosphoglycolate to glycolate | RuBP, O2 | 3-PGA, Glycolate |
| Peroxisome | Oxidation of glycolate to glyoxylate, transamination to glycine, conversion of serine to hydroxypyruvate and glycerate | Glycolate, Glyoxylate, Serine | Glycine, H2O2, Hydroxypyruvate, Glycerate |
| Mitochondria | Conversion of two glycine molecules to serine, CO2, and NH3 | Glycine | Serine, CO2, NH3 |
Significance of Photorespiration
While often considered a wasteful process, photorespiration has several important functions:
- Protection against Photooxidative Damage: When CO2 levels are low, RuBisCO is more likely to fix oxygen, leading to the formation of reactive oxygen species (ROS). Photorespiration helps to dissipate excess energy and prevent damage to the photosynthetic apparatus.
- Metabolic Intermediates: Photorespiration provides metabolic intermediates that can be used in other biosynthetic pathways. For example, glycine is a precursor for purines and heme.
- Nitrogen Recovery: The ammonia released during glycine decarboxylation is reassimilated into amino acids, preventing nitrogen loss.
- Evolutionary Relic: Photorespiration is thought to be a remnant of an earlier evolutionary period when atmospheric CO2 levels were much higher and oxygen levels were lower. RuBisCO evolved in this environment and did not initially need to discriminate effectively between CO2 and O2.
However, the drawbacks of photorespiration are significant. It reduces photosynthetic efficiency by approximately 25-50% in C3 plants. This is why C4 and CAM plants have evolved mechanisms to minimize photorespiration by concentrating CO2 around RuBisCO.
Conclusion
Compartmentation is fundamental to the efficient operation of photorespiration, allowing for the coordinated execution of reactions across different organelles. While energetically costly, photorespiration serves crucial protective and metabolic roles, particularly in C3 plants. Understanding the intricacies of this pathway is vital for improving crop productivity, especially in the context of rising global temperatures and changing atmospheric CO<sub>2</sub> concentrations. Future research focusing on enhancing RuBisCO specificity or engineering alternative CO<sub>2</sub> concentrating mechanisms could significantly reduce photorespiration and boost photosynthetic efficiency.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.