Photorespiration in Plants: Rubisco Oxygenase Activity, 2-Phosphoglycolate Formation, Pathway, CO₂ and NH₃ Release
Photorespiration is an important metabolic pathway in plants that occurs when the enzyme Rubisco acts as an oxygenase instead of a carboxylase. During normal photosynthesis, Rubisco adds carbon dioxide (CO₂) to ribulose-1,5-bisphosphate (RuBP). However, Rubisco can also react with molecular oxygen (O₂).
When Rubisco uses oxygen, RuBP is converted into one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate (2-PG). The 2-phosphoglycolate molecule cannot directly enter the Calvin cycle and therefore has to be recovered through a series of reactions involving three organelles: the chloroplast, peroxisome and mitochondrion.
The photorespiratory pathway converts glycolate-derived carbon through several intermediates, including glycolate, glycine and serine. During the pathway, CO₂ and NH₃ (ammonia) are released. The pathway also recovers much of the carbon skeleton and returns useful metabolites to the photosynthetic machinery.
Table of Contents
- Introduction to Photorespiration
- Rubisco: Carboxylase vs Oxygenase Activity
- Formation of 2-Phosphoglycolate
- Why Does Photorespiration Occur?
- Overview of the Photorespiratory Pathway
- Step 1: Chloroplast – Formation of Glycolate
- Step 2: Peroxisome – Glycolate to Glyoxylate
- Step 3: Glyoxylate and Glycine Formation
- Step 4: Mitochondrion – Glycine to Serine
- CO₂ Release During Photorespiration
- NH₃ Release During Photorespiration
- Step 5: Return to the Peroxisome
- Step 6: Return to the Chloroplast
- Why Is Photorespiration Considered Wasteful?
- Is Photorespiration Completely Useless?
- Factors Increasing Photorespiration
- Photorespiration in C3 Plants
- Photorespiration and C4 Plants
- Photorespiration and CAM Plants
- Animated SVG Pathway
- Important Exam Points
- Common Mistakes
- 10 Practice MCQs
- Final Summary
1. Introduction to Photorespiration
Photosynthesis is the primary process through which plants convert light energy into chemical energy. During the Calvin cycle, the enzyme Rubisco fixes CO₂ into organic carbon. This process is essential for producing carbohydrates and supporting plant growth.
However, Rubisco has an unusual characteristic: it can use both CO₂ and O₂ as substrates. This dual activity is partly related to the chemical similarity between carbon dioxide and oxygen and the active-site properties of Rubisco.
When CO₂ is abundant relative to O₂, Rubisco more frequently performs its carboxylase reaction. When the concentration of CO₂ near Rubisco decreases and O₂ becomes relatively more available, oxygenase activity becomes more important.
The oxygenase reaction initiates photorespiration.
Photorespiration is therefore closely connected with photosynthesis and is especially significant in C3 plants.
Photorespiration is a light-associated metabolic pathway initiated by the oxygenase activity of Rubisco, in which RuBP reacts with O₂ to produce 3-PGA and 2-phosphoglycolate, followed by metabolic recycling through chloroplasts, peroxisomes and mitochondria.
2. Rubisco: Carboxylase vs Oxygenase Activity
Rubisco is one of the most important enzymes in the global carbon cycle. Its full name is ribulose-1,5-bisphosphate carboxylase/oxygenase.
The name itself tells us that Rubisco has two important catalytic activities:
- Carboxylase activity – uses CO₂.
- Oxygenase activity – uses O₂.
Carboxylase reaction
During the normal Calvin cycle, Rubisco combines RuBP with CO₂. The unstable six-carbon intermediate formed during this reaction rapidly breaks down into two molecules of 3-phosphoglycerate.
This reaction results in net carbon fixation and supports carbohydrate synthesis.
Oxygenase reaction
When Rubisco uses O₂ instead of CO₂, RuBP undergoes oxygenation.
The 3-PGA molecule can re-enter the Calvin cycle. However, 2-phosphoglycolate is not directly useful in the Calvin cycle and must be metabolically recovered.
Rubisco + CO₂ → Calvin cycle
Rubisco + O₂ → Photorespiration
3. Formation of 2-Phosphoglycolate
The formation of 2-phosphoglycolate is one of the most important steps in understanding photorespiration.
During Rubisco oxygenase activity, one carbon of RuBP is incorporated into 3-PGA while the other two-carbon product is converted into 2-phosphoglycolate.
2-phosphoglycolate contains a phosphate group and is not directly compatible with the Calvin cycle. Therefore, the plant must convert it into other metabolites.
The pathway begins in the chloroplast, where 2-phosphoglycolate is processed toward glycolate.
4. Why Does Photorespiration Occur?
One major reason is the dual substrate specificity of Rubisco.
The ratio of CO₂ to O₂ around Rubisco influences which reaction is favored. Several environmental conditions can reduce the relative availability of CO₂ or increase the relative importance of O₂.
For example, during hot and dry conditions, plants often partially close their stomata to reduce water loss. Stomatal closure decreases the entry of atmospheric CO₂ into the leaf. At the same time, photosynthesis continues to consume CO₂ internally.
As a result, the internal CO₂ concentration can fall, increasing the probability of Rubisco oxygenase activity.
High temperature can also favor photorespiration because Rubisco's relative affinity and reaction balance with CO₂ and O₂ are temperature dependent.
5. Overview of the Photorespiratory Pathway
The photorespiratory pathway is unusual because it involves several organelles. The major compartments are:
- Chloroplast
- Peroxisome
- Mitochondrion
The pathway can be summarized as follows:
2-Phosphoglycolate → Glycolate
↓
Peroxisome
Glycolate → Glyoxylate → Glycine
↓
Mitochondrion
2 Glycine → Serine + CO₂ + NH₃
↓
Peroxisome
Serine → Hydroxypyruvate → Glycerate
↓
Chloroplast
Glycerate → 3-PGA → Calvin cycle
This three-organelle pathway is one of the most frequently tested concepts in plant physiology.
6. Step 1: Chloroplast – Formation of Glycolate
Photorespiration begins in the chloroplast.
Rubisco oxygenase activity generates 2-phosphoglycolate. Because this compound contains a phosphate group, it is converted into glycolate.
The enzyme 2-phosphoglycolate phosphatase removes the phosphate group from 2-phosphoglycolate.
Glycolate can then be transported from the chloroplast to the peroxisome.
This is the first major organelle transition in the photorespiratory pathway.
7. Step 2: Peroxisome – Glycolate to Glyoxylate
Once glycolate enters the peroxisome, it undergoes oxidation.
The enzyme glycolate oxidase converts glycolate into glyoxylate.
In many plant systems, glycolate oxidase transfers electrons to oxygen, producing hydrogen peroxide (H₂O₂) as a by-product.
Glyoxylate is then converted toward glycine through aminotransferase reactions.
8. Step 3: Glyoxylate and Glycine Formation
Glyoxylate is an important intermediate because it can receive an amino group through transamination.
Amino acids such as glutamate can participate in these transamination reactions.
The result is formation of glycine.
The exact amino-transfer reactions involve interconnected amino-acid metabolism, but the major point for understanding photorespiration is that glyoxylate is converted into glycine in the peroxisome.
Glycine is then transported to the mitochondrion.
9. Step 4: Mitochondrion – Glycine to Serine
The mitochondrion is the central organelle for the decarboxylation step of photorespiration.
Two molecules of glycine are converted into one molecule of serine.
This reaction is catalyzed by the glycine decarboxylase complex together with serine hydroxymethyltransferase.
This reaction is extremely important because it explains the release of both CO₂ and NH₃ during photorespiration.
Glycine decarboxylase complex
The glycine decarboxylase system is a multienzyme complex located in mitochondria. It removes a carbon atom from glycine as CO₂ and transfers other molecular groups through its component reactions.
Serine hydroxymethyltransferase
Serine hydroxymethyltransferase participates in the conversion of glycine units into serine.
10. CO₂ Release During Photorespiration
One major consequence of photorespiration is the release of carbon dioxide.
During the mitochondrial conversion of glycine to serine, CO₂ is released.
This means that photorespiration partially reverses the benefit of carbon fixation by releasing CO₂ that had previously been incorporated into organic metabolism.
This is one reason why photorespiration is traditionally described as a carbon-loss pathway.
However, the pathway is more accurately understood as a recovery pathway that evolved around the unavoidable oxygenase activity of Rubisco.
11. NH₃ Release During Photorespiration
Photorespiration also results in the release of NH₃.
NH₃ originates from the nitrogen-containing amino acid metabolism associated with glycine conversion in the mitochondrion.
Because free ammonia can be toxic and represents a valuable nitrogen resource, plants rapidly reassimilate much of the released NH₃.
This reassimilation is closely connected to the GS-GOGAT pathway.
Glycine metabolism → NH₃ release → NH₃ reassimilation → GS/GOGAT → amino acid metabolism
Thus, photorespiration causes nitrogen recycling as well as carbon recycling.
12. Step 5: Return to the Peroxisome
The serine produced in mitochondria returns to the peroxisome.
In the peroxisome, serine is converted into hydroxypyruvate.
Hydroxypyruvate is then reduced to glycerate.
The enzyme hydroxypyruvate reductase plays an important role in the conversion of hydroxypyruvate to glycerate.
The glycerate is then transported back into the chloroplast.
13. Step 6: Return to the Chloroplast
The final major stage occurs in the chloroplast.
Glycerate is phosphorylated to form 3-phosphoglycerate (3-PGA).
The enzyme responsible for this reaction is glycerate kinase.
3-PGA can then re-enter the Calvin cycle.
Therefore, although photorespiration results in loss of CO₂ and requires energy for metabolite recovery, it also salvages much of the carbon skeleton originally diverted into the pathway.
14. Live Animated SVG Photorespiration Pathway
The following SVG is designed for Blogger. The moving dashed arrows represent the direction of metabolite flow between the three organelles. The pulsing circles highlight important intermediates such as glycolate, glycine and serine, while CO₂ and NH₃ release are shown at the mitochondrial stage.
Note: The animation uses SVG/CSS only. If Blogger's editor strips animation-related CSS, the pathway will still remain visible as a static diagram.
15. Why Is Photorespiration Considered Wasteful?
Photorespiration has historically been called a wasteful process because it consumes energy and results in the release of previously fixed carbon as CO₂.
Several costs are associated with the pathway.
- CO₂ is released.
- NH₃ is released and must be reassimilated.
- ATP and reducing power are required for recovery reactions.
- Carbon that could otherwise contribute directly to net photosynthesis is temporarily diverted.
- The pathway reduces the efficiency of photosynthetic carbon fixation under many conditions.
However, calling it simply “waste” is incomplete. Photorespiration also performs important metabolic functions and is closely integrated with plant stress physiology.
16. Is Photorespiration Completely Useless?
No. Modern plant biology recognizes that photorespiration is not simply an accidental waste pathway.
Photorespiration is essential for processing 2-phosphoglycolate generated by Rubisco oxygenase activity. Without this recycling pathway, 2-phosphoglycolate could accumulate and interfere with photosynthetic metabolism.
The pathway also interacts with nitrogen metabolism, redox balance and cellular signaling.
Photorespiration can therefore be viewed as a metabolic recovery system that allows plants to cope with the oxygenase side reaction of Rubisco.
17. Factors Increasing Photorespiration
High temperature
Higher temperatures can increase the relative importance of Rubisco oxygenase activity and can therefore increase photorespiration in C3 plants.
Low internal CO₂
When internal CO₂ concentration decreases, Rubisco has greater opportunity to interact with O₂.
Stomatal closure
During drought, stomata close to reduce water loss. This limits CO₂ diffusion into the leaf and can increase photorespiration.
High O₂ relative to CO₂
A higher O₂-to-CO₂ ratio around Rubisco favors oxygenase activity.
Environmental stress
Drought, heat, salinity and other stresses can change photosynthetic and respiratory metabolism and may alter photorespiratory flux.
18. Photorespiration in C3 Plants
Photorespiration is especially significant in C3 plants.
In C3 plants, the initial stable product of CO₂ fixation by Rubisco is 3-PGA, a three-carbon compound.
Examples of C3 plants include wheat, rice, soybean, potato and many other species.
Under hot and dry conditions, C3 plants can experience significant photorespiratory carbon loss because stomatal closure decreases internal CO₂ availability.
19. Photorespiration and C4 Plants
C4 plants have evolved a CO₂-concentrating mechanism that reduces the oxygenase activity of Rubisco.
In C4 plants, atmospheric CO₂ is initially fixed by PEP carboxylase in mesophyll cells. The resulting four-carbon compounds are transported and processed so that CO₂ is released near Rubisco in bundle sheath cells.
This produces a high CO₂ concentration around Rubisco and suppresses its oxygenase activity.
CO₂ concentration around Rubisco ↑ → Rubisco oxygenase activity ↓ → Photorespiration ↓
C4 photosynthesis is therefore particularly advantageous under high temperature, high light and water-limited conditions.
20. Photorespiration and CAM Plants
CAM plants also possess a CO₂-concentrating strategy, but they separate carbon fixation and Calvin-cycle CO₂ release primarily in time rather than space.
CAM plants open their stomata mainly at night, when evaporative water loss is lower. CO₂ is incorporated into organic acids and stored.
During the day, stored organic acids release CO₂ internally, helping supply Rubisco while stomata remain relatively closed.
This can reduce photorespiration while conserving water.
21. Organelles Involved in Photorespiration
| Organelle | Major Events |
|---|---|
| Chloroplast | Rubisco oxygenase activity; 2-phosphoglycolate formation; conversion toward glycolate; final conversion of glycerate to 3-PGA. |
| Peroxisome | Glycolate oxidation to glyoxylate; glycine formation; serine conversion to hydroxypyruvate and glycerate; H₂O₂ detoxification by catalase. |
| Mitochondrion | Conversion of two glycine molecules into serine with release of CO₂ and NH₃. |
22. Major Metabolites in Photorespiration
| Metabolite | Importance |
|---|---|
| 2-Phosphoglycolate | Immediate product of Rubisco oxygenase activity. |
| Glycolate | Produced from 2-phosphoglycolate and transported to the peroxisome. |
| Glyoxylate | Produced by oxidation of glycolate. |
| Glycine | Formed from glyoxylate and transported to mitochondria. |
| Serine | Produced from two glycine molecules in mitochondria. |
| Hydroxypyruvate | Produced from serine in the peroxisome. |
| Glycerate | Converted to 3-PGA in chloroplasts. |
| 3-PGA | Returns to the Calvin cycle. |
23. Important Exam Points for CSIR-NET, GATE and DBT-BET
- Rubisco has both carboxylase and oxygenase activities.
- Rubisco oxygenase activity acts on RuBP and O₂.
- The oxygenase reaction produces 3-PGA + 2-phosphoglycolate.
- 2-phosphoglycolate is the key compound that initiates the photorespiratory recovery pathway.
- Photorespiration involves chloroplast, peroxisome and mitochondrion.
- 2-Phosphoglycolate is converted to glycolate in the chloroplast.
- Glycolate enters the peroxisome.
- Glycolate is oxidized to glyoxylate.
- Glyoxylate is converted toward glycine.
- Glycine enters the mitochondrion.
- Two glycine molecules are converted into one serine molecule.
- The mitochondrial reaction releases CO₂ and NH₃.
- Serine returns to the peroxisome.
- Serine → hydroxypyruvate → glycerate.
- Glycerate returns to the chloroplast.
- Glycerate is converted into 3-PGA.
- 3-PGA can re-enter the Calvin cycle.
- Photorespiration consumes metabolic energy.
- Photorespiration is generally higher in C3 plants than in C4 plants.
- C4 plants suppress photorespiration through a CO₂-concentrating mechanism.
- CAM plants reduce photorespiration through temporal separation of carbon fixation and CO₂ availability.
24. Common Mistakes Students Make
Mistake 1: Rubisco oxygenase produces only 2-phosphoglycolate
Not exactly. The oxygenase reaction produces one molecule of 3-PGA and one molecule of 2-phosphoglycolate.
Mistake 2: Photorespiration occurs only in mitochondria
Incorrect. Photorespiration is a multi-organelle pathway involving chloroplasts, peroxisomes and mitochondria.
Mistake 3: Glycolate is formed directly in the mitochondrion
Incorrect. Glycolate is generated from 2-phosphoglycolate in the chloroplast before entering the peroxisome.
Mistake 4: CO₂ is released in the chloroplast
For the major photorespiratory decarboxylation reaction, CO₂ is released during glycine metabolism in the mitochondrion.
Mistake 5: NH₃ is never released
Incorrect. Photorespiration includes release of NH₃ during mitochondrial glycine metabolism. The released nitrogen is then rapidly reassimilated.
Mistake 6: Photorespiration occurs in complete darkness
Photorespiration is closely associated with photosynthetic metabolism and is strongly stimulated under illuminated conditions because the oxygenase reaction of Rubisco competes with photosynthetic carbon fixation.
Mistake 7: C4 plants completely lack photorespiration
C4 plants strongly suppress photorespiration under normal conditions, but it is more accurate to say that their CO₂-concentrating mechanism greatly reduces the oxygenase reaction rather than saying photorespiration is absolutely impossible.
25. Practice MCQs – Photorespiration
- N₂
- O₂
- NH₃
- H₂O₂
- Two molecules of 3-PGA
- 3-PGA and 2-phosphoglycolate
- Glycine and serine
- Glycerate and glycolate
- 3-PGA
- RuBP
- 2-Phosphoglycolate
- Serine
- Nucleus
- Peroxisome
- Golgi body
- Vacuole
- Chloroplast
- Peroxisome
- Mitochondrion
- Nucleus
- Two molecules of glycolate
- One molecule of serine plus CO₂ and NH₃
- Two molecules of 3-PGA
- One molecule of RuBP
- O₂ only
- CO₂
- N₂
- CH₄
- NO₂
- NH₃
- N₂
- NO₃⁻
- Glycine
- Serine
- Glycerate
- Glyoxylate
- Mitochondrion → chloroplast → peroxisome
- Chloroplast → peroxisome → mitochondrion → peroxisome → chloroplast
- Peroxisome → chloroplast → mitochondrion only
- Chloroplast → mitochondrion only
26. One-Minute Revision
Rubisco oxygenase:
RuBP + O₂
↓
3-PGA + 2-Phosphoglycolate
Chloroplast:
2-Phosphoglycolate
↓
Glycolate
Peroxisome:
Glycolate
↓
Glyoxylate
↓
Glycine
Mitochondrion:
2 Glycine
↓
Serine + CO₂ + NH₃
Peroxisome:
Serine
↓
Hydroxypyruvate
↓
Glycerate
Chloroplast:
Glycerate
↓
3-PGA
↓
Calvin cycle
27. Final Summary
Photorespiration is a major metabolic pathway associated with the oxygenase activity of Rubisco. Rubisco is unusual because it can catalyze both carbon fixation and oxygenation. During oxygenase activity, RuBP reacts with O₂ to produce one molecule of 3-PGA and one molecule of 2-phosphoglycolate.
The 2-phosphoglycolate cannot directly enter the Calvin cycle, so it is recovered through a complex pathway involving the chloroplast, peroxisome and mitochondrion.
In the chloroplast, 2-phosphoglycolate is converted into glycolate. Glycolate moves into the peroxisome, where it is oxidized to glyoxylate and subsequently converted toward glycine.
Glycine enters the mitochondrion. Two molecules of glycine are converted into one molecule of serine, with the release of CO₂ and NH₃. This mitochondrial step is one of the most important facts to remember for competitive examinations.
Serine then returns to the peroxisome, where it is converted through hydroxypyruvate to glycerate. Glycerate returns to the chloroplast and is converted into 3-PGA, allowing carbon to re-enter the Calvin cycle.
Photorespiration is energetically expensive and results in CO₂ release, which is why it is often described as reducing photosynthetic efficiency. Nevertheless, the pathway is essential for processing 2-phosphoglycolate and contributes to the recycling of carbon and nitrogen metabolites.
Photorespiration is particularly important in C3 plants, especially under conditions such as high temperature, drought and low internal CO₂. C4 and CAM plants possess mechanisms that reduce the oxygenase activity of Rubisco and therefore reduce photorespiratory losses.
C → P → M → P → C
Chloroplast → Peroxisome → Mitochondrion → Peroxisome → Chloroplast
2-PG → Glycolate → Glyoxylate → Glycine → Serine → Glycerate → 3-PGA
Mitochondrion = Glycine → Serine + CO₂ + NH₃
For CSIR-NET, GATE Biotechnology, DBT-BET and other life-science examinations, the highest-yield points are the Rubisco oxygenase reaction, 2-phosphoglycolate formation, three-organellar pathway, glycolate-glycine-serine sequence, mitochondrial CO₂ and NH₃ release, and return of glycerate to the chloroplast as 3-PGA.
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