1. What do you mean by photooxidation in plants?
Photooxidation is a process where high light results in the production of reactive oxygen species that cause damage to plant cells.
2. What is photorespiration and why is it considered wasteful?
The process by which the enzyme RuBisCO oxygenates RuBP leading to loss of energy and lowered photosynthetic efficiency in plants is called photorespiration.
3. How does photooxidation differ from photorespiration?
Photooxidation is damage by reactive oxygen species because of high light, and photorespiration is the oxygenation of RuBP by RuBisCO leading to loss of energy.
4. Why is photorespiration important to understand for agriculture?
Knowledge of photorespiration is useful in devising methods for improving crop yield and photosynthetic efficiency.
5. What are reactive oxygen species and their role in photooxidation?
Reactive oxygen species, ROS is a highly reactive molecule formed in photooxidation that inflicts damage to the cellular component, thus influencing plant health.
6. What is the main difference between photooxidation and photorespiration?
Photooxidation is a damaging process where excess light energy leads to the production of reactive oxygen species, potentially harming cellular components. Photorespiration, on the other hand, is a metabolic process that occurs when RuBisCO fixes oxygen instead of carbon dioxide, resulting in energy loss but protecting the photosynthetic apparatus from damage.
7. How do CAM plants regulate photorespiration?
CAM (Crassulacean Acid Metabolism) plants minimize photorespiration by temporally separating CO2 fixation and the Calvin cycle. They fix CO2 at night when stomata are open, storing it as malic acid. During the day, CO2 is released from malic acid and concentrated around RuBisCO, reducing oxygen fixation and thus photorespiration.
8. How do cyanobacteria minimize photorespiration?
Cyanobacteria minimize photorespiration through carbon concentrating mechanisms (CCMs). These involve actively pumping bicarbonate ions into the cell and converting them to CO2 near RuBisCO, effectively increasing local CO2 concentration and reducing oxygen fixation.
9. What is the relationship between photorespiration and photosynthetic light reactions?
Photorespiration is indirectly linked to light reactions as it consumes ATP and NADPH produced during the light-dependent phase of photosynthesis. This consumption can help prevent over-reduction of the electron transport chain when CO2 fixation is limited.
10. How does the presence of heavy metals in soil affect photorespiration and photooxidation?
Heavy metals in soil can increase both photorespiration and photooxidation by inducing oxidative stress in plants. They can interfere with photosynthetic electron transport, leading to the production of more reactive oxygen species and potentially increasing photorespiratory activity as a protective mechanism.
11. Why does photorespiration occur in C3 plants but not in C4 plants?
Photorespiration occurs in C3 plants because their RuBisCO enzyme can bind to both CO2 and O2. In C4 plants, a mechanism concentrates CO2 around RuBisCO, minimizing oxygen binding and thus reducing photorespiration.
12. What is the energy cost of photorespiration compared to photosynthesis?
Photorespiration is an energy-consuming process that reduces the efficiency of photosynthesis. It uses ATP and NADPH without producing carbohydrates, effectively decreasing the overall energy yield of photosynthesis by about 25% in C3 plants under normal conditions.
13. How do plants protect themselves against photooxidation?
Plants protect themselves against photooxidation through various mechanisms, including:
14. What is the relationship between photorespiration and drought stress?
During drought stress, plants close their stomata to conserve water. This reduces CO2 availability inside the leaf, increasing the likelihood of oxygen fixation by RuBisCO and thus enhancing photorespiration rates.
15. How does the C4 photosynthetic pathway minimize photorespiration?
The C4 pathway minimizes photorespiration by spatially separating initial CO2 fixation and the Calvin cycle. CO2 is concentrated around RuBisCO in bundle sheath cells, reducing oxygen competition and thus decreasing photorespiration.
16. How does the presence of UV radiation affect photooxidation in plants?
UV radiation can increase photooxidation in plants by directly damaging cellular components and by generating reactive oxygen species. This can lead to the degradation of photosynthetic pigments, proteins, and lipids, reducing overall photosynthetic efficiency.
17. How does photooxidation affect photosynthetic efficiency?
Photooxidation reduces photosynthetic efficiency by damaging photosynthetic components, particularly photosystem II. This damage can lead to a decrease in electron transport and overall photosynthetic rate.
18. What is the role of catalase in protecting against photooxidation?
Catalase is an enzyme that breaks down hydrogen peroxide, a reactive oxygen species produced during photooxidation. By neutralizing this harmful compound, catalase helps protect cellular components from oxidative damage.
19. What is the evolutionary significance of photorespiration?
Photorespiration is thought to be an evolutionary relic from a time when Earth's atmosphere had higher O2 and lower CO2 levels. It may have evolved as a protective mechanism against photooxidation and continues to play a role in maintaining photosynthetic efficiency under varying environmental conditions.
20. What is the relationship between photorespiration and plant productivity?
Photorespiration generally reduces plant productivity because it consumes energy without producing carbohydrates. However, it also plays a protective role against photooxidation and helps maintain electron flow under high light conditions, indirectly supporting overall plant health and productivity.
21. What are the main cellular locations where photorespiration occurs?
Photorespiration occurs in three cellular compartments:
22. How does temperature affect the rate of photorespiration?
Higher temperatures increase photorespiration rates because they decrease the solubility of CO2 more than O2 in the cellular environment. This shifts the balance towards oxygen fixation by RuBisCO, leading to increased photorespiration.
23. What role does the enzyme RuBisCO play in photorespiration?
RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the key enzyme in photorespiration. Its dual function allows it to fix both CO2 (carboxylation) and O2 (oxygenation). When it fixes oxygen, it initiates the photorespiratory pathway.
24. What are the primary products of the photorespiratory pathway?
The primary products of the photorespiratory pathway are glycine and serine. These amino acids are produced as a result of the oxidation of the 2-carbon compound formed when RuBisCO fixes oxygen instead of carbon dioxide.
25. How does the concentration of CO2 in the atmosphere affect photorespiration rates?
Higher atmospheric CO2 concentrations reduce photorespiration rates because they increase the likelihood of CO2 fixation by RuBisCO relative to O2 fixation. This is why elevated CO2 levels can enhance plant growth, especially in C3 plants.
26. How does photorespiration contribute to nitrogen metabolism in plants?
Photorespiration contributes to nitrogen metabolism by producing glycine and serine, which are important amino acids. This process can be particularly significant under conditions of nitrogen limitation, as it provides a source of organic nitrogen compounds.
27. How does the structure of RuBisCO contribute to its dual function in photosynthesis and photorespiration?
RuBisCO's active site can accommodate both CO2 and O2 molecules due to their similar size and shape. This structural feature allows RuBisCO to catalyze both carboxylation (photosynthesis) and oxygenation (photorespiration) reactions, depending on which molecule binds.
28. How does the pH of the cellular environment affect photorespiration rates?
The pH of the cellular environment can affect photorespiration rates by influencing the activity of RuBisCO. Generally, a slightly alkaline pH favors carboxylation (CO2 fixation) over oxygenation (O2 fixation), potentially reducing photorespiration rates.
29. What role does glycolate play in the photorespiratory pathway?
Glycolate is the first product formed when RuBisCO fixes oxygen instead of carbon dioxide. It is then transported from the chloroplast to the peroxisome, where it is oxidized to glyoxylate, initiating the series of reactions that characterize the photorespiratory pathway.
30. What is the role of glycine decarboxylase in photorespiration?
Glycine decarboxylase is a key enzyme in the photorespiratory pathway, located in the mitochondria. It catalyzes the conversion of glycine to serine, releasing CO2 and NH3 in the process. This step is crucial for recycling carbon back into the Calvin cycle.
31. What is the role of serine hydroxymethyltransferase in the photorespiratory pathway?
Serine hydroxymethyltransferase is an enzyme involved in the final step of the photorespiratory pathway in the mitochondria. It catalyzes the conversion of glycine to serine, playing a crucial role in recycling carbon and nitrogen compounds back into cellular metabolism.
32. How does the leaf anatomy of C4 plants contribute to reduced photorespiration?
C4 plants have a specialized leaf anatomy called Kranz anatomy, where bundle sheath cells surround the vascular bundles. This structure allows for the spatial separation of initial CO2 fixation in mesophyll cells and the Calvin cycle in bundle sheath cells, effectively concentrating CO2 around RuBisCO and reducing photorespiration.
33. What is the significance of the glycine-serine ratio in understanding photorespiratory activity?
The glycine-serine ratio is an indicator of photorespiratory activity. A higher ratio suggests increased photorespiration, as glycine accumulates faster than it can be converted to serine. This ratio can be used to assess the relative rates of photorespiration under different environmental conditions.
34. How does the presence of air pollutants like ozone affect photorespiration and photooxidation?
Air pollutants like ozone can increase both photorespiration and photooxidation. Ozone enters leaves through stomata and breaks down to form reactive oxygen species, leading to oxidative stress. This can damage photosynthetic machinery, potentially increasing photorespiration as a protective mechanism and enhancing photooxidation.
35. What is the role of hydroxypyruvate reductase in the photorespiratory pathway?
Hydroxypyruvate reductase is an enzyme in the peroxisome that catalyzes the reduction of hydroxypyruvate to glycerate in the photorespiratory pathway. This step is important for regenerating 3-phosphoglycerate, which can then re-enter the Calvin cycle.
36. How does the presence of mycorrhizal fungi affect photorespiration in plants?
Mycorrhizal fungi can indirectly affect photorespiration by improving plant nutrient uptake, particularly phosphorus. Better-nourished plants may have more efficient photosynthetic machinery, potentially reducing the relative rate of photorespiration. Additionally, improved water relations due to mycorrhizal associations may help maintain higher internal CO2 concentrations, further reducing photorespiration.
37. What is the relationship between photorespiration and nitrogen assimilation in plants?
Photorespiration and nitrogen assimilation are interconnected processes. The photorespiratory pathway produces ammonia, which needs to be reassimilated. This reassimilation process is linked to the plant's overall nitrogen metabolism, potentially influencing the efficiency of nitrogen use in the plant.
38. How does the Calvin cycle enzyme phosphoribulokinase indirectly affect photorespiration rates?
Phosphoribulokinase catalyzes the regeneration of RuBP (ribulose-1,5-bisphosphate) in the Calvin cycle. The activity of this enzyme indirectly affects photorespiration rates by influencing the availability of RuBP for RuBisCO. Higher phosphoribulokinase activity can lead to more RuBP, potentially increasing both photosynthesis and photorespiration rates.
39. What role do heat shock proteins play in protecting against photooxidation?
Heat shock proteins act as molecular chaperones, helping to protect and repair proteins damaged by oxidative stress during photooxidation. They assist in maintaining the structure and function of photosynthetic proteins under stress conditions, thereby helping to preserve photosynthetic efficiency.
40. How does the presence of heavy shade affect the balance between photorespiration and photosynthesis?
In heavy shade, the rate of photosynthesis decreases due to limited light availability. This can lead to a relative increase in photorespiration as the CO2:O2 ratio at RuBisCO's active site may decrease. However, the absolute rate of photorespiration may also decrease due to lower overall metabolic activity in low light conditions.
41. What is the role of glycerate kinase in the photorespiratory pathway?
Glycerate kinase is an enzyme that catalyzes the phosphorylation of glycerate to 3-phosphoglycerate in the final step of the photorespiratory pathway. This reaction occurs in the chloroplast and is crucial for returning carbon to the Calvin cycle, completing the photorespiratory carbon recovery process.
42. How does the presence of salinity stress affect photorespiration and photooxidation in plants?
Salinity stress can increase both photorespiration and photooxidation. High salinity leads to stomatal closure, reducing CO2 availability and potentially increasing photorespiration. It also induces oxidative stress, enhancing photooxidation. Both processes can be part of the plant's stress response mechanism under saline conditions.
43. What is the relationship between photorespiration and the production of reactive oxygen species in plants?
Photorespiration can both produce and scavenge reactive oxygen species (ROS). The glycolate oxidase reaction in peroxisomes produces hydrogen peroxide, a type of ROS. However, the overall photorespiratory pathway can also help dissipate excess energy and reduce ROS production in chloroplasts, acting as a protective mechanism against oxidative stress.
44. How does the structure and function of carbonic anhydrase relate to photorespiration in C4 plants?
Carbonic anhydrase plays a crucial role in C4 photosynthesis by catalyzing the rapid conversion of CO2 to bicarbonate and vice versa. In C4 plants, this enzyme helps concentrate CO2 around RuBisCO in bundle sheath cells, effectively reducing oxygen competition and minimizing photorespiration.
45. What is the significance of the CO2 compensation point in understanding photorespiration?
The CO2 compensation point is the CO2 concentration at which the rate of photosynthesis exactly balances the rate of respiration and photorespiration, resulting in no net CO2 fixation. It provides insight into the relative rates of carboxylation and oxygenation by RuBisCO, with a lower compensation point indicating less photorespiration.
46. How does the presence of elevated CO2 levels affect the expression of genes related to photorespiration?
Elevated CO2 levels generally lead to a downregulation of genes involved in the photorespiratory pathway. This is because higher CO2 concentrations favor carboxylation over oxygenation by RuBisCO, reducing the need for photorespiratory enzymes. However, the response can vary among plant species and growth conditions.
47. What is the role of peroxisomal catalase in the photorespiratory pathway?
Peroxisomal catalase plays a crucial role in detoxifying hydrogen peroxide produced during the oxidation of glycolate to glyoxylate in the photorespiratory pathway. By breaking down hydrogen peroxide into water and oxygen, catalase helps protect the cell from oxidative damage associated with photorespiration.
48. How does the presence of high light intensity affect the balance between photooxidation and photoprotection mechanisms?
High light intensity can increase the risk of photooxidation by generating excess excitation energy. Plants respond by activating various photoprotection mechanisms, such as non-photochemical quenching and the xanthophyll cycle. The balance between photooxidation and photoprotection depends on the plant's capacity to dissipate excess energy effectively.
49. What is the relationship between photorespiration and drought tolerance in plants?
Photorespiration can contribute to drought tolerance in plants by helping to dissipate excess light energy when CO2 fixation is limited due to stomatal closure. It also produces glycine and serine, which may act as compatible solutes, helping in osmotic adjustment. However, the increased energy cost of photorespiration can also negatively impact plant productivity under drought conditions.
50. How does the activity of phosphoglycolate phosphatase affect the rate of photorespiration?
Phosphoglycolate phosphatase catalyzes the first step of the photorespiratory pathway in the chloroplast, converting 2-phosphoglycolate to glycolate. The activity of this enzyme directly influences the flux through the photorespiratory pathway. Higher activity can lead to increased photorespiration rates, while lower activity may slow down the process.
