Photorespiration: Definition, Diagram, Process, Cycle, Topics, Examples
What Is Photorespiration?
Photorespiration is considered to be one of the key metabolic pathways of plants, which links in with the Calvin Cycle of photosynthesis. It is thought to be an energy-wasting route, as it brings down the net efficiency of photosynthesis with the consumption of energy and the evolution of carbon dioxide. Photorespiration is a very important process in understanding how plants adapt to different environmental settings, particularly in terms of carbon fixation.
Commonly Asked Questions
Q: What is photorespiration and why is it considered a wasteful process?
A:
Photorespiration is a process that occurs in plants when the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) fixes oxygen instead of carbon dioxide. It's considered wasteful because it consumes energy and reduces the efficiency of photosynthesis by not producing glucose. Instead, it releases previously fixed carbon dioxide and ammonia, which the plant must then re-assimilate.
Q: What is the significance of the glycerate kinase enzyme in the photorespiratory pathway?
A:
Glycerate kinase is an important enzyme in the final stages of the photorespiratory pathway. Located in the chloroplast, it catalyzes the ATP-dependent phosphorylation of glycerate to 3-phosphoglycerate. This step is crucial for returning the carbon from the photorespiratory pathway back into the Calvin cycle, allowing for its potential re-fixation and preventing net carbon loss.
Q: What is the role of peroxisomal catalase in protecting against oxidative stress during photorespiration?
A:
Peroxisomal catalase plays a crucial protective role during photorespiration by detoxifying hydrogen peroxide (H2O2). H2O2 is produced as a byproduct when glycolate is oxidized to glyoxylate in the peroxisome. Catalase rapidly converts H2O2 to water and oxygen, preventing oxidative damage to cellular components. This protection is essential for maintaining the function of peroxisomes and allowing the
Conditions Favouring Photorespiration
Stomatal Closure
Photorespiration primarily occurs on hot, dry days when the stomata of the plants close to prevent the excessive loss of water. The closing process diminishes the amount of carbon dioxide intake while oxygen is continuously produced and accumulated in the leaf.
Low Carbon Dioxide Concentration
When the concentration of carbon dioxide internally becomes less than the threshold of 5% or 50 ppm, RuBisCO starts fixing the oxygen; this would instead form a phosphoglycolate, which is a 2-carbon molecule.
Commonly Asked Questions
Q: Why does photorespiration occur more frequently at higher temperatures?
A:
At higher temperatures, the solubility of CO2 in water decreases more rapidly than that of O2. This change in relative concentrations favors the oxygenase activity of RuBisCO over its carboxylase activity. Additionally, the enzyme's specificity for CO2 decreases at higher temperatures, making it more likely to bind with O2 and initiate photorespiration.
Q: How do environmental factors other than temperature affect photorespiration rates?
A:
Besides temperature, factors like light intensity and CO2 concentration significantly affect photorespiration rates. High light intensity can increase photorespiration by saturating the photosynthetic apparatus with energy, favoring RuBisCO's oxygenase activity. Low CO2 concentrations, such as those caused by closed stomata during water stress, also increase photorespiration by shifting the CO2:O2 ratio.
Q: How does atmospheric CO2 concentration affect photorespiration rates?
A:
Atmospheric CO2 concentration inversely affects photorespiration rates. Higher CO2 levels increase the CO2:O2 ratio around RuBisCO, favoring carboxylation over oxygenation and thus reducing photorespiration. Conversely, lower CO2 levels increase photorespiration. This relationship is why C3 plants generally show increased growth and productivity in elevated CO2 environments, as photorespiration is suppressed.
Q: How does photorespiration interact with the light reactions of photosynthesis?
A:
Photorespiration interacts with the light reactions by consuming some of their products. It uses ATP and NADPH generated by the light reactions, potentially competing with the Calvin cycle for these resources. However, by acting as an energy sink, photorespiration may also help prevent over-reduction of the photosynthetic electron transport chain under high light conditions, potentially protecting against photoinhibition.
Q: How does the rate of photorespiration compare between C3 and C4 plants?
A:
C3 plants typically have much higher rates of photorespiration compared to C4 plants. In C3 plants, photorespiration can reduce photosynthetic efficiency by 20-50%. C4 plants, however, have evolved mechanisms to concentrate CO2 around RuBisCO, effectively suppressing its oxygenase activity. As a result, C4 plants have very low rates of photorespiration, contributing to their higher efficiency in hot, dry environments.
Overview Of Photorespiration
Photorespiration comprises a few significant steps that add up to the entire process. These are:
Carbon Fixation
At low carbon dioxide, RuBisCO catalyzes the reaction of oxygen with ribulose bisphosphate, RuBP, to produce one molecule of 3-phosphoglycerate, PGA, and one molecule of phosphoglycolate.
Conversion Of Phosphoglycolate
To prevent the toxic accumulation of phosphoglycolate, it gets converted into glycolic acid by the plant. This occurs rather rapidly to avoid any possible harmful effects.
Transformation In Peroxisomes
Transport of glycolic acid to the peroxisomes for further conversion to glycine, a 2-carbon amino acid. This step is important in detoxifying the plant and making it ready for further metabolism.
Conversion In Mitochondria
Glycine is again transported to mitochondria where it undergoes a conversion into serine, a 3-carbon amino acid. This process is energy-consuming with carbon dioxide being formed as an end-product.
Commonly Asked Questions
Q: What are the main cellular compartments involved in photorespiration?
A:
Photorespiration involves three cellular compartments: chloroplasts, peroxisomes, and mitochondria. The process begins in chloroplasts with the oxygenation of RuBP, continues in peroxisomes with the oxidation of glycolate, and concludes in mitochondria with the conversion of glycine to serine. This multi-compartment process requires coordinated enzyme activities and metabolite transport.
Q: What is the significance of the 2-phosphoglycolate phosphatase enzyme in photorespiration?
A:
2-phosphoglycolate phosphatase is a crucial enzyme in the initial stages of the photorespiratory pathway. Located in the chloroplast, it catalyzes the hydrolysis of 2-phosphoglycolate (the product of RuBisCO's oxygenase activity) to glycolate. This step is essential for removing the phosphate group, allowing glycolate to be exported from the chloroplast to the peroxisome for further metabolism in the photorespiratory pathway.
Q: What is the role of catalase in the photorespiratory pathway?
A:
Catalase is a key enzyme in the photorespiratory pathway, located in peroxisomes. It catalyzes the breakdown of hydrogen peroxide (H2O2), a toxic byproduct formed during the oxidation of glycolate to glyoxylate. By converting H2O2 to water and oxygen, catalase protects the cell from oxidative damage and allows the photorespiratory process to continue safely.
Q: How does the glycine decarboxylase complex function in photorespiration?
A:
The glycine decarboxylase complex is a multi-enzyme system located in plant mitochondria. It catalyzes the oxidative decarboxylation and deamination of glycine, converting two glycine molecules into one serine molecule, releasing CO2 and NH3 in the process. This step is crucial in the photorespiratory pathway, linking the metabolism in peroxisomes to that in mitochondria.
Q: What is the role of serine hydroxymethyltransferase in the photorespiratory pathway?
A:
Serine hydroxymethyltransferase (SHMT) is a key enzyme in the photorespiratory pathway, located in mitochondria. It catalyzes the reversible conversion of glycine to serine by transferring a one-carbon unit from tetrahydrofolate. This reaction is crucial for completing the photorespiratory cycle and regenerating 3-phosphoglycerate, which can re-enter the Calvin cycle.
Impact Of Photorespiration On Plants
Photorespiration has various influences on the metabolism of plants:
Reduced Photosynthetic Efficiency
The energy used to convert the phosphoglycolate to serine is not returned for sugar production, so there is a net loss of carbon. Because of this inefficiency, it may limit the growth and productivity of plants.
Energy Loss
Photorespiration requires ATP and NADPH, the same substrates needed for photosynthesis. This depletes supplies for photosynthesis, and further impacts the general capacity of plants to capture energy efficiently.
Protective Role
Under certain conditions, photorespiration may serve as an energy sink, thus protecting plants from oxidative stress. Under high light intensity or conditions of drought, this protective mechanism may be of value.
Commonly Asked Questions
Q: How does photorespiration affect nitrogen metabolism in plants?
A:
Photorespiration significantly impacts nitrogen metabolism. It releases ammonia during the conversion of glycine to serine, which must be reassimilated to prevent nitrogen loss. This reassimilation, occurring via the GS-GOGAT cycle, requires energy and reducing power. Additionally, photorespiration produces glyoxylate, which can be used in photorespiratory nitrogen cycling, potentially aiding in nitrate assimilation.
Q: How does photorespiration affect crop productivity?
A:
Photorespiration can significantly reduce crop productivity by decreasing the efficiency of photosynthesis. It can lower crop yields by 20-50% in C3 plants, which include important crops like wheat, rice, and soybeans. This inefficiency increases under conditions that favor photorespiration, such as high temperatures and drought.
Q: What is the energetic cost of photorespiration to the plant?
A:
Photorespiration is energetically costly to plants. It consumes ATP and reducing power (NADPH) without producing useful products like glucose. Additionally, it releases CO2 that was previously fixed, requiring energy to re-fix this carbon. Overall, it's estimated that photorespiration can reduce the efficiency of photosynthesis by 20-50% in C3 plants.
Q: What are some potential benefits of photorespiration to plants?
A:
While often considered wasteful, photorespiration may have some benefits:
Q: What is the relationship between photorespiration and photosynthetic efficiency?
A:
Photorespiration decreases photosynthetic efficiency by competing with carbon fixation. When RuBisCO fixes oxygen instead of CO2, it initiates a process that consumes energy and releases previously fixed CO2. This reduces the net carbon gain from photosynthesis. In C3 plants, photorespiration can decrease photosynthetic efficiency by up to 25-50%, especially under conditions of high temperature and low CO2 concentration.
Photosynthesis: Relation With Photorespiration
Photosynthesis and photorespiration can occur in a plant at the same time. During photosynthesis, oxygen is a by-product, but in photorespiration, carbon dioxide is produced. The gases formed in these processes are somewhat interrelated because the oxygen formed during photosynthesis may increase photorespiration when the concentration of carbon dioxide is low.
Commonly Asked Questions
Q: How does the C3 cycle differ from photorespiration?
A:
The C3 cycle (Calvin cycle) is the primary carbon fixation pathway in photosynthesis, where RuBisCO fixes CO2 to produce glucose. Photorespiration occurs when RuBisCO fixes O2 instead, leading to energy loss and no net carbon fixation. While the C3 cycle is productive, photorespiration is generally considered counterproductive for the plant.
Q: How does the CO2 compensation point relate to photorespiration?
A:
The CO2 compensation point is the CO2 concentration at which the rate of photosynthesis exactly matches the rate of respiration and photorespiration combined, resulting in no net CO2 fixation. Plants with high rates of photorespiration have higher CO2 compensation points, indicating they require more CO2 to overcome the effects of photorespiration and achieve net carbon gain.
Q: What is the role of glycolate in the photorespiration pathway?
A:
Glycolate is the first stable product formed when RuBisCO fixes oxygen instead of carbon dioxide. It's toxic to the plant and must be quickly metabolized. The glycolate is transported to peroxisomes where it's oxidized to glyoxylate, starting the series of reactions that characterize the photorespiratory pathway.
Q: What is the evolutionary significance of photorespiration?
A:
While often viewed as wasteful, photorespiration may have evolutionary significance. It's thought to be a vestigial process from when Earth's atmosphere had higher O2 and lower CO2 levels. It may also serve as a "safety valve" to prevent the accumulation of excess energy in the photosynthetic apparatus, protecting against photo-oxidative damage.
Q: How does the structure of RuBisCO contribute to photorespiration?
A:
RuBisCO's structure allows it to bind both CO2 and O2 at its active site. The similarity in size and shape between CO2 and O2 molecules means RuBisCO can't always discriminate between them. This lack of specificity is a key factor in the occurrence of photorespiration, as about 25% of the time, RuBisCO will fix O2 instead of CO2.
Adaptations To Reduce Photorespiration
To avoid or reduce the losses through photorespiration, some plants have evolved with alternate mechanisms of carbon fixation :
C4 Pathway
Plants like maize and sugarcane are of the C4 type that have evolved a means of concentrating carbon dioxide in the bundle sheath cells. This minimises the chance of photorespiration. They fix carbon dioxide into a 4-carbon compound—first oxaloacetic acid—before it enters the light-independent reactions.
CAM Pathway
Plants which follow Crassulacean Acid Metabolism such as cacti and succulents assimilate carbon dioxide in the dark. This prevents loss of water during the daytime and reduces the extent of photorespiration.
Commonly Asked Questions
Q: How do C4 plants minimize photorespiration?
A:
C4 plants have evolved a mechanism to concentrate CO2 around RuBisCO, reducing the likelihood of oxygen fixation. They use PEP carboxylase to initially fix CO2 in mesophyll cells, then transport the fixed carbon to bundle sheath cells where RuBisCO is located. This spatial separation and CO2 concentration minimize photorespiration.
Q: How do plants recycle ammonia produced during photorespiration?
A:
During photorespiration, ammonia is released when glycine is converted to serine in mitochondria. Plants recycle this ammonia through the glutamine synthetase-glutamate synthase (GS-GOGAT) cycle. This process reassimilates the nitrogen into amino acids, preventing nitrogen loss and maintaining the plant's nitrogen balance.
Q: How do CAM plants deal with photorespiration?
A:
CAM (Crassulacean Acid Metabolism) plants, like cacti, temporally separate CO2 fixation and the Calvin cycle. They fix CO2 at night when temperatures are cooler and stomata are open, storing it as malic acid. During the day, they release this CO2 internally for use in the Calvin cycle, maintaining a high CO2 concentration around RuBisCO and minimizing photorespiration even when stomata are closed.
Q: What strategies are scientists exploring to reduce photorespiration in crops?
A:
Scientists are exploring several strategies to reduce photorespiration in crops:
Q: What is the connection between photorespiration and drought stress?
A:
Drought stress exacerbates photorespiration. When plants are water-stressed, they close their stomata to conserve water, reducing CO2 intake. This lowers the CO2:O2 ratio in leaves, favoring the oxygenase activity of RuBisCO and increasing photorespiration. This further reduces the plant's water use efficiency and photosynthetic productivity under drought conditions.
Recommended video on Photorespiration
Frequently Asked Questions (FAQs)
Q: How does the phosphate translocator contribute to the photorespiratory pathway?
A:
The phosphate translocator is a membrane protein that facilitates the exchange of phosphorylated C3 compounds between the chloroplast and the cytosol. In photorespiration, it plays a crucial role in exporting 3-phosphoglycerate (3-PGA) produced by the oxygenase activity of RuBisCO out of the chloroplast. This export is balanced by the import of phosphate, maintaining the phosphate balance in the chloroplast and allowing the continuation of the photorespiratory cycle.
Q: What is the role of hydroxypyruvate reductase in photorespiration?
A:
Hydroxypyruvate reductase is an enzyme located in peroxisomes that catalyzes a key step in the photorespiratory pathway. It reduces hydroxypyruvate to glycerate using NADH as a cofactor. This reaction is important for regenerating 3-phosphoglycerate, which can then re-enter the Calvin cycle. The activity of this enzyme helps complete the carbon recovery process in photorespiration.
Q: How does photorespiration affect water use efficiency in plants?
A:
Photorespiration negatively impacts water use efficiency in plants. As it competes with carbon fixation, plants need to keep their stomata open longer to acquire sufficient CO2 for adequate photosynthesis. This extended stomatal opening leads to increased water loss through transpiration. Consequently, plants experiencing high rates of photorespiration typically have lower water use efficiency, which can be particularly problematic in water-limited environments.
Q: How does photorespiration interact with the nitrogen assimilation pathway in plants?
A:
Photorespiration interacts closely with nitrogen assimilation in plants:
Q: What is the role of transamination reactions in the photorespiratory pathway?
A:
Transamination reactions play a crucial role in the photorespiratory pathway by facilitating the transfer of amino groups between amino acids. Two key transamination reactions occur:
Q: How does photorespiration affect the carbon isotope composition of plant tissues?
A:
Photorespiration affects the carbon isotope composition of plant tissues by influencing the discrimination against 13C during carbon fixation. RuBisCO preferentially fixes 12C over 13C, and this discrimination is enhanced when CO2 concentrations are low and photorespiration is high. As a result, plants with higher rates of photorespiration (like C3 plants) tend to have tissues more depleted in 13C compared to plants with lower photorespiration rates (like C4 plants).
Q: What is the connection between photorespiration and cellular redox status?
A:
Photorespiration influences cellular redox status in several ways:
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