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Photorespiration is a process that occurs when the concentration of carbon dioxide in a leaf decreases. Usually, respiration takes place in the mitochondria of plants in general, but in the case of photorespiration, it is mostly seen in the C3 plants.
- Photorespiration requires sunlight, while respiration does not.
- It is also known as oxidative photosynthetic carbon cycle or C2 cycle.
- The most abundantly found enzyme on the globe is RuBisCO, which can bind both CO2 and O2 in its active state.
- However, it possesses more affinity towards CO2.
- The process is an important step in the Calvin–Benson cycle.
- Photorespiration affects the growth and productivity of plants.
- It is a process that competes with photosynthesis.
- The process involves exchange between metabolites, chloroplasts, mitochondria and leaf peroxisomes.
Key Terms: Photorespiration, C3 Plants, C4 Plants, Photosynthesis, Respiration, Oxygen, Carbon Dioxide, Mitochondria, Calvin-Benson Cycle, Cells, Plants, Sunlight, Leaves, ATP
What is Photorespiration?
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Photorespiration is defined as a process which involves the loss of fixed carbon in the presence of light that occurs during the Calvin Cycle. The process occurs in three locations that include chloroplast, mitochondria, and peroxisomes.
- The parameters that affect photorespiration include high temperature, high oxygen level, and higher CO2 concentration.
- In the process, there is a release of CO2 due to the oxidation of glycolate.
- When RuBisCo binds to oxygen, it is termed ribulose bisphosphate oxygenase.
- Photorespiration and photosynthesis are two different processes, as one yields oxygen and the other yields carbon dioxide.
RuBisCO, instead of carbon dioxide, reacts with oxygen when there is a decrease in the leaf's carbon dioxide level. To be precise, it happens when the carbon dioxide level in the leaf goes below 50 ppm.
- To get rid of phosphoglycolate, the plant first removes the cluster of phosphate.
- It is done by converting it to the units of glycolic acid.
- Then, the glycolic acid is shifted to the peroxisome and then to the glycine.
- It is then converted to serine, which occurs in the mitochondria of the plant cell.
- The result is that serine is helpful in the making of organic units.
- However, this process results in the loss of carbon dioxide as it requires the energy of the plant.
Photorespiration
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Photorespiration in C3 plants
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When in any C3 plant, the RuBisCO binds with O2, it results in the declination of CO2 fixation. During the photorespiration process, the RuBP does not form 2 PGA molecules.
- It reacts with oxygen and gives results to the molecules of one phosphoglycerate and phosphoglycolate.
- In the process of photorespiration, there is no production of sugars or ATP.
- Instead, ATP helps in releasing carbon dioxide.
- The process of photorespiration is costly and does not even produce any NADPH or ATP.
- This plant uses the standard mode of carbon fixation.
Photorespiration in C4 Plants
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Photorespiration in C4 plants does not take place as it consists of a mechanism that results in the increased concentration of CO2 at the location of the enzyme.
- It occurs because there is mesophyll C4 acid, which breaks down in the bundle sheet.
- This happens to release the CO2, which in turn increases the intracellular concentration of CO2.
- Finally, it takes care that RuBisCO tends to be a carboxylase, which can decrease oxygenase activity.
- So, C4 plants do not undergo photorespiration and have a higher tolerance towards the temperature.
Crassulacean Acid Metabolism (CAM)
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Crassulacean Acid Metabolism, also known as CAM, is a carbon fixation process that involves plants photosynthesising during the day, but only exchanging gases at night.
- It is mostly carried out in dry and arid environments.
- The process focuses on water loss and photorespiration.
- It undergoes the process of photosynthesis under stress.
- CAM involves the conversion of oxaloacetate into malate.
- Carbon dioxide is released during the process of decarboxylation.
- Orchids, Aloe, Cacti and Pineapple are some examples of Crassulacean Acid Metabolism.
Things to Remember
- Photorespiration is a process that is mainly found in the C3 plants, where RuBisCo acts as a catalyst.
- The process is carried out at the mitochondria, peroxisomes, and chloroplasts.
- Photorespiration does not require sunlight and yields carbon dioxide in reaction with oxygen.
- RuBisCo is an important agent in the process of photorespiration and forms phosphoglycolate and phosphoglycerate.
- The process does not give rise to ATP or NADPH.
- Photorespiration does not take place in C4 plants as they increase the level of carbon dioxide.
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Sample Questions
Ques: Is it possible for C4 plants to undergo photorespiration? (2 marks)
Ans: When the Calvin cycle enzyme RuBisCO works on oxygen rather than that of CO2, photorespiration takes place. In C4 plants, there is a splitting of CO2 fixation and the Calvin cycle through which they are less into respiration. Therefore, it can be said that the C4 plants can't undergo the process of photorespiration as depending on the location of the enzyme, the concentration of CO2 gets higher.
Ques: Mention the demerits of photorespiration. (4 marks)
Ans: Photorespiration is the reason for the declining carbon dioxide content in a leaf. The demerits of photorespiration are:
- It is an inefficient process as it does not produce NADPH or ATP.
- There is a 25% loss in the fixed value of CO2 present.
- To prevent the failure caused by photorespiration, sugarcane has established its mechanism.
- With the help of ATP, it releases CO2.
Ques: Briefly explain the process of photorespiration. (2 marks)
Ans: Photorespiration is an inefficient process that takes place when oxygen absorbs carbon dioxide. It is increasing in the plants with time and it is due to the rising oxygen levels in the atmosphere. At the times when the weather becomes too hot or too dry, plants get to prevent the loss of water which can be done due to closing the stomata which in turn restricts the CO2 intake of a leaf.
Ques: In what location of plants does photorespiration occur? (3 marks)
Ans: The process of photorespiration mostly occurs in the chloroplast of the plant. It is respiration that is light-activated. It needs glycolate metabolism and thus is different from normal or dark breathing. However, the other part of the plant where the process of photorespiration takes place is in the mitochondria and the peroxisomes. It intakes the ATP and NADPH and loses the CO2 content. It occurs when the content of CO2 decreases to around 50 ppm.
Ques: What is responsible for the lower affinity and higher affinity of RuBisCO to O2? (2 marks)
Ans: The factor responsible for the lower affinity of RuBisCO to O2 is the reduced water levels in the plants. The low affinity is the nutrient uptake system which the plants follow at the higher concentration while the high affinity is the nutrient uptake system which the plants follow at the lower concentration. The factors that are responsible for the higher affinity of RuBisCO to O2 include the increased level of oxygen, the increase in the concentration of CO2 in cells, and the high temperature.
Ques: Mention the contrast between photorespiration and photosynthesis. (4 marks)
Ans: The contrast between photorespiration and photosynthesis is:
- Photorespiration is a process that involves the reaction of oxygen and RuBisCO and in turn, completes the Calvin cycle. While photosynthesis, the RuBisCO is responsible for the carbon fixation.
- Photorespiration takes place in the C4 plants while the process of photosynthesis takes place in C3 plants.
- Photorespiration occurs in the absence of light while photosynthesis occurs in the presence of light.
- Photorespiration requires the presence of carbon dioxide while photosynthesis requires the presence of oxygen.
Ques: Explain the role of RuBisCO in the process of Photorespiration. (3 marks)
Ans: RuBisCo stands for Ribulose Bisphosphate Oxygenase and plays an important role in the process of photorespiration. In the process of photorespiration, RuBisCo acts as a catalyst to the oxygen of the RuBP molecule to form a molecule of phosphoglycolate and phosphoglycerate. It produces a soluble protein that is present in the leaves in around 20-25% quantity. It tends to have a higher affinity for carbon dioxide but can bind to both carbon and oxygen.
Ques: What are the benefits of photorespiration? (3 marks)
Ans:
The benefits of photorespiration are as follows:- The process provides plant-resistant protection.
- It helps in maintaining the redox equilibrium of the cell.
- Photorespiration improves the defence mechanism of plants.
- It improves the plant's immunological system.
Ques: What is the difference between C3 and C4 plants? (4 marks)
Ans: The difference between C3 and C4 plants are as follows:
| C3 Plants | C4 Plants |
|---|---|
| C3 plants will produce 3- carbon acid and phosphoglyceric acid as product. | C4 plants will produce 4 carbon acid:- oxaloacetate as by-product. |
| It requires cool and wet environments. | It requires tropical and dry environments. |
| In this carbon fixation occur only once in a row. | In this carbon fixation occur only twice in a row. |
| The process does not suppress photorespiration. | The process suppress photorespiration. |
Ques: What is the significance of the process of photorespiration? (3 marks)
Ans: The significance of the process of photorespiration are as follows:
- Photorespiration is totally opposite of photosynthesis.
- It is used to decrease the viability of photosynthesis.
- The process is considered an inefficient cycle.
- ATP or NADPH molecules are not delivered.
Ques: Mention the factors influencing the process of photorespiration? (2 marks)
Ans: The factors influencing the process of photorespiration are as follows:
- High temperature
- High intensity
- Concentration of oxygen
- Formation of glycolate
- Concentration of light
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