18 Questions
What is the primary product of the Calvin cycle in C3 plants?
Glycerate-3-phosphate (3-PGA)
What unstable molecule is formed as a result of Rubisco binding to oxygen in C3 plants?
2-phosphoglycolate (2-PG)
Which phase of the photorespiratory pathway involves the conversion of glycolate back to 2-phosphoglycolate?
Peroxisomal phase
What happens to glycolate after it is converted back to 2-phosphoglycolate in peroxisomes?
It is recycled through the chloroplasts into glycerate-3-phosphate
What is the main drawback of photorespiration on the Calvin cycle?
Reduces efficiency by competing with CO2 fixation
Which adaptive strategy helps plants offset losses due to photorespiration?
C4 photosynthetic pathway
Why is photorespiration considered a 'necessary evil' in C3 photosynthesis?
It can limit plant growth and productivity
Which plant process separates carbon fixation and oxygenation reactions to minimize the effects of photorespiration?
CAM (Crassulacean Acid Metabolism) photosynthesis
How does lower carbon dioxide concentration impact photorespiration?
Enhances Rubisco's affinity for oxygen
What effect does closed stomata have on the likelihood of oxygen binding to Rubisco?
Increases the likelihood
Under high light conditions, what generally happens to the speed of photorespiration?
Speeds up
What is a benefit of photorespiration for plants?
Enhances photosynthesis efficiency
How does photorespiration impact plant productivity?
Reduces plant productivity
What is the toxic compound produced when Rubisco binds to oxygen instead of carbon dioxide in the photorespiratory pathway?
2-phosphoglycolate (2-PG)
What is the full form of the enzyme Rubisco discussed in the text?
Ribulose-1,5-bisphosphate carboxylase/oxygenase
Which factor increases the rate of photorespiration by promoting oxygen binding to Rubisco?
High temperatures
What critical process does Rubisco catalyze in photosynthesis?
Carboxylation of RuBP
How do higher temperatures influence the occurrence of photorespiration?
Increase photorespiration due to more oxygen binding
Study Notes
Photorespiration: A Closer Look at the Photorespiratory Pathway
Photosynthesis, the process by which plants harness sunlight to convert carbon dioxide and water into glucose, is a vital source of energy for life on Earth. However, not all is perfect in the world of photosynthesis. In the case of plants and algae that use C3 photosynthesis, there exists a less efficient byproduct: photorespiration.
The Photorespiratory Pathway
Photorespiration is a process that occurs in C3 plants, which regulate their photosynthesis through the Calvin cycle. The Calvin cycle is intrinsically linked to the pathway of carbon fixation in C3 plants, where the primary product is 3-carbon molecules (3-C) such as glycerate-3-phosphate (3-PGA).
Photorespiration occurs when ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the enzyme responsible for carbon fixation, inadvertently binds to oxygen (O₂) instead of carbon dioxide (CO₂). This leads to the formation of an unstable molecule called 2-phosphoglycolate (2-PG), which eventually results in a loss of fixed carbon and the expenditure of energy.
The photorespiratory pathway can be divided into two phases: the mitochondrial phase and the peroxisomal phase. The mitochondrial phase begins when 2-PG is converted into glycolate, which is then transported to peroxisomes. In peroxisomes, glycolate is converted back to 2-PG and released, which is then recycled through the chloroplasts into glycerate-3-phosphate by the Calvin cycle.
The Cost of Photorespiration
The photorespiratory pathway may account for up to 25% of a plant's daily photosynthetic capacity. This seemingly inefficient process ultimately results in a loss of carbon, water, and energy. The key drawback of photorespiration is that it reduces the overall efficiency of the Calvin cycle and thus the net photosynthetic rate, which can limit plant growth and productivity.
Adaptations to Photorespiration
To offset the losses due to photorespiration, plants have developed various adaptive strategies, including the C4 photosynthetic pathway, which separates the carbon fixation and oxygenation reactions, and CAM (Crassulacean Acid Metabolism) photosynthesis, where stomatal opening and CO₂ fixation occur during the night.
Conclusions
Photorespiration is a necessary evil in C3 photosynthesis, but it can limit plant growth and productivity. Despite its limitations, photorespiration plays an essential role in maintaining the balance of the Calvin cycle and preventing the accumulation of toxic intermediates in the chloroplasts. Understanding the photorespiratory pathway is crucial for developing strategies to optimize photosynthesis in crops, ultimately leading to increased food production and a healthier planet.
Test your knowledge on the photorespiratory pathway in plants and how it impacts photosynthesis. Explore the process of photorespiration, its consequences, and the adaptations developed by plants to minimize its effects.
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