The Calvin cycle and photorespiration are distinct metabolic pathways in plants, differing primarily in their outcome regarding carbon fixation. Specifically, the Calvin cycle results in a gain of fixed carbon atoms, whereas photorespiration results in a loss of fixed carbon atoms.
Key Differences Between Calvin Cycle and Photorespiration
| Feature | Calvin Cycle | Photorespiration |
|---|---|---|
| Net Effect | Gain of fixed carbon atoms | Loss of fixed carbon atoms |
| Primary Enzyme | RuBisCO (Carboxylase activity) | RuBisCO (Oxygenase activity) |
| Substrates | CO2 and RuBP | O2 and RuBP |
| Products | G3P (glyceraldehyde-3-phosphate) | Phosphoglycolate and Glycerate |
| Energy Requirement | Requires ATP and NADPH | Requires ATP |
| Cellular Location | Chloroplast Stroma | Chloroplast, Peroxisome, Mitochondria |
| Favored Conditions | High CO2, low O2, moderate temperatures | Low CO2, high O2, high temperatures |
| Purpose | Carbon fixation, sugar production | To recycle phosphoglycolate, dissipate excess energy |
Detailed Explanation:
The Calvin cycle is the series of biochemical reactions in which carbon dioxide is fixed to produce sugars, using the enzyme RuBisCO to catalyze the carboxylation of RuBP (ribulose-1,5-bisphosphate). The resulting G3P molecules are then used to synthesize glucose and other organic molecules.
Photorespiration, on the other hand, occurs when RuBisCO binds to oxygen instead of carbon dioxide. This process is favored when CO2 levels are low and O2 levels are high, often due to high temperatures causing stomata to close and prevent CO2 entry. In this case, RuBisCO catalyzes the oxygenation of RuBP, yielding one molecule of phosphoglycolate and one molecule of glycerate. Phosphoglycolate undergoes a series of reactions in the peroxisome and mitochondria to regenerate glycerate, which can then re-enter the Calvin cycle. However, this process releases CO2 and consumes ATP, leading to a net loss of fixed carbon and reducing photosynthetic efficiency.
Impact on Plants:
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Calvin Cycle: Essential for plant growth and survival, providing the building blocks and energy for all cellular processes.
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Photorespiration: Considered a wasteful process because it reduces photosynthetic efficiency. However, some evidence suggests it may also play a protective role by dissipating excess energy when CO2 is limiting, preventing damage to the photosynthetic apparatus.
