Model Answer
0 min readIntroduction
Crassulacean Acid Metabolism (CAM) is a water-saving adaptation employed by plants in arid and semi-arid environments. Unlike C3 and C4 plants, CAM plants exhibit a unique temporal separation of initial CO₂ uptake and the Calvin cycle. This mechanism allows them to minimize water loss by opening their stomata during the cooler, more humid night and closing them during the hot, dry day. This strategy is particularly advantageous for plants facing severe water stress, enabling them to survive and photosynthesize efficiently in challenging conditions. Understanding the intricacies of CAM is crucial for comprehending plant adaptation and resilience in the face of climate change.
Mechanism of CO₂ Fixation in CAM Plants
The CAM pathway can be broadly divided into two phases: nocturnal (nighttime) and diurnal (daytime). These phases are characterized by distinct physiological processes.
Nocturnal Phase (Night)
- Stomatal Opening: CAM plants open their stomata at night when the air is cooler and more humid, reducing transpirational water loss.
- CO₂ Uptake & Fixation: CO₂ enters the leaves through the open stomata and is fixed by Phosphoenolpyruvate carboxylase (PEP carboxylase), an enzyme with a high affinity for CO₂. This reaction occurs in the mesophyll cells.
- Formation of Malic Acid: PEP carboxylase catalyzes the addition of CO₂ to phosphoenolpyruvate (PEP), forming oxaloacetate. Oxaloacetate is then reduced to malic acid, which is stored in the vacuole of the mesophyll cells. This accumulation of malic acid lowers the vacuolar pH.
- No Calvin Cycle: Importantly, the Calvin cycle does *not* operate at night. The malic acid serves as a temporary CO₂ store.
Diurnal Phase (Day)
- Stomatal Closure: During the day, the stomata remain closed to minimize water loss.
- Malic Acid Transport: Malic acid is transported from the vacuole to the cytoplasm.
- Decarboxylation: Malic acid undergoes decarboxylation, releasing CO₂. This process is catalyzed by malic enzyme.
- Calvin Cycle: The released CO₂ is then fixed by RuBisCO and enters the Calvin cycle, leading to the production of sugars. This occurs within the chloroplasts.
- PEP Regeneration: Pyruvate, a byproduct of the malic enzyme reaction, is transported back to the mesophyll cells and converted back to PEP, completing the cycle.
The efficiency of CAM photosynthesis is influenced by factors such as temperature, water availability, and light intensity. Some CAM plants exhibit ‘crassulacean acid metabolism idling’ where the pathway is less distinct under favorable conditions.
| Phase | Stomata | CO₂ Fixation Enzyme | Primary Product | Calvin Cycle |
|---|---|---|---|---|
| Nocturnal (Night) | Open | PEP Carboxylase | Malic Acid | Inactive |
| Diurnal (Day) | Closed | Malic Enzyme (Decarboxylation) & RuBisCO | Sugars | Active |
Variations in CAM
CAM is not a fixed pathway; there are variations. Some plants exhibit constitutive CAM (always using the pathway), while others switch to CAM under water stress (facultative CAM). The degree of temporal separation can also vary.
Conclusion
In conclusion, the CAM pathway represents a remarkable adaptation enabling plants to thrive in arid environments by temporally separating CO₂ uptake and fixation. This mechanism minimizes water loss through nocturnal stomatal opening and daytime closure, coupled with the efficient use of PEP carboxylase and subsequent decarboxylation of malic acid to fuel the Calvin cycle. Understanding CAM is vital for addressing challenges related to plant productivity and survival in a changing climate, and for developing strategies for sustainable agriculture in water-limited regions.
Answer Length
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