Dalvoy Logo
Dalvoy
UPSC MainsAGRICULTURE-PAPER-II201510 Marks150 Words
Q5.

What do you mean by C3, C4 and CAM mechanisms?

How to Approach

This question requires a concise explanation of three distinct photosynthetic mechanisms. The approach should be to first define each mechanism (C3, C4, and CAM) clearly, then highlight the key differences in their biochemical pathways and adaptations to different environmental conditions. A comparative table summarizing the features of each mechanism will be crucial for clarity and demonstrating a comprehensive understanding. Focus on the ecological significance and examples of plants utilizing each pathway.

Model Answer

0 min read

Introduction

Photosynthesis, the cornerstone of life on Earth, is not a uniform process. While all plants utilize light energy to convert carbon dioxide into sugars, the specific mechanisms employed vary significantly. These variations are primarily driven by adaptations to different environmental conditions, particularly temperature and water availability. C3, C4, and CAM photosynthesis represent three major photosynthetic pathways, each with unique biochemical and physiological characteristics. Understanding these pathways is crucial for appreciating plant diversity and agricultural advancements aimed at improving crop yields, especially in a changing climate.

C3 Photosynthesis

C3 photosynthesis is the most common photosynthetic pathway, found in approximately 85% of plant species. It's named after the initial three-carbon compound (3-phosphoglycerate) produced during carbon fixation. During C3 photosynthesis, CO2 is directly fixed by RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) in the mesophyll cells.

Limitations: RuBisCO also binds to oxygen, leading to photorespiration, a wasteful process that reduces photosynthetic efficiency, particularly in hot, dry conditions.

Examples: Wheat, rice, soybeans, and most trees.

C4 Photosynthesis

C4 photosynthesis is an adaptation to hot, dry environments. It involves a two-step carbon fixation process. Initially, CO2 is fixed by PEP carboxylase (PEPcase) in mesophyll cells, forming a four-carbon compound (oxaloacetate). This compound is then transported to bundle sheath cells, where it's decarboxylated, releasing CO2 which is then fixed by RuBisCO. This concentrates CO2 around RuBisCO, minimizing photorespiration.

Advantages: Higher photosynthetic efficiency in hot, dry conditions; reduced photorespiration.

Examples: Sugarcane, maize (corn), sorghum.

CAM Photosynthesis

CAM (Crassulacean Acid Metabolism) photosynthesis is another adaptation to arid environments. It’s similar to C4 photosynthesis in that it involves initial fixation of CO2 by PEP carboxylase, but with a temporal separation of steps. CAM plants open their stomata at night to take in CO2, fix it into organic acids, and store them. During the day, the stomata close to conserve water, and the stored organic acids are decarboxylated, releasing CO2 for use in the Calvin cycle.

Advantages: Extremely water-use efficient; allows survival in very arid conditions.

Examples: Cacti, succulents (like *Aloe vera*), pineapple.

Feature C3 Photosynthesis C4 Photosynthesis CAM Photosynthesis
Initial CO2 Fixation Enzyme RuBisCO PEP Carboxylase PEP Carboxylase
Cell Type of Initial Fixation Mesophyll Mesophyll Mesophyll
Cell Type of Calvin Cycle Mesophyll Bundle Sheath Mesophyll
Timing of CO2 Uptake & Fixation Daytime Daytime Nighttime (uptake), Daytime (fixation)
Water Use Efficiency Low Moderate High
Photorespiration Rate High Low Low

Conclusion

In conclusion, C3, C4, and CAM photosynthesis represent evolutionary adaptations to varying environmental conditions. C3 photosynthesis, while the most prevalent, suffers from photorespiration. C4 and CAM pathways, by employing different strategies for carbon fixation and minimizing photorespiration, enable plants to thrive in hot, dry environments. Understanding these photosynthetic mechanisms is crucial not only for comprehending plant physiology but also for developing strategies to improve crop resilience and productivity in the face of climate change.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

Evaluate your handwritten answer in under a minute

Free trial available • Trusted by 1,00,000+ aspirants

Additional Resources

Key Definitions

RuBisCO
Ribulose-1,5-bisphosphate carboxylase/oxygenase – an enzyme critical for carbon fixation in the Calvin cycle; it can also bind to oxygen, leading to photorespiration.
Photorespiration
A metabolic pathway that occurs when RuBisCO binds to oxygen instead of carbon dioxide, consuming energy and releasing CO<sub>2</sub>, effectively reducing photosynthetic efficiency.

Key Statistics

Approximately 85% of land plants utilize C3 photosynthesis.

Source: Raven, J. A. (2002). The evolution of C4 photosynthesis.

C4 plants are estimated to contribute to about 20% of terrestrial carbon fixation, despite comprising only 5% of plant species.

Source: Stillman, J. R. (2003). C4 photosynthesis: adaptations for a hot, dry world.

Examples

Sugarcane - A C4 Plant

Sugarcane's efficient photosynthesis allows it to thrive in tropical climates and contributes significantly to global sugar production.

Aloe Vera - A CAM Plant

Aloe Vera, a succulent, exemplifies CAM photosynthesis, enabling it to survive in arid desert environments by storing CO<sub>2</sub> at night and using it during the day.

Frequently Asked Questions

▶What is the main difference between C4 and CAM photosynthesis?

The key difference is the timing of carbon fixation. C4 photosynthesis separates the initial fixation and the Calvin cycle spatially (different cells), while CAM photosynthesis separates them temporally (different times of day).

Topics Covered

BiologyBotanyPlant PhysiologyPhotosynthesisPlant AdaptationCarbon Fixation
Back to all AGRICULTURE-PAPER-II 2015 questions