C3, C4, and CAM pathways, factors affecting photosynthesis

1. Introduction to Photosynthesis

1. Photosynthesis is the process by which plants convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂) using light energy.
2. It takes place in the chloroplasts of plant cells.
3. The reaction is: 
   6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂
4. There are three main types of photosynthetic pathways:
   • C3 Pathway (Calvin Cycle)
   • C4 Pathway (Hatch-Slack Pathway)
   • CAM Pathway (Crassulacean Acid Metabolism)

2. C3 Pathway (Calvin Cycle)

1. Most plants use the C3 pathway for photosynthesis.
2. Occurs in plants living in cool and moist environments.
3. The first stable product is a 3-carbon compound called 3-phosphoglycerate (3-PGA).
4. Key enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
5. Major steps:
   • Carbon fixation: CO₂ is fixed into RuBP by RuBisCO.
   • Reduction: ATP and NADPH convert 3-PGA into G3P (glyceraldehyde-3-phosphate).
   • Regeneration: Some G3P molecules regenerate RuBP.
6. Drawback: RuBisCO can bind with oxygen instead of CO₂, leading to photorespiration and energy loss.

3. C4 Pathway (Hatch-Slack Pathway)

1. Found in plants adapted to hot and dry climates (e.g., maize, sugarcane).
2. The first stable product is a 4-carbon compound called oxaloacetate (OAA).
3. Occurs in two types of cells:
   • Mesophyll cells: CO₂ is initially fixed.
   • Bundle sheath cells: CO₂ is released and enters the Calvin cycle.
4. Key enzyme: PEP carboxylase (Phosphoenolpyruvate carboxylase), which has a high affinity for CO₂.
5. More efficient than the C3 pathway because it minimizes photorespiration.

4. CAM Pathway (Crassulacean Acid Metabolism)

1. Found in desert plants like cacti and pineapples.
2. Helps plants conserve water in arid environments.
3. CO₂ is fixed at night and stored as malic acid in vacuoles.
4. During the day, stored CO₂ is released and used in the Calvin cycle.
5. CAM plants keep their stomata closed during the day to prevent water loss.
6. More energy-consuming but highly water-efficient.

5. Comparison of C3, C4, and CAM Pathways

6. Factors Affecting Photosynthesis

1. Light intensity: Higher light intensity increases photosynthesis up to a saturation point.
2. Carbon dioxide concentration: More CO₂ enhances the rate.
3. Temperature: Optimal temperature is required; extreme heat or cold reduces the rate.
4. Water availability: Water shortage slows down the process.
5. Chlorophyll content: More chlorophyll increases light absorption.

7. Conclusion

1. Plants use C3, C4, or CAM pathways depending on environmental conditions.
2. C3 plants are common but suffer from photorespiration.
3. C4 plants are more efficient in hot climates.
4. CAM plants are adapted to arid environments.
5. Various factors like light, CO₂, temperature, and water influence photosynthesis.