C3, C4, and CAM Plants 

The mechanisms of photosynthesis vary across plant species, with C3, C4, and CAM (Crassulacean Acid Metabolism) being the three main pathways. These pathways describe the different ways plants fix carbon dioxide (CO₂) and adapt to their environment.

C3 Photosynthesis (The Calvin Cycle)

• C3 plants are the most common plants that perform photosynthesis using the Calvin Cycle. This process occurs in the stroma of chloroplasts.
• Carbon Fixation: In C3 photosynthesis, carbon dioxide (CO₂) is directly fixed by ribulose bisphosphate (RuBP), a 5-carbon molecule, forming an unstable 6-carbon compound that splits into two molecules of 3-phosphoglyceric acid (PGA) (a 3-carbon compound).
• Enzyme: The enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is responsible for fixing CO₂ into RuBP. However, RuBisCO is inefficient because it also catalyzes a reaction with oxygen, leading to photorespiration, a process that wastes energy and carbon.
• Efficiency: This pathway is inefficient under high temperatures and low CO₂ conditions due to photorespiration, which results in energy and carbon loss.
• Examples of C3 Plants: Wheat, rice, barley, and most temperate plants.

C4 Photosynthesis (Hatch and Slack Pathway)

• C4 plants use an alternative mechanism for carbon fixation known as the C4 pathway, which minimizes photorespiration. The Hatch and Slack pathway was discovered by Hatch and Slack in 1966, explaining how these plants perform photosynthesis.
• Leaf Anatomy: C4 plants have a distinct Kranz anatomy, where chloroplasts are located in two types of cells:
  • Mesophyll cells: Where the initial fixation of CO₂ occurs.
  • Bundle sheath cells: Where the Calvin Cycle occurs, and the CO₂ is fixed into sugars.
• Carbon Fixation: CO₂ is initially fixed into a 4-carbon compound, oxaloacetate, by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase). This enzyme has a higher affinity for CO₂ than RuBisCO. Oxaloacetate is then converted into malate or aspartate, which is transported into the bundle sheath cells, where it decarboxylates to release CO₂. The released CO₂ is then fixed in the Calvin Cycle.
• Enzyme: PEP carboxylase in the mesophyll cells has a higher affinity for CO₂ than RuBisCO, ensuring that CO₂ is captured efficiently, reducing photorespiration.
• Efficiency: C4 plants are more efficient in hot, dry conditions, as they can concentrate
• CO₂ in bundle sheath cells, preventing photorespiration.
• Examples of C4 Plants: Sugarcane, maize (corn), sorghum, and millet.

CAM Photosynthesis (Crassulacean Acid Metabolism)

• CAM plants are adapted to arid conditions where water conservation is crucial. These plants open their stomata at night (instead of during the day, like most plants) to minimize water loss.
• Carbon Fixation: At night, CAM plants fix CO₂ into malic acid (C4 compound), which is stored in vacuoles. During the day, the stored malic acid is decarboxylated to release CO₂, which is then used in the Calvin Cycle to produce sugars.
• Enzyme: Similar to C4 plants, PEP carboxylase is responsible for fixing CO₂ into a 4-carbon compound at night. The CO₂ is stored as malate and is used during the day.
• Water Conservation: By fixing CO₂ at night, CAM plants reduce water loss since stomata are closed during the day to conserve water. This makes CAM plants ideal for environments with limited water availability.
• Efficiency: CAM plants are extremely efficient in water use but are less efficient in terms of overall photosynthesis compared to C3 and C4 plants, as they only fix CO₂ at night.
• Examples of CAM Plants: Cacti, succulents (like aloe vera), pineapple, and orchids.

Comparison of C3, C4, and CAM Photosynthesis

Conclusion

• C3 Plants are the most widespread and perform photosynthesis using the Calvin Cycle. However, they are less efficient in hot and dry climates due to photorespiration.
• C4 Plants have evolved to be more efficient in hot and dry climates by concentrating CO₂ in bundle sheath cells, preventing photorespiration.
• CAM Plants are adapted to arid environments by fixing CO₂ at night, minimizing water loss and conserving energy for photosynthesis during the day.