Core Differences in Photosynthetic Pathways
Photosynthesis in C3 and C4 plants differs primarily in the initial carbon fixation step and adaptations to environmental conditions. In C3 plants, the enzyme RuBisCO directly fixes CO2 into a three-carbon compound (3-phosphoglycerate) during the Calvin cycle. In contrast, C4 plants use a preliminary step where phosphoenolpyruvate (PEP) carboxylase fixes CO2 into a four-carbon compound (oxaloacetate), which is then transported to bundle sheath cells for the Calvin cycle. This C4 mechanism minimizes photorespiration, especially in hot, dry environments.
Key Biochemical and Anatomical Components
C3 plants lack specialized anatomy for CO2 concentration, making them prone to photorespiration when oxygen competes with CO2 at RuBisCO's active site, reducing efficiency. C4 plants have Kranz anatomy, with mesophyll cells performing initial fixation and bundle sheath cells hosting the Calvin cycle, creating a high CO2 environment that suppresses photorespiration. Biochemically, C4 plants invest energy in the C4 cycle (ATP for PEP regeneration), but this cost is offset by higher net photosynthesis under stress.
Practical Example: Crop Plants
Wheat and rice are C3 plants, thriving in temperate climates with adequate water, where photorespiration is minimal. Maize and sugarcane, as C4 plants, excel in tropical regions; for instance, in hot, arid fields, maize maintains higher photosynthetic rates by concentrating CO2, leading to greater yields compared to C3 crops under similar conditions, demonstrating C4's adaptation for water-efficient farming.
Ecological and Agricultural Importance
These differences influence plant distribution and agriculture: C3 plants dominate cooler, moist areas, while C4 plants are vital in warmer, drier ecosystems, contributing significantly to global productivity (e.g., C4 grasses in savannas). Understanding these pathways aids in developing drought-resistant crops through genetic engineering, enhancing food security amid climate change.