Photosynthesis converts light energy into chemical energy. Carbon fixation, where atmospheric carbon dioxide (CO2) is incorporated into organic molecules, is a crucial step. Plants have developed different strategies for this, with the C3 and C4 pathways being two prominent examples that allow them to thrive in various environmental conditions.
The Basics of Carbon Fixation: C3 Pathway
The C3 pathway is the most common and ancient method of carbon fixation, utilized by approximately 85% of all plant species. CO2 enters the leaf and is directly fixed by RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). RuBisCO combines CO2 with ribulose-1,5-bisphosphate (RuBP) to form an unstable intermediate that breaks down into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound, giving the C3 pathway its name.
RuBisCO can react with oxygen instead of CO2, a process known as photorespiration, especially in warm conditions. This reduces photosynthetic efficiency by causing a net carbon loss. C3 plants flourish in moderate sunlight and temperature, with ample water and CO2 concentrations of 200 ppm or higher. Common C3 plants include wheat, rice, and soybeans.
An Alternative Approach: C4 Carbon Fixation
The C4 pathway evolved as an adaptation to hot, dry, high-light environments, where photorespiration challenges C3 plants. This two-stage process spatially separates initial CO2 capture from the main carbon-fixing cycle. Initial fixation occurs in mesophyll cells, closer to the leaf surface.
In mesophyll cells, PEP carboxylase fixes CO2 with phosphoenolpyruvate (PEP). This produces a four-carbon compound, transported to specialized bundle sheath cells surrounding vascular bundles. This arrangement of mesophyll cells surrounding bundle sheath cells is known as Kranz anatomy.
Inside bundle sheath cells, four-carbon compounds release CO2, creating a concentrated CO2 environment around RuBisCO. This high CO2 concentration suppresses photorespiration, allowing RuBisCO to operate more efficiently. The released CO2 then enters the Calvin cycle to produce sugars.
Distinguishing Features: C3 vs. C4
C3 and C4 carbon fixation differ in primary CO2-fixing enzymes, initial products, leaf anatomy, and environmental responses. In C3 plants, RuBisCO fixes atmospheric CO2, forming 3-PGA. C4 plants use PEP carboxylase for initial CO2 fixation in mesophyll cells, yielding a four-carbon compound before CO2 is delivered to RuBisCO in bundle sheath cells.
Leaf anatomy distinguishes these pathways. C3 plants lack specialized bundle sheath cells, with photosynthesis occurring primarily in mesophyll cells. C4 plants exhibit Kranz anatomy, characterized by prominent bundle sheath cells arranged around vascular bundles, encircled by mesophyll cells. This arrangement enables spatial separation of fixation steps.
CO2 uptake efficiency varies. C4 plants are more efficient at lower CO2 concentrations due to PEP carboxylase’s high affinity for CO2 and its ability to concentrate CO2 in bundle sheath cells. This mechanism allows C4 plants to maintain efficient photosynthesis even when stomata are partially closed to conserve water.
Photorespiration is negligible in C4 plants, but substantial in C3 plants, particularly under hot, dry conditions. C4 plants also have higher water-use efficiency. Optimal temperature ranges differ, with C3 plants performing best in cooler temperatures (20-30°C) and C4 plants thriving in warmer conditions.
Ecological Significance and Plant Distribution
The physiological advantages of C3 and C4 pathways influence their global distribution. C3 plants, more efficient in cooler, wetter conditions with moderate light, dominate temperate regions, forests, and grasslands. This group includes most trees, shrubs, and crops such as wheat, rice, potatoes, and soybeans.
C4 plants, efficient in hot, dry, high-light environments, are prevalent in tropical and subtropical grasslands, savannas, and arid regions. Their ability to minimize water loss and photorespiration provides a competitive edge. Major C4 crops include maize (corn), sugarcane, sorghum, and millet. While C3 plants constitute the majority of Earth’s plant biomass, C4 plants, despite being only about 3% of flowering species, contribute significantly to global primary production, accounting for approximately 23%.