Askiitians Tutor Team
Last Activity: 4 Months ago
The C3 and C4 cycles are two different pathways that plants use to fix carbon dioxide (CO2) during photosynthesis. These cycles differ in their efficiency, biochemical pathways, and adaptations to environmental conditions. Here are the key differences between the C3 and C4 cycles:
Biochemical Pathway:
C3 Cycle (Calvin Cycle): In the C3 cycle, the initial fixation of CO2 occurs through the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). This enzyme fixes CO2 to ribulose-1,5-bisphosphate, producing a three-carbon compound (3-phosphoglycerate or 3-PGA) as the first stable product.
C4 Cycle: The C4 cycle involves two types of cells in the leaf: mesophyll cells and bundle sheath cells. In the mesophyll cells, CO2 is initially fixed into a four-carbon compound called oxaloacetate by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase). The four-carbon compound is then shuttled to bundle sheath cells, where it releases CO2 and enters the Calvin cycle. This spatial separation of CO2 fixation and Calvin cycle reduces the oxygen competition with RuBisCO, leading to higher efficiency.
Anatomical and Physiological Adaptations:
C3 Cycle: C3 plants lack the anatomical and physiological adaptations found in C4 plants. They generally have a higher sensitivity to high temperatures and water stress because they don't have the same CO2 concentration mechanism as C4 plants.
C4 Cycle: C4 plants have adapted to overcome the oxygen inhibition of RuBisCO by spatially separating the initial CO2 fixation from the Calvin cycle. This separation occurs due to the arrangement of mesophyll and bundle sheath cells in their leaves. This adaptation allows C4 plants to thrive in higher temperatures and lower CO2 concentrations.
CO2 Concentration:
C3 Cycle: C3 plants initially fix CO2 directly from the atmosphere, which means they have to compete with oxygen for RuBisCO's active site. This can lead to a phenomenon called photorespiration, which reduces the efficiency of carbon fixation.
C4 Cycle: C4 plants concentrate CO2 in the bundle sheath cells, effectively reducing the oxygen competition for RuBisCO. This minimizes photorespiration and improves carbon fixation efficiency, especially in environments with high temperatures and intense sunlight.
Efficiency:
C3 Cycle: The C3 cycle is less efficient in terms of carbon fixation under high temperatures and intense light due to the higher likelihood of photorespiration.
C4 Cycle: The C4 cycle is more efficient in terms of carbon fixation under high-temperature and high-light conditions because of its CO2 concentrating mechanism, which reduces photorespiration.
Examples of Plants:
C3 Cycle: Common C3 plants include wheat, rice, soybeans, and most trees.
C4 Cycle: Common C4 plants include maize (corn), sugarcane, sorghum, and certain types of grasses.
In summary, the main differences between the C3 and C4 cycles lie in their biochemical pathways, anatomical adaptations, CO2 concentration mechanisms, and efficiency under different environmental conditions. C4 plants have evolved to address some of the limitations of C3 plants, particularly in warm and arid environments.