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# C₃ and C₄ Pathways This document discusses the differences between C₃ and C₄ photosynthetic pathways in plants. ## Photorespiration Photorespiration is a process where oxygen competes with carbon dioxide for the active site on RuBisCO, an enzyme crucial for the first step of the Calvin cycle, w...

# C₃ and C₄ Pathways This document discusses the differences between C₃ and C₄ photosynthetic pathways in plants. ## Photorespiration Photorespiration is a process where oxygen competes with carbon dioxide for the active site on RuBisCO, an enzyme crucial for the first step of the Calvin cycle, which is CO₂ fixation. This leads to reduced efficiency in photosynthesis, as instead of producing simple sugars from CO₂, the plant is losing carbon in the form of phosphoglycolate. ## C₃ pathway In C₃ plants, CO₂ fixation occurs directly in the mesophyll cells. RuBisCO is the enzyme that catalyzes this reaction. Its affinity for CO₂ is not extremely high, so when O₂ concentrations are high, photorespiration can take place. ## C₄ plants C₄ plants have a specialized mechanism to reduce photorespiration. They initially fix CO₂ in the mesophyll cells using PEP carboxylase. This enzyme has a high affinity for CO₂ and a low affinity for O₂, effectively allowing them to concentrate CO₂ near RuBisCO in the bundle sheath cells. This high concentration of CO₂ minimizes O₂ competition and consequently photorespiration. ## Summary of C₄ Pathway advantages * Reduced photorespiration * Higher photosynthetic efficiency, especially under high temperatures or low CO₂ levels. * Increased productivity compared to C3 plants, possibly due to the tolerance of high temperatures. The image also includes a diagram of a plant cell, but it is not explained in the document.