C3 And C4 Photosynthesis Pathways — a labelled NEET Biology diagram with a definitions lexicon.
C3 and C4 Photosynthesis Pathways Labelled parts: C3 Photosynthesis, C4 Photosynthesis, Mesophyll cell, Bundle sheath cell, Calvin cycle, Photorespiration, RuBisCO, PEP carboxylase, Oxaloacetate (OAA), Malate, Pyruvate, Kranz anatomy, Vascular bundle, CO2, O2, Sugars. This is the standard photosynthetic pathway where the initial fixation of CO₂ occurs directly in the mesophyll cells using the enzyme RuBisCO. The first stable product formed is a 3-carbon compound (3-PGA). FYI: In C3 plants, the initial CO₂ acceptor is Ribulose-1,5-bisphosphate (RuBP), and the enzyme responsible for fixation is RuBisCO. This mechanism spatially separates initial CO₂ fixation from the Calvin cycle. CO₂ is first fixed in the mesophyll cells and then transported to the bundle sheath cells for the Calvin cycle. FYI: C4 plants use two enzymes: PEP carboxylase (in mesophyll) and RuBisCO (in bundle sheath), which minimizes photorespiration. These are the primary photosynthetic cells located in the leaf interior, where the initial capture and concentration of atmospheric CO₂ occurs in both C3 and C4 plants. FYI: In C4 plants, the mesophyll cell is the site where CO₂ is initially fixed by PEP carboxylase. These are the specialized cells surrounding the vascular bundles (veins) in the leaf. In C4 plants, they are the site where the concentrated CO₂ is released and the Calvin cycle takes place. FYI: The bundle sheath cell acts as a CO₂ pump in C4 plants, concentrating CO₂ around RuBisCO to suppress photorespiration. This cycle occurs in the stroma of the chloroplast and uses the ATP and NADPH generated during the light reactions to fix CO₂ into stable organic molecules (G3P). It is the primary pathway for sugar synthesis. FYI: The Calvin cycle has three phases: CO₂ fixation (using RuBP), reduction (using NADPH), and regeneration (of RuBP). This wasteful process occurs when RuBisCO fixes O₂ instead of CO₂ onto RuBP, consuming energy (ATP) and releasing CO₂. It is prevalent in C3 plants under high temperature and low CO₂ conditions. FYI: Photorespiration reduces the efficiency of photosynthesis because it consumes energy (ATP) and releases previously fixed CO₂. This enzyme catalyzes the initial fixation of atmospheric CO₂ onto Phosphoenolpyruvate (PEP). It has a high affinity for CO₂ and does not react with O₂, making it key to C4 photosynthesis. FYI: PEP carboxylase is highly efficient at low CO₂ concentrations and is the enzyme responsible for the first fixation step in C4 plants. A four-carbon dicarboxylic acid that serves as a key intermediate in the Citric Acid Cycle (Krebs Cycle). It condenses with acetyl-CoA to form citrate. FYI: OAA is regenerated at the end of the Krebs cycle, allowing the cycle to continue, and it is crucial for gluconeogenesis. Malate is a four-carbon organic acid that acts as a shuttle molecule in C4 plants. It transports the fixed CO₂ from the mesophyll cell to the bundle sheath cell. FYI: The movement of malate (or aspartate) between mesophyll and bundle sheath cells is the defining feature of the C4 mechanism.