C4 Pathway: PEP and Bundle Sheath — NEET Biology
✅ Asked in NEET 2024Now study the pathway This pathway that has been named the Hatch and Slack Pathway, is again a cyclic process. Let us study the pathway by listing the steps. The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase. It is important to register that the mesophyll cells lack RuBisCO enzyme. The C4 acid OAA is formed in the mesophyll cells. It then forms other 4-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, which are transported to the bundle sheath cells. In the bundle sheath cells these C4 acids are broken down to release CO2 and a 3-carbon molecule. The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle. The CO2 released in the bundle sheath cells enters the C3 or the Calvin pathway, a pathway common to all plants. The bundle sheath cells are
The C4 pathway (Hatch-Slack) uses phosphoenol pyruvate (PEP) as the primary CO2 acceptor in mesophyll cells, not RuBP like in C3 plants. The enzyme PEPcase catalyzes this fixation to form OAA, which is converted to malic or aspartic acid and transported to bundle sheath cells where CO2 is released. The critical trap: students confuse which CO2 acceptor belongs to which pathway—PEP for C4 (mesophyll), RuBP for C3 (stroma). Remember: C4 plants have a spatial separation (mesophyll vs. bundle sheath) and temporal separation of carbon fixation and Calvin cycle, making them more efficient in hot, dry conditions. This concept tests understanding of photosynthetic adaptations.
This paragraph was tested 3 times in NEET.
(NEET 2024) Match List-I with List-II: List-I: A. C4 Pathway B. Light Reaction C. Photorespiration D. Calvin Cycle List-II: I. Water splitting (2H₂O → 4H⁺ + O₂ + 4e⁻) II. Ribulose 1,5-bisphosphate III. Phosphoenolpyruvate IV. Phosphoglycolate Choose the correct answer from the options given below:
Given below are two statements: (NEET 2022 Phase 1) Statement I: The primary CO₂ acceptor in C₄ plants is phosphoenolpyruvate and is found in the mesophyll cells. Statement II: Mesophyll cells of C₄ plants lack RuBisCO enzyme.
Phosphoenol pyruvate (PEP) is the primary CO₂ acceptor in: (NEET 2017)
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