C4 Photosynthesis: Bundle Sheath Cells and PEP Carboxylase
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C4 plants fix carbon dioxide twice: first with PEP carboxylase in mesophyll cells, then again by RuBisCO in bundle sheath cells.
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Corn's bundle sheath cells wrap each leaf vein, keeping local carbon dioxide high enough for RuBisCO to avoid oxygen.
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PEP carboxylase does not replace RuBisCO — it concentrates carbon dioxide so RuBisCO fixes it without binding oxygen.