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Carbon dioxide enters the C4 cycle in mesophyll cells. An intermediate of this cycle diffuses to the bundle-sheath cell, where it releases carbon dioxide. Carbon dioxide is used by bundle-sheath cells in the Calvin-Benson cycle. This process "pumps" carbon-dioxide to the bundle-sheath cells. Rubisco is found only in the bundle sheath cells, and not in the mesophyll cells, so this effectively eliminates the ability of Rubisco to use oxygen as a substrate (since there is so much more CO₂ present in the bundle sheath cells than oxygen).