In photosynthesis; is first fixed into acid i.e., oxaloacetate in mesophyll cells. It then forms other 4-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, which are then transported to bundle sheath cells, where it undergoes decarboxylation to release and a 3-carbon molecule. This is used in the Calvin cycle within bundle sheath cells, followed by the regeneration of the acceptor molecule in the mesophyll cells.

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