Transport of the metabolites that move between the two cell types occurs by diffusion through plasmo-desmata. The concentration gradient between the mesophyll and bundle sheath cells is sufficiently high to allow diffusion at a rate that readily sustains photosynthesis, with the exception of that of pyru-vate. How can we account for rapid transport of pyruvate from the bundle sheath to the mesophyll if there is no concentration gradient?
Uptake of pyruvate in the chloroplasts of the mesophyll cells is a light-dependent process, requiring a specific energy-dependent carrier. Active uptake of pyruvate into the chloroplast reduces the pyruvate concentration in the cytosol of these mesophyll cells to a low level, creating a concentration gradient that drives diffusion from the bundle sheath cells (Flügge et al. 1985).
In the chloroplasts of the mesophyll cells, pyru-vate is converted into PEP, which is exported to the cytosol in exchange for Pi. The same translocator that facilitates this transport is probably also used to export triose-phosphate in exchange for PGA. This translocator operates in the reverse direction in mesophyll and bundle sheath chloroplasts, in that PGA is imported and triose-phosphate is exported in the mesophyll chloroplasts, while the chloro-plasts in the bundle sheath export PGA and import triose phosphate.
The chloroplast envelope of the mesophyll cells also contains a translocator for the transport of dicar-boxylates (malate, oxaloacetate, aspartate, and glutamate). Transport of these carboxylates occurs by exchange. The uptake of oxaloacetate, in exchange for other dicarboxylates, is competitively inhibited by these other dicarboxylates, with the values for Ki being in the same range as those for Km. [Ki is the inhibitor (i.e., dicarboxylate) concentration at which the inhibition of the transport process is half that of the maximum inhibition by that inhibitor; Km is the substrate (oxaloacetate) concentration at which the transport process occurs at half the maximum rate.] Such a system does not allow rapid import of oxaloacetate. A special transport system, transporting oxa-loacetate without exchange against other dicarboxylates, takes care of rapid import of oxaloa-cetate into the mesophyll chloroplasts.
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