FIG. 5.

The ascorbate transporters and ascorbate fluxes of mitochondria and chloroplast. The ultimate step of ascorbate biosynthesis is localized in the mitochondria. Hence, ascorbate must be transported into the other compartments of the plant cell. The chloroplast envelope contains a low affinity (K_m∼20 mM) ascorbate translocator, which mediates the facilitated diffusion of ascorbate from the cytosol to the stroma. The thylakoid membranes appear to have no transport system for ascorbate. This system is supported by the recently determined intracellular distribution of ascorbate. In the investigated plants, highest levels of ascorbate specific labeling were found in the cytosol (21.7 mM), whereas mitochondria (10.4 mM) and chloroplasts (10.8 mM) contained intermediate levels. In concordance with the transport experiments within chloroplasts and mitochondria, ascorbate was only detected in the stroma and matrix, respectively, but not within the lumen of thylakoids or cristae (58). To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars