# Metabolism and Regulation of Ascorbic Acid in Fruits
## 2 Metabolic Pathway of Ascorbic Acid
### Ascorbic Acid Transport  
Although AsA is known to be synthesized in the mitochondrial inner membrane, AsA can also be detected in other subcellular structures, such as apoplast, chloroplast and vacuole, indicating the presence of AsA transmembrane transport [66; 67; 68] (Figure 2). At the outset, it is often assumed that AsA may be released from mitochondria into the cytosol via simple diffusion [69]. However, at physiological pH values, AsA has no membrane permeability [70]. Interestingly, it has been reported that mitochondrial ascorbic acid transporter from potato has unidirectional AsA transport activity, demonstrating the existence of transport proteins in mitochondria [71]. Currently, it is known that AsA is transferred from cytosol to chloroplast in a carrier-mediated manner [69]. In plants, AtPHT4;4 is the first chloroplast-localized ascorbate transporter identified from _Arabidopsis_[72]. In addition, some MDHAR and DHAR have also been shown to localize at chloroplast, suggesting that DHA can be taken up by chloroplast [73; 74]. However, the mechanism of DHA uptake is still unknown. AsA utilizes concentration and pH gradients to enter the vacuole and thylakoid by passive diffusion [75]. Due to the missing AsA regenerated enzymes in the apoplast, AsA transport between protoplast and apoplast is necessary [66]. In mammals, AsA and DHA crossing the plasma membrane are mediated by Na-dependent transporters and glucose transporters [76; 77]. However, the carriers that help AsA and DHA across the plasma membrane have not yet been identified [3]. Some nucleobase/ascorbate transporter (NAT) homologous genes have been identified and have functional redundancy according to the phenotype of multiple mutants. However, the regulation mechanism of _NAT_ genes on AsA transmembrane transport is still unclear [78]. Furthermore, MDHA can reduce to AsA by accepting electrons from cytochrome b in the apoplast. MDHA can reduce to AsA by cytochrome b, which provides electrons from protoplast AsA oxidation [79].  
AsA biosynthesis occurs in various tissues of plants, but some evidence shows that AsA has long-distance transport between tissues. AsA biosynthesis-related enzymes can be detected in the phloem of _Cucurbita pepo_ fruits, indicating that AsA may be transported to other tissues through the phloem [80]. By labeling AsA with C-14, [PERSON] and [PERSON] find that AsA can be transported from leaf to flower and root tip [81]. In addition, the AsA in the phloem and tubers of potato will be enriched with the increase in the AsA content in the leaves [82]. The inter-tissue transport of AsA still needs more evidence to support it.  
Figure 2: The AsA transport in plant cell. AsA is synthesized on the inner mitochondrial membrane and transported into the cytoplasm. AsA in the cytoplasm can enter organelles, such as vacuoles, chloroplasts, and nuclei, through diffusion or carriers. In addition, AsA can also be transported outside the cell membrane by simple diffusion or transport proteins. DHA in the apoplast can also enter the cell membrane and participate in the regeneration of AsA. Arrows indicate the direction of material transport. NAT, nucleobase/ascorbate transporter; Cytb, cytochrome b.