**2. Vitamin C: chemical structure and synthesis**

Chemically known as ascorbic acid (C6H8O6), vitamin C is a six-carbon lactone synthesized from glucose by many animals [9]. Abundantly present in many fruits and vegetables, vitamin C is essential for robust immune function. It is found in a reduced form as ascorbic acid and in an oxidized form as dehydroascorbic acid [10]. According to the International Union of Pure and Applied Chemistry (IUPAC), the systematic name is "2,3-enediol-L-gluconic acid-β-lactone" [11].

While plants and some animals, mainly in their livers, can synthesize ascorbic acid, humans lack the ability to produce this vitamin internally. Consequently, humans must acquire this vitamin through their diet. Many animals and plants, on the other hand, can synthesize vitamin C using a pathway involving glucuronic acid derived from D-glucose or D-galactose. In certain animals, the synthesis of ascorbic acid occurs in their liver cells (**Figure 1**) [13]. Similarly, since humans also lack the ability to synthesize vitamin C, it is advisable to supplement with synthetic vitamin C in cases of inadequate or unbalanced nutrition, or obtain it naturally. Most mammals,

**Figure 1.** *L-ascorbic acid [12].*

*Understanding Vitamin C: Comprehensive Examination of Its Biological Significance… DOI: http://dx.doi.org/10.5772/intechopen.114122*

including mice, are capable of synthesizing vitamin C from glucose, while humans and other primates meet their daily requirements for it through their diet because they lack the L-gulono-g-lactone oxidase enzyme involved in vitamin C synthesis [14–16].

Vitamin C is transported into cells in its oxidized form, dehydroascorbic acid (DHA), through glucose facilitative transporters (GLUT), and as ascorbic acid via sodium-dependent vitamin C transporters [14, 17, 18]. Specifically, sodiumdependent vitamin C transporters (SVCT) encoded by the SLC23 family consist of two sodium-dependent transporters, SVCT1 and SVCT2. Both SVCT1 and SVCT2 transport ascorbic acid in a sodium-dependent manner with high affinity. SVCT1 is responsible for the absorption of dietary vitamin C along the apical membrane of enterocytes, while SVCT2 plays a role in providing vitamin C to cells for metal ion-dependent enzymatic reactions and protecting cells from oxidative stress [18–21]. In a study, it was reported that vitamin C transport does not occur in the absence of calcium (Ca) and magnesium (Mg) minerals, even in the presence of sodium, and SVCT2 becomes inactive under such conditions [22].
