## 2 Molecules, Properties, and Sources  
### Rosmarinic Acid  
The plant kingdom is the source of various phytochemicals that are used in a wide range of applications, from food preservatives to pharmaceutical and cosmetic industries [20]. A broad class of phytochemicals are antioxidants defined as compounds that inhibit or delay the oxidation of a target molecule [21]. Rosmarinic acid (RA) is a typical active antioxidant constituent and in the last 25 years, the RA attracted a large amount of research due to its presence in many spices and its use in food preservation. RA is extensively distributed in the plant kingdom (26 families); it accumulates in many official plants, belonging to the _Lamiaceae_ family (and their subfamily _Nepeotoidae_) and _Boraginaceae_, but it is also found in others such as _Blechnaceae_, Bryophytes (horn-worts), and other species of dico and monotypedonous plants, part of the sea grass family _Zosteraceae_[22; 23]. RA is stored in the vacuoles of plant cells; the chemical structure is an ester of caffeic acid and 3,4-hydroxyphenillactic acid and is one of the most frequently occurring caffeic acid ester in plants besides chlorogenic acid (Figure 2). Its name originated from the plant _Rosmarinus officinalis_ L., from which RA was initially isolated [24]. The biosynthetic pathway has been first elucidated in _Coleus blumei_ cell cultures [25] and the eight enzymes involved in the chemical pathway were deeply described in several _Lamiaceae_ species. RA biosynthesis follows two pathways: a phenylpropanol branch, that drives to the caffeic acid moiety, and a tyrosine-derived branch that leads to the dihydroxyphenylactic acid (DHPL) [26] (Figure 3). In the phenylpropanol pathway, L-phenylalanine is transformed to 4-coumaroylCoA by coupling the reactions catalyzed by phenylalanine ammonia lyase (PAL), cinnamic acid 4-hydroxylase (C4H), and 4-coumamate CoA ligase (4 CL). For the tyrosine-derived moiety, the first step is the transmination of L-tyrosine by 2-oxoglutarate as a second substrate to form 4-hydroxyphenylpyruvate by tyrosine aminotransferase (TAT). Then, 4-hydroxyphenylpyruvate is transformed by hydroxyphenylpyruvate reductase (HPPR) to 4-hydroxyphenyllactate [27]; this product is then coupled with 4-coumaroyl CoA to form an intermediate by the enzyme rosmarinic acid synthase (RAS), whereby the aromatic 3-and 3'-hydroxyl positions are hydroxylated via cytochrome P450 to form the RA molecule [20]. Figure 1: Scheme followed by the present review to discuss rosmarinic acid and ulvan potentialities in bionanosystems and final biobased products. RA is constitutively accumulated in the aerial parts of rosemary, sage, thyme, spearmint, lemon balm and lavender with different yield according to the vegetative or reproductive phase (reaching up to 11 mg g\({}^{-1}\) of dry weight, [28]). However, plant cell cultures are an alternative and efficient source to produce this biologically important metabolite; the concentration of RA is thus enhanced and can reach up to 36% of dry weight (_S. officinalis_, [29]), much higher than in the plant itself. Other good sources of RA are hairy root cultures, e.g., from _Oximum busilium_ (14.1% DW; [30]). RA is noted for its high antioxidant capacity, principally due to the high redox potential of its four phenolic hydrogens that support the ability to modulate free radical scavenging activity. A complementary support is given by the two catechol moieties that provide the suitable polarity for RA to penetrate the lipid bilayers and protect them against oxidation [31]. However, the biological activities of plant extracts containing RA have received considerable attention, and scientific reports have explored the various properties. As antioxidant it is used in several applications within the food industry as a result of its role in scavenging free radicals and ability to chelate pro-oxidant ions, and thus prevent lipid peroxidation [32].