Aloe vera is a perennial succulent xerophyte;
…it has elongated and pointed leaves that are joined at the stem in a rosette pattern and that grow to about 30–50 cm in length and 10 cm in breadth at the base in the adult plant. The leaf is protected by a thick, green epidermis layer (skin or rind), which surrounds the mesophyll. Immediately beneath the rind are located the vascular bundles, which are composed of three types of tubular structures: the xylem (transports water and minerals from roots to leaves), the phloem (transports starch and other synthesized products to the roots), and the large pericyclic tubules (contains the yellow leaf exudate commonly referred to as “aloes,” “sap,” or “latex”). The pericyclic portion of the vascular bundle is adherent to the rind, whereas the remainder of the vascular bundle protrudes into the mesophyll layer. The mesophyll can be differentiated into chlorenchyma cells and thinner-walled parenchyma cells. The parenchyma (filet or pulp), which is the major part of the leaf by volume, contains a clear mucilaginous gel (known as Aloe vera gel).
Aloe vera is considered to be the most biologically active of the Aloe species. More than 75 potentially active constituents have been identified in the plant including vitamins, minerals, saccharides, amino acids, anthraquinones, enzymes, lignin, saponins, and salicylic acids. The leaf exudate contains anthraquinones, particularly barbaloin, which appear to be responsible for its bitter taste and cathartic effect. Barbaloin and other products of the phenylpropanoid pathway are commonly referred to as polyphenolic compounds. These are derived from the precursor phenolic acids, and they may act as antioxidants to inhibit free radical–mediated cytotoxicity and lipid peroxidation. Aloe vera also contains products of the isoprenoid pathway, including carotenoids, steroids, terpenes, and phytosterols. Isoprenoids can be regarded as sensory molecules because they contribute to the color and fragrance of the products in which they exist.
Aloe vera gel is rich in polysaccharides, including acemannan (partially acetylated glucomannans), which has been reported as the primary active substance in the parenchyma. However, given the number of other potentially active compounds in the plant, it is possible that the biological activities of Aloe vera result from the synergistic action of a variety of compounds, rather than from a single defined component. Equally, the potential for constituents to exhibit antagonistic and competitive activities also influences the overall biological activity of particular Aloe vera preparations.
(Note: This article is quoted by permission from Chapter 3: Evaluation of the Nutritional and Metabolic Effects of Aloe vera. In I. F. Wachtel-Galor. (Ed.), Herbal Medicine: Biomolecular and Clinical Aspects, 2011)