Many components of resin are referred to as oils. For example, mono- and sesquiterpenes are called essential oils, cupressaceous resins with large quantities of sesquiterpenes are called cedarwood oil, and some leguminous resins are known as copaiba oil (Chapter 7). Oils and fats (and waxes), however, are distinguished chemically from terpenes in being alcoholic esters of fatty acids. Oils and fats are formed by synthesis of fatty acids from carbohydrates, followed by the combination of these fatty acids through enzymatic action with glycerol to form esters (triglycerides). Fatty acids are long hydrocarbon chains that carry a terminal carboxyl group, giving them the characteristics of a weak acid. Glycerol forms a link with the carboxyl groups, serving as a carrier for fatty acids.
Plant fats differ only slightly from oils in having fatty acid constituents that are more or less solid rather than liquid at ordinary temperatures. Fatty acids do occur in some conifer resins. For example, fatty acids (predominantly C18 oleic and linoleic) are co-isolated with rosin as by-products from pine resins in the kraft pulping process (Chapter 7). They also occur in abundance in the resin of glandular trichomes on the leaves of some Australian shrubs (Chapter 3). The utility of oils and fats to the plant is a bit obscure, although those in the seed constitute an abundant reservoir of stored energy for early growth of the seedling.
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