Biosynthesis of plant oils

Plants produce starch from glucose to store energy for future requirements. Plants and animals have a second energy-storage mechanism, synthesis of fats and oils. Plants may have evolved two mechanisms for energy storage because of differences in energy density. Heat of combustion values for starch are –17.5 MJ/kg or –26.2 MJ/l. Comparable values for peanut oil are –33.7 MJ/kg and –33.5 MJ/l. In the main body of the living plant, mass or volume are not usually crucial parameters, and storage of energy as starch, with relatively low energy density, does not become an issue. In storing energy for future generations, the amount of energy that can be packed into the limited volume available in nuts or seeds is critical. Here oils provide a significant advantage relative to starch. Fats and oils represent a subdivision of the broad category of biologically important compounds called lipids. Lipids are characterized as being insoluble in water, but generally soluble in common organic solvents, such as chloroform or diethyl ether. The family of lipids includes many diverse kinds of compound, including beeswax, cholesterol, and oil of turpentine.

Fats and oils are esters. In the special case of fats and oils, the alcohol fragment is 1,2,3-propanetriol, almost always known by its common name glycerol, or by the even older common name, glycerine. The acid fragments are long-chain aliphatic acids, collectively called fatty acids. These acids could be almost any aliphatic acid larger than butyric acid, but in biologically important materials they typically contain 12 or more carbon atoms. Esters of fatty acids with glycerol are called glycerides. Three families of glycerides exist, depending on whether one, two, or all three of the hydroxyl groups in glycerol have been esterified. These families would be known, respectively, as mono-, di-, or tri-glycerides. In plants, the overwhelmingly dominant form is the triglyceride [A]. Mono- and di-glycerides are important only in the digestion of fats. Simple triglycerides are ones in which all three fatty acid segments are identical; in mixed triglycerides, the –OH positions in the glycerol are esterified by different fatty acids. Mixed triglycerides dominate among plant products. Triglycerides solid at 20 °C are classified as fats; those liquid at this temperature are oils. Generally, fats contain saturated fatty acid chains, while oils contain unsaturated ones. Animals usually store energy as fats, while plants store energy as oils.