Lipids are a diverse group of compounds sharing the common property of being hydrophobic (insoluble in water). Lipids include fatty acids, fats, oils, steroids (sterols), waxes, cutin, suberin, glycerophospholipids (phospholipids), glyceroglycolipids (glycosylglycerides), terpenes, and tochopherols.
Lipids are ubiquitous in plants, serving many important functions, including storage of metabolic energy, protection against dehydration and pathogens, the carrying of electrons, and the absorption of light. Lipids also contribute to the structure of membranes. In addition, plant lipids are agricultural commodities important to the food, medical, and manufacturing industries.
Fatty acids, the simplest of the lipids, are highly reduced compounds with a hydrophilic (watersoluble) carboxylic acid group and a hydrophobic hydrocarbon chain. Hundreds of different fatty acids have been isolated from plants.
Fatty acids differ from one another in the length of the hydrocarbon chain and degree of saturation (number of carbon-carbon double bonds). The most common fatty acids have chain lengths ranging from sixteen to twenty carbon atoms, but many less common fatty acids are longer or shorter.
Saturated fatty acids have no double bonds, whereas unsaturated fatty acids have one or more double bonds. Naturally occurring unsaturated fatty acids have cis double bonds, in which the two hydrogen atoms bonded to the carbon atoms of the double bond are on the same side of the fatty acid molecule. Fatty acids with one double bond and two or more double bonds are referred to as monounsaturated, diunsaturated, and polyunsaturated, respectively.
Fats and Oils
Fatty acids rarely occur free in the cell. Instead they are attached by ester linkages to glycerol, a three-carbon sugar alcohol, to form fats and oils, the most abundant lipids.
Glycerol molecules with one, two, and three fatty acids attached are referred to as monoglycerides, diglycerides, and triglycerides (often called triacylglycerols), respectively. The fatty acids attached to a triglyceridemay be the same, in which case it is referred to as a simple triglyceride, or the fatty acids may be different, in which case it is referred to as a mixed triglyceride.
The degree of saturation and hydrocarbon chain length of fatty acids in triglycerides affects their melting point. Common fats, such as those from palms and coconuts, are triglycerides that contain a high proportion of saturated fatty acids and are solid at room temperature (22 degrees Celsius).
Common oils, such as those from corn, peanuts, soybeans, sunflowers, and olives, are triglycerides that contain a high proportion of unsaturated fatty acids and are liquid at room temperature.
Because plants cannot control their temperatures, they contain much more oil than fat so their membranes will be fluid at ambient temperatures. The most common mono unsaturated fatty acid is oleic acid, while the most common polyunsaturated fatty acids are linoleic acid and linolenic acid.
Some plants have a high proportion of saturated fats, containing such fatty acids as palmitic acid, the most common saturated fatty acid found in plants. Plants also contain lesser amounts of other saturated fatty acids, such as lauric and myristic acid. Phytanic acid, a product of chlorophyll metabolism, is a saturated, branched chain fatty acid.
Plant oils are a mixture of triglycerides and are used as a storage form of energy in seeds. Because fats are more highly reduced than starch, they provide almost twice the energy on a per-weight basis. When fatty acids are removed from glycerol they can undergo oxidation to yield energy.
Waxes, Cutin, and Suberin
Waxes are long-chain fatty acids attached to long-chain alcohols by ester linkages. Cutin is a complex of hydroxylated fatty acids (fatty acids with hydroxyl groups attached to them) cross-linked to one another.
Waxes and cutin are found in the cuticle, the outermost layer of plant surfaces exposed to the air, and provide protection from dehydration and pathogens. Suberin is a complex compound of unknown structure. It is the major component of the walls of cork cells, the outer most layer of bark. Like cutin and waxes, suberin provides protection from dehydration and pathogens.
Glycerophospholipids, or phosphoglycerides, contribute substantially to cellular plasma membranes. Thus, they form one of the most important classes of lipids. Glycerophospholipids are composed of glycerol phosphate to which is attached two fatty acids by ester linkages. Molecules, such as ethanol-amine, choline, inositol, and serine, may be bonded to the phosphate, resulting in an even greater variety of glycerophospholipids.
Known as glyceroglycolipids, or glycosylglycerides, these lipids are primarily found in chloroplast membranes, are widespread in plants, and consist of glycerol to which is attached one or two sugar molecules and two fatty acids. The sugars attached to the glycerol are either glucose, galactose, or a digalactose unit.
Steroids, also called sterols, comprise another class of lipids that are important constituents of the plant plasma membrane. They also function as plant hormones. Steroids are formed by a conjugated ring system.
Side chains and groups attached to the rings result in a variety of steroids with many biological activities. Stigmasterol, beta-sitosterol, lanosterol, and ergosterol are plant steroids. Cholesterol, a common steroid in the plasma membranes of animal cells, is rarely present in plants.
Terpenes are lipids that are composed of two or more five-carbon isoprene units. More than twenty-two thousand terpenes have been described. The familiar flavor and aroma of many plants are due to their characteristic terpenes.
Plant terpenes and terpenoid derivatives include phytol, a constituent of chlorophyll; beta-carotene, a photosynthetic pigment that is a precursor of vitamin A in animals; paclitaxel, an anticancer agent; and rubber. The blue haze often seen in the air on summer afternoons is due, in part, to terpenes emitted from leaves.
Tocopherols contain an aromatic ring and a long isoprene side chain. Plant tocopherols include vitamin E, a biological antioxidant that protects unsaturated fatty acids from damage from free radical attack; vitamin K, which plays an essential blood clotting role in higher animals; and ubiquinone and plastoquinone, which are essential electron carriers in the reactions leading to the synthesis of adenosine triphosphate (ATP).