All the aliphatic components of plant waxes are synthesized in the epidermal cells from saturated very long-chain fatty acids (commonly C20–C34). and. Plant waxes are complex mixtures of hydrocarbons, alcohols, aldehydes, ketones , esters, acids, and combinations of these that are deposited in a layer outside. Plants secrete waxes into and on the surface of their cuticles as a way to control evaporation, wettability and hydration.
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The polar heads of the component molecules form the two faces of the lamella, while the hydrophobic moieties form its interior. Each lamella is thus two molecules in thickness, with the long axis of the component molecules perpendicular to the plane of the bilayer. Other types of aggregates are also formed in water by certain amphipathic lipids. For example, liposomes are artificial collections of lipids arranged in a bilayer, having an inside and an outside surface. The lipid bilayers form a sphere that can trap a molecule inside.
The liposome structure can be useful for protecting sensitive molecules that are to be delivered orally. Lipids of this class are the most abundant in biological membranes. In glycerophospholipids, fatty acids are linked through an ester oxygen to carbons 1 and 2 of glycerol , the backbone of the molecule. Phosphate is ester-linked to carbon 3, while any one of several possible substituents is also linked to the phosphate moiety. Glycerophospholipids are amphipathic—glycerol and phosphate form the polar end of the molecule, while hydrocarbon chains form the nonpolar end.
Although the fatty acids can be any of those common in biological systems, usually those attached to carbon 1 are saturated and those attached to carbon 2 are unsaturated. The various combinations of two fatty acids give rise to many different molecules bearing the same substituent group. Since this is true for each head group, there are altogether about a thousand possible types of glycerophospholipids.
The great majority are found in biological membranes. From the standpoint of physical properties, the greatest difference among various molecules lies in the particular substituent. This is due in part to the different sizes of the various types and in part to differences in their electric charges. The phosphatidylcholines and phosphatidylethanolamines are zwitterionic, meaning they have one negative and one positive charge on the substituent group. Phosphatidic acid, phosphatidylserine, and phosphatidylinositol have a net negative charge.
Differences in fatty acid composition also contribute to differences in physical properties of a series of molecules with the same substituent. In the presence of an excess of water, the molecules form aggregates with a variety of geometries, the most common of which is the bilayer.
In bilayers many glycerophospholipids as well as sphingomyelin discussed below can be in either one of two states, gel or liquid-crystalline. In the solidlike gel state, the lipid molecules in each half of the bilayer are arranged in a two-dimensional lattice, with their two acyl chains in the extended form.
With the application of heat, the gel converts into the liquid-crystalline state at some temperature characteristic of the lipid mixture. In this state the molecules in each half of the bilayer remain in a fairly regular two-dimensional lattice but are free to rotate about their long axes and slide laterally through the layer. Their acyl chains now undergo considerable motion, leading to transiently kinked conformations. These motions give the bilayer a quasi-liquid behaviour that is characteristic of the bilayers in all biological membranes.
A second major class of lipids usually associated with the membranes surrounding cells is sphingolipids. Sphingolipids are based on an carbon amine alcohol, sphingosine, and to a much lesser extent on a carbon analog , phytosphingosine. All but one generic member of this class have a simple or complex sugar linked to the alcohol on carbon 1. The single deviant member is sphingomyelin, a molecule with a phosphorylcholine group the same polar head group as in phosphatidylcholine instead of the sugar moiety, making it an analog of phosphatidylcholine.
All sphingolipids have, in addition to the sugar, a fatty acid attached to the amino group of sphingosine. Among the sphingolipids, only sphingomyelin, a phospholipid , is a major component of biological membranes.
The principal factor determining the physical properties of sphingolipids is the substituent group attached to carbon 1 of sphingosine. Minor variations in properties depend upon the particular fatty acid component.
The glycosphingolipids, all containing a sugar attached to carbon 1 of sphingosine, have physical properties that depend primarily on the complexity and composition of this substituent. Two generic types of glycosphingolipids are recognized: Many hundreds of different glycosphingolipids have been isolated, and many more unidentified types probably exist.
Glycosphingolipids are found exclusively on the external surface of the cell membrane, where their sugar moieties often act as antigens and as receptors for hormones and other signaling molecules. Cholesterol may be the most intensely studied small molecule of biological origin. Not only are its complex biosynthetic pathway and the physiologically important products derived from it of scientific interest, but also the strong correlation in humans between high blood cholesterol levels and the incidence of heart attack and stroke diseases that are leading causes of death worldwide is of paramount medical importance.
The study of this molecule and its biological origin have resulted in more than a dozen Nobel Prizes. Cholesterol is a prominent member of a large class of lipids called isoprenoids that are widely distributed in nature. The class name derives from the fact that these molecules are formed by chemical condensation of a simple five-carbon molecule, isoprene.
Isoprenoids encompass diverse biological molecules such as steroid hormones , sterols cholesterol, ergosterol , and sitosterol , bile acids, the lipid-soluble vitamins A, D, E, and K , phytol a lipid component of the photosynthetic pigment chlorophyll , the insect juvenile hormones, plant hormones gibberellins , and polyisoprene the major component of natural rubber. Many other biologically important isoprenoids play more-subtle roles in biology.
The sterols are major components of biological membranes in eukaryotes organisms whose cells have a nucleus but are rare in prokaryotes cells without a nucleus, such as bacteria. Cholesterol is the principal sterol of animals, whereas the major sterol in fungi is ergosterol and that in plants is sitosterol. The characteristic feature of each of these three important molecules is four rigidly fused carbon rings forming the steroid nucleus and a hydroxyl OH group attached to the first ring.
One molecule is distinguished from another by the positions of the carbon-carbon double bonds and by the structure of the hydrocarbon side chain on the fourth ring. Cholesterol and its relatives are hydrophobic molecules with exceedingly low water solubility. The overall hydrophobicity is negligibly affected by the hydrophilic OH group. The structure of cholesterol is such that it does not form aggregates in water, although it does shoehorn between the molecules of biological membranes, with its OH group located at the water-membrane interface.
Carnauba wax has a shiny finish, so it is often used in polishes for cars, shoes, floors and furniture. Soy wax comes from soybean oil. After the beans are harvested, they are cleaned, cracked and rolled into flakes. The oil is then extracted from the flakes. Candlemakers commonly use soy wax to make unique candle creations. Paraffin wax is another common candle making wax, but it comes from petroleum. Some candle makers choose to use soy wax because it is completely renewable.
Jojoba wax is harvested from jojoba plant seeds. Jojoba wax is commonly referred to as jojoba oil, even though its chemical makeup makes it a wax not an oil. This wax is used in many beauty products due to its protective and moisturizing properties. Candelilla wax comes from the small leaves of Candelilla shrubs native to northern Mexico and the southwestern U. Journal of Biological Chemistry Journal of Applied Botany — Angewandte Botanik Plant Cell and Physiology Philosophical Transactions of the Royal Society A Kolattukudy PE Biosynthesis of paraffins in Brassica oleracea: Kolattukudy PE Species specificity in the biosynthesis of branched paraffins in leaves.
Kunst L and Samuels L Plant cuticles shine: Current Opinion in Plant Biology Plant Cell Reporter Last R ed The Arabidopsis Book, pp. American Society of Plant Biologists Journal of Experimental Botany Nantes, France, September 2—4. Dimerize in different combinations. Plant Cell Physiology Mikkelsen J The effects of inhibitors on the biosynthesis of the long chain lipids with even carbon numbers in barley spike epicuticular wax.
Carlsberg Research Communications Applied and Environmental Microbiology Journal of Experimental Botany. Journal of Molecular Biology Canadian Journal of Botany Journal of the American Chemical Society Wen M and Jetter R Composition of secondary alcohols, ketones, alkanediols, and ketols in Arabidopsis thaliana cuticular waxes. Hamilton RJ ed Waxes: Chemistry, Molecular Biology and Functions, pp.
The use of plant waxes as templates for micro- and nanopatterning of surfaces.
Waxes, found primarily in the cuticle of vascular plants, prevent uncontrolled water loss. They comprise a diverse mixture of aliphatics, triterpenoids, flavonoids. The range of lipid types in plant waxes is highly variable, both in nature and in composition, and Table 1 illustrates some of this diversity in the main components. PDF | Waxes, found primarily in the cuticle of vascular plants, prevent uncontrolled water loss. They comprise a diverse mixture of aliphatics.