Waxes are similar to fats except that waxes are composed of only one long-chain fatty acid bonded to a long-chain alcohol group attached. Because of their long, nonpolar carbon chains, waxes are extremely hydrophobic meaning they lack an affinity for water. Both plants and animals use this waterproofing characteristic as part of their composition.
Plants most noticeably use waxes for a thin protective covering of stems and leaves to prevent water loss. Similarly, animals employ waxes for protective purposes; for instance, earwax in humans prevents foreign material from entering and possibly injuring the ear canal area. Phospholipids are similar to fats except they have two fatty acid chains bonded to a glycerol plus they contain the element phosphorus.
Phospholipids are unique because they have a hydrophobic and a hydrophilic water-soluble end. Phospholipids are biologically important because they are the main structural components of cell membranes. The cell membrane is called a phospholipid bilayer because it consists of two phospholipid layers oriented so that the hydrophyllic?
Therefore, water and other cellular fluids are contained. The hydrophobic ends for both molecules face each other on the inside and allow for passage of acceptable, and some objectionable, materials through the cell membrane.
Steroids are structurally different from the other lipids. The carbon skeleton of steroids is bent to form four fused rings that do not contain fatty acids. The most common steroid, cholesterol, is needed to make both the male testosterone and female estrogen sex hormones, and it is a component of cell membranes and is needed for the proper function of nerve cells.
Excessive amounts of cholesterol, however, have been linked to heart disease. Another popular steroid group is the anabolic steroids that are man-made and mimic the effect of the male hormone, testosterone. The presence of unsaturation or double bonds in fatty acids is represented by denoting the number of carbons of the molecule followed by an indication of the number of double bonds, thus: C corresponds to a fatty acid of 18 carbons and one unsaturation, it will be a MUFA.
C 4 correspond to a molecule having 20 carbons and four double bonds, being a PUFA. However, in the cell the metabolic utilization of fatty acids occurs by the successive scission of two carbon atoms from the C1 to the Cn mitochondrial or peroxisomal beta oxidation.
This means that as the fatty acid is being metabolized oxidized in beta position , the number of each carbon atom will change, creating a problem for the identification of the metabolic products formed as the oxidation progress. For this reason R. Holman, in , proposed a new type of notation that is now widely used for the biochemical and nutritional identification of fatty acids [ 24 ].
This nomenclature lists the carbon enumeration from the other extreme of the fatty acid molecule. The latter notation is the most often used in nutrition and refers to the last letter of the Greek alphabet [ 25 ]. What happens with fatty acids having more than a double bond?
Double bonds are not randomly arranged in the fatty acid structure. Nature has been "ordained" as largely incorporate them in well-defined positions. This particular structural disposition of double bonds, i. Oleic acid is highly abundant both in vegetable and animal tissues. ALA is a less abundant fatty acid, almost exclusively present in the vegetable kingdom and specifically in land-based plants [ 31 ].
The increase of double bonds in fatty acids significantly reduces its melting point. Thus, for a structure of the same number of carbon atoms, if it is saturated may give rise to a solid or semisolid product at room temperature, but if the same structure is unsaturated, may originate a liquid or less solid product at room temperature. Different fatty acids, showing the C nomenclature, their systematic name, their common name and the respective melting point.
The structural organization of fatty acids in food and in the body is mainly determined by the binding to glycerol by ester linkages. The reaction of a hydroxyl group of glycerol, at any of its three groups, with a fatty acid gives rise to a monoacylglyceride. The linking of a second fatty acid, which may be similar or different from the existing fatty acid, gives rise to a diacylglyceride.
If all three hydroxyl groups of glycerol are linked by fatty acids, then this will be a triacylglyceride [ 33 ]. Monoacylglycerides, by having free hydroxyl groups two are relatively polar and therefore partially soluble in water.
Different monoacylglycerides linked to fatty acids of different lengths are used as emulsifiers in the food and pharmaceutical industry [ 34 ]. The less polar diacylglycerides which have only one free hydroxyl group are less polar than monoacylglycerides and less soluble in water. Finally, triacylglycerides, which lack of free hydroxyl groups are completely non-polar, but highly soluble in non-polar solvents, which are frequently used for their extraction from vegetable or animal tissues, because constitutes the energy reserve in these tissues [ 35 ].
Diacylglycerides and monoacylglycerides are important intermediates in the digestive and absorption process of fats and oils in animals. In turn, some of these molecules also perform other metabolic functions, such as diacylglycerides which may act as "second messengers" at the intracellular level and are also part of the composition of a new generation of oils nutritionally designed as "low calorie oils" [ 36 ].
When glycerol forms mono-, di-, or triacylglycerides, its carbon atoms are not chemically and structurally equivalent. This spatial structure or conformation of mono-, di- and triacyglycerides is relevant in the digestive process of fats and oils ref. Figure 2 shows the structure of a monoacylglceride, a diacylglyceride and a triacylglyceride, specifying the "sn-" notation.
Structure of a monoacylglyceride, a diacylglyceride and a triacylglyceride, specifying the "sn-" notation. The capability of an organism to metabolically introduce double bonds in certain positions of a fatty acid or the inability to do this, determines the existence of the so-called non-essential or essential fatty acids EFAs.
According to the distribution of double bonds in a fatty acid and to its spatial structure, unsaturated fatty acids may have two types of isomerism: geometrical isomerism and positional isomerism.
By isomerism it is referred to the existence two or more molecules having the same structural elements atoms , the same chemical formula and combined in equal proportions, but having a different position or spatial distribution of some atoms in the molecule [ 40 ].
Carbon atoms forming the structure of the fatty acids possess a three-dimensional spatial structure which forms a perfect tetrahedron. However, when two carbons having tetrahedral structure are joined together through a double bond, the spatial conformation of the double bond is modified adopting a flat or plane structure [ 41 ].
Figure 4 shows the cis — trans geometric isomerism of fatty acids. The cis or trans isomerism of fatty acids confers them very different physical properties, being the melting point one of the most relevant [ 43 ]. Table 2 shows the melting point of various cis — trans geometric isomers of different fatty acids. It can be observed substantial differences in the melting point of cis - or trans isomers for the same fatty acid.
Melting point differences bring to the geometrical isomers of a fatty acid very different biochemical and nutritional behavior. Fatty acids having trans isomerism, especially those of technological origin such as generated during the partial hydrogenation of oils , have adverse effect on humans, particularly referred to the risk of cardiovascular diseases [ 44 ].
It is noteworthy that the majority of naturally occurring fatty acids have cis isomerism, although thermodynamically is more stable the trans than the cis isomerism, whereby under certain technological manipulations, such as the application of high temperature frying process or during the hydrogenation process applied for the manufacture of shortenings, cis isomers are easily transformed into trans isomers [ 45 ].
Geometric isomerism of fatty acids. Positional isomerism refers to the different positions that can occupy one or more double bonds in the structure of a fatty acid.
Changes in the melting point of various cis — trans geometrical isomers of different fatty acids. In general, all fatty acids naturally present positional isomerism of their more frequent molecular structure. However, these isomers occur in very low concentrations. Such as geometrical isomerism, the technological manipulation of fatty acids i. Figure 5 summarizes the positional and geometric isomers of unsaturated fatty acids. Ernest Z. Jun 10, See below. Explanation: All lipids contain carbon, hydrogen, and oxygen.
The four main classes of lipids are fats, waxes, sterols, and phospholipids. Fats Fats are triglycerides. Waxes Waxes are organic compounds that usually consist of long hydrocarbon chains. Sterols Sterols are derivatives of cholesterol. They all have the basic structure Examples are desmosterol, lathosterol. Fats, waxes, and sterols contain only carbon, hydrogen, and oxygen.
Phospholipids Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline. A typical phospholipid is phosphatidylcholine.
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