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20.3 FATS AND LIPIDS
The term lipid applies to any water-insoluble substance which can be extracted from cells by organic solvents such as chloroform, ether, or benzene. Two major categories may be identified. Nonpolar lipids have molecular structures which contain no electrically charged sites, few polar groups, and large amounts of carbon and hydrogen. They are similar to hydrocarbons in being almost completely insoluble in water, and so they are said to be hydrophobic (from the Greek, meaning water-hater). On the other hand, polar lipids consist of molecules which have polar groups (such as —OH) or electrically charged sites at one end, and hydrocarbon chains at the other. Since polar or charged groups can hydrogen bond to or electrostatically attract water molecules, one end of a polar lipid molecule is said to be hydrophilic (water-loving). Some typical structures of both types of lipids are shown in Fig. 20.2.
Nonpolar Lipids
A good example of a nonpolar lipid is the neutral fat glycerol tristearate. This most-common form of animal fat serves as a storehouse for energy and as insulation against heat loss. On a molecular level it is constructed from three molecules of stearic acid and one of glycerol:
(20.1)
A great many nonpolar lipids can be made by combining different long- chain acids with glycerol. Because these acids were originally derived from fats, they are collectively referred to as fatty acids.
Notice that for each stearic or other fatty acid molecule which combines with one of the —OH groups of glycerol, a molecule of water is given off, and so the reaction is a condensation. It turns out that a great many important biological molecules are put together by condensation reactions during which water is given off. The reverse of Eq. (20.1), in which water reacts with a large molecule and splits it into smaller pieces, is called hydrolysis. By carrying out hydrolysis living organisms can break down molecules manufactured by other species. The simple building blocks obtained this way can then be recombined by condensation reactions to form structures appropriate to their new host.
By contrast with the glycerol tristearate found in animals, vegetable fats contain numerous double bonds in their long hydrocarbon chains. This polyunsaturation introduces “kinks” in the hydrocarbon chains because of the barrier to rotation and the 120° angles associated with the double bonds. Consequently it is more difficult to align the chains side by side (see Fig. 20.2), and the unsaturated fats do not pack together as easily in a crystal lattice. Most unsaturated fats (like corn oil) are liquids at ordinary temperatures, while saturated fats (like butter) are solids. Vegetable oils can be converted by hydrogenation to compounds which are solids. This process involves adding H2 catalytically to the double bonds:
Hydrolysis of fats [the reverse of Eq. (20.1)] is important in the manufacture of soaps. It can be speeded up by the addition of a strong base like NaOH or KOH, in which case the reaction is called saponification. Since saponification requires that the pH of the reaction mixture be high, the fatty acid that is produced will dissociate to its anion. When glycerol tristearate is saponified with NaOH for example, sodium stearate, a relatively water-soluble substance and a common soap, is formed.
The ability of soaps to clean grease and oil from soiled surfaces is a result of the dual hydrophobic-hydrophilic structures of their molecules. The stearate ion, for example, consists of a long nonpolar hydrocarbon chain
with a highly polar —COO– group at one end. The hydrophobic hydrocarbon chain tries to avoid contact with aqueous media, while the anionic group readily accommodates the dipole attractions and hydrogen bonds of water molecules.
The two main ways that the hydrophobic portions of stearate ions can avoid water are to cluster together on the surface or to dissolve in a small quantity of oil or grease (see Fig. 20.3).
In the latter case the hydrophilic heads of the soap molecules contact the water outside the grease, forming a structure known as a micelle. Since the outsides of the micelles are negatively charged, they repel one another and prevent the grease droplets from recombining. The grease is therefore suspended (emulsified) in the water and can be washed away easily.
Natural soaps, such as sodium stearate, were originally made in the home by heating animal fat with wood ashes, which contained potash, K2CO3. Large quantities are still produced industrially, but to a considerable extent soaps have been replaced by detergents. This is a consequence of the undesirable behavior of soaps in hard water. Calcium, magnesium, and other hard-water cations form insoluble compounds when combined with the anions of fatty acids. This produces scummy precipitates and prevents the soap molecules from emulsifying grease unless a large excess is used.
Detergents such as alkylbenzenesulfonates (ABS) and linear alkylbenzenesulfonates (LAS) have structures very similar to sodium stearate except that the charged group in their hydrophilic heads is —SO3– attached to a benzene ring. The ABS detergents also have methyl (CH3) groups branching off their hydrocarbon chains. Such molecules do not precipitate
with hard-water cations and therefore are more suitable for machinewashing of clothes. The LAS detergents replaced ABS during the mid-nineteen-sixties when it was discovered that the latter were not biodegradable. They were causing rivers and even tap water to become covered with detergent suds and foam. Apparently the enzymes in microorganisms which had evolved to break down the unbranched hydrocarbon chains in natural fats and fatty acids were incapable of digesting the branched chains of ABS molecules. LAS detergents, though manufactured by humans, mimic the structures of naturally occurring molecules and are biodegradable.
Polar Lipids
As was true of most nonpolar lipids, the structures of polar lipids are based on condensation of fatty acids with glycerol. The main difference is that only two of the three OH groups on glycerol are involved. The third (as shown by the structures in Fig. 20.2b) is combined with a highly polar molecule. In one sense the polar lipids are like the anions of fatty acids, only more so. They contain two hydrophobic hydrocarbon tails and a head which may have several electrically charged sites. As in the case of soap and detergent molecules, the tails of polar lipids tend to avoid water and other polar substances, but the heads are quite compatible with such environments.
The polar lipids are most commonly found as components of cell walls and other membranes. Nearly all hypotheses regarding membrane structure take as a fundamental component a lipid bilayer (Fig. 20.4). Bilayers made in the laboratory have many properties in common with membranes. Ions such as Na+, K+, and Cl– cannot pass through them, but water molecules can. The hydrocarbon core of such a bilayer should have large electrical resistance, as does a membrane. Certain carrier molecules can transport K+ and other ions across a bilayer, apparently by wrapping a hydrophobic cloak around them to disguise their charges.
Bioamplification
One important consequence of the hydrophobic character of lipids is the concentration of nonpolar substances along ecological food chains, a process known as bioamplification. As an example, consider organisms in an aqueous environment such as a river or lake. Any substance which is more soluble in living tissues than in the surrounding water will tend to concentrate in even the simplest plants and animals. These plants and animals are often the food supply for more complex life forms—a food supply which contains a greater concentration of the substance in question. As we proceed up the food chain to larger predatory animals, the concentrations of some substances can be increased by factors of 10 000 or more.
What kinds of substances are likely to undergo bioamplification? Clearly those which are more soluble in living systems. Since the surroundings are dilute aqueous solutions while organisms contain both aqueous and lipid phases, nonpolar substances which dissolve in lipids are most likely to be concentrated. This is a problem with DDT, polychlorinated biphenyls (PCB’s), and other long-lived synthetic organic compounds. It also applies to metal ions which can combine with organic groups to form uncharged molecules. A good example of such an organometallic compound involves mercury. Certain aquatic microorganisms can convert relatively inert mercury metal into chloromethylmercury(II) (commonly known as methylmercuric chloride):
Since the charge of Hg2+ is neutralized by Cl3– and Cl–, this organometallic compound is quite soluble in lipid tissues. Concentration of mercury along aquatic food chains has led to several episodes where hundreds of Japanese people whose diet consisted mainly of fish became ill and many died. It also accounts for the relatively high level of mercury observed in some species of fish taken from the Great Lakes of the United States and Canada.
Humans and other organisms do have mechanisms for eliminating toxic and otherwise undesirable organic substances. A wide variety of enzymes in the human liver can convert hydrophobic molecules to more polar forms which are less soluble in lipid tissues and more readily excreted. However, there is often a problem with synthetic substances which have been synthesized by chemists and whose structures are quite different from any the liver enzymes are equipped to handle. As in the case of the ABS detergents, the reactions which decompose such substances may be relatively slow. If they are not converted into structures which can be eliminated easily, even small quantities may remain in the body for long periods, sometimes causing chronic illness. It is for this reason that toxic effects of synthetic organic or organometallic chemicals should be thoroughly tested before large quantities are released to the environment.
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