3 Биологические мембраны. Обмен веществом (1160072), страница 2
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This explains why oil-water mixtures (oil-and-vinegar salad dressing, for example) have two phases. Because lipids have lower specific gravitiesthan water, the oil floats on the aqueous phase.Triacylglycerols Provide Stored Energy and InsulationIn most eukaryotic cells, triacylglycerols form a separate phase of microscopic, oily droplets in the aqueous cytosol, serving as depots ofmetabolic fuel. Specialized cells in vertebrate animals, called adipocytes, or fat cells, store large amounts of triacylglycerols as fat droplets, which nearly fill the cell (Fig. 9-3).
Triacylglycerols are alsostored in the seeds of many types of plants, providing energy and biosynthetic precursors when seed germination occurs.NucleusLipid droplets(b)Figure 9-3 Fat stores in cells, (a) Cross-section offour guinea pig adipocytes, showing huge fat droplets that virtually fill the cells. Also visible are several capillaries in cross-section, (b) Two cambialcells from the underground stem of the plantIsoetes muricata, a quillwort. In winter, these cellsstore fats as lipid droplets.244Part II Structure and CatalysisBOX 9-1Sperm Whales: Fatheads of the DeepStudies of sperm whales have uncovered anotherway in which triacylglycerols are biologically useful. The sperm whale's head is very large, accounting for over one-third of its total body weight (Fig.1).
About 90% of the weight of the head is made upof the spermaceti organ, a blubbery mass that contains up to 3,600 kg (about 4 tons) of spermacetioil, a mixture of triacylglycerols and waxes containing an abundance of unsaturated fatty acids.This mixture is liquid at the normal resting bodytemperature of the whale, about 37 °C, but it begins to crystallize at about 31 °C and becomes solidwhen the temperature drops several more degrees.The probable biological function of spermacetioil has been deduced from research on the anatomyand feeding behavior of the sperm whale. Thesemammals feed almost exclusively on squid in verydeep water.
In their feeding dives they descend1,000 m or more; the record dive is 3,000 m (almost2 miles). At these depths the sperm whale has nocompetitors for the very plentiful squid. The spermwhale rests quietly, waiting for schools of squid toFigure 1 Silhouette ofa sperm whale, showing the spermacetiorgan, a huge enlargement of the snout thatlies above the upperjaw.Spermacetiorganpass. For a marine animal to remain at a givendepth, without a constant swimming effort, it musthave the same density as the surrounding water.The sperm whale can change its buoyancy tomatch the density of its surroundings—from thetropical ocean surface to great depths where thewater is much colder and thus has a greater density.The key to the sperm whale's ability to changeits buoyancy is the freezing point of spermaceti oil.When the temperature of liquid spermaceti oil islowered several degrees during a deep dive, it congeals or crystallizes and becomes more dense, thuschanging the buoyancy of the whale to match thedensity of seawater.
Various physiological mechanisms promote rapid cooling of the oil during adive. During the return to the surface, the congealed spermaceti oil is warmed again and melted,decreasing its density to match that of the surfacewater. Thus we see in the sperm whale a remarkable anatomical and biochemical adaptation, perfected by evolution. The triacylglycerols synthesized by the sperm whale contain fatty acids of thenecessary chain length and degree of unsaturationto give the spermaceti oil the proper melting pointfor the animal's diving habits.Unfortunately for the sperm whale population,spermaceti oil is commercially valuable as a lubricant. Several centuries of intensive hunting ofthese mammals have depleted the world's population of sperm whales.As stored fuels, triacylglycerols have two significant advantagesover polysaccharides such as glycogen and starch.
The carbon atoms offatty acids are more reduced than those of sugars, and oxidation oftriacylglycerols yields more than twice as much energy, gram for gram,as that of carbohydrates. Furthermore, because triacylglycerols arehydrophobic and therefore unhydrated, the organism that carries fatas fuel does not have to carry the extra weight of water of hydrationthat is associated with stored polysaccharides. In humans, fat tissue,which is composed primarily of adipocytes, occurs under the skin, inthe abdominal cavity, and in the mammary glands. Obese people mayhave 15 or 20 kg of triacylglycerols deposited in their adipocytes, sufficient to supply energy needs for months.
In contrast, the human bodycan store less than a day's energy supply in the form of glycogen. Carbohydrates such as glucose and glycogen do offer certain advantages asquick sources of metabolic energy, one of which is their ready solubilityin water.In some animals, triacylglycerols stored under the skin serve notonly as energy stores but as insulation against very low temperatures.Seals, walruses, penguins, and other warm-blooded polar animals are245Chapter 9 Lipidsamply padded with triacylglycerols.
In hibernating animals (bears, forexample) the huge fat reserves accumulated before hibernation alsoserve as energy stores (see Box 16-1). The low density of triacylglycerols is the basis for another remarkable function of these compounds.
Insperm whales, a store of triacylglycerols allows the animals to matchthe buoyancy of their bodies to that of their surroundings during deepdives in cold water (Box 9-1).Many Foods Contain TriacylglycerolsMost natural fats, such as those in vegetable oils, dairy products, andanimal fat, are complex mixtures of simple and mixed triacylglycerols.These contain a variety of fatty acids differing in chain length anddegree of saturation (Table 9-2).
Vegetable oils such as corn and oliveoil are composed largely of triacylglycerols with unsaturated fattyacids, and thus are liquids at room temperature. They are convertedindustrially into solid fats by catalytic hydrogenation, which reducessome of their double bonds to single bonds. Triacylglycerols containingonly saturated fatty acids, such as tristearin, the major component ofbeef fat, are white, greasy solids at room temperature.Table 9-2 Fatty acid composition of three natural food fats*Fatty acidsState atroom temperature(25 °C)SaturatedUnsaturatedOOlive oilButterBeef fatLiquidSolid (soft)Solid (hard)<211<2<210<213262931121804046* These fats consist of mixtures of triacylglycerols, differing in their fatty acid composition andthus in their melting points.+Values are given as percentage of total fatty acids.When lipid-rich foods are exposed too long to the oxygen in air,they may spoil and become rancid.
The unpleasant taste and smellassociated with rancidity result from the oxidative cleavage of the double bonds in unsaturated fatty acids to produce aldehydes and carboxylic acids of shorter chain length and therefore higher volatility.CH2—O—C—R1O||TriacylglycerolCH —O —C—R2OUHo-O-C-R3saponification3KOHOK+CH2—OHHydrolysis of Triacylglycerols Produces SoapsCH—OHThe ester linkages of triacylglycerols are susceptible to hydrolysis byeither acid or alkali. Heating animal fats with NaOH or KOH producesglycerol and the Na + or K+ salts of the fatty acids, known as soaps(Fig. 9-4). The usefulness of soaps is in their ability to solubilize ordisperse water-insoluble materials by forming microscopic aggregates(micelles). When used in "hard" water (having high concentrations ofCa2+ and Mg 2+ ), soaps are converted into their insoluble calcium ormagnesium salts, forming a residue.
Synthetic detergents such as sodium dodecylsulfate (SDS; see p. 141) are less prone to precipitation inhard water, and have largely replaced natural soaps in many industrial applications.CH 2 -OHO-C-R10K+O—C—R2O+K "0—C—R3GlycerolSoaps (K+ saltsof fatty acids)Figure 9—4 Triacylglycerol breakdown by alkalinehydrolysis: the process of saponification. R1, R2, R3represent long alkyl chains. Household soap ismade by hydrolyzing a mixture of triacylglycerols(animal fat, for example) with KOH. The K+ saltsof the fatty acids are collected, washed free ofKOH, and pressed into cakes.Part II Structure and Catalysis246At neutral pH, a variety of lipases catalyze the enzymatic hydrolysis of triacylglycerols.
Lipases in the intestine aid in the digestion andabsorption of dietary fats. Adipocytes and germinating seeds containlipases that break down stored triacylglycerols, releasing fatty acidsfor export to other tissues where they are required as fuel.Waxes Serve as Energy Stores andWater-Impermeable CoatingsBiological waxes are esters of long-chain saturated and unsaturatedfatty acids (having 14 to 36 carbon atoms) with long-chain alcohols(having 16 to 30 carbon atoms) (Fig. 9-5). Their melting points (60 to100 °C) are generally higher than those of triacylglycerols.
In marineorganisms that constitute the plankton, waxes are the chief storageform of metabolic fuel.Waxes also serve a diversity of other functions in nature, related totheir water-repellent properties and their firm consistency. Certainskin glands of vertebrates secrete waxes to protect the hair and skinand to keep them pliable, lubricated, and waterproof. Birds, particularly waterfowl, secrete waxes from their preen glands to make theirfeathers water-repellent. The shiny leaves of holly, rhododendrons,poison ivy, and many tropical plants are coated with a layer of waxes,which protects against parasites and prevents excessive evaporation ofwater.OICH 3 (CH 2 ) 1 4 -C-O-CH 2 -(CH 2 ) 2 8 -CH 31-TriacontanolPalmitic acid(a)Figure 9-5 (a) Triacontanylpalmitate, the majorcomponent of beeswax.
It is an ester of palmiticacid with the alcohol triacontanol. (b) A honeycomb, constructed of beeswax, is firm at 25 °C andcompletely impervious to water. The term "wax"originates in the Old English word weax, meaning"the material of the honeycomb."Biological waxes find a variety of applications in the pharmaceutical, cosmetic, and other industries. Lanolin (from lamb's wool), beeswax (Fig. 9-5), carnauba wax (from a Brazilian palm tree), and spermaceti oil (from whales) are widely used in the manufacture of lotions,ointments, and polishes.Structural Lipids in MembranesThe central architectural feature of biological membranes is a doublelayer of lipids, which constitutes a barrier to the passage of polar molecules and ions.
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