Triacylglycerols are highly concentrated stores of metabolic energy because they are reduced and anhydrous. The yield from the complete oxidation of fatty acids is about 38 kJ g⁻¹ (9 kcal g⁻¹), in contrast with about 17 kJ g⁻¹ (4 kcal g⁻¹) for carbohydrates and proteins. The basis of this large difference in caloric yield is that fatty acids are much more reduced than carbohydrates or proteins. Furthermore, triacylglycerols are nonpolar, and so they are stored in a nearly anhydrous form, whereas much more polar carbohydrates are more highly hydrated. In fact, 1 g of dry glycogen binds about 2 g of water. Consequently, a gram of nearly anhydrous fat stores 6.75 times as much energy as a gram of hydrated glycogen, which is likely the reason that triacylglycerols rather than glycogen were selected in evolution as the major energy reservoir. Consider a typical 70-kg man, who has fuel reserves of 420,000 kJ (100,000 kcal) in triacylglycerols, 100,000 kJ (24,000 kcal) in protein (mostly in muscle), 2500 kJ (600 kcal) in glycogen, and 170 kJ (40 kcal) in glucose. Triacylglycerols constitute about 11 kg of his total body weight. If this amount of energy were stored in glycogen, his total body weight would be 64 kg greater. The glycogen and glucose stores provide enough energy to sustain physiological function for about 24 hours, whereas the triacylglycerol stores allow survival for several weeks.

In mammals, the major site of triacylglycerol accumulation is the cytoplasm of adipose cells (fat cells). This fuel-rich tissue is located throughout the body, notably under the skin (subcutaneous fat) and surrounding the internal organs (visceral fat). Droplets of triacylglycerol coalesce to form a large globule, called a lipid droplet, which may occupy most of the cell volume (Figure 22.1). The lipid droplet is surrounded by a monolayer of phospholipids and numerous proteins required for triacylglycerol metabolism. Originally believed to be inert-lipid deposits, lipid droplets are now understood to be dynamic organelles essential for the regulation of lipid metabolism. Adipose cells are specialized for the synthesis and storage of triacylglycerols and for their mobilization into fuel molecules that are transported to other tissues by the blood. Muscle also stores triacylglycerols for its own energy needs. Indeed, triacylglycerols are evident as the “marbling” of expensive cuts of beef.

The utility of triacylglycerols as an energy source is dramatically illustrated by the abilities of migratory birds, which can fly great distances without eating after having stored energy as triacylglycerols. Examples are the American golden plover and the ruby-throated hummingbird. The golden plover flies from Alaska to the southern tip of South America; a large segment of the flight (3800 km, or 2400 miles) is over open ocean, where the birds cannot feed. The ruby-throated hummingbird flies nonstop across the Gulf of Mexico. Fatty acids provide the energy source for both these prodigious feats.

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Most lipids are ingested in the form of triacylglycerols and must be degraded to fatty acids for absorption across the intestinal epithelium. Intestinal enzymes called lipases, secreted by the pancreas, degrade triacylglycerols to free fatty acids and monoacylglycerol (Figure 22.3). Lipids present a special problem because, unlike carbohydrates and proteins, these molecules are not soluble in water. They exit the stomach as an emulsion, particles with a triacylglycerol core surrounded by cholesterol and cholesterol esters. How are the lipids made accessible to the lipases, which are in aqueous solution? First, the particles are coated with bile acids (Figure 22.4), amphipathic molecules synthesized from cholesterol in the liver and secreted from the gallbladder. The ester bond s of each lipid are oriented toward the surface of the bile salt-coated particle, rendering the bond more accessible to digestion by lipases in aqueous solution. However, i n this form, the particles are still not substrates for digestion. The protein colipase (another pancreatic secretory product) must bind the lipase to the particle to permit lipid degradation. The final digestion products are carried in micelles to the intestinal epithelium where they are transported across the plasma membrane (Figure 22.5). If the production of bile salts is inadequate due to liver disease, large amounts of fats (as much as 30 g day⁻¹) are excreted in the feces. This condition is referred to as steatorrhea, after stearic acid, a common fatty acid.

In the intestinal mucosal cells, the triacylglycerols are resynthesized from fatty acids and monoacylglycerols and then packaged into lipoprotein transport particles called chylomicrons, stable particles approximately 2000 Å (200 nm) in diameter (Figure 22.5). These particles are composed mainly of triacylglycerols, with apoliprotein B-48 (apo B-48) as the main protein component. Protein constituents of lipoprotein particles are called apolipoproteins. Chylomicrons also transport fat-soluble vitamins and cholesterol.

The chylomicrons are released into the lymph system and then into the blood. These particles bind to membrane-bound lipases, primarily at adipose tissue and muscle, where the triacylglycerols are once again degraded into free fatty acids and monoacylglycerol for transport into the tissue. The triacylglycerols are then resynthesized inside the cell and stored. In the muscle, they can be oxidized to provide energy.

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