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Berg JM, Tymoczko JL, Stryer L. Biochemistry. Fifth edition. Brand-new York: W H Freeman; 2002.

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Triacylglycerols are highly focused stores the metabolic energy because they room reduced and anhydrous. The productivity from the complete oxidation of fat acids is around 9 kcal g-1 (38 kJ g-1), in comparison with around 4 kcal g-1 (17 kJ g-1) for carbohydrates and also proteins. The communication of this big difference in caloric productivity is that fatty acids room much more reduced. Furthermore, triacylglycerols space nonpolar, and so they are stored in a almost anhydrous form, conversely, much an ext polar proteins and carbohydrates are much more highly hydrated. In fact, 1 g of dried glycogen binds about 2 g of water. Consequently, a gram of practically anhydrous fat stores much more than six times together much power as a gram of sign language glycogen, i m sorry is likely the factor that triacylglycerols rather than glycogen were selected in advancement as the significant energy reservoir. Think about a usual 70-kg man, who has fuel make reservation of 100,000 kcal (420,000 kJ) in triacylglycerols, 25,000 kcal (100,000 kJ) in protein (mostly in muscle), 600 kcal (2500 kJ) in glycogen, and 40 kcal (170 kJ) in glucose. Triacylglycerols constitute around 11 kg the his total body weight. If this quantity of energy were save in glycogen, his full body weight would certainly be 55 kg greater. The glycogen and glucose stores administer enough energy to sustain biological duty for around 24 hours, conversely, the triacylglycerol stores enable survival for numerous weeks.

In mammals, the significant site of build-up of triacylglycerols is the cytoplasm of adipose cells (fat cells). Droplets the triacylglycerol coalesce to form a huge globule, which may occupy many of the cabinet volume (see figure 22.1). Adipose cells are specialized for the synthesis and also storage of triacylglycerols and also for your mobilization right into fuel molecule that space transported to other tissues by the blood.

The utility of triacylglycerols together an energy resource is dramatically shown by the abilities of migratory birds, which have the right to fly great distances without eating. Examples are the American golden plover and also the ruby-throated sparrow. The golden plover paris from Alaska to the southern reminder of south America; a large segment of the trip (3800 km, or 2400 miles) is over open up ocean, where the birds cannot feed. The ruby-throated hummingbird have the right to fly nonstop throughout the Gulf the Mexico. Fat acids provide the energy resource for both these prodigious feats.



Triacylglycerols fuel the long migration flights of the American golden Plover (Pluvialis dominica).

22.1.1. Diet Lipids room Digested by Pancreatic Lipases

many lipids room ingested in the kind of triacylglycerols but must it is in degraded to fat acids because that absorption across the intestinal epithelium. Recall that lipids are not easily solubilized, yet they must be in order to it is in degraded. Triacylglycerols in the intestinal lumen are integrated into micelles formed with the help of bile salts (Figure 22.3), amphipathic molecules synthesized indigenous cholesterol in the liver and secreted indigenous the bile bladder. Organization of lipids right into micelles orients the ester bond of the lipid toward the surface of the micelle, calculation the bonds much more susceptible to cradle by pancreatic lipases that are in aqueous solution. If the manufacturing of bile salts is poor owing come liver disease, large amounts of fat (as lot as 30 g day-1) are excreted in the feces. This problem is described as steatorrhea, native the Greek steato, “fat.”


Figure 22.3

Glycocholate. Bile salts, such together glycocholate, facilitate lipid digestion in the intestine.

The lipases digest the triacylglycerols into cost-free fatty acids and monoacylglycerol (Figure 22.4). These digestion products are carried in micelles come the minister epithelium where they are took in across the plasma membrane.


Figure 22.4

Action of Pancreatic Lipases. Lipases secreted by the pancreas transform triacylglycerols into fatty acids and also monoacylglycerol for absorption right into the intestine.

22.1.2. Dietary Lipids space Transported in Chylomicrons

In the intestinal mucosal cells, the triacylglycerols room resynthesized from fat acids and monoacylglycerols and then packaged into lipoprotein transfer particles dubbed chylomicrons, steady particles varying from approximately 180 come 500 nm in diameter (Figure 22.5). These particles space composed largely of triacylglycerols, with apoprotein B-48 together the main protein component. Protein constituents of lipoprotein particles are called apolipoproteins. Chylomicrons also duty in the move of fat-soluble vitamins and also cholesterol.


Figure 22.5

Chylomicron Formation. Complimentary fatty acids and monoacylglycerols are soaked up by intestinal epithelial cells. Triacylglycerols are resynthesized and also packaged with other lipids and apoprotein B-48 to form chylomicrons, which are then released right into the lymph (more...)

The chylomicrons are released into the lymph system and then into the blood. These particles bind to membrane-bound lipoprotein lipases, primarily at adipose tissue and also muscle, wherein the triacylglycerols are as soon as again degraded into totally free fatty acids and also monoacylglycerol because that transport right into the tissue. The triacylglycerols room then resynthesized inside the cell and stored. In the muscle, they deserve to be oxidized to administer energy, as will certainly be questioned shortly.

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