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You are watching: The most highly concentrated source of energy in the body is

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. Brand-new York: W H Freeman; 2002.


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Triacylglycerols space highly focused stores the metabolic energy since they room reduced and anhydrous. The yield from the complete oxidation of fat acids is around 9 kcal g-1 (38 kJ g-1), in contrast with around 4 kcal g-1 (17 kJ g-1) for carbohydrates and also proteins. The basis of this huge difference in caloric yield is that fatty acids are much much more reduced. Furthermore, triacylglycerols are nonpolar, and also so they are stored in a practically anhydrous form, whereas much much more polar proteins and also carbohydrates are an ext highly hydrated. In fact, 1 g of dry glycogen binds about 2 g of water. Consequently, a gram of practically anhydrous fat stores more than six times as much power as a gram of character language glycogen, i m sorry is likely the factor that triacylglycerols rather than glycogen were selected in advancement as the significant energy reservoir. Consider a usual 70-kg man, who has actually fuel to 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 on computer in glycogen, his full body weight would be 55 kg greater. The glycogen and also glucose stores carry out enough energy to sustain biological duty for about 24 hours, conversely, the triacylglycerol stores permit survival for numerous weeks.

In mammals, the major site of build-up of triacylglycerols is the cytoplasm the adipose cell (fat cells). Droplets the triacylglycerol coalesce to kind a huge globule, which might occupy many of the cabinet volume (see number 22.1). Adipose cells are specialized for the synthesis and storage the triacylglycerols and also for your mobilization into fuel molecule that are transported to other tissues through the blood.

The utility of triacylglycerols together an energy source is dramatically portrayed by the abilities of migratory birds, which deserve 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 tip of southern America; a huge segment that the flight (3800 km, or 2400 miles) is over open up ocean, wherein the birds cannot feed. The ruby-throated hummingbird can fly nonstop across the Gulf the Mexico. Fat acids administer the energy resource for both these prodigious feats.


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Figure

Triacylglycerols fuel the lengthy migration flights that the American gold Plover (Pluvialis dominica).


22.1.1. Diet Lipids are Digested by Pancreatic Lipases

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many lipids space ingested in the kind of triacylglycerols however must be degraded to fat acids for absorption across the minister epithelium. Recall that lipids room not conveniently solubilized, however they need to be in bespeak to it is in degraded. Triacylglycerols in the intestinal lumen are included into micelles created with the help of bile salts (Figure 22.3), amphipathic molecules synthesized indigenous cholesterol in the liver and secreted native the bile bladder. Organization of lipids right into micelles orients the ester bond of the lipid towards the surface of the micelle, calculation the bonds more susceptible to cradle by pancreatic lipases that room in aqueous solution. If the production of bile salt is poor owing come liver disease, big amounts of fat (as much as 30 g day-1) room excreted in the feces. This problem is referred to as steatorrhea, native the Greek steato, “fat.”


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Figure 22.3

Glycocholate. Bile salts, such as glycocholate, facilitate lipid cradle in the intestine.


The lipases digest the triacylglycerols into complimentary fatty acids and also monoacylglycerol (Figure 22.4). This digestion commodities are lugged in micelles to the intestinal epithelium wherein they are took in across the plasma membrane.


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Figure 22.4

Action of Pancreatic Lipases. Lipases secreted through the pancreas convert triacylglycerols into fatty acids and monoacylglycerol for absorption right into the intestine.


22.1.2. Diet Lipids room Transported in Chylomicrons

In the intestinal mucosal cells, the triacylglycerols are resynthesized from fatty acids and monoacylglycerols and also then packaged into lipoprotein transport particles dubbed chylomicrons, secure particles ranging from around 180 come 500 nm in diameter (Figure 22.5). This particles are composed mostly of triacylglycerols, through apoprotein B-48 as the key protein component. Protein ingredient of lipoprotein corpuscle are referred to as apolipoproteins. Chylomicrons also duty in the carry of fat-soluble vitamins and cholesterol.


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Figure 22.5

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


The chylomicrons are released right into the lymph system and then right into the blood. This particles tie to membrane-bound lipoprotein lipases, mostly at adipose tissue and muscle, wherein the triacylglycerols are when again degraded into cost-free fatty acids and monoacylglycerol because that transport into the tissue. The triacylglycerols room then resynthesized within the cell and also stored. In the muscle, they deserve to be oxidized to provide energy, as will certainly be disputed shortly.

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