Triacylglycerols Are Highly Concentrated Energy Stores

II. Transducing and Storing Energy
22. Fatty Acid Metabolism
22.1. Triacylglycerols Are Highly Concentrated Energy Stores
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 9 kcal g-1 (38 kJ g-1), in contrast with about 4 kcal g-1 (17 kJ g-1) for
carbohydrates and proteins. The basis of this large difference in caloric yield is that fatty acids are much more reduced.
Furthermore, triacylglycerols are nonpolar, and so they are stored in a nearly anhydrous form, whereas much more polar
proteins and 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 more than six 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 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 about 11 kg of his total body weight. If this amount of energy were stored in glycogen, his
total body weight would be 55 kg greater. The glycogen and glucose stores provide enough energy to sustain biological
function for about 24 hours, whereas the triacylglycerol stores allow survival for several weeks.
In mammals, the major site of accumulation of triacylglycerols is the cytoplasm of adipose cells (fat cells). Droplets of
triacylglycerol coalesce to form a large globule, which may occupy most of the cell volume (see Figure 22.1). 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.
The utility of triacylglycerols as an energy source is dramatically illustrated by the abilities of migratory birds, which can
fly great distances without eating. Examples are the American golden plover and the ruby-throated sparrow. 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 can fly nonstop across the Gulf of
Mexico. Fatty acids provide the energy source for both these prodigious feats.
22.1.1. Dietary Lipids Are Digested by Pancreatic Lipases
Most lipids are ingested in the form of triacylglycerols but must be degraded to fatty acids for absorption across
the intestinal epithelium. Recall that lipids are not easily solubilized, yet they must be in order to be degraded.
Triacylglycerols in the intestinal lumen are incorporated into micelles formed with the aid of bile salts (Figure 22.3),
amphipathic molecules synthesized from cholesterol in the liver and secreted from the gall bladder. Incorporation of
lipids into micelles orients the ester bonds of the lipid toward the surface of the micelle, rendering the bonds more
susceptible to digestion by pancreatic lipases that are in aqueous solution. If the production of bile salts is inadequate
owing 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, from the Greek steato, "fat."
The lipases digest the triacylglycerols into free fatty acids and monoacylglycerol (Figure 22.4). These digestion products
are carried in micelles to the intestinal epithelium where they are absorbed across the plasma membrane.
22.1.2. Dietary Lipids Are Transported in Chylomicrons
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 ranging from approximately 180 to 500
nm in diameter (Figure 22.5). These particles are composed mainly of triacylglycerols, with apoprotein B-48 as the main
protein component. Protein constituents of lipoprotein particles are called apolipoproteins. Chylomicrons also function
in the transport of fat-soluble vitamins and cholesterol.
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 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, as will be discussed shortly.
II. Transducing and Storing Energy
22. Fatty Acid Metabolism
22.1. Triacylglycerols Are Highly Concentrated Energy Stores
Triacylglycerols fuel the long migration flights of the American Golden Plover (Pluvialis dominica). [Gerard
Fuehrer/Visuals Unlimited.]
II. Transducing and Storing Energy
22. Fatty Acid Metabolism
22.1. Triacylglycerols Are Highly Concentrated Energy Stores
Figure 22.3. Glycocholate. Bile salts, such as glycocholate, facilitate lipid digestion in the intestine.