A triacylglycerol
Synthesis SR SR Degradation SR
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 calori 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 hydratedglycogen,which is likely the reason that triacylglycerols rather than glycogen were selected in evolution as the major energy reservoir.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
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 calori 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 hydratedglycogen, which is likely the reason that triacylglycerols rather than glycogen were selected in evolution as the major energy reservoir.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. Triacylglycerols Are Highly Concentrated Energy Stores
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
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 rubythroated hummingbird can fly nonstop across the Gulf of Mexico. Fatty acids provide the energy source for both these prodigious feats
In mammals,the major site ofaccumulation 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
In mammals, the major site ofaccumulation 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