James Richard Fromm
A lipid is defined as a water-insoluble biomolecule which has a high solubility in nonpolar organic solvents such as chloroform. The simplest lipids are the fats, which are triesters made up of one glycerol and three fatty acids. The term fats is also used as a general synonym for lipids, so the more precise terms triacylglycerols or triglycerides are preferable for the simplest lipids. Triacylglycerols are used primarily for energy storage in animals. More complex lipids, the phospholipids, glycolipids, and cholesterol, are the major constituents of biological cell membranes.
In mammals, the major reservoir of triacylglycerols is in the cytoplasm of adipose cells (fat cells). A mammalian fat cell consists of a small droplet of condensed triacylglycerols surrounded by a thin cell membrane with the cell nucleus bulging out to one side. Most of the energy reserve of animals is stored in fat cells. The total energy reserve of a standard 70 kg man consists of about 0.17 MJ in glucose, 2.5 MJ in glycogen, 105 MJ in protein (mostly in the muscles), and 420 MJ in triacylglycerols. The triacylglycerols constitute about 11 kg of his total body mass of 70 kg. The proportion of protein is lower and that of triacylglycerols higher, on average, in women.
The processing of glucose to triacylglycerols rather than glycogen, and its reverse, require metabolic energy and one would expect, since energy storage as triacylglycerols is clearly preferred, that there is a major advantage to this form of energy storage. There is such an advantage; one gram of fatty acids yields on complete oxidation almost double the energy of one gram of anhydrous carbohydrate (such as glycogen) or protein. Moreover, the triacylglycerols are hydrophobic and are stored in nearly anhydrous form by living organisms, while glycogen is hydrophilic; one gram of dry glycogen binds, under physiological conditions, about two grams of water. As a consequence, a gram of fat stores more than six times as much energy as does a gram of glycogen under physiological conditions. If most of the energy of the 70 kg standard man were not stored as triacylglycerols, his mass would have to be nearly doubled to 125 kg.
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