There are probably no definite rules as to the selection of fatty acids that make up a triglyceride molecule. One triglyceride molecule may be composed of one saturated, one monounsaturated, and one polyunsaturated fatty acid, all of the same or varying lengths. However, the types of fatty acids found within triglyceride molecules will depend on the plant or the animal source. For instance, the triglycerides in olive oil largely contain the MUFA oleic acid (18:1 ra-9) (about 82 percent), while about two-thirds of the fatty acids in butter are SFAs of varying length.
The presence of certain types of fatty acids in either a plant or an animal largely depends upon the nature of the plant or animal and the purpose of the fat for that life-form. For instance, fish that live in deeper water tend to be better sources of ra-3 PUFA because these fatty acids are found in the cell membranes of these fish and play a protective role against the increased pressure and decreased temperatures at greater depths as well as help regulate their buoyancy. Land animals create storage fat that is largely composed of saturated fatty acid. Since these fat molecules can pack tightly in fat cells it minimizes the necessary space.
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