392 FAT METABOLISM 



Linoleic acid (CigHooOo) is an example of a fatty acid which contains 

 two double bonds. This acid is an important constituent of cottonseed oil. 

 Linolenic acid (C^sHsoOo), found in linseed oil, contains three double 

 bonds. A number of other less common unsaturated fatty acids have been 

 isolated from the tissues of plants and animals. 



All of the unsaturated fatty acids combine with hydrogen, oxygen, or the 

 halogens. The "drying" properties of linseed, sunflower, and certain other 

 oils are due to the combination of the highly unsaturated fatty acid radicals 

 of the oil with oxygen of the air resulting in the formation of solid, waxy 

 compounds. 



None of the fat-forming fatty acids — saturated or unsaturated — is appre- 

 ciably soluble in water. The lower members of the saturated series are 

 liquids at ordinary temperatures, while those containing ten or more carbon 

 atoms are solids. Most of the unsaturated fatty acids found in plants arc 

 liquids at ordinary temperatures. In general the fatty acids resemble the 

 fats proper in most of their properties. 



Fat Synthesis. — It is generally considered that fats, being insoluble in 

 water, cannot readily diffuse from cell to cell, although the actual evidence 

 for this supposition is not very substantial. It seems probable, therefore, 

 that fats are usually synthesized in the cells in which they occur. Fats con- 

 tain the same three elements — carbon, hydrogen, and oxygen — as the carbo- 

 hydrates, but the proportion of oxygen to carbon is less in fats than in the 

 carbohydrates. In other words the carbon in fats is in a more highly reduced 

 form than in the carbohydrates. When in a liquid state fats are often called 

 oils. Regardless of the physical state of these compounds, however, all are 

 included under the chemical term of fats. 



Fats are synthesized in living organisms by the condensation of one mole- 

 cule of the trihydric alcohol glj'cerol (glycerine) with three molecules of the 

 same or different fatty acids. Both the glycerol and the fatty acid molecules 

 are synthesized from carbohydrates. The general scheme of fat sj-nthesis 

 may be represented as follows: 



Glycerol 

 Carbohydrates ^ ^^ Fats + Water 



Fatty Acids '^ 



y 



During the maturation of many oily seeds an increase in oil content occurs 

 concurrently with a decrease in the quantity of carbohydrate present (Table 

 36). This indicates that the carbohydrates in the seed are being converted 

 into fats, and is in accord with the generally accepted theory of fat sj^nthesis. 



