310 ANIMAL BIOCHEMISTRY 



RESERVE TISSUES 



The tissues discussed in earlier sections in this chapter constitute the 

 working tissues of the body. Their functions are structure, transporta- 

 tion, and control. Certain other cells in the body function as storage 

 areas for reserve foods to be used in time of stress. Small amounts of 

 carbohydrates are stored as glycogen in the liver, muscle, and, to a 

 much lesser extent, in the nervous tissue. The principal energy-storage 

 form, however, is fats. 



Fatty Tissues 



Practically every tissue in the animal body contains lipides in some 

 form, although certain tissues are characterized as fatty tissties. If an 

 animal is starved for a long time, much of the lipide fraction is uti- 

 lized, except for the phosphatides, which remain almost intact. Thus 

 this latter fraction cannot be considered an energy reserve. 



Reserve lipides are stored in areas characteristic of the animal in 

 question. All animals store subcutaneous fats to a degree, and in cattle 

 and hogs this is the major storage area. Man accumulates most of 

 the adipose tissue in the abdominal region. Each species tends to 

 store a characteristic mixture of fats, although the composition can 

 be changed by feeding large quantities of a specific lipide, for example, 

 linoleic acid-rich soybean oil. Pork fat contains triglycerides high in 

 oleic acid with smaller amounts of stearic acid. However, if pigs are 

 fed quantities of unextracted soybeans, they tend to accumulate greater 

 amounts of linoleic acid, and the pork becomes flabby or oily. Other 

 experiments have shown that similar changes can be produced in a 

 number of species. When animals are fed ordinary rations, they tend 

 to accumulate depot fats with melting points close to their own body 

 temperature. 



Subcutaneous fats usually contain more oleic acid than the visceral 

 fats, which accumulate a higher percentage of saturated acids. It is 

 obvious that an animal must oxidize unwanted fatty acids or alter 

 them before storage in the particular depot in forms characteristic of 

 the animal. 



Although fatty tissues are quite inert from the metabolic standpoint, 

 they are not entirely inactive. Experiments with fats containing 

 deuterium (heavy hydrogen) in place of normal hydrogen have shown 

 that there is a turnover of storage fats. Even small amounts of tagged 



