CH. XL!.] METABOLISM OF FAT 617 



about 11 grammes; the storage of the same amount of potential 

 heat as glycogen is never effected in less than ten times that bulk 

 of liver tissue weighing 130 grammes, and rarely in less than 

 double that amount. 



Another use of fat, and one concerning which we know less, is 

 to participate in the actual construction of protoplasm, a role which 

 is not exclusively, though it is mainly, that of the proteins. In this 

 direction it is probable that the simple fats such as are found in 

 adipose tissue are less concerned than the more complex fats, which 

 contain phosphorus and nitrogen and are known as the phosphatides. 

 Many years ago Hoppe-Seyler pointed out that these lipoids are 

 as universally distributed as are the proteins wherever the 

 phenomena of life are observable, and the more recent work of 

 bio -chemists has fully confirmed the view, that though present 

 usually in small quantities, they are nevertheless indispensable. 



We have on pp. 541, 542 seen that the liver is important from 

 both these aspects. It not only prepares the fats for combustion 

 into their final products (carbon dioxide and water), with the 

 simultaneous liberation of heat, but it is also of use in synthesising 

 the more complex fatty compounds, the importance of which has 

 just been alluded to. 



Just as the carbohydrate of the food is the usual source of the 

 carbohydrate of the body, so the fat of the food is the usual source of 

 the fat of the body. But, again, fat may arise from something which 

 is not fat in the food. 



During absorption, the fatty acid and glycerin components of the 

 fat undergo a temporary separation, but they soon reunite, and the 

 fat which is not needed for immediate use, passes via the chyle and 

 blood to the cells of adipose tissue, where it is stored. The com- 

 position of the body fat depends to some extent on the composition 

 of the fat in the food. The proportion of the olein, stearin, and 

 palmitin in the fat of an animal can be varied by variations in 

 their proportion in the food, and if unusual glycerides or unusual 

 fatty acids (such as linolein from linseed oil, erucic acid from rape- 

 seed oil, or iodised fats) are administered, they will be discoverable 

 in the storage fat of the body. 



How the fat which is transported to the adipose tissue by the 

 blood enters the cells of that tissue is probably to be explained by 

 the existence of an enzyme in the cells ; this intracellular lipase acts 

 as the pancreatic enzyme acts in the intestine, cleaving fat into its 

 soluble constituents, fatty acid and glycerin ; these pass through the 

 cell membrane and are once more re-synthesised into fat, the action 

 of lipase being a reversible one. When the fat leaves the cells of 

 adipose tissue for utilisation, that is, combustion in the tissues, lipase 

 unties the knot once more, and so fat is rendered soluble in the 



