SIGNIFICANCE OF LIPOIDS 49 



is provided by the decomposition of the proteins by means of 

 a tryptic enzyme. But however this may be, in view of our 

 ignorance of these substances, and the fact that vegetable 

 lecithins apparently have seldom or never been obtained in a 

 state of purity, and the uncertainty relating to some of their 

 cleavage products, it does not appear profitable further to 

 consider here the theories which have been advanced to 

 explain their formation. 



Physiological Significance. 



Nothing of a very definite nature is known of the physio- 

 logical significance of the lipoids. Overton points out that 

 under certain conditions lecithin and similar substances have 

 the power of absorbing water, and suggests that the ectoplasm 

 may consist of layers of these substances which thus play an 

 important role in absorption and secretion. Green and Jackson 

 also consider that it exercises considerable influence on the 

 transport of materials from cell to cell. This view of the 

 lipoid nature of the plasmatic membrane is greatly supported 

 by the work of Czapek * on the surface tension of the external 

 limiting layer of the protoplasm. 



Lipoids may of course represent an intermediate product 

 between the fats and proteins, for it is a well-known fact that 

 fats may develop in cheese, but according to Nierenstein f in 

 such cases the fats are derived not from the proteins, but from 

 other substances, such as cholesterol. 



Also the view has been put forward that they are the 

 means of setting up the change in zymogens which leads to the 

 formation of enzymes. More recently Palladin \ has suggested 

 that there is a relationship between lipoids and respiration, 

 for the more of these substances extracted from seedlings the 

 more was the respiration depressed. Possibly the lipoids, 

 which contain phosphorus, act in a similar way as the phos- 

 phates in alcoholic fermentation. 



* Czapek : " Ueber eine Methode zur direkten Bestimmung der Oberflachen- 

 spannung der Plasmahaut von Pflanzenzellen," Jena, igi2. 



t Nierenstein : " Proc. Roy. Soc, Lond.," B., igii, 83, 301. 



J Palladin: " Ber. deut. bot. Gesells.," 1910,28, 120; Palladin and Stane- 

 witsch : " Biochem. Zeit.," 1910, 26, 351. 



