394 FAT METABOLISM 



Reduction reactions in the cells of living organisms are not brought about by 

 the action of free hydrogen, but reducing systems whereby hydrogen is 

 transferred from one kind of molecule to another are known to occur in 

 plant cells (Chap. XXX). Synthesis of glycerol in plant cells may therefore 

 occur in essentially the manner which has just been described. The produc- 

 tion of glycerol from the hexose sugar involves reduction of the carbon atoms 

 which requires energy. The energy with which this process is accomplished 

 comes from the process of respiration. 



2. Fatty Acid Synthesis. — Practically nothing is actually known of the 

 manner whereby fatty acids are synthesized from carbohydrates in plant cells, 

 although several theories of the possible mechanism of this process have been 

 advanced. It is generally considered that the fatty acids do not arise directly 

 from the carbohydrates, but from some simpler compound, such as acetalde- 

 hyde (CH3CHO), which is produced by decomposition of the carbohydrates. 

 By the combination of molecules of acetaldehyde, a fatty acid with a longer 

 chain of carbon atoms could be built up, as follows: 



CH3 CH3 



2 CH3CHO -. HCOH —'^-^, lu2 



Acetaldehyde 



CH2 CH2 



I I 



CHO COOH 



Butyric acid 



By similar reactions fatty acids of still longer carbon chains could be con- 

 structed. There is little positive evidence, however, in favor of this view of 

 fatty acid synthesis, except the fact that acetaldehyde is commonly found in 

 plant cells. The conversion of carbohydrates to fatty acids, by whatever 

 series of reactions this is accomplished, involves reduction of the carbon 

 atoms and hence requires energy. The energy utilized in fatty acid synthesis, 

 as in the synthesis of glycerol, is furnished by the process of respiration. 



3. Condensation of Fatty Acids and Glycerol. — There is little doubt 

 that this is the final stage in the process of fat synthesis. Using palmitic 

 acid for purposes of illustration the equation for this reaction can be written 

 as follows : 



CisHsiCOOv 



C3H5(OH)3 -h 3 C15H31COOH lipase C15H31COO 7 CaHs-f 3 H2O 



glycerol palmitic acid "^^ 



C15H31COO' 



palmitin 



The condensation of fatty acids and glycerol in plant cells is supposed to be 



