154 PLANT PHYSIOLOGY 



First, the hydroxyl groups must be reduced to hydrogen and, 

 secondly, the terminal aldehyde must be changed into an acid 

 group. Simple as it looks, it is very hard to do, and how the plant 

 does it we have no exact idea. It is possible that hexoses are trans- 

 formed into six carbon acids and these acids then linked together, 

 or the acids might possibly be made directly from 18 carbon sug- 

 ars (trisaccharides). The significance of the fact that the com- 

 mon sugars and the common acids contain multiples of six car- 

 bon atoms has been mentioned above. 



Smedley (1912) has prepared fatty acids from carbohydrates 

 by first breaking the hexoses into three-carbon compounds and 

 then oxidizing these into pyruvic acid (CH 3 .CO.COOH), which 

 is then condensed into acids with longer carbon chains. The diffi- 

 culty with which this is accomplished is in sharp contrast to the 

 ease with which the plant accomplishes these transformations, 

 and that the plant uses this method is doubtful, although Neu- 

 berg has isolated an enzyme which he calls carboligase that can 

 unite two molecules of pyruvic acid or of acetaldehyde. 



A somewhat similar theory (which, however, has met with 

 more success) is that of Raper (1907) and Euler (1909), who pro- 

 posed that the synthesis of fatty acids from sugar takes place 

 by breaking the sugars down into acetaldehyde and then forming 

 the acids from this substance. The fact that acetaldehyde has 

 been found in the products of sugar decomposition in cells lends 

 decided support to this hypothesis, according to which two mole- 

 cules of acetaldehyde unite first to form an aldol : 



H OH O HHHO 



I • I / III/ 



H— C— C + H— C— C = H— C— C— C— C +H 2 



I \ I \ I I I \ 



H H H H H OHH H 



which changes to crotonic aldehyde by removal of water 



HHHO HHHO 



III/ 111/ 



H— C— C— C— C -* H— C— C=C— C +H2O. 



I I I \ I \ 



H OHH H H H 



Another molecule of acetaldehyde is added to this with the sub- 

 sequent removal of water, and the product sorbic aldehyde is 



