PLANT METABOLISM 229 



are reversible, various amino acids can be converted by such reactions 

 into keto acids for respiratory metabolism. Probably not all amino 

 acids are formed or decomposed directly by this process, but many 

 are, including glycine, alanine, leucine, isoleucine, tyrosine, and per- 

 haps others. Once these amino acids have been synthesized by trans- 

 aminations, others are made from them. Starting with one or more 

 of these substances, most of the other nitrogen compounds of the cells 

 are made (see Chapter 20). 



METABOLISM OF UPIDES 



Once again more work has been done with animals than with 

 plants. However, some general features may be presented for plants. 

 Triglycerides accumulate in higher plants, where they appear to serve 

 as energy reserves, especially in seeds. Since little is known of the 

 reactions involved in the utilization of fats by germinating seeds, 

 attention will be limited here to the biosynthesis of fats in plants. 



All the tissues of plants contain fats, but the amounts are small 

 in most species except in the fruits and especially the seeds. Long 

 ago it was believed that fats, or perhaps the component parts, were 

 synthesized in the leaves and growing parts of plants and translocated 

 to the fruits. Later, however, it was found that fruits detached from 

 the plant coidd still make fats and that most if not all of the fat of 

 the fruit was synthesized therein. As a result, studies of the biosynthe- 

 sis of fats in plants have involved fruits and seeds. 



Present opinion holds that the carbon of fats comes from carbo- 

 hydrate. Radioactive carbon from glucose appears in the oil of seeds 

 from maturing cotton bolls and in that from castor beans. It is 

 logical to assume that any substance whose metabolism is connected 

 to that of carbohydrates could supply carbon to the fats. This logical 

 possibility must depend upon the occurrence of intermediates that 

 can be transported to the fruit. Amino acids can be converted in some 

 cases to keto acids of the tricarboxylic acid cycle and thus become 

 available for synthesis. The branchcd-chain amino acids actually 

 supply part of the carbon skeletons for branched-chain fatty acids 

 now known to occur in small amounts. These and related speculations 

 await examination. 



Assuming that carbohychates or their breakdown products are 

 transported from leaves to fruits and converted to fats, the mechanism 

 of this conversion becomes of interest for several reasons. First, the 

 seeds of different species are specific in the kinds of fatty acids formed. 



