278 PRINCIPLES OF GENERAL PHYSIOLOGY 



fat of the dog which had received mutton suet was solid at 50 C., whereas that 

 of the dog fed on oil was still liquid at C. Although fats are hydrolysed in the 

 lumen of the intestine, their constituents are resynthesised in the wall of the 

 intestine, and are carried in the chyle to the blood as neutral fats. The part 

 played by enzymes in this process will be discussed later. 



2. From Carbohydrate in the food. Although the ^formation of fat from 

 carbohydrate in the process of fattening animals for food seems obvious in the 

 ordinary practice of farmers, complete evidence was wanting until the experiments 

 of Lawes and Gilbert (1852, p. 350) on young pigs, fed on barley, in which it 

 was shown that the amount of fat laid on was .considerably greater than could 

 possibly have come from the protein in the food, after deducting that used for 

 tissue formation in the body. The amount of fat in barley food is very small. 



Until the recent work of Miss Smedley (1912) the chemical mechanism of 

 such a transformation was unknown. 



As regards the glycerol component, the facts described in the previous section 

 show how it is readily obtained from glucose. 



As regards the fatty acid component, the first fact to be noticed is that the 

 fats of the organism are limited to those whose fatty acids contain an even number 

 of carbon atoms. There must obviously be some reason for this. Another fact 

 is that the process of formation from carbohydrate is, as Leathes has shown (1906, 

 p. 85), an exothermic one. This might, at first sight, seem surprising, since the 

 heat of combustion of a gram-molecule of fat is so much higher than that of a 

 gram-molecule of glucose. But we must remember that several molecules of sugar 

 are required to form one of a higher fatty acid ; for example, stearic acid contains 

 eighteen carbon atoms. 



Miss Smedley and Miss Lubrynzka (1913, 1 and 2) have brought forward good 

 evidence to show that the process of fat synthesis in the organism takes place in 

 the following way : We have seen that pyruvic acid is formed as a stage in the 

 oxidation of glucose. Moreover, there is reason to believe that pyruvic acid is 

 converted by a further process, probably enzymic, into acetaldehyde and carbon 

 dioxide. This aldehyde may then condense with another molecule of pyruvic acid 

 to form a higher ketonic acid, thus : 



CH 3 CHO + CH 3 CO.COOH = CH 3 CH:CH.CO.COOH + H 2 O. 



The investigators named have succeeded in obtaining this reaction with butyl 

 aldehyde and pyruvic acid. 



The next stage is the conversion of the ketonic acid thus obtained into its 

 aldehyde and carbon dioxide, by a similar process to that by which the acet- 

 aldehyde was originally obtained from pyruvic acid : 



CH 3 .CH:CH.CO.COOH = CH 3 .CH:CH.CHO + CO 2 . 



This aldehyde, which has two more carbon atoms than that from which we 

 started, condenses with another molecule of pyruvic acid, forming a ketonic 

 acid of still longer carbon chain, and so on. 



From the unsaturated ketonic acid formed at any stage we obtain, by 

 oxidation, an unsaturated fatty acid with one less carbon atom, thus : 



CH,.CH:CH.CO.COOH + O = CH,.CH:CH.COOH + CO,, 



o O 



and from this, by reduction, the fatty acid containing two more carbon at'm> 

 than the aldehyde from which we started in this particular stage. For example : 



CH 3 .CH:CH.COOH + H 2 = CH 3 .CH 2 .CH 2 .COOH, 



that is, we have obtained butyric acid starting from acetaldehyde. 



The process may be repeated many times, and fatty acids with a long chain 

 of carbon atoms obtained. It is necessary to remember, however, that branched 

 chains are not formed in this comparatively simple manner. 



The reverse change from fat to carbohydrate is known to occur in the 

 germination of fatty seeds, where starch and cellulose are formed from the fat. 



In hibernating mammals, as Pembrey (1903) has shown, there is an extraordinarily low 

 respiratory quotient of 0'3-0'4. This means that there is a conversion of substances 

 containing a small amount of oxygen into others containing a larger amount of oxygen, that 



