SYNTHESIS OF FATS ACCOMPANYING INTESTINAL ABSORPTION 45 
Discussion of the Synthetic Activity of Enzymes and of Living Cells 
One of the most important results obtained from the work described above, 
is that neither cell-free extracts of the intestinal mucosa nor detached cells are capable 
of synthesizing neutral fats from their constituents ; the positive results in this 
direction obtained by previous workers being shown to be due to incomplete analysis 
of the products obtained. On the other hand, it is quite as clearly demonstrated, by 
the experiments on the nature of the fatty constituents contained in the mesenteric 
lymphatic vessels during fat absorption, that all the neutral fat decomposed during 
digestion is resynthesized during the process of absorption back to neutral fat. 
This result is in full accordance with the facts known regarding cellular activity 
as contrasted wich enzymic activity throughout the body generally, and naturally leads 
the mind to enquire what are the usual respective limits of enzymic and cellular 
activity, and to a contrast between the types of chemical reaction induced respectively 
by isolated enzymes and by living cells. 
It is at the present time becoming increasingly fashionable to attribute all the 
chemical processes occurring in living cells to the presence in the cell substance of 
soluble enzymes, which, if we possessed sufficient knowledge of the necessary 
manipulation, might be satisfactorily isolated, and made to perform their work apart 
from their natural laboratory, the cell ; so that if all the contained enzymes had been 
isolated in common solution, the chemical changes occurring in the cell would be 
capable of being brought about by that solution. 
There exists, however, no sufficient experimental basis for such a generalization; 
there is no proof, in the first place, that all chemical changes in the cell are due to 
enzymic action ; nor, in the second place, even admitting that the process is carried 
out by a number of enzymes acting in consort, that the presence of the living 
protoplasm is not essential for correlating the work of the enzymes concerned, and 
regulating the extent of activity of each, so that a number of complex chemical 
reactions may be combined in such a fashion as to yield definite end-products in 
definite proportions. When a comparison is made of the products elaborated by the 
living cells of the body, and those produced by the activity of enzymes, certain very 
important differences become apparent, which may here be reviewed. 
In the first place, the type of reaction induced by enzymes is very simple compared 
with that brought about by cells. The vast majority of enzymic reactions are simple 
hydrolyses, such as can readily be imitated by dilute acids or alkalies ; in other cases there 
is merely a simple molecular rearrangement, or at most, the detachment of the elements 
of water and carbon-dioxide, as in the case of the yeast enzyme. On the other hand, 
when the activity of living protoplasm is called into play, there occurs a building up 
of most complex chemical products from simple constituents, such as has only been 
imitated in a few of the less complex instances in the chemical laboratory, and then 
