Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/32

 deeper insight into the working of the metabolic processes of the cells, and obtain important clues as to the means by which the cells of the animal organism produce, from compounds of a definite kind, substances that belong to a different group. We may mention here, for instance, the conversion of amino-acids into grape sugar, and of carbohydrates into fats.

Some of the unicellular forms of life, and some organisms consisting of a few groups of cells, are, at least in part, equipped with agents (ferments) which are not so precisely directed against certain substrates as the ferments of the higher species, such as plants and animals. While the ferments of the latter, so far as we know, principally decompose substrates consisting of units which are found in the cell-constituents that constantly recur in Nature, cases have been observed amongst the lower organisms (i.e., morphologically lower) where the latter split up compounds, which have been prepared in the laboratory from units which are not known to exist in a free state in Nature. Owing to this wider independence these organisms are assured of better conditions of existence. These cells can live where others, being unable to secure the energic contents of the material supplied to them, and being also unable to form from this substrate the elements required for their bodies, are bound to perish through lack of nourishment. In