364 LIFE : OUTLINES OF GENERAL BIOLOGY 



comparable to pepsin dissolves the hard protein stores in sprouting 

 seeds. 



The chemical nature of enz^Tnes or ferments is not yet known. 

 They are definite organic substances, but they do not seem to 

 belong to any of the great groups — namely, the proteins (like white 

 of egg), the carbohydrates (like starch), or the fats. It is not difficult 

 to obtain concentrated essence of digestive glands like the sweet- 

 bread or pancreas, but the extract in such cases is a complex 

 mixture, including for the gland mentioned at least three different 

 ferments. Heat and powerful chemical reagents destroy these, and, 

 though some progress has been made, no ferment has yet been 

 isolated in a pure state, though there are rumours of a crystalline 

 urease from certain plants. According to Willstatter an enzyme prob- 

 ably consists of two parts. One of these may be a protein or a carbo- 

 hydrate in a colloidal state, but it serves mainly as the carrier of 

 a simpler, chemically active substance of unknown nature, which 

 is the ferment in the strict sense, though it cannot do its work 

 without its carrier. Some of the properties of ferments depend on 

 the carrier, but the particular power depends on the mysterious 

 "active substance". And here it should be noted that the power is 

 very particular, or as is said specific; thus an enzyme, which will 

 cause the rapid fermentation of one kind of sugar, may exert no 

 influence at all on another sugar closely related. 



Most of the ferments are soluble in water or salt solutions and in 

 glycerine. Each has a temperature at which it works best, and a 

 particular medium that suits it better than any other. 



But in what way are ferments jx'culiar? They quicken chemical 

 changes, often prodigiously, yet they do not enter into combination 

 with the substance that is formed as the outcome of the fermenta- 

 tion; a small amount, given sufficient time, is as effective as a large 

 amount ("a little leaven leaveneth the whole lump"); a very minute 

 quantity will often serve: thus a spoonful of rennet will clot 400,000 

 times its own weight of the cheese-forming substance in milk. When 

 a substance acts as a quickener of chemical changes without being 

 used up in so doing, it is called a catalyst, and all ferments seem to 

 act as catalysts. But there are many substances, such as spongy 

 platinum, which act as catalysts, though one would not call them 

 ferments or enzymes. All enzymes act as catalysts, but all catalysts 

 are not enzymes. 



Another ]x?culiarity of ferments is that in most cases their actions 

 are "reversible". This requires some explanation. In many a mixture 

 when a ferment is splitting up a substance — untying a knot of 

 molecules — there is another change in progress working in the 

 opposite way, tying the knot once more. At a certain point the 

 splitting-up and the building-up attain the same velocity and no 

 further change occurs, a condition of balance or equilibrium having 



