LECTURE IV. 



YEAST, then, lives in a mixture of proteins, carbohydrates and salts 

 in water. Evidently the soluble salts and carbohydrates being 

 free to penetrate the cellulose wall of the cell by diffusion come 

 into contact with the protoplasm and are by it taken up and in- 

 corporated. Not so, however, with the proteins and insoluble 

 carbohydrates. Their dispersed colloidal particles are, for the 

 most part, too large to penetrate the colloidal cell-wall and con- 

 sequently cannot come directly into contact with the protoplasm. 



As was mentioned before, proteins are capable of partial dis- 

 integration furnishing substances of less molecular complexity and 

 smaller molecular weight. These substances, the peptones and 

 amino-acids, exhibit some of the properties of crystalloids and 

 appear to be intermediate in their characteristics between crystal- 

 loids and colloids. Chemically the change is one of hydrolysis, 

 and may be effected by means of acids. The plant, however, 

 effects it by the agency of bodies called proteolytic enzymes. 



The enzymes, of which these are a special group, are substances 

 which are produced by organisms to hasten or facilitate a change. 

 The change which is thus facilitated would otherwise take place 

 imperceptibly slowly, at ordinary temperatures. In the presence 

 of the enzyme, however, the change proceeds with moderate speed. 

 At the end of the change the enzyme is unaltered, except so far 

 as secondary reactions may affect it. The enzymes resemble in 

 their action catalysts, which in a similar way determine and hasten 

 slow reactions. But these organic catalysts differ from the in- 

 organic, e.g. acids, platinum black, etc., in being limited in their 

 action, and each will catalyse only one, or a group of similar re- 

 actions. For this reason they are said to be "specific". There 

 is, in fact, reason to believe that the atomic groups in the enzyme 

 have a specific relation to those in the substance on which it acts. 

 In other words, the molecule of the enzyme " fits " the molecule 

 of the substrate, as the substance to be changed is called. 



Enzymes are in the colloidal state but their degree of dispersion 

 varies, hence some of them can penetrate a cell-wall and others 



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