CHAPTER VIII 



THE PROTEINS AND PROTEASES 



No class of compounds is of more fundamental significance than the 

 proteins. The matrix of protoplasm largely consists of proteins in the 

 colloidal state, and, without doubt, they occur to some extent in the 

 same condition in the cell-sap. They are also found in the cell in the 

 solid state, in the form of either amorphous granules, termed aleurone, 

 or crystalline or semi-crystalline bodies, termed crystalloids. Both solid 

 forms constitute " reserve material" and are often found in seeds, tubers, 

 bulbs, buds and roots. 



Plant proteins may be classified on the following plan : 



1. The simple proteins. 



(a) Albumins. 



(b) Globulins. 



(c) Prolamins (Gliadins). 

 {d) Glutelins. 



2. Conjugated proteins. 



(a) Nucleoproteins. 



3. Derived proteins. 



(a) Metaproteins. 



(6) Proteoses (Albumoses). 



(c) Peptones. 



(d) Polypeptides. 



Although they are present in every cell in all parts of plants, little, 

 however, is known of plant proteins, except of those in seeds, because 

 of the difficulties of obtaining them in sufficiently large quantities, and 

 of separating them from each other. 



Proteins are in the colloidal state when in so-called solution, and are 

 unable to diffuse through parchment membranes. The proteoses and 

 peptones, however, which have simpler molecules, can diffuse through 

 such membranes. 



The vegetable proteins are soluble in various solvents according to 

 the nature of the protein ; some are soluble in water, others in dilute 

 salt solutions, others, again, in dilute alkalies, and a few in dilute 

 alcohol. Vegetable albumins are coagulated from solution on boiling. 



