166 PLANT PHYSIOLOGY 



percentage composition the formulae have been calculated. The 

 following calculations for typical proteins give some conception 

 of their size and complexity. 



Protein Source Formula 



Albumin eggs C696H1125N175O220S8 



Gliadin wheat C685H1068N196O211S5 



Zein corn C736H1161N184O208S3 



These are the formulae for simple proteins; others like the conju- 

 gated proteins are much more complex. 



Since the molecules are so large, the passing of protein through 

 the pores in a parchment or animal membrane is prohibited, al- 

 though some of the derived proteins (albumoses and peptones), 

 which are obtained by partly breaking down the complex proteins, 

 may pass through such membranes. 



Coagulation. — When in a colloidal solution, the natural or 

 native proteins may be turned into a semisolid gel by heat, by 

 alcohol, or by certain enzymes known as coagulases. As an ex- 

 ample of the first named may be cited the coagulation of an egg 

 during boiling or frying; and the coagulation by enzymes is 

 utilized in the manufacture of cheese, during which the protein 

 is coagulated by the enzyme in rennet. The action is not revers- 

 ible and the protein cannot be made soluble again unless de- 

 composed into simpler substances. 



Optical Activity. — All solutions of plant and animal proteins 

 are optically active and rotate the plane of polarized light to 

 the left. 



Precipitation without Change. — In strong salt solutions of the 

 ordinary alkaline earths such as NaCl, CaCl 2 , Na 2 S0 4 , etc., the 

 proteins are precipitated out of solution, which simply means that 

 the proteins are not soluble in such solutions. Whether this is 

 due to the fact that the salts in some way rob the protein of water 

 and thus precipitate it or to an electrical effect of the salt is not 

 known. The first hypothesis assumes that the salt holds the water 

 from the protein and thus precipitates it, while the second hypoth- 

 esis assumes that the proteins are held in a colloidal solution 

 because they are mutually repelled by the similar electric charges 

 which they carry. If the salt neutralizes these charges, the force 

 holding the molecules in solution is no longer operative and they 

 settle out. 



This type of precipitation differs from chemical precipitation 



