THE CELL-MEMBRANE. 355 



the alkaline earths, especially calcium ; when in the free 

 state it is insoluble in water. Metapectic acid is soluble in 

 water without forming a jelly. The two series are closely 

 related to each other, for by the action of heat, acids and 

 alkalies the various members of both can be prepared from 

 pectose. The final product of the action of the reagents is 

 the freely soluble nietapectic acid. 



Mangin gives their distinctive reactions as under : — 



Pectose. — ^The actual properties of this substance are not 

 at all easy to ascertain, nor can they be said to be well 

 known. The material is so closely associated with cellulose 

 that it cannot be prepared pure at present. The reagents 

 that separate it from cellulose convert it either into pectine or 

 into pectic acid, the former being soluble in water, the latter 

 in alkalies. The membrane can be shown to contain the 

 two constituents by the action of Schweizer's reagent, which 

 when used with the proper precautions dissolves out the 

 cellulose and leaves the framework of the cell apparently 

 unaltered ; it then consists, however, not of pure pectose, 

 but of a compound of pectic acid with the copper of the re- 

 agent. 



Pectine. — This body swells up and dissolves in water, 

 giving a viscid liquid which is very difficult to filter and which 

 soon forms a jelly. This is the body which Fremy found in 

 ripe fruits, and it exists in many mucilages. It gives no 

 precipitate with neutral acetate of lead, but is thrown down 

 by the basic acetate in the form of white flocks. If boiled 

 for several hours in water it is converted into an isomer, 

 parapectine, which is precipitated by the neutral acetate. 

 Further boiling with dilute acids converts it into metapectine 

 which is precipitated by barium chloride. 



Pectic acid. — This body is insoluble in water, alcohol 

 and acids ; it forms soluble pectates with alkalies and in- 

 soluble ones with the metals of the alkaline earths, of which 

 calcic pectate is most widely distributed. It dissolves in 

 solutions of alkaline salts, such as the carbonates of sodium 

 and potassium, stannates, alkaline phosphates, and most or- 

 ganic ammoniacal salts, forming with them double salts which 

 gelatinise more or less freely with water. Its solutions in 



