348 



PROTOPLASM 



potential of cells. Wurmser also refers to certain derivatives of 

 sugar. 



In conclusion, it seems that the most that can be said of the 

 work done on oxidation-reduction potentials in Uving systems 

 is that the potential value obtained is at least an indication of 

 relative metabolic activity. 



Membrane Potentials. — Membranes, such as parchment 

 paper, collodion, skin (of plant or animal), the Hning of intestines 

 and the protoplasmic membrane of cells, have the extraordinary 

 property of allowing only certain ions to pass through or certain 



(■Agar bridge 



\ .^Membrane 



52 



^; 



^Mkci 



Fig. 152. 



-Chain of connections for measuring the difference in potential between 

 the two sides of a membrane. 



ions to pass through more rapidly than others. Such membranes 

 are said to be differentially, selectively, or semipermeable. 

 Most nonliving and all Uving membranes are selectively perme- 

 able. Thus, parchment paper and collodion membrahes permit 

 positive ions to pass through more readily than negative ones, 

 and the apple skin is likewise more freely permeable to the cation 

 potassium than to the anion chlorine. If a selectively permeable 

 membrane separates electrolytic solutions, there will result a 

 different distribution of the ions on the two sides of the mem- 

 brane from that which existed at the beginning of the experi- 

 ment. A membrane potential results which may be measured 

 with the aid of electrodes and a potentiometer (Fig. 152). The 

 values are usually of the order of 50 mv. (In Fig. 152, two 

 calomel electrodes are shown. Here they do not serve as 

 reference electrodes, as in pH measurements, but are used in 

 going from metallic to liquid conduction so that they may 



