PHYSICO-CHEMICAL ASPECTS 19 



the analysis of drug action, because as a result of this 

 polarisation, the cell membrane becomes a highly labile 

 system capable of giving almost explosive responses to 

 certain drugs such as acetylcholine, or alternatively of 

 losing its labile character in the presence of other drugs. 

 Possibly connected with the lability of polarised mem- 

 branes is the presence in many cells of small patches 

 which are selectively permeable to certain substances. 

 It may be that the action of acetylcholine is not an action 

 generalised over the whole surface of the responding 

 cell, but is confined to small active patches. Similarly 

 the stimulating action of light, which is mediated by 

 visual purple, may involve a sensitive system localised 

 in active patches in the cell membrane. 



The Dynamic Condition of Cell Constituents. Studies 

 made with isotopes, though still very far from complete, 

 have shown that practically all the atoms present in the 

 apparently stable structures of a cell are being fairly 

 rapidly exchanged with other atoms of the same type. 

 Thus, although many features of cell architecture when 

 examined by histological or cytological methods may 

 appear stable, in fact every part of the cell appears to be 

 in a dynamic condition. Every enzyme molecule, every 

 protein molecule, every nucleic acid molecule, and prob- 

 ably every part of a gene is in a state of constant change 

 on the chemical level. It is very easy to see that drugs 

 may find sites of action of the greatest importance in 



Cell Physiology 2 



