108 MEMBRANES (PLASMAHAUT) 



or excretion from the cell. In plants, it generally consists of 

 cellulose. Certain animals develop an exoskeleton of the excreted 

 salt of lime, of silica or of chitin. These excreted membranes 

 should not be confounded with the true cell membrane or plas- 

 mahaut, which term connotes the layer of cell protoplasm which, 

 in animal and plant cells alike, lies between cell and environment. 

 Only through this layer or membrane can the cell be influenced 

 by changes in the surrounding medium. 



The presence of a covering for animal cells cannot be proved 

 in quite the same way. Animal cell membranes are more elastic 

 than those of plants. Microscopic examination shows generally 

 a difference in refractive index (page 103) round the border of 

 cells. Free cells like red blood corpuscles may be submitted to 

 experiments similar to the plasmolytic one detailed above. If 

 corpuscles are put into a solution of lower osmotic pressure 

 (hypotonic solution) than their contents, they will swell up because 

 of the passage inwards of water, i.e. endosmosis, and will probably 

 burst. This is called haemolysis, and may be brought about in 

 other ways, which are, however, all obviously methods for de- 

 stroying a membrane (page 243). Artificial membranes may be 

 made which act in a similar way to animal cell coverings. These 

 experiments, together with the fact that it is quite impossible to 

 conceive of energy changes taking place in naked protoplasm, are 

 sufficient evidence of the need for cell membranes. 



Much research and much speculation has been published on the 

 nature of these membranes. What conditions have they to 

 fulfil ? At least four qualities are essential : 



(1) The membrane must prevent the outward passage of cell 



substance while allowing water to pass freely in and 

 out. 



(2) The membrane must permit of the intake of nutrient 



material and of the output of undigested and non- 

 utilised material. 



(3) The waste products of metabolism, gaseous and liquid, must 



find a way out, while oxygen must find a way in. 



(4) Finally, the membrane must be of such a nature as to allow 



of expansion. Mere elasticity will not answer this end. 



The membrane must be capable of almost instantaneous 



growth. 



The animal cell membrane must not be considered as a box 

 or container in which the cell protoplasm has been placed. It is 

 not something apart from the cell like an eggshell. It is not even 



