THE BIOLOGY OF THE CELL SURFACE 



they show the effect of compression; thus cells which, were 

 they free, would be spheres, become polyhedral. The 

 varied shapes of cells depend upon many diverse factors in 

 addition to compression, such as presence of a supporting 

 frame-work, as in Foraminifera, the greater or lesser fluidity 

 of the cytoplasm, the structure of the limiting surface, and 

 most of all the little understood intrinsic structure of the 

 cell itself. 



Cells, like all matter, are chemical in composition and 

 display physical properties. Their form, structure, and 

 morphology, however varied, are molecular in make-up. 

 Hence, as we have seen, there is, in this respect, no dif- 

 ference between living and non-living matter. All the 

 protoplasms known to us have in common a certain 

 basic chemical structure. Since living things are part of 

 the natural world it is not astonishing that they are com- 

 posed chiefly of those chemical elements which are most 

 frequently found in the world of non-living things: Carbon 

 (C), nitrogen (N), oxygen (O), hydrogen (H), sulphur (S), 

 phosphorus (P), iron (Fe), sodium (Na), potassium (K), 

 calcium (Ca), magnesium (Mg) and chlorine (CI). In 

 some cells are found other elements as lithium, barium and 

 strontium; copper and zinc; fluorine, arsenic, bromine and 

 iodine. Thus the difference between non-living and living 

 systems does not reside in the basic elements of which they 

 are composed. A difference obtains in the compounds into 

 which these elements enter. 



The elements, C, H, N, O, combined build up the protein 

 molecules; out of C, O, H carbohydrates are formed; and 

 C, O, H makes up most lipins; other lipins in addition 

 contain N or P. These are the "organic" compounds of 

 the living substance and exist in nature only in living sub- 

 stance; non-living things do not contain proteins or the 

 oils found in animals and plants; sugars and starches 

 (carbohvdrates) occur naturally only in animal and plant 



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