LIFE AND PROTOPLASM 



i.r, 



selectively permeable. Most living cells allow oxygen and carhon 

 dioxide to pass freely through their mcmijranes, while diss(jlved 

 sugars and digested proteins in the form of amino acids dilTu.sc 

 through more slowly. Water of course passes through, acting as a 

 vehicle for other substances. Such membranes are impernicablc to 

 certain salts and not to others. The permeability of living cells 

 to dissolved substances differs with the cell, and naturally with tlu; 

 organism. Salt- and fresh-water fishes are examples of types, the 

 cells of whose gills exhibit different permeabilities. Dead cell mem- 

 branes are usually permeable to crystalloid solutes, while living 

 cell membranes permit but few salts to enter. In general, cells are 

 not permeable to colloids, because of the large size of the particles 

 constituting the colloid. 



Osmosis and Its Significance to Living Cells 



We have already seen that if a membrane is sc^lectiveiy permeable, 

 then some substances, such as water or certain solutes, will pass 

 through readily, but other 

 solutes may not. because 

 their molecules are too large 

 to pound their way through 

 the ultramicroscopic "pores" 

 of the membrane. The 

 process by which substances 

 diffuse through membranes 

 is known as osmosis. It is 

 of the greatest importance 

 to living cells, as it is by this 

 means that dissolved gases, 

 such as oxygen, and dis- 

 solved food substances get 

 into the cell, as well as the 

 process by which waste ma- 



- sugar 



^littion... 



lJ» selectively 

 lJ.pcrroeab\^ 



t 



-'-'4* 



molcciclc 



^ • •wat«r'mo\eculti =^ °'TS^^ 



-penTKoBe 



=1. "WCCt&V 



vater- 



.XLi» 



Diagram to explain osmotic pressure. Sii>:ar 

 solution is of equal density in ea<h tube. 

 Explain rise of solution in left hand tube. 



terials pass out. Perhaps a further word of oxplana .on ,s u, orcler. 

 Other things being equal, if two soh.tions «' '''«""": ™"''™'7,;" 

 are separated by a permeable membrane, the chffus.on ^v,ll ^fll , 

 in the direetion of the greater to the lesser ™';'-"- "' ; V^ 

 it a sugar solution be .separated by a pern,eabIo men,b,a,.o 

 another more dilute sugar solution, diffusion -l'/^' ;;■'," 

 the more coneentrated. If, however, we separate «at,. from .. 



H. W. H. — 10 



