124 



Living Things Are Basically Alike unit ii 



Fig. 152 Year after year the 

 pieces of rock are pushed 

 farther apart by the growth 

 of the tree. (American mu- 

 seum OF NATURAL HISTORY) 



are said to be insoluble; thev do not dis- 

 solve. Among many other compounds, 

 starches and proteins and fats, for ex- 

 ample, are insoluble in water. Therefore 

 they do not diffuse through a membrane. 

 You can convince yourself of this by 

 doing Exercise 13. 



But there are some substances that are 

 soluble in water and yet fail to pass 

 through some kinds of membranes. Or 

 let us state it the other way: some mem- 

 branes allow certain soluble substances to 

 pass through but keep out other soluble 

 substances. The word pcrDieable means 

 "allowing substances to pass through." 

 Then we can say that some membranes 

 are permeable to some soluble substances 

 and are not permeable or less permeable 

 to other substances. They are not per- 

 meable t(; certain very large molecules 

 or groups of molecules. 



Whicli substances enter living cells? 

 The cell membrane, made up of proto- 

 plasm, is a good example of a membrane 

 which differs in its permeability to dif- 

 ferent substances. Some compounds pass 

 through; other solui)lc compounds ^ith 

 molecules of the same size cannot pass 

 through. And what is more, the cell mem- 

 brane changes in its permeability. Vari- 



ations in light and temperature or the 

 presence or absence of a great number 

 of substances make it more or less per- 

 meable from time to time. 



There is much that is not understood 

 about diffusion of substances into a liv- 

 ing cell and much that cannot be ex- 

 plained here. But you must remember 

 that many, though not all, soluble sub- 

 stances which come in contact with a 

 cell enter it. Insoluble substances do not 

 enter. Starches, fats, and proteins are 

 insoluble and certainly cannot diffuse 

 into or out of a cell. But you will learn 

 later how they are made soluble and how 

 they enter in their soluble form. Oxygen 

 is soluble in water and diffuses readily 

 into a living cell through the membrane. 



Thus you can see that as substances 

 are used up in oxidation in the cell they 

 are constantly replaced by the substances 

 that diffuse through the membrane into 

 the living cell. 



Cell activities defined. Now with an 

 understanding of oxidation in cells and 

 how the necessary fuel and oxygen enter 

 them, let us go back and examine again 

 the activities of a cell. 



Food, and certain other necessary as 

 well as unnecessary materials diffuse into 



