HOW DIFFUSION TAKES PLACE 



If we throw balls of different sizes at a tennis net, we may expect most of the smaller 

 balls to go through the net, and all or most of the larger ones to be stopped. In 

 much the same way, we imagine, some of the rapidly moving molecules of dissolved 

 substances pass through the pores of an osmotic membrane, while larger molecules 

 move through in smaller numbers or not at all 



pens in roots and in other parts of living things if we think of this work as 

 the action of the rapidly moving molecules. But there is still the problem 

 of understanding how roots work, since they seem to be raising water 

 against gravity, and they seem, at any rate, to be taking more out of the soil 

 than they might be giving off. 



The cell walls of the root, and of practically all plant parts, consist of 

 cellulose, a substance that does not dissolve in water, but does absorb water 

 in the same way as glue or gelatin. Now, we must imagine that wherever 

 there is water, substances dissolved in it will diffuse in it. When the cellulose 

 walls of root-hair cells are saturated with water, the molecules of dissolved 

 substances diffuse through this water. This kind of "diffusion through a 

 membrane" is called osmosis, from a Greek word meaning "to push". We 

 conceive osmosis to be taking place through the walls of all cells, those of 

 animals as well as those of plants. 



Since the liquid or solution inside the root hair is different from the soil 

 water surrounding the cell, we should expect that some of the substances 

 would be diffusing into the cell, and other substances moving out of the cell. 



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