50 



GENERAL CONCEPTS 



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B 



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• • • • • 



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r 



3u^: 



ar 



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D 

 • • • C)^» o^» 



o o o 0^0^. 



f' ^■- II ■ II ■ - - ■■ ■ *^ 



Su^ar# Salt O 



Figure 3.9. Diffusion. When a cube of sugar is placed in water (A) it dissolves and 

 its molecules become uniformly distributed throughout the water as a result of the 

 molecular motion of both sugar and water molecules (B). When lumps of sugar and salt 

 are placed in water (C), each type of molecule diffuses independently of the other and 

 both salt and sugar become uniformly distribiUed in the water (D). 



ber of molecules ot oxygen and nutrients that can reach an organism 

 by diffusion alone. Only a very small organism that requires relatively 

 few molecules per second can survive it it remains in one place and 

 allows molecules to come to it by diffusion. A larger organism must 

 have some means of moving to a new region or some means of stirring 

 its environment to bring molecules to it, or it may live in some spot 

 where the environment is constantly moving past it— in a river, for ex- 

 ample, or in the intertidal region at the seashore. The larger land plants 

 have solved this problem by developing an extensively branched system 

 of roots which can tap a large area of the surrounding environment for 

 the needed raw materials. 



16. Exchanges of Material between Cell and Environment 



All nutrients and waste products must pass through the plasma 

 membrane to enter or leave the cell. Cells are almost invariably sur- 

 rounded by a watery medium— the fresh or salt water in which an organ- 

 ism lives, the tissue sap of a higher plant, or the plasma or extracellular 



