Movements of Materials Into and Out 

 of the Cell 



All cells are surrounded by a plasma membrane, and in most cases 

 all of the materials which enter and leave a cell must pass through this 

 membrane. There are a few cells, such as amoebae and white blood 

 cells, which can engulf food in large masses, but the vast majority of 

 cells have no such methods. The plasma membrane is composed of 

 molecules of fats and proteins and is somewhat porous because of the 

 openings between the molecules. Water, with its small H 2 molecules, 

 is able to pass through this membrane readily. Also, certain substances 

 which are dissolved in water exist as ions, atoms, and molecules which 

 are small enough to pass through this membrane. Larger molecules 

 and aggregations of molecules cannot penetrate this barrier. The cell, 

 therefore, must obtain all its needed food and other materials in a form 

 which is reduced to particles small enough to pass through the plasma 

 membrane. Also, all of the waste products of the cell must be expelled 

 through this membrane in the same state of solution. But from whence 

 comes the force to cause these particles to move in the necessary direc- 

 tions? There is an important principle to explain it. 



Diffusion 



If we drop a crystal of some soluble dye in a beaker of water, the 

 color will gradually spread out from the region of the crystal until the 

 water is colored at the top of the beaker. If we leave it undisturbed for 

 a long period of time the dye will become uniformly distributed through- 

 out the water. Some force has caused this dispersion of the dye. What 

 is it? It is the force of diffusion which can be explained as follows. 

 As the dye is dropped into the water it begins to dissolve — the outer 

 molecules of the crystal break off and float free in the water. We 

 learned in Chapter 3 that molecules are always in motion, and in a 

 liquid they are free to move about one another. This movement we 

 remember comes from the energy of heat. In the immediate area of 

 the crystal the number of molecules of the dye is greater than it is in 



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