MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 55 



standing of how a differentially permeable membrane allows passage of 

 dissolved particles, but holds back larger particles. Many dissolved 

 substances can pass in and out of the cell because of their small particle 

 size, but the larger particles cannot. 



Selective Nature of Living Membranes 



In our discussion up to this point we have assumed that diffusion 

 through a membrane was a purely physical process, with the passage 

 of materials governed solely by the size of particles in comparison with 

 the size of the openings in the membrane. This is generally true of 

 nonliving membranes, but experiments show that living membranes 

 exert some selectivity over the particles which diffuse through them. 

 The plasma membrane is a sheet made of protein and fat molecules. 

 The openings between these molecules permit water and certain small 

 dissolved particles to pass through. We know that the molecules of 

 dissolved glucose (a simple sugar) can pass through, yet some of the 

 ions of salts which are smaller than the molecules of glucose do not 

 pass through. The ions which do pass through will vary from time to 

 time according to the need of the cell for these ions. How can such a 

 selectivity be accomplished? 



We learned in Chapter 3 that ions are either positively or negatively 

 charged particles of matter. The living plasma membrane is also 

 charged, but its charge may vary. When the charge is negative it 

 will attract and admit positively charged ions, such as those of sodium 

 and potassium, but will repel negatively charged ions, such as those of 

 chlorine and carbonate. These negatively charged ions cannot even 

 approach the membrane and thus have no opportunity to pass through. 

 When the protoplasm has need of the negative ions to achieve a proper 

 balance, however, the membrane is changed to a positive charge and 

 conditions are just reversed. 



It has also been found that molecules which are soluble in fat pass 

 into the cell more readily than molecules of a similar size which are 

 not soluble in fat. We have already mentioned the fact that the cell 

 membrane contains molecules of fatty compounds. It is thought that 

 the fat-soluble molecules first dissolve in the fatty portions of the mem- 

 brane and then pass out on the other side of the membrane. If fat- 

 soluble molecules are present in large quantities, they may dissolve out 

 so much of the fat from the cell membrane as to cause a disruption 

 of normal functioning. Alcohol and ether are fat-soluble compounds 

 which act as anesthetics because they pass through the membranes of 

 the cells of the nervous system so freely. When these substances reach 



