EARLY HISTORY OF LIFE 



31 



Na* ^ 



cr 



* glucose 



OO fot 



o 



*^* cm'mo 

 acids 



Fig. 2-21. A diagrammatic view of a cell showing how particles may enter and leave through the semi-permeable 

 membrane. Glucose, amino acids, water, and some ions pass readily through the membrane due to its apparent 

 porosity; fats enter by dissolving in the membrane itself. Ions with charges opposite to that of the membrane 

 cannot pass through. Large protein molecules are denied entrance because of their large size. 



pie, if an amoeba is placed in a solution 

 high in potassium and low in calcium, a 

 new membrane will fail to form if a rent is 

 made in it, whereas in a solution with the 

 proper proportions of these ions a new 

 membrane forms at once. If the concentra- 

 tion is high in calcium and low in potassium, 

 the entire organism gels, in other words, it 

 loses all of its fluidity and becomes a con- 

 gealed corpse. Therefore, a careful balance 

 must be maintained between these two ions 

 if a normal membrane is to form around the 

 protoplasmic mass. 



Such a sheet of protein and fat molecules 

 must possess some unique properties if it is 

 to perform the necessary functions of keep- 

 ing the protoplasm from disintegrating. This 

 is accomplished by the physical nature of 

 the membrane itself. The lipid and protein 

 molecules cling together but between them 

 are small openings where they do not fit 

 quite snugly, and through these water and 



other small molecules may pass freely from 

 one side to the other. Fat-like particles can 

 dissolve in the lipid molecules forming the 

 membrane, pass inside, then slowly move 

 out of the other side to the interior of the 

 cell. Larger protein molecules cannot pene- 

 trate the membrane because the openings 

 are too small to admit them (Fig. 2-21). 

 Glucose and amino acid molecules pass 

 through the openings readily but some of 

 the ions, even though much smaller, are un- 

 able to get through because of the electrical 

 charge which they bear. The membrane is 

 charged either positively or negatively de- 

 pending on the surrounding conditions of 

 the environment. If it is negative, such ions 

 as chlorine and carbonate are repelled and 

 cannot get by this electrical barrier. On the 

 other hand, sodium and potassium ions are 

 attracted to the membrane and pass through 

 readily. A constant flow of these ions both 

 ways satisfies the needs of the internal pro- 



