116 - The Cell 



Fig. 6-4. The diffusional exchange of oxygen (the circle.) and carbon dioxide (the dots) between the protoplasm 

 of an amoeba and the surrounding water. Both substances obey the same law— passing toward the region of 

 lowest concentration; i.e., into the protoplasm, in the case of the oxygen; and into the environment, in the case 

 of the carbon dioxide. 



this water remains at equilibrium with the 

 20 percent of oxygen in the supernatant air. 

 Likewise carbon dioxide, in leaving the cell, 

 follows a similar gradient. In the protoplasm, 

 carbon dioxide is produced continuously by 

 metabolism. Consequently CO L , reaches a 

 higher concentration relative to the outside 

 water, which maintains equilibrium with the 

 small amount (0.03 percent) of carbon diox- 

 ide in the atmosphere. 



Passive Permeability of the Plasma Mem- 

 brane. Nonprotoplasmic membranes such as 

 the pellicle and cell wall are freely permeable 



to most of the substances that normally im- 

 pinge upon the cell, and it is the plasma 

 membrane that exerts a main control over all 

 diffusional and osmotic processes. The per- 

 meability of the plasma membrane, which is 

 very complex and variable, depends not only 

 upon the nature of the impinging particles, 

 but also upon the changing conditions in- 

 side and outside the cell. Permeability differs 

 in dillerent cells and sometimes on the differ- 

 ent sides of the same cell. Nevertheless, cer- 

 tain generalizations can be made, and these 

 are summarized in Table 6-1. 



Table 6-1— Capacity of Various Substances to Penetrate (Passively) the Plasma Membrane 



Very Rapid 



Gases 



Carbon dioxide 



Oxygen 



Nitrogen 

 Fat solvents 



Alcohol 



Ether 



Chloroform 



Rapid 



Slow 



Very Slow 



Simple organic Strong electrolytes 

 substances Inorganic salts 



Glucose 

 Amino acids 

 Glycerol 

 Fatty acids 



Acids 

 Bases 

 Disaccharides 

 Sucrose 

 Maltose 

 Lactose 



Virtually Xo 

 Penetration 



Complex (colloidal) 

 compounds 

 Proteins 

 Polysaccharides 

 Phospholipids, etc. 



