752 General and Applied Biology 



pH. Distilled water has a pH of 7.0 or neutrality. Acids extend in pH 

 from to 7, while bases (alkalies) extend in the scale from 7 upward. 



Protoplasm is a mixture of electrolytes. The acids, bases, and salts 

 of the protoplasm are dissociated into ions. These substances confer 

 charges on any surfaces on which they accumulate. Colloidal particles, 

 each bearing a minute charge, may be changed as chemical reactions 

 take place in the protoplasm or as ionizing substances are introduced 

 from the outside. The effects on colloidal particles of the protoplasm 

 by organic and inorganic substances brought to the protoplasm may ex- 

 plain the reasons for the invariable variations in all living protoplasm. 

 The hydrogen ion acts as a catalyzer by hastening hydrolysis (double 

 decomposition involving water) in the digestion of foods. The pH of 

 human blood is slightly alkaline (about 7.4), which is of great impor- 

 tance in counteracting the acidity of other tissues. The body tissues, 

 by constantly giving carbon dioxide to the blood, produce carbonic acid 

 which causes only a very slight change in pH of the blood because of 

 the buffer action of the carbonates, phosphates, and proteins in the 

 blood. The carbonic acid also stimulates the respiratory centers of the 

 nervous system to increase respiration to eliminate the excess carbon 

 dioxide. 



Permeability of Membranes and Osmotic Pressure 



Permeability may be defined as the property of a membrane or parti- 

 tion that determines its penetrability. The permeability of a membrane 

 depends on (1) the size of the pores of the membrane, (2) the size of 

 the particles of the substance attempting to pass through that mem- 

 brane, and (3) the solubility of the substance in the membrane. A mem- 

 brane may be permeable to small molecules but impermeable to large 

 molecules (Fig. 367). Another membrane may be permeable to ions 

 but impermeable to even the smaller molecules. The boundary of cells 

 consists of fatty substances and other aqueous materials which influence 

 its solubility properties, which in turn, at least partially, determine its 

 permeability. Living membranes, such as the plasma membrane of cells, 

 which have a selective permeability are known as semipermeable mem- 

 branes. Living membranes usually permit the passage of small molecules 

 and certain ions, while larger molecules, like protein molecules, and 

 colloidal particles are restrained. Diflferent cells vary in the permeability 

 of their boundaries. The cells of the lining of the lung allow certain 

 gases to pass, while the cells of the intestine permit certain other sub- 

 stances to pass. Each has its specific type of permeability, and the 



