The Chemical and Physical Structure of Protoplasm - 73 



The origin of the electric charges, which 

 are characteristic of all ions, can be seen by 

 inspecting the atomic structure of the NaCl 

 molecule (Fig. 4-5). The sodium atom in unit- 

 ing with chlorine gives up one electron, 

 becoming the sodium ion (Na+); and the 

 chlorine atom becomes the chloride ion 

 (Cl~), by accepting an electron. So long as 

 these ions remain united as the NaCl mole- 

 cule, the electric charges neutralize each 

 other; but if the ions separate — as they do 

 when the salt is dissolved in water — the 

 ionic charges become effective. 



Not all ions are as simple as Na+ and CI - , 

 as may be seen in the following cases: 



dissociation 



1. NaNQ 3 ( > Na+ -f NOj 



sodium association sodium nitrate 



nitrate, ton ion 



in aqueous 

 solution 



2. MgCI 2 ; 



magnesium 

 chloride 



± Mg 2 + -f- 2CI- 



magnesium chloride 



Generally speaking, an ion is an atom, or 

 group of atoms, that bears one or more elec- 

 tric charges. Positively charged ions are 

 called cations, and negatively charged ions 

 are called anions. 



Electrolytes and Nonelectrolytes. Some 

 substances, such as sugars, starches, and fats, 

 display no appreciable tendency to ionize, 

 and such compounds are called nonelectro- 

 lytes; but other substances, such as salts and 

 proteins, dissociate more or less strongly, and 

 these are called electrolytes. Among the elec- 

 trolytes, some are strong and others are weak, 

 depending upon the proportion of the mole- 

 cules of the given substance that undergo 

 dissociation in aqueous solution. 



Experimentally, electrolytes are distinguished 

 from nonelectrolytes by measuring the electrical 

 conductivity of the substances dissolved in water. 

 Water itself conducts electricity very poorly, and 

 when pure water is placed between the poles in 

 an electric circuit, scarcely any current flows. But 

 if a strong electrolyte, such as sodium chloride, is 

 added to the water, the resulting solution is a 



conductor. The current, in fact, is carried by 

 the ions of the solution. The cations (in this case 

 Na + ) migrate toward the cathode, or negative 

 pole; and the anions (in this case Cl — ) pass to- 

 ward the anode, or positive pole. Accordingly, 

 the conductivity of an aqueous solution is a good 

 index to the degree of dissociation of the solute 

 molecules. In the case of nonelectrolytes, such as 

 sugar, the solution, as compared to pure water, 

 displays little or no increase in conductivity. 



Many compounds in the cell are strong 

 electrolytes, which dissociate freely in the 

 aqueous parts of the protoplasm. On this 

 account, protoplasm itself will conduct elec- 

 tricity quite freely, and this fact is of con- 

 siderable importance in every cell (Chap. 11). 



Dissociation of Water: Hydrogen and 

 Hydroxyl Ions. The conductivity of pure 

 water is very small, but even this small con- 

 ductivity is significant. It indicates that water 

 dissociates in small degree, as is shown in 

 the following equation: 



H-OHCHgO) £ 



molecular 



± H+ + OH- 



hydrogen hydroxyl 



Despite the fact that the ratio of dissoci- 

 ated to undissociated molecules in pure water 

 is very small (1:555,000,000), the dissociation 

 of water cannot be overlooked. Hydrogen 

 ions (H + ) and hydroxyl ions (OH - ) are both 

 extremely active ions, which participate, di- 

 rectly or indirectly, in many metabolic re- 

 actions of every cell. 



Acid, Basic, and Neutral Solutions. All 

 solutions are classified as acid, basic, or neu- 

 tral— depending upon the proportion of hy- 

 drogen (H + ) and hydroxyl (OH - ) ions they 

 contain. In acid solutions, the hydrogen ion 

 is more abundant than the hydroxyl; in basic 

 (alkaline) solutions, the hydroxyl ion is more 

 abundant than the hydrogen; and in neutral 

 solutions, the hydrogen and hydroxyl ions 

 are present in equal numbers. 



Pure water (HOH) displays a neutral re- 

 action, because each dissociated water mole- 

 cule liberates H+ and OH" ions in equal 



