The Basis oj Life 27 



Atoms which vary in atomic weight, but not in chemical properties, 

 are known as isotopes. Some isotopes are radioactive, that is, they 

 emit certain radiations from their nuclei. This radiation ceases when 

 a stable atomic weight is reached. Only certain combinations of neu- 

 trons and protons appear to be stable. Since this decay or loss of nu- 

 clear particles proceeds at a constant rate, it is possible to use these 

 radioactive isotopes for the study of various biological processes. Radio- 

 active isotopes of most elements have been prepared artificially. These 

 isotopes are known as labeled elements. Their pathway through the 

 plant or animal body can be traced. This new technique is resulting 

 in many rapid advances in our understanding of biological activities, 

 particularly those concerned with the details of metabolism. 



Ions. — The number of electrons in the outer ring is of utmost im- 

 portance in determining the combining properties of the atom. If the 

 outer ring has less than one-half the number required to fill it, it has 

 a tendency to lose those that it has. Inasmuch as the number of pro- 

 tons in the nucleus remain the same — despite this loss — the atom be- 

 comes positively charged. If the outer ring has more than one-half 

 the number necessary to saturate it, the atom tends to pick up electrons. 

 Again, the number of protons in the nucleus remains constant, despite 

 the increased number of electrons in the outer ring. Thus the atom 

 becomes negatively charged. These modified atoms are known as 

 ions. Positively charged atoms are called cations; negatively charged 

 ones are anions. In general, these charged atoms move toward areas 

 carrying the opposite charge. 



This property of ionization is very important in the functioning 

 of biological systems. As ions can pass readily through membranes, 

 their movements are the basis of many of the physiological activities of 

 an organism. 



Molecules.— Although the atom is the smallest possible particle 

 of a substance, it seldom if ever exists alone. Rather atoms combine 

 with one another to form molecules. At times, a molecule is formed of 

 but a single type of atom ; thus two atoms of oxygen combine to form a 

 molecule. This is the manner in which oxygen exists in the air. More 

 often, a single molecule is formed of more than a single kind of atom 

 and is known as a compound. A molecule, thus, is the smallest unit 

 of any substance that can exist alone. 



