PROTOPLASM 2 1 



by new molecules of that substance. This tracing is possible because, 

 although the several isotopes of an element have the same chemical 

 properties, they have different physical properties. Some are radioactive, 

 that is, they emit rays or particles of some sort which can be detected 

 by an instrument such as the Geiger counter. Others are differentiated 

 in a mass spectrometer by the slight difference in the mass of the atomic 

 nucleus which residts from the presence there of an extra neutron. Thus, 

 with radioactive calcium one can study the rate of formation of bone 

 (and the effects of a host of variables such as vitamin D intake or rate 

 of parathyroid activity on this process), or the rate of secretion of shell 

 by a clam or oyster. Or, one can prepare sugar labeled with radioactive 

 carbon (C" or C'^) or heavy carbon (C^-^), inject it into an experimental 

 animal, and determine the metabolic paths of glucose— its conversion to 

 glycogen, fat and protein— and their respective amounts. Many problems 

 in zoology and the other biological sciences which could be attacked in 

 no other way have been solved by this method. 



The analysis of the human body reveals that it contains about 50 

 per cent carbon, 20 per cent oxygen, 10 per cent hydrogen, 9 per cent 

 nitrogen, 4 per cent calcium, 2.5 per cent phosphorus (P), 1 per cent 

 potassium (K), 0.8 per cent sulfur (S), 0.4 per cent sodium (Na), and 

 0.4 per cent chlorine (CI). Analyses of other animals would yield com- 

 parable results. Such analyses are not very informative unless the animal 

 has some unusual element. Tunicates, for example, are unusual in con- 

 taining a large amount of the element vanadium (V). 



Chemical Compounds. Most elements are present in protoplasm as 

 chemical compounds, substances composed of two or more different 

 kinds of atoms. The smallest particle of a substance having the compo- 

 sition and properties of a larger part of the substance is called a 

 molecule. The molecules of a pure compound are always composed of 

 two or more elements combined in a fixed ratio. Water molecules, for 

 example, always contain two atoms of hydrogen and one of oxygen. 

 Chemists state this fact by writing the formula of water as HoO. A chemi- 

 cal formula represents both the kinds and the relative proportions of 

 the atoms present in a molecule. 



A large part of any kind of protoplasm is simply water. In an ani- 

 mal such as man, the water content of protoplasm varies from about 20 

 per cent in bone to 85 per cent in brain cells. The water content is 

 greater in embryonic and young cells and decreases as aging occurs. 

 About 70 per cent of our total body weight is water; as much as 95 per 

 cent of jellyfish protoplasm is water. Water has a number of important 

 functions in protoplasm. Most of the other chemicals present are dis- 

 solved in it; they must be dissolved in water in order to react. Water 

 aids in the removal of the waste products of metabolism by dissolving 

 them so they can be excreted. Water has a great capacity for absorbing 

 heat with a minimal change in its own temperature; thus it protects 

 protoplasm against sudden thermal changes. Since water absorbs a large 

 amount of heat as it changes from a liquid to a gas, the mammalian 

 body can dissipate excess heat by the evaporation of sweat. Water's 

 high heat conductivity makes possible the even distribution of heat 



