32 Introduction 



portion to the hydrogen and carbon. One common animal fat has the 

 formula C51H98O6. 



Proteins. — Proteins are the most complex and varied of all the 

 organic compounds. The molecules are much larger than those of 

 the fats. In addition to containing the usual carbon, hydrogen, and 

 oxygen, the protein molecule may also have sulfur, nitrogen, phos- 

 phorus, or other elements. 



Protein molecules can be broken into constituent parts which ?.re 

 known as amino acids. These acids vary in complexity, but all are 

 characterized by the presence of NH2 radicals, the amine group. 



In the animal body, the proteins are of great importance in the 

 growth and repair of tissues. During digestion the complex proteins 

 which are consumed are broken down into their component amino acids. 

 These are then distributed through the body and then rebuilt into the 

 proteins characteristic of the animal's body. 



Enzymes. — All the complex reactions which occur within the liv- 

 ing organism are made possible by the activity of special substances 

 known as enzymes. Enzymes are protein in nature and are true cata- 

 lysts ; that is, they bring about or mediate reactions between other sub- 

 stances without themselves appearing in the final products. 



Enzymes make life possible inasmuch as they bring about re- 

 actions at the comparatively low temperatures at which organisms exist. 

 The duplication of even simple biological reactions outside an organism 

 may require the use of high temperatures and strong acids or bases. 

 Obviously such extreme measures would be fatal within any organism. 



Enzymes are usually specific in their action ; that is, they act only 

 upon certain definite substances under only certain specific conditions 

 of pH and temperature. The reactions which they mediate may be 

 reversible. Perhaps most characteristic of all is the fact that enzymes 

 are produced only by living organisms. 



PHYSICAL CHARACTERISTICS OF MATTER 



Solutions. — When a compound such as table salt, NaCl, is dis- 

 solved in water, an equilibrium is set up between the sodium and 

 chlorine ions and the salt molecules. It is illustrated thus : 



NaCl ^ Na+ -\- Cl- 

 in this example, the water is the solvent and the salt is the solute. If 

 this solution is evaporated, the salt crystals will again appear. 



