INTRODUCTION o 



here. Frequently large classes of substances can be separated from 

 each other because of their different degrees of solubility. For example, 

 a mixture of fat and sugar can easily be separated by shaking with 

 ether and water. These two solvents, being immiscible, on standing form 

 layers, one of which contains the sugar and the other the fat. Two types 

 of materials bcith dissolved in the same solvent may be separated by 

 causing one to precipitate. For example, a boiling water extract of a 

 fresh fruit or vegetable will contain, among other things, both sugars 

 and proteins. This mixture can be separated by adding a soluble salt 

 of a heavy metal such as lead acetate and filtering, since the proteins 

 are thereby rendered insoluble. Again, some substances are held on 

 the surface of adsorbents, e.g., activated charcoal, while others are not; 

 certain substances can be volatilized (distilled) leaving others behind. 

 By progressively applying such fractionation procedures, a particular sub- 

 stance can be gradually separated from the compounds which are origi- 

 nally mixed with it in the living material, and thus brought nearer 

 to a state of purity. 



Once an individual chemical substance has been isolated, it can be 

 analyzed by standard chemical methods, broken down into simpler frag- 

 ments, which are also analyzed, and in general examined to see just how 

 it is constituted chemically. If the compound is not of too great com- 

 plexity, e.g., has a molecular weight of a few hundred or less, its structure 

 is usually established within a few years. The results of this work are 

 expressed as a structural formula, which shows just what the substance 

 is and how it may be expected to react with other substances. Since 

 most compounds isolated from living things are organic (carbon) com- 

 pounds, such studies fall into the realm of organic chemistry. 



Nutrition. A large part of biochemical research for the past fifty 

 years has been concerned with the nutrition of animals, plants, and micro- 

 organisms. The objectives of this work have been to find out just what 

 chemical substances are needed in the food of living organisms to nourish 

 them properly and to determine w^iat purpose each nutrient serves. In 

 the earlier days of the twentieth century, attention was focused mainly 

 on the energy-yielding and body-building materials which constitute the 

 bulk of the food, namely the carbohydrates, fats and proteins. More 

 recently, substances required in smaller amounts such as the mineral 

 elements, vitamins, and other gro^\'tll factors have been intensively studied. 

 These remarks apply particularly to animals and microorganisms, as 

 plants need only mineral elements besides carbon dioxide and water for 

 nourishment. 



The experimental methods of investigating these questions are similar 

 in principle regardless of the type of organism being studied, and may 

 be illustrated for the case of animals. The general approach has been 

 to feed animals a diet prepared from purified ingredients and to observe 



