16 Inside the Living Cell 



fact, only during the last twenty or thirty years has much progress 

 been made in this study. 



It has been known for a long time that living things are made of the 

 same elementary substances as we are familiar with in ordinary life — 

 carbon, oxygen, hydrogen, nitrogen and phosphorus and calcium are 

 the commonest elements, but there are smaller quantities of many 

 others. 



A great profusion of ^organic' substances, like oils, fats, waxes, 

 starch, resins, alcohols, sugars, had been obtained from living things. 

 They were termed 'organic' to distinguish them from the 'inorganic' 

 substances found in the earth, such as salt, ores, rocks, etc. An exact 

 chemistry of 'organic' substances is a fairly recent development of 

 science, going back only about one hundred years. Before that time, 

 they were thought to be an entirely different type of material from 

 'inorganic' substances. But the distinction broke down when a Ger- 

 man chemist, Wohler, prepared in his laboratory from purely 'in- 

 organic' materials (ammonia and carbon dioxide) the substance urea, 

 which had previously been regarded as a typical product of life. Since 

 then chemists have been able to synthesize in their laboratories an 

 enormous number of 'organic' substances, including not only many 

 which occur in nature, but also many thousands of others for which 

 no natural counterpart exists. They were guided in this work by a 

 knowledge of the rules of combination of atoms which were estab- 

 lished by Van't Hoff and Kekule. On the basis of these rules, it is pos- 

 sible to derive a formula for a compound which shows how its atoms 

 are joined together. The formula is usually established by studying 

 the reactions of the compound and by breaking it down into its com- 

 ponent parts. It can be confirmed by building up a compound as rep- 

 resented by the formulae, to see if it corresponds with the original 

 compound. 



It soon appeared that it would be only a matter of time before all 

 the compounds present in living cells could be synthesized in this way. 

 However, it was eventually found that the more characteristic com- 

 pounds of living cells, such as proteins, had a much greater com- 

 plexity for which chemists were hardly prepared. 



It had been recognized early in the nineteenth century that the most 

 characteristic of the substances of living things were a type of nitro- 

 genous compound called 'proteins'. These are familiar to everyone. 

 They constitute a large part of animal tissues, e.g. they are the prin- 

 cipal constituent of muscle, skin, nails, hair, the red cells of blood and 

 the soluble substances present in blood serum, the casein of milk. 



Every form of life — even plants, corals, jellyfish and bacteria — 

 are rich in proteins of many kinds, and no organism has been found 



