18 Inside the Living Cell 



acids, according to the proportions of the amino acids which they 

 contain and the order in which they come. 



The study of proteins was delayed because their molecules were 

 found to be very much larger than the molecules with which the 

 chemist had been accustomed to deal. This presented problems of 

 purification of a novel kind, and for a long time it was not known 

 whether typical proteins were composed of many molecules of one 

 kind or were mixtures of numerous different molecules. This could 

 not be settled until ways were discovered of analysing mixtures of 

 proteins into their component parts. 



This analysis was greatly assisted by the discovery that some 

 soluble proteins could be induced to crystallize. It had been dis- 

 covered by Hofmeister, in 1890, that egg albumin, the principal con- 

 stituent of the white of eggs, could be obtained as crystals. The red 

 pigment of the blood of animals had also been obtained in a crystalline 

 state by very simple means as early as 1867. 



Now a crystalline form is only possible when all the molecules are 

 either identical or have very similar shapes. So the crystallization of a 

 protein is by itself an act of purification and means that we have ob- 

 tained a substance in which all the particles are either the same, or at 

 least very similar in size and shape. 



Physical methods of studying the molecules of proteins were also 

 developed. Professor The Svedberg of Uppsala, Sweden, invented an 

 instrument called the *ultracentrifuge' in which a solution of the 

 protein was whirled in a small tube at a very high speed — about one 

 thousand revolutions per second. The heavier the particle, the more 

 rapidly does it move away from the centre of rotation under the 

 influence of the centrifugal force. From the rate of movement, which 

 could be observed by a rather complicated optical arrangement, it 

 was possible to deduce the weights of the molecules of proteins. 



In these experiments, it was found first that well-purified proteins 

 (and especially those which had been crystallized) were composed 

 of molecules of the same size. Secondly, the weights of the molecules 

 of proteins were much greater than those of simple organic sub- 

 stances like sugar, which chemists were accustomed to handle. 

 Taking the weight of the hydrogen atom as unity, simple organic 

 substances like alcohol or sugar have molecular weights between 

 50 and 200. Larger molecules had been made with weights up to a 

 thousand or even more, but with proteins molecular weights of many 

 thousands are common. One of the simplest protein-like substances 

 known is insulin, with a molecular weight of about 5,000. But most 

 protein molecules are much larger than this and have weights of 

 60,000-100,000, while in others the molecular weight is a million or 



