48 INTRODUCTION TO GENERAL PHYSIOLOGY 



properties of the emulsoids. One of these, as we saw, is that of 

 taking up more or less water, according to surrounding conditions : 

 an important fact. They have also the properties due to the 

 possession of surface, as well as those more definitely due to their 

 chemical composition. Of these we may note that, owing to 

 terminal free NH 2 and COOH groups, as well as those of some 

 amino-acids attached as side branches, they act either as acids or 

 bases towards strong bases and acids respectively. 



Optical Activity 



We have seen that, when there are alternatives in chemical 

 compounds of the same general structure, such as the a- and other 

 series of amino-acids, the living organism has been evolved in such a 

 way as to be able to make use of one kind only. This applies to the 

 proteins and to the carbohydrates. Of the eight possible forms of 

 the aldo-hexoses, only three are utilised, namely, glucose, galactose, 

 and mannose. 



There is, moreover, in addition to this exclusiveness, a further 

 one to which we must give a little attention. 



If we write the formula of methane thus : 



H 

 H C H 



H 



we see that the carbon atom is symmetrical on all sides. On the 

 other hand, writing alanine thus : 



CH, 

 H,N C H 



COOH 



the central carbon atom is obviously differently weighted on all 

 sides. By representing such an arrangement in space (p., p. 282), 

 it can be seen that by interchanging positions of two of the groups 

 a compound is obtained which is different in space arrangement, 

 and cannot by any turning about be changed into the first one. It 

 is, in fact, the image of it as seen in a mirror. All compounds 

 which contain asymmetrical carbon atoms, that is, attached to four 

 different groups, show the same characteristic, which is, indeed, 

 a geometrical necessity. There are then two " isomers " of each 

 of these compounds. How can we distinguish them ? It is by 



