PHYSIOLOGICAL 383 



carbon, hydrogen, oxygen, and nitrogen, and some (e.g. cystein) 

 also contain sulphur. They range from simple straight-chain forms, 

 such as glycine (amino-acetic acid) NH2 . CH2 . COOH, to forms 

 containing the benzene ring such as tyrosine or even more complex 

 rings, as in the case of tryptophane. The particular amino-acid called 

 tyrosine, from the Greek word for cheese, was obtained by Liebig 

 by heating up cheese with potash, and the fundamental fact that 

 proteins are built up of interlinked amino-acids may be re-empha- 

 sised by noting that tyrosin is the most readily isolated of the 

 amino-acids into which casein, the chief protein of cheese, breaks 

 up. As amino acids play a very important part in the life of the 

 body, we must dwell on them for a moment longer, while referring 

 the student for details to the luminous Fundamentals of Bio- 

 Chemistry by T. R. Parsons (Cambridge, 1923). Amino-acids have 

 great capacity for uniting with other substances, and they all act 

 not only as acids, but as bases by virtue of the nitrogen-containing 

 part of the molecule. This power of uniting is characteristic of pro- 

 teins also, and they too can form salts with either acids or alkahes. 

 Indeed, it is only at a point in the region of neutrality that they exist 

 by themselves. Moreover, quite apart from the salts they form, 

 proteins have a great power of combining with other organic sub- 

 stances, such as sugars and pigments. For the biological student, the 

 important fact is that the proteins and their amino-acids, the two 

 kinds of substances pre-eminently characteristic of protoplasm, have 

 extraordinary powers of entering into combinations. 



The second class of materials includes the carbohydrates, such 

 as the sugars, and they present fewer difficulties. The simpler 

 sugars are chains of five or six carbon atoms united with hydrogen 

 and oxygen, thus glucose or grape sugar (CeHijOe) may be repre- 

 sented as 



OH OH OH OH OH 



I I I I I 

 H— C C C C C C = O 



I I I I I I 



H H H H H H 



But there are higher sugars (disaccharides), like cane sugar, malt 

 sugar, and milk sugar, whose molecules contain twelve carbon 

 atoms instead of five or six. In still more complex carbohydrates, 

 such as starch and glycogen, which form very important stores 

 in plant and animal cells respectively, the structure is so complex 

 that it has not yet been exactly determined, and it may be 

 useful to linger over the fact. The empirical formula of starch 

 is CeHjoOs, indicating the proportions in which the elements 

 carbon, hydrogen, and oxygen occur, but this does not inform us 

 as to the structure of the starch molecule. "When it is attacked by 



