INTRODUCTION 3 



Among the derivatives of proteins, the most important are those 

 already mentioned as being produced in protein-hydrolysis, viz. : 



(a) Meta-proteins. 



(b) Proteoses, including albumose, the proteose derived from 

 albumin ; globulose, that derived from globulin ; gelatose, that 

 derived from gelatin, etc. The proteoses may be further subdivided, 

 according to the order in which they are formed in digestion into 

 proto-proteoses, hetero-proteoses, and deutero-proteoses. 



(c) Peptones. 



(d) Polypeptides. The majority of these are artificial products, 

 formed by the synthesis of amino-acids, although some can be 

 obtained from proteins by hydrolysis. Only a few of those hitherto 

 prepared give the biuret test. 



However formidable the above list may appear to the student, it 

 gives an inadequate idea of the extreme complexity of the protein 

 class and its richness in individuals. For, apart from the fact that 

 the list has been purposely left incomplete, especially as regards the 

 numerous vegetable proteins, there is the best evidence that proteins 

 of the same name from different animal species have certain pro- 

 perties which distinguish them from each other. The serum- 

 albumins can be crystallized much more easily in some animals than 

 in others. The same is conspicuously true of the haemoglobins, 

 which differ also in certain animals in the relative proportion of 

 sulphur and iron in the molecule, as well as in the crystalline form. 

 Even when no chemical or physical differences have as yet been made 

 out, proteins of the same name from the blood or organs of different 

 species show notable ' specific ' differences when subjected to certain 

 biological tests (see, e.g., the paragraph on ' Precipitins, ' p. 30). 



Carbo-hydrates. The most important carbo-hydrates in their 

 physiological relations are dextrose, levulose, galactose, lactose, 

 maltose, sucrose (cane-sugar), starch, and glycogen. As regards their 

 chemical constitution, the simplest carbo-hydrates are aldehydes or 

 ketones that is, the first oxidation products of primary and 

 secondary alcohols respectively. Thus dextrose is the aldehyde of 

 sorbite, a hexatomic alcohol (an alcohol containing six OH groups), 

 while levulose is the ketone of the isomeric alcohol called mannite, 

 and galactose the aldehyde of the isomeric alcohol called dulcite. 

 The sugars containing six carbon atoms are termed hexoses. They 

 include dextrose, levulose, and galactose. The empirical formula 

 of these three simple sugars (or monosaccharides) is the same 

 (C C H 12 O ), but, owing to the different arrangement of the atoms 

 or groups of atoms, they have each their characteristic properties 

 by which they can be easily distinguished. For example, dextrose 

 rotates the plane of polarization to the right, levulose to the left. 

 By the union or ' condensation ' of two molecules of a monosac- 

 charide, with loss of a molecule of water, a disaccharide is formed. 

 Cane-sugar, maltose, and lactose, all with the same empirical formula, 

 (C^H.^OH), are disaccharides. Cane-sugar yields on hydrolysis a 

 mixture of equal parts of dextrose and levulose ; lactose, a mixture 

 of dextrose and galactose ; while maltose is converted into dextrose. 

 By the condensation of more than two molecules of monosaccharide 

 polysaccharides are formed, such as starch, dextrin, and glycogen. 

 The exact molecular weights of these substances are unknown. 

 Their general formula can be written (C 6 H 10 O 5 ), where n represents 

 the number of monosaccharide molecules condensed to form the 

 polysaccharide, in the case of 'starch probably some hundreds. 



I 2, 



