NUTRITION 



249 



The Protozoa are apt to be considered as very primitive organisms, rudimentary 

 ancestors of higher animals, because they are unicellular. But, although there is 

 no doubt that higher animals have arisen in the course of evolution from simple 

 creatures of this kind, one must admit that the protozoa, as we have them now, are 

 complex, highly differentiated organisms. The Amoeba, apparently, cannot be grown 

 on a culture medium, unless it is supplied with bacteria, although dead ones suffice. 



As a general rule, we may say that animals require food which has been 

 previously built up by the plant. They feed either on vegetable matter or on 

 other animals. 



It was believed at one time that animals, at all events the higher ones, required 

 nitrogen in the form of more or less complex proteins, but we have now definite 

 proof that the products of hydrolysis of proteins, amino-acids, will prevent loss of 

 nitrogen from the adult animal. 



Optical Activity. In connection with the remark made above as to the 

 preference of one form of carbon or nitrogen food before another, it is interesting 

 to note that, of the amino-acids, it is the /-series only which is utilised for the 

 building up of the tissue proteins, although there is evidence that the opposite 

 optical isomers can be used for energy purposes, although not so readily. In 

 the case of carbohydrates, again, it is only the (/-series that is easily utilised. 

 The mistake is sometimes made, however, of stating this use of one series only as 

 an absolute fact, whereas it is only relative. Pasteur (1860, p. 33 of the reprint in 

 Ostwald's "Klassiker") in his classical work on the separation of the d and /-tartrates, 

 used moulds to consume the dextro acid and leave the other intact. As soon, how- 

 ever, as all the c?-acid was exhausted, the mould proceeded to consume the /-acid, 

 so that the rotatory power of the solution passed through a maximum. Other 

 instances will be referred to when enzymes are under discussion, and the general 

 question is treated in a later section of the present chapter. There is also 

 preference for certain disaccharides, and here the utilisation is connected with the 

 possession of particular enzymes which hydrolyse the disaccharide. The facts have 

 been chiefly studied in the case of different species of yeasts. Emil Fischer has 

 also shown (1884-1908) that, of all the possible carbohydrates of the general 

 formula C u H 2n O n , ordinary yeasts can only act upon those in which the number 

 of carbon atoms is three or a multiple of three ; moreover, of those of the same 

 constitution, but of different stereochemical configuration, a particular yeast will 

 ferment one at a much greater rate than another. 



One of the most striking examples is that of the sorbose bacterium, as studied by Bertrand 

 (1896). Acting only on glycerol or on sugars with a terminal alcohol group (CH 2 OH), it 

 attacks a CHOH group near this one, transforming it into CO and thus producing a ketone. 

 Moreover, the OH of the group attacked must not be next to the H of a neighbouring CHOH. 

 Glycerol is thus oxidised into dihydroxyacetone. 



Salts. As already stated, these are necessary for all organisms, but the 

 requirements as to particular salts vary considerably. This is one of the problems 

 with which the agriculturist has to deal. Apart from nitrates, which do 

 not come under the head of salts as such, potassium and calcium seem to be 

 indispensable and other salts are more or less favourable. The reader is referred 

 to the monograph by E. J. Russell (1912) for further information. As an 

 illustration, we may refer to the pioneer work of Raulin (1870), some aspects 

 of which have been mentioned above. By numerous experiments with different 

 salts in different concentrations, it was found that for the growth of Aspergillus, 

 a medium of the following composition gave better results than one in which 

 any one of the constituents was omitted or present in another concentration : 



Water - - 1,500 



Cane sugar, cryst. - 70 



Tartaric acid 4 



Ammonium nitrate 4 



Ammonium phosphate - 0*60 



Potassium carbonate - - - 60 



Magnesium carbonate - 40 



Ammonium sulphate 0'25 



Zinc sulphate 0'07 



Ferrous sulphate - 0'07 



Potassium silicate - 0*07 



Note that, while some of these substances are foods in the narrow sense of the 



