THE FROG 113 



pounds known as carbohydrates that are more or less chemically 

 complex, and which we term organic. This power is termed photo- 

 synthesis, since the building up is dependent on light. Further 

 changes then occur, probably involving the utilisation of energy 

 obtained by the oxidation of previously manufactured organic 

 compounds, and these result in the final production of substances 

 termed proteins, from which protoplasm itself is built, and they are 

 among the most complex bodies known. The typical animal, as 

 we have seen, requires for food organic matter in the form of carbo- 

 hydrates or proteins. These it breaks up by digestion into some- 

 what simpler compounds, by means of the chemically active 

 enzymes ; but afterwards, by means of energy obtained by oxida- 

 tion, it recombines them into proteins and protoplasm. The building 

 up processes in both animals and plants, involving the manipulation 

 of organic compounds and their actual incorporation into the living 

 protoplasm, are distinguished as assimilation. 



The constitution of living protoplasm is not known with any 

 certainty, since as soon as we start to analyse it we must kill it, and 

 then we have only a mixture of proteins, etc. It may possibly 

 always be a system of such unstable compounds interacting one upon 

 another, and is certainly in a state of continual molecular change, 

 which forms the chemical basis of the vital phenomena, for when they 

 stop life also ceases. 



The katabolic or energy-release changes are probably brought 

 about in the main by the action of enzymes, and result in the break- 

 ing down of either the carbohydrate food reserves or the proteins, 

 with the consequent production of waste substances, and these 

 reactions are ultimately dependent upon the oxygen obtained by 

 respiration. The dissimilation of the proteins takes place in succes- 

 sive stages, which we may for convenience distinguish as the forma- 

 tion of the first katastates (i.e. peptones, albumoses, amino- acids, 

 etc.), and the production of the intermediate bodies like urea, etc.. 

 still moderately complex chemically and still termed organic. 

 These occur in both animals and plants. Just as we find plants 

 at the beginning of the energy-building series, so they come in again 

 at the end, and, as Bacteria, etc., are mainly responsible for the 

 disintegration of the intermediate bodies to form simple inorganic 

 end-products (e.g. CO 2 , NH 3 , P 2 O 6 , H 2 S, CH 4 , etc.) ready to be distri- 

 buted again in the water and air, and serve as food for a new 

 generation of green plants. 



This wonderful cycle of metabolic changes is infinitely more 

 complex than has been outlined above, and in spite of the vast 

 amount of work that has been done upon it, many of its details still 

 remain to be discovered. The general idea, however, that underlies 

 it all may be expressed somewhat crudely in a diagramatic manner. 



I 



