METABOLISM 



13 



has an alkaline reaction, while the liquid in the larger vacuoles, 

 at least, is acid, especially in Plant-cells. 1 



Metabolism. The chemical processes that go on in the 

 organism are termed metabolic changes, and were roughly 

 divided by Gaskell into (1) '-anabolic," in which more complex 

 and less stable substances are built up from less complex and 

 more stable ones with the absorption of energy; and (2) " cata- 

 bolic" changes in which the reverse takes place. Anabolic 

 processes, in all but the cells containing plastids or chromato- 

 phores (see p. 36) under the influence of light, necessarily imply 

 the furnishing of energy by concurrent catabolic changes in the 

 food or reserves, or in the protoplasm itself. 



Again, we have divided anabolic processes into " accumulative,''" 

 where the substances formed are merely reserves for the future 

 use of the cell, and " assimilative," where the substances go to 

 the building of the protoplasm itself, whether for the purpose of 

 growth or for that of repair. 



Catabolic processes may involve (1) the mere breaking of 

 complex substances into simpler ones, or (2) their combination 

 with oxygen ; in either case waste products are formed, which 

 may either be of service to the organism as " secretions " (like 

 the bile in Higher Animals), or of no further use (like the urine). 

 When nitrogenous substances break down in this way they give 

 rise to " excretions," containing urea, urates, and allied substances ; 

 other products of catabolism are carbon dioxide, water, and 

 mineral salts, such as sulphates, phosphates, carbonates, oxalates, 

 etc., which if not insoluble must needs be removed promptly 

 from the organism, many of them being injurious or even 

 poisonous. The energy liberated by the protoplasm being derived 

 through the breakdown of another part of the same or of the food- 



1 The specific gravity of living protoplasm has been estimated by determining 

 the density of a solution of gum in which certain Infusoria float freely at any 

 depth. It was found by the concurrent results of Julia B. Piatt and Stephen R. 

 Williams (see Amer. Natural, xxxiii. 1899, p. 31, xxxiv. 1900, p. 95) to be from 

 1-014 to 1-019, while the Metazoon Hydra was found to give a density of only 1 -0095 

 to 1-0115. The difference of about '006, it is easy to show, is of the correct "order 

 of magnitude," if we admit that the actual substance of the Hydra has about the 

 same specific gravity as the Infusorian, while the density of the whole is lightened 

 by the watery contents of the internal cavity, etc. Jensen obtained a much higher 

 result for Paramecium, using a solution of the crystalloid substance, potassium 

 carbonate; but it is almost certain that this would be readily absorbed by the 

 organism, and so raise its density in the course of the experiment. 



