RESPIRATION 295 
only go on in the living cell. Other similar instances 
could be quoted. 
The probable course of events in respiration is that the 
oxygen in some way unites with the protoplasm, rendering 
it unstable, and initiating a series of decompositions which 
result in the successive formation of many bodies of less 
complex composition, each successive decomposition pro- 
ducing simpler ones, till finally carbon dioxide and water are 
formed. Simultaneously, reconstruction of the protoplasm 
goes on, many of these residues being in whole or in great 
part built up again into its substance, together with new 
material supplied to it in the shape of food. If the 
temperature is low, the breaking down of the protoplasm 
proceeds but slowly, and reconstruction is rapid, so that 
under these conditions the quantity of oxygen absorbed 
or fixed as intramolecular oxygen by the protoplasm is 
greater than the quantity of carbon dioxide formed by its 
decomposition. Ata higher temperature decomposition is 
much more easily carried on, and its products are more 
numerous and simpler. The decomposition and recom- 
position go on side by side, simpler bodies being gradually 
produced, either by their splitting from the protoplasm 
directly, or by their being formed at the expense of the 
more complex decomposition-products during processes of 
slow oxidation in the substance of the protoplasm, till 
finally a certain production of carbon dioxide and water is 
arrived at. So long as the protoplasm remains alive the 
amount of these is relatively small, reconstruction con- 
tinually taking place. When, however, the protoplasm 
dies, simple bodies, such as carbon dioxide, water, and 
possibly ammonia, in addition, are produced abundantly 
from the decomposition which attends its death. 
If the self-decomposition of protoplasm during life 
involved such a splitting-up as would lead to the formation 
of nothing but these, nearly all the potential energy of the 
cell would be liberated. We have seen, however, that this 
is not the case, but that a good deal of the energy set free 
