PRODUCTS OF METABOLISM 173 



between* the extra-cellular and the intra-cellular products. 

 Many products are formed that are of such a nature that they 

 can pass through the cell wall readily and are excreted. These 

 are known as extra- cellular products and are to be distin- 

 guished sharply from the split products which are referred to 

 above. Many of the products of metabolism, however, are 

 of such a nature that they do not readily pass through the 

 cell walls and are liberated only on the death of the cell. 

 These are the intra-cellular products. All cells in their 

 metabolism produce certain by-products, such as carbon 

 dioxide, water, nitrogen, waste products, and energy. But in 

 addition to these, many organisms produce in considerable 

 amount definite by-products by which they are character- 

 ized. Many organisms, for instance, produce substances 

 w T hich have the power of reducing other substances. These 

 reduction substances are found among the denitifying bac- 

 teria where the nitrates are reduced to the nitrites, and also 

 where colored substances are reduced to colorless or leuco- 

 products. An example of this is seen in the growth of many 

 bacteria in litmus milk. The color of the litmus is frequently 

 lost completely, due to the formation of some reducing sub- 

 stance. Culture media containing methylene blue are also 

 reduced by certain bacteria. Neutral red, when added to 

 culture media, is reduced by certain forms, and this reaction 

 is very useful in the differentiation of various bacteria. It 

 was formerly supposed that this reduction was due to the split- 

 ting off of oxygen from the reducible substance, but it is now 



