188 EACTERIOLOaiCAL CHEMISTRY 



These reactions should be regarded as the life processes 

 of the cell, providing both the energy required and the 

 raw materials for building up new cells. 



As far as we knoAv only two sources of energy are 

 available for living cells, light and chemical energy. Of 

 these, light can only be utilised by chlorophyll-containing 

 plants, by the blue-green algae and by a few autotrophic 

 bacteria ; for all other forms of life the requisite energy 

 must be derived from chemical reactions . Heat, electricity 

 and mechanical energy cannot be utilised by organisms, 

 probably because they lack appropriate " transformers " ; 

 the only transformers we know are chlorophyll and 

 similar pigments for light. Heat, or in other words a 

 rise in temperature, may cause increased growth and 

 metabolic activity of a cell, but it is only in so far as 

 the chemical changes (which supply the essential energy) 

 are speeded up by a rise in temperature. A cell cannot 

 economise on food by using the heat energy of the medium; 

 a starving cell, for instance, derives no benefit from a 

 rise in temperature. 



It follows that the energy liberated in one cell is 

 of no use to any other cells ; neighbouring cells, even 

 those closely linked as in tissues, have no direct energy 

 exchange system. Moreover, the chemical energy neces- 

 sary for growth must be liberated within the cell, since 

 if it were produced outside the cell it would have to 

 take the form of heat or electricitj^ which cannot be 

 utilised by the cell. As a result of this the only foods of 

 value to the organism are those which can diffuse into 

 the cell. Thus complex proteins, fats and carbohydrates 

 like starch and cellulose, are not directly available to the 

 organism, but first have to be broken down or hydrolysed 

 to appropriate smaller, soluble, diffusible compounds. 

 This is the work of the class of exo -cellular enzymes or 

 hydrolases which are secreted into the medium by the 

 organism. 



