CHAPTER VII 

 GROWTH AND METABOLISM 



Metabolism and growth are interdependent. It has already 

 been noted that abiUty to assimilate a particular substrate 

 may depend on the previous history of an alga, but the 

 clearest examples of this interdependence are to be found 

 in the relation which exists between the physiological con- 

 dition of the organism and the products of its metabolism 

 and it is with this relationship that this chapter must be 

 chiefly concerned. 



An alga varies in chemical composition, most notably in 

 its content of major fractions such as protein, fat and carbo- 

 hydrate, according to the stage of growth which it has 

 reached and to the conditions under which growth has 

 taken place. Growth and multiplication in any organism are 

 contingent upon the development of particular structures 

 which in turn depends on particular chemical conditions 

 which are likely to be more circumscribed the more complex 

 the structures involved. The greatest variation in chemical 

 composition is accordingly found in the simplest organisms. 

 In the fungi and bacteria this variation is less evident than 

 it otherwise might be because products of metabolism which 

 are not required for protoplasmic synthesis may be liberated 

 into the surrounding medium. In algae, in which excretion 

 appears generally to be reduced to a minimum, changes in 

 intracellular and extracellular conditions are reflected to a 

 greater extent in the composition of the cell material. In 

 Chlorella, for example, growth of which is possible under 

 a wide range of conditions, the chemical composition of the 

 cells may vary to an extreme extent (see Table ii). In the 

 structurally more complex algae the range within which 

 variation of this sort is possible is reduced as requirements 

 for growth and reproduction become more exacting. The 

 cases of simple and structurally complex algae are for 



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