GROWTH AND METABOLISM III 



gible, an over-all equation for the metabolism at this stage 

 can be written as follows: 



i-oNH4++57C02+3'4H20 -> 



C5.7H9.802.3Ni.o+6-2502+PoH+ (26) 



The value of the photosynthetic quotient required by this 

 equation, i.e. i-io, is in good agreement with that observed, 

 i.e. I -06.^^^ The relative growth rate and composition of 

 cells in exponential growth alter, of course, according to the 

 conditions of culture, e.g. increase in light intensity pro- 

 duces an increase in the proportion of protein. ^^'^' ^* 



Cells of the type just described show characteristic changes 

 in metabolism when subjected to conditions other than those 

 under which growth was taking place. ^^^ Chlorella pyre- 

 noidosa cells starved of carbohydrate by aerobic incubation 

 in a complete inorganic nutrient medium in the dark lose 

 all detectable starch in the course of about ten hours and 

 corresponding with this the rate of respiration falls and 

 approaches a constant value. The respiratory quotient 

 characteristic of such starved cells indicates that carbo- 

 hydrate rather than protein or fat forms the substrate for 

 respiration. The rate of photosynthesis in starved cells is 

 only about 20 per cent less under conditions of light and 

 carbon dioxide saturation than it is under similar conditions 

 in normal cells.®* It is algae in this starved condition that 

 have usually been used in studies on oxidative assimilation 

 and further information regarding their reactions may be 

 found under this heading in Chapter III. Cells starved 

 of carbohydrate are incapable of assimilating nitrate or 

 ammonium nitrogen (see pp. 72 and 76) and on return to 

 conditions permitting growth only show nitrogen assimi- 

 lation after a period of photosynthesis which presumably 

 restores the original carbon /nitrogen balance. ^^^ 



Nitrogen-deficient cells of Chlorella may be produced by 

 exposure to intensities of light saturating photosynthesis or, 

 more effectively, by transference to a medium containing 

 no source of nitrogen. Such cells, although showing a re- 

 duced capacity for photosynthesis^** continue to increase 

 in dry weight if exposed to light and come to have a low 



