CHAPTER 3 



The Energetics of Photosynthesis 



A. ENERGY TURNOVER 



§ 15 Chlorella and Its Cultivation 



To study photosynthesis use is made of a unicellular alga Chlorella pyre- 

 noidosa which can be easily cultured in a nutrient solution (Fig. 13). The 

 algal cells are 3 to 5m in diameter and are resistant to mechanical and chem- 

 ical influences. They can be centrifuged off from the nutrient solution and 

 suspended in other solutions. They maintain their power of assimilation in 

 solutions of the most varied composition and with widely differing pH values, 

 and even in distilled water. In carbonate-bicarbonate solution, at pH 9, 

 Chlorella cells remain intact for days, exhibiting constant pho'tosynthetic 

 activity. Organisms such as Chlorella offer the simplest conditions for the 

 diffusion of metabolic gases. 



Ever since Chlorella has been used in the study of photosynthesis it has been 

 known that there are cells which use light efficiently and cells which do not. 

 One of the most important conditions is the intensity of light at which the 

 cells are cultured. If use is made of artificial light burning constantly at 

 the same degree of brightness, the conditions are too far removed from the 

 natural conditions under which Chlorella has thriven for over 500 million years. 

 Such methods of culture are highly unphysiological, and it is astonishing that 

 some botanists who have cultivated Chlorella for about 40 years still do not 

 take this into consideration. As Warburg (24, 35, 36) pointed out, under 

 these conditions the cells are forced to produce organic substance uninter- 

 ruptedly and even to produce much more than is actually needed for normal 

 anabolic processes. It is clear that the natural defence reaction of the cells is 

 a very inefficient transformation of light energy into chemical energy. 



When the light intensity is varied so that day and night, dusk and dawn are 

 reproduced, the culture gives rise to cells— named A-cells by Warburg— which 

 use light most efficiently. Figure 14 depicts the equipment used for the 

 cultivation of A-cells in a dark room. Light is proved by a 300 watt, 220 

 volt metal filament lamp. The resistance is changed automatically so that 

 the voltage increases from 50 to 220 and then decreases to 50 within a 24- 

 hour period. Not only is the light intensity changed but also, as under 

 natural conditions, the spectral composition of the light produced. Although 

 it is, of course, possible to produce any variation in light intensity that may be 

 required, Warburg prefers the method depicted in Figure 15. 



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