ENERGY EFFICIENCY IN PHOTOSYNTHESIS 267 



minute. It is not necessary to get the dissolved oxygen out of the solu- 

 tion into the gas phase, as is the case with gas analysis or manometric 

 methods. The electrode is placed in the center of the beam of light, and 

 stirring is not essential. 



Oxygen can be determined polarigraphically also Avith a platinum 

 anode as well as with the dropping-mercury electrode. Brackett (Olson, 

 Brackett, and Crickard, 1949) has developed a very effective electronic 

 circuit which he uses for the analysis of oxygen in algal suspensions with 

 a time lag of less than 10 sec. 



The storage of energy in the process of photosynthesis can be deter- 

 mined calorimetrically (Arnold, 1949; Magee et al., 1939; Tonnelat, 1944). 

 The number of ergs of radiation absorbed by the algal suspension is deter- 

 mined in a small glass cell with clear flat windows at front and back. A 

 thermopile is placed back of the cell, and the energy passing through the 

 cell is measured first with the clear nutrient solution filling it and then 

 again after it is filled with algae. Multiple thermocouples are placed all 

 around the photocell so that a slight increase in temperature of the algal 

 suspension can be measured. In this way the photocell acts also as a 

 microcalorimeter. It is calibrated with an opaque solution of India ink 

 or other inert material. When the heat evolved in the cell is subtracted 

 from the amount of radiant energy absorbed, a measure is obtained of 

 the amount of energy stored as chemical energy in the form of carbo- 

 hydrates or other plant material. The percentage of energy stored chemi- 

 cally can then be calculated in terms of photons per molecule. Stirring 

 of the algae is difficult in these calorimetric experiments, and the dissolved 

 oxygen and carbon dioxide must be relied upon to supply the necessary 

 chemicals for photosynthesis and respiration. This calorimetric method 

 offers an independent means for determining the energy conversion in 

 photosynthesis. 



Photosynthesis leads to growth of plants and to an increase in the num- 

 ber of algal cells. The increase in dry weight following synthesis should 

 be a direct measure of the amount of photosynthesis. For the determi- 

 nation of the total amount of energy stored in photosynthesis, the heat 

 of combustion as measured in a bomb calorimeter would seem to offer an 

 excellent method. One gram of material is sufficient for determination 

 of the heat of combustion, but this method has not yet been used. Went 

 (1950) determined the increase in dry weight of tomato plants grown 

 under optimum conditions. The light absorption was estimated, and 

 the best conditions, involving low temperature during illumination and 

 higher temperatures during dark periods, were determined empirically. 

 He found, on the basis of the dry weight of the tomato plants and the 

 calculated heat of combustion, that a maximum of about 20 per cent of 

 the radiant energy absorbed could be stored chemically in the plants. 

 This amounts to a quantum yield of about one molecule per 15 photons 



