ENERGY EFFICIENCY IN PHOTOSYNTHESIS 265 



receiver is placed to the back of the photocell, the smaller will be the 

 scattering losses. 



The scattering of light by algal suspensions has been studied also bj^ 

 Emerson and Lewis (1942) and by Kok (1948). Probably it was the 

 difficulty of allowing for the scattered light which led Warburg, in his 

 early work with Negelein (Warburg and Negelein, 1923) and in much of 

 his later work, to use such a high concentration of algal cells that the 

 light was totally absorbed. All the incident light is then used in photo- 

 synthesis. But in this procedure the price paid for the precise measure- 

 ment of energy is high, because the large excess of algal cells causes 

 respiration to overbalance photosynthesis, and the algal cells are being 

 exposed to light of all difTerent intensities from full intensity at the front 

 of the solution to zero at the back. Moreover, if the photocell is shaken, 

 the algal cells are exposed to varying and uncertain alternating periods 

 of light and dark which can complicate the relation between the primary 

 photoprocess and the thermal reactions that follow. 



The light-scattering difficulty, which is most serious in concentrated 

 solutions, can be solved with the help of an integrating reflecting Ulbricht 

 sphere, which catches all light that enters in. The glass cell in which 

 photosynthesis takes place is set in the center of a large internally reflect- 

 ing sphere, which also contains the thermopile or other energy-measuring 

 instrument. Light is admitted to the reaction chamber through a hole 

 in the sphere. Calibrations may be made with the cell filled with totally 

 absorbing ink solution, and with a reflecting suspension of calcium car- 

 bonate having particles of the same size as the algal cells. Rieke (1939) 

 and Kok (1948) have used this method successfiiUy. 



4. MEASUREMENT OF CHEMICAL CHANGE IN PHOTOSYNTHESIS 

 BY NONMANOMETRIC METHODS 



Equally as important as the measurement of the energy consumption 

 is the measurement of the chemical changes brought about by photo- 

 synthesis in the reaction 



CO2 + H2O + Ught -> (HoCO) + O2. 



Obviously there are many different ways in which changes in these four 

 chemical compounds can be measured by chemical or physical means so 

 as to follow the progress of the reaction, remembering always that the 

 reverse reaction in the form of respiration is going on continuously in the 

 Uving material whether or not photosynthesis is taking place. 



The exact measurement of chemical change is handicapped by the fact 

 that the amount of the change is usually very small. In most of the 

 investigations of energy requirements thus far, it has been the aim to 

 obtain as high an efficiency as possible, and the experiments have been 



