CHLOROPHYLL AND ENERGY 177 



by this potential difference of 1-2 volt. The energy used by 

 life proceeds from this difference in the level of potential — 

 from this provision of electrons. 



The Stages of Photosynthesis 



A reduction is a much more complex operation than an 

 oxidation because an oxidation produces energy and con- 

 sequently can be effected entirely without demanding anything 

 from the environment. But a reduction needs energy and can 

 develop only if this is suppUed ; it therefore involves two stages 

 — the first to capture the necessary energy and the second to 

 utilize it in the reaction. 



These two stages are found in photosynthesis. The first, the 

 luminous phase, which the Americans call the Hill reaction, 

 captures solar energy; the second, the dark phase, which is 

 sometimes called the Blackman reaction, uses this energy in 

 chemical syntheses. 



The existence of these two phases has been shown by a 

 number of observations. In 1932, Emerson and Arnold studied 

 the speed of photosynthesis, using as the luminous source very 

 brief flashes of a hundred thousandth of a second. The 

 efficiency is higher if the flashes are spaced to give the dark 

 phase time to be completed. At a temperature of 25° C, the 

 maximum efficiency is attained with an interval of four 

 hundredths of a second, while at 1° C. the same efficiency is 

 attained, but only when the interval between flashes is ten 

 times longer. Since the efficiency is not altered by it, the 

 temperature does not affect the capture of energy but only 

 the chemical reactions which take place during the dark phase 

 and are slowed down by cold. 



The influence is of the same order if a small quantity of 

 hydrocyanic acid or heavy water is used — the chemical reactions 

 are effected more slowly. If the interval between the flashes is 

 lengthened, the reactions will have time to be completed and 

 the process will continue unchecked. A low temperature, 

 heavy water or hydrocyanic acid hinders only the dark 

 phase. 



