4 PROBLEMS IN PHOTOSYNTHESIS 



the formation of carbohydrates, resuking in an assimilatory quotient greater 

 than 1. The quotient may also be smaller than 1, e.g., growing Chlorella 

 reduces the nitrate of the culture medium, resulting in a greater O.- 

 production : 



NO,- + 2H2O -> NH3 + OH- + 20,. 



If the quantity of CO. reduced is indicated by x^o^ and the quantity of Oo 

 produced by .Vq,, the general formula for the assimilatory quotient is: 



7 = -— (1) 



Uptake of gas is regarded as negative and removal of gas as positive, so that 

 Xco. and consequendy 7 are always negative. The quantities a'co, and Xq, 

 are expressed in /^mole or, more usually, in ix\ whereby 1 /xmole = 22.4 lA. 



The reversal of the photosynthetic over-all reaction is the over-all com- 

 bustion reaction of glucose (respiration) : 



CeHi.Oe + 6O2 > 6C0.2 + 6H,0 + 674000 cal 



In respiration the ratio of the CO2 produced to the O. consumed is called the 

 respiratory quotient. According to the combustion reaction given above, it 

 equals 1 . In § 54 we shall see whether or not it is correct to consider photo- 

 synthesis as the reversal of respiration, not only with respect to the over-all 

 reactions, but also with regard to the different intermediate steps in the two 

 processes : 



respiration 

 (CH2O) + 0.2 \ ^ CO.. + H,0 + 112000 cal 



photosynthesis 



Photosynthesis is composed of two distinct types of process: a photo- 

 chemical process and a series of dark reacdons which were originally called 

 the Blackman reaction. The photochemical reaction or light reaction is 

 either the photolysis of water or the photolysis of CO2. The dark reactions 

 are far more complex than Blackman (5) presumed. 



When green cells of Chlorella at constant temperature and in the presence 

 of sufficient CO2 are illuminated with various light intensities /, we obtain 

 various rates v of photosynthesis. Figure 1 shows an assimilation curve 

 obtained by plotting v against i (44, 45, 48). At low intensities the curve is 

 nearly rectilinear and dv/di is constant. As / increases, dv/di decreases and 

 finally becomes zero. Blackman found that the rate of photosynthesis at 

 low intensities is independent of temperature; at higher intensities, however, 

 it increases when the temperature is increased. It thus seems that various 

 processes influence the rate of photosynthesis, depending on the light intensity. 

 One of these processes is of a photochemical nature, dv/di being constant. 

 As this process is independent of temperature, dv/dT = 0. The other 

 process— the Blackman reaction— is not influenced by illumination. It is an 



