PHYSIOLOGICAL STUDIES 63 



involving a second substance present in an amount com- 

 parable to the chlorophyll concentration. This might 

 correspond to the hypothetical substance A which combines 

 to form the carbon dioxide complex of the simple mechanism 

 given on p. 50. The longer dark time will then be determined 

 by the velocity constant either of process i or of the re- 

 action between the products of processes i and 2. 



QUANTUM EFFICIENCY 



If we assume that the product of photosynthesis is carbo- 

 hydrate, then in the standard state 115,000 cals. free energy 

 will be required for the reduction of each gram molecule of 

 carbon dioxide. With the concentrations of oxygen and 

 carbon dioxide normally present in air and assuming the 

 concentration of sugar to be o-o5M. the figure becomes 

 118,000 cals. This energy must be obtained from light and 

 in monochromatic red light, say 6,78oA, the maximum of 

 chlorophyll absorption, it will be available in units or mole 

 quanta each of 41,000 cals. Thus for each gram molecule of 

 carbon dioxide reduced at least three mole quanta will be 

 required. As we have seen, the maximum efficiency will only 

 be observed when the concentration of carbon dioxide is so 

 high that it does not limit the rate and when the light 

 intensity is small, i.e. in that region of the light curve where 

 the slope is a maximum. With leaves of higher plants it is 

 not easy to determine the light energy absorbed by the 

 plant owing to the difficulty of determining the total reflected 

 and transmitted light. Briggs (1929) and Gabrielson (1935) 

 both obtained values of about 12 quanta red light/molecule 

 oxygen. With suspensions of algae more precise measure- 

 ments are possible by the use of an integrating sphere. 

 This consists of a totally reflecting surface completely sur- 

 rounding the algal suspension, except for a very small hole 

 through which the incident beam enters and a second hole 

 in which the detecting instrument is placed. The scattered 

 light is reflected round the sphere until an average sample 

 falls on the detector. A zero reading on the detector is 

 obtained by replacing the algal suspension by a totally 

 absorbing system such as a suspension of Indian ink. With 



