Chapter 29 



THE LIGHT FACTOR. II. MAXIMUM QUANTUM YIELD OF 



PHOTOSYNTHESIS* 



In analyzing the light curves of photosynthesis in chapter 28, we did 

 not discuss the slope of the initial linear part. This is a particularly im- 

 portant quantity, since it determines the maximuni quantum yield of photo- 

 synthesis and the maximum conversion of light into chemical energy achieved 

 in this process. The present chapter will deal with these two subjects. 



The definition of maximum quantum yield as the limiting slope of the light curve 

 at low light intensities implies that this curve has no inflection. An inflection has often 

 been observed in the light curves of purple bacteria, but it has usually been assumed that 

 the light curves of algae and higher plants show no such complication. Certain recent 

 observations (Kok, van der Veen) lead, however, to new doubts concerning the shape of 

 the light ciu-ves below the compensation point; we will discuss these observations and 

 their possible significance for the determination of the maximum quantum yield later in 

 this chapter (page 1113). 



If the yield of photosynthesis is expressed in moles of reduced carbon 

 dioxide (or liberated oxygen), P, and the light absorption, la, is given in 

 einsteins of absorbed photons, the ratio y = P/Ia is the quantum yield. 

 (In many papers on photosynthesis, and photochemistry in general, the 

 quantum yield is designated by the letter cp, which we reserved — (c/. Vol. I, 

 page 546) — for the quantum yield oi fluorescence. 



If the yield is measured by the energy content (heat of combustion) of 

 the produced carbohydrates, — AHc, and /„ is given in calories, the ratio 

 e = — AHc/Ia can be called the energy conversion factor. The relation be- 

 tween 7 and e is: 

 (29.1) 6 = -AH,ny/NAhv = 3.9() X IQ-'' X„^7 — 4 X lO'^ X,„^7 



where AH„, is the molar heat of combustion of one {CH2OI group in carbo- 

 hydrates (approximately 112 kcal, or 4.69 X 10^^ erg); Na, Avogadro's 

 number (6.02 X lO^^); h, Planck's constant (6.55 X 10""); and p, the 

 frequency of light (3.00 X 10^'' /K J- 



The concept of the "quantum yield" of a photochemical process arose 

 from Einstein's application of quantum theory to photochemistry in 1913. 

 Einstein suggested, in elaboration of Planck's concept of vibrational energy 

 quanta of electrons in atoms and molecules, that light energy, too, consists 



* Bibliography, page 1139. 



1083 



