1084 THE LIGHT FACTOR. II. QUANTUM YIEL CHAP. 29 



of finite quanta (photons); from this, he deduced that the number of 

 molecules, N, changed photochemically by the absorption of a certain 

 amount of light, must be equal to the number of absorbed photons, N^^ 

 (Einstein's law of photochemical equivalency). In the following years, 

 rapidly accumulating rate measurements of photochemical reactions made 

 it clear that Einstein's principle applies only to the primary photochemical 

 process, while the observed over-all rates of photochemical reactions usu- 

 ally depend on the efficiency of secondary reactions, which follow the pri- 

 mary photochemical step. Over-all rate measurements therefore only 

 seldom lead to straightforward confirmation of the equivalency law (in 

 other words, the empirical quantum yields usually are smaller — or larger — 

 than unity). 



Photosynthesis, as the most important photochemical process in na- 

 ture, naturally came under scrutiny from the point of view of its quantum 

 yield. The problem appeared particularly intriguing because of the 

 strongly endothermal character of the photosynthetic reaction. It could 

 easily be calculated that one quantum of visible light (energy available: 

 40-60 kcal/einstein) is insufficient to convert one molecule of carbon di- 

 oxide (and one molecule of water) into a link in the carbohydrate chain 

 and a molecule of oxygen (energy needed: about 112 kcal/mole). It was 

 obvious that several quanta must cooperate in the reduction of one mole- 

 cule of carbon dioxide. The question was: how many (or rather: how 

 few — since, from the point of view of reaction mechanism, we are above all 

 interested in the maximum quantum yield obtainable under the most fav- 

 orable conditions). 



According to equation (29.1), the answer to this question meant also 

 the determination of the maximum efficiency of plants as converters of 

 light energy into chemical energy. Over a century ago, in 1845, Robert 

 Mayer recognized that storage of light energy by conversion into chemical 

 energy is a most important aspect of plant activity on earth (c/. Vol. I, 

 chapter 2). We have described in the preceding chapter several investiga- 

 tions in which the yield of this conversion was measured over considerable 

 periods of time, and concluded that under natural conditions it is rather 

 low — of the order of 2-5%. 



It was known, however, since Reinke's investigation in 1883 (c/. page 

 964) that the light curves of photosynthesis are convex; the curvature 

 sometimes becomes apparent even at very low light intensities (c/. chapter 

 28, section A2). This means that the energy conversion efficiency and 

 the quantum yield increase as light intensity decreases. Warburg and 

 Negelein (1922) set out to determine the maximum quantum yield by 

 measuring the yield in very Ioav light. Their work marked the beginning 

 of a new stage in the quantitative study of photosynthesis, 



