A Critical Survey of the Physical Background of Photosynthesis

Abstract

During the last 30' years the methods employed to study photosynthesis have alternated between application of pure organic chemistry and the use of physical and physico-chemical procedures. The former, in the hands of Willstatter et at. (1 ) and later of Fischer (2), gave us a full understanding of the structure of chlorophyll, while the latter, used for quantitative measure­ ment of the over-all gas exchange of photosynthesis, gave us an abundance of information about the influence of external conditions on the photosyn­ thetic rate, quantum yield, etc. Warburg (3), in his pioneer work in this field, introduced the use of manometers. Later, other methods for the meas­ urement of the gas exchange were developed, i .e., calorimetry, electrolytic conductivity measurements, measurements of the oxygen electrode potential, spectroscopic methods, measurements of the heat conductivity of gases, and the use of the quenching effect of oxygen on phosphorescence of dyes. Some of the more important earlier studies, mostly by the manometric method, are: Warburg's (3) early work; van Niel's (4) and Gaffron's (5) studies of purple bacteria and Gaffron's discovery of photoreduction in green plants; Emerson & Arnold's (6) results on photosynthesis in flashing light; Hill's (7) discovery that isolated chloroplasts are able, when illumi­ nated, to reduce substances of high oxidation potential ; Kautsky's (8) observation that the chlorophyll fluorescence in plants shows transient changes of its intensity during the induction period and that the intensity of the steady state fluorescence depends upon external conditions. Because radioactive isotopes (which could be used to follow the chemical course of photosynthesis) have become available, the main progress in the last few years has again been made by the use of organic chemistry. The first studies of Ruben & Kamen (9) , who worked with small quantities of the short-lived CIl, showed how powerful radioactive isotopes could be used as tools for solving chemical problems of photosynthesis. After long-lived 04 became available, Calvin & Benson (10) in California successfully used it to show that phosphoglyceric acid is the first stable intermediate of photo­ synthesis. The confirmation and implications of these results are discussed in the article by Gaffron & Fager ( 1 1 ) in this volume. However, since the main point of disparity remaining between the results and interpretations of the California group and those of the Chicago group plays a major role in the discussions of this article, it needs to be discussed here. The fundamental question concerning the photochemical part of photo-