Photochemistry of Carbon Assimilation

K. Ch. Baly and Isidor M. Heilbronn. Photokatalysis p. 1, Journ. of Chem. Society, July 1921, p. 1025.

One of the most interesting photochemical processes is undoubtedly the process in which, in the leaf of a plant, under the action of light, the simplest organic substances are formed from carbon dioxide and water; these serve as material for the construction of the complex substances of the plant and, indirectly, as material for the construction of the animal. As might be supposed, in this process, from carbon dioxide and water, with the mediation of the green pigment, plants first form formaldehyde, \(CH_2O\), which then forms starch in the leaf. This view of the course of the process was developed on the basis of a number of chemical and botanical considerations; it also formed the basis of the above-mentioned work by the authors, published a year ago. First of all, the authors show that the reaction of formation of \(CH_2O\), proceeding according to the equation

\[ CO_2 + H_2O = CH_2O + O_2, \]

can proceed in ultraviolet light if \(CO_2\) is passed into water placed in a quartz vessel and the solution is vigorously stirred. Stirring is necessary, since otherwise the formaldehyde formed during insolation of the vessel under the action of short waves (\(\lambda\) about \(200\mu\mu\)) will be decomposed by the action of long waves (wavelength \(290\mu\mu\)) and converted into sugar.

Instead of stirring, light filters may be used, which will absorb waves \(\lambda > 290\mu\mu\), and for this purpose the author adds to the solution paraldehyde, sodium phenolate, and other substances which, without entering into the reaction and without being changed by ultraviolet light, absorb waves \(\lambda = 290\mu\mu\). On the other hand, by filtering the rays through thin glass plates, which filter out waves \(\lambda < 290\mu\mu\), the reaction of conversion of \(CH_2O\) into sugar can be effected in the light, and thus the process of carbohydrate formation may be regarded as a double photochemical reaction, in which 1) formaldehyde is formed from carbon dioxide and water, and 2) formaldehyde is condensed into sugar according to the equation

\[ nCH_2O = C_nH_{2n}O_n. \]

Thus the possibility of photochemical synthesis has been fully established by these works. It remains to decide the question: what is the role of chlorophyll, and can the reaction also be carried out outside the leaf in the visible spectrum?

Baly shows that, at least, the first part of the reaction, namely

\[ CO_2 + H_2O = CH_2O + O_2 \]

can be transferred into the visible part of the spectrum if we mix in a number of colored substances—malachite green, methyl orange, ferric hydroxide, uranium hydroxide, etc. In Baly’s opinion, the polymerization of formaldehyde into carbohydrates may be

also been carried out in the visible spectrum, although the reaction in this respect has not yet been sufficiently studied.

Thus, these remarkable experiments pose, with complete clarity, the question of the cardinal significance of photocatalysts, among which chlorophyll must also be included, replaced in Baly’s experiments by malachite green and other substances; and these investigations undoubtedly greatly advance the question of the most complex process in the plant organism.

P. Lazarev.