The Path of Carbon in Photosynthesis

Abstract

Factors influencing the dark reduction and early stages of photosynthesis in Chlorella and Scenedesmus were studied. The dark uptake curve of CO2 has 2 parts, the importance of each in Chlorella being dependent upon the earlier history of the algae. An initial rapid fixation after pre-illumination is closely connected with the effect of the light. A 30-sec. pre-illumination brings the reducing power to over 80% of its maximum value. The maximum is a steady state concn. of reducing power resulting from a balance betw. the rate of its photochem. production and the rate of its natural decay. The max. amt. of CO2 fixed is 10-15 times that fixed by the cells without pre-illumination. After complete decay, reducing power is restorable by illumination on the same culture. Since all CO2 fixation is complete in < 2 min., expts. were done in which the dark fixation time was reduced to 1 min. Fraction I in these fixations and in short photosynthetic fixations was 75% malic acid. Fraction II was over 50% alanine for Chlorella. Dark fixation by Scenedesmus and photosynthetic fixations by both algae produced 15% alanine in this fraction. Selective elution of the anion exchange resin gave 2 fractions. III-A (NH4OH eluate) was made resistant to ammonia elution by cold evaporation of the eluate in air, suggesting that this radioactive product was a reduced form of that obtained in III-B (NaOH eluate). The radioactive subst. in III-B of the 10-min. pre-illuminated Scenedesmus was phosphoglyceric acid. Hydrolysis gave radioactive glyceric acid. The radio-active substances in III-B, formed during the 30-sec. photosynthesis in Scenedesmus, were also over 80% phosphoglyceric acid. III-A was found to contain over 90% phosphoglyceric acid and 4% sugar phosphate. It appeared that the major original component of III-A which produced phosphoglyceric acid was triose phosphate. Fraction IV was glucose and fructose in about equal amts. Degradation was performed on several of the identified compds. Large amts. of radioactive triose phosphate and phosphoglyceric acid occurred in very short photosynthetic expts. as well as in the dark fixation. Hexose synthesis probably proceeds by reversal of the usual glycolytic split of fructose diphosphate. The alanine distribution may be an index of the distribution in pyruvic acid, and the problem is to determine the path from CO2 to pyruvic acid which gives the characteristic distribution. It is suggested that the 3-carbon compounds are produced through 4-carbon dicarboxylic acids. The acceptors for carboxylation reactions must be regenerated continuously and must contain an increasing amt. of isotopic carbon, approaching the isotopic concn. of CO2. The amt. of radio-activity in any compounds or carbon atom in a specific compound will depend upon the size of its actively functioning reservoir and the relative rates of all reactions leading to and from that reservoir under any set of exptl. conditions. The exact nature of the reducing agents cannot yet be stated.