A. Fundamentals

The historical development of our understanding of the photosynthesis of biomass began in 1772 when the English scientist Joseph Priestley discovered that green plants expire a life-sustaining substance (oxygen) to the atmosphere, while a live mouse or a burning candle removes this same substance from the atmosphere. A variety of suggestions were offered by the scientific community during the ensuing 30 years to explain these observations until in 1804, the Swiss scientist Nicolas Theodore de Sausseure showed that the amount of C02 absorbed by green plants is the molecular equivalent of the oxygen expired. From that point on, the stoichiometry of the process was developed and major advancements were made to detail the chemistry of photosynthesis and how the assimilation of C02 takes place. Much of this work paralleled the develop­ment of research done to understand the biochemical pathways of the cellular metabolism of foodstuffs. Indeed, there is much overlap in the chemistry of both processes.

About 75% of the energy in solar radiation, after passage through the atmosphere where much of the shorter wavelength, high-energy radiation is filtered out, is contained in light of wavelengths between the visible and near­infrared portions of the electromagnetic spectrum, 400 to 1100 nm. The light­absorbing pigments effective in photosynthesis have absorption bands in this range. Chlorophyll a and chlorophyll b, which strongly absorb wavelengths in the red and blue regions of the spectrum, and accessory carotenoid and phycobilin pigments participate in the process. Numerous investigations have established many of the parameters in the complex photosynthetic reactions
occurring in biomass membrane systems which contain the necessary pigments and electron carriers. Tracer studies to establish the chemical structures of the intermediates in photosynthesis were initiated in the 1940s by the U. S. investigators Melvin Calvin, J. A. Bassham, and Andrew A. Benson. These studies showed that the oxygen evolved in photosynthesis comes exclusively from water and not C02. With green algae (Chlorella), reducing power is accumulated during illumination in the absence of C02 and can later be used for the reduction of C02 in the absence of light (Calvin and Benson, 1947). After a short 30- to 90-second exposure to light, the main portion of the newly reduced, labeled C02 was found to be distributed in a dozen or more organic compounds. By progressively shortening the light exposure to 2 seconds before killing the cells, almost all of the 14C in the labeled C02 was found to be incorporated in 3-phosphoglyceric acid, a compound that occurs in practically all plant and animal cells. Such experiments led to elaboration of the biochemi­cal pathways and the essential compounds required for photosynthesis. It was found that the pentose ribulose-1,5-diphosphate is a key intermediate in the process. It reacts with C02 to yield 3-phosphoglyceric acid. Equimolar amounts of ribulose-1,5-diphosphate and C02 react to form 2 mol of 3-phosphoglyceric acid, and in the process, inorganic carbon is transformed into organic carbon.

CH2OP03H2

C=0

1

H-C-OH + COo

I

H-C-OH

I

ch2opo3h2

Ribulose-1,5-diphosphate 3-Phosphoglyceric acid

These reactions of course occur in the presence of the proper enzyme catalysts and cofactors. Glucose is the primary photosynthetic product. As will be shown later in the discussion of the structures of the organic intermediates in the various pathways, the dark reactions take place in such a manner that ribulose- 1,5-diphosphate is regenerated.