PHOTOSYNTHESIS (CHLOROPHYLL). — The suggestive process of leaf nutrition in plants was clarified in its essential features as early as the classic experiments of De Saussure (1740–99), which established that, in the presence of sunlight, a green plant emits a volume of oxygen equal to that of the carbon dioxide absorbed. Thus, three fundamental conditions for the occurrence of the phenomenon can be identified: a) the presence of carbon dioxide in the environment; b) the presence of sunlight; c) the presence of green pigment.
Therefore, the process of leaf nutrition was definitively recognized as limited to carbon nutrition, proceeding under the influence of light and in the presence of chlorophyll; hence it acquired the name of the process of photosynthesis or chlorophyllian photosynthesis.
The transformation of inorganic carbon into organic carbon—an organization of carbon effected by photosynthesis—has long been regarded as a reduction process in which substances that are fully oxidized and therefore non-combustible (carbon dioxide and water) are converted into substances with a lower degree of oxidation and hence combustible. This is an endoenergetic process, i.e., one that proceeds with the absorption of energy, specifically light energy (photons) supplied by the sun. The plant absorbs and stores this energy as chemical energy through the construction of organic matter. It is for this reason that plants may be defined as accumulators of solar energy.
The fruitful experimental investigations that have followed in great number since the mid-19th century have shed light on the importance of the individual factors regulating the photosynthetic process, namely: light (intensity and quality of radiation), the raw materials involved (carbon dioxide and water), temperature, chlorophyll content, mineral content (macro- and microelements), and finally other so-called internal factors linked to enzymatic and protoplasmic activity within the cell (in vitro, outside the chloroplasts, chlorophyll loses its characteristic action).
Less convincing, however, are the numerous hypotheses concerning the first organic product that arises from the photochemical reduction of carbon dioxide. The hypothesis advanced about a century ago—that formaldehyde is the first product of carbon assimilation—has been discarded. The hypotheses of Baeyer and of Willstätter and Stoll, which posit formaldehyde as the primary reduction product, and that of Maquenne, which excludes formaldehyde and instead posits that the first product of photosynthesis is directly a monosaccharide, i.e., a simple sugar, remain in contention.
Recently, in California, several researchers—Ruben, Randall, Kamen, and Hyde—administered to green algae of the genus *Chlorella* ordinary carbon dioxide and water containing the isotope O18 in place of the usual O16. They found that the oxygen released during photosynthesis derives entirely from the water and not from carbon dioxide. Thus, water undergoes dehydrogenation in the photosynthetic process, while carbon dioxide or one of its derivatives serves as the corresponding hydrogen acceptor.
Also in California, Calvin and his colleagues pursued further research using the isotope C14 and concluded that the appearance of glucose as the first stable product of chlorophyllian synthesis is preceded by the formation of organic acids, which are progressively hydrogenated through photochemical action and also by reactions that proceed without the need for light.
According to these most recent findings, photosynthesis may be regarded as the reverse of respiration, not only in an energetic sense but especially in biochemical and enzymatic terms (Pratolongo).
Regarding the solar energy utilized by plants in photosynthesis, recent calculations by Putter—taking into account losses due to respiration—indicate that the efficiency ranges between 2% and 4%. The percentage of solar energy utilized is 2.1 in beet, 2.6 in barley, 3.0 in potatoes, 3.2 in oats and wheat, and 3.7 in cabbage.
The quantities of carbon fixed annually by plants through photosynthesis are enormous. It is estimated that the land vegetation currently carries out an annual assimilatory activity equivalent to 1.9 × 10¹⁰ metric tons of organic carbon, which corresponds on average to 13 quintals of carbon per year per hectare over the entire emerged surface. The chlorophyllian synthesis occurring within the oceans exceeds that on land; it has been estimated at 13.1 × 10¹⁰ metric tons annually, equivalent on average to 37.5 quintals of carbon fixed per year per hectare of sea surface (Riley, 1941).