Plant Physiology - Musienko M. M. 2001

Photosynthesis: Physiological, Biochemical, and Ecological Aspects
General Equation of Photosynthesis and the Origin of Oxygen

The discovery of Photosynthesis is generally dated back to 1771, when the English scientist J. Priestley Observed changes in air composition caused by animal activity. In the presence of green plants, the air once again became suitable for both Respiration and combustion. Subsequent studies by A number of researchers (J. Ingenhousz, J. Senebier, N. de Saussure, and J. Boussingault) established that green plants absorb carbon dioxide from the air, which, in the Presence of Water and light, is converted into organic matter:

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It was precisely this process that the German scientist W. Pfeffer termed photosynthesis in 1877. The law of conservation of energy, formulated by R. Mayer in 1845, played a crucial role in unraveling The Essence of photosynthesis. According to this law, the energy utilized by plants is solar energy, which they convert into chemical energy during photosynthesis. This principle was later confirmed by the experimental research of K.A. Timiryazev. The General equation of photosynthesis does not quite accurately convey the essence of the process; based on it, one might assume that part of the released O₂ originates from CO₂, whereas in reality, all the oxygen is produced from water through its photolysis.

For nearly a century after the discovery of photosynthesis in 1771, scientists investigated its nature, yet initially established only the substrates and end products resulting from the process.

For a long time, it was widely accepted that CARBOHYDRATES are formed from carbon and hydrogen, while oxygen is released from carbon dioxide. As was later clarified, this hypothesis was entirely incorrect.

In 1893, A.M. Bach was the first to predict that the assimilation of CO₂ during photosynthesis has nothing to do with the Cleavage of oxygen from CO₂, but is instead associated with a redox process involving hydrogen and hydroxyl groups from water, with oxygen being released from the water.

The theory that oxygen originates from carbon dioxide was refuted by Cornelis van Niel. While still an undergraduate at Stanford University (USA) studying the METABOLISM of various photosynthetic Bacteria, he established that purple sulfur bacteria require hydrogen sulfide for photosynthesis, resulting in the accumulation of sulfur inside the bacterial Cells:

Following a series of experiments between 1937 and 1941, he made the bold generalization that The primary photochemical reaction of photosynthesis actually consists of the photodissociation of water rather than the decomposition of CO₂, and proposed the following overall equation for photosynthesis:

In this equation, H₂A can represent water or other reducing agents. Thus, bacteria capable of photosynthetic CO₂ assimilation—with the exception of cyanelles—utilize H₂S, H₂, or CH₃, and therefore do not release oxygen during photosynthesis. This type of phototrophic Nutrition became known as photoreduction. For Higher Plants and Algae, H₂A is water. Van Niel hypothesized that water, rather than carbon dioxide, is split during photosynthesis; therefore, the equation for photosynthesis should be:

or in stages:

Van Niel's concept regarding THE ORIGIN OF oxygen was confirmed by experiments using the ¹⁸O isotope, independently conducted in 1941 by S. Ruben (USA) and A.P. Vinogradov and R.V. Teis (USSR). Mass spectrometric analysis demonstrated that The ratio of ¹⁶O to ¹⁸O in the oxygen produced during photosynthesis corresponds to the ratio found in water rather than in carbon dioxide:

When the ¹⁸O isotope is introduced into carbon dioxide, the equation takes on a different form:

This indicates that the newly formed water molecule differs from the two molecules that entered the reaction:

Comparative Analysis of the isotopic composition of water, carbon dioxide, and the O₂ released during the photosynthetic process confirmed the affinity of photosynthetic oxygen with water.

Finally, in 1939, R. Hill demonstrated that isolated Chloroplasts, in the presence of appropriate electron acceptors, are capable of splitting water in the light with the release of O₂ without the participation of carbon dioxide:

The absence of a reaction utilizing the electrons is compensated for, as seen from the equation, by introducing an electron acceptor (A), or "Hill reagent," into the system. These can be iron salts (potassium ferrioxalate, potassium ferricyanide), benzoquinone, or the natural electron acceptor NADP⁺. This reaction is referred to as the Hill reaction.

Thus, for algae and higher plants, in which water serves as the electron donor, the net equation of photosynthesis is as follows:

In addition, free Energy is stored in The amount of 470 kJ/mol. Green plants are uniquely capable of capturing solar energy and storing it as chemical bond energy. During photosynthesis, A wide variety of Organic compounds are synthesized from simple inorganic precursors, namely water and carbon dioxide. This process drives a rearrangement of chemical bonds: =C=O and H-O- bonds are replaced by =C=C= and ≡C-H bonds, in which electrons occupy higher energy levels. Consequently, the energy-rich organic molecules that serve as food and Energy Sources for animals and humans—released through cellular respiration—are originally synthesized by green plants. It has been established that nearly all atmospheric oxygen is of photosynthetic origin. Hence, respiration and combustion became possible only after The Emergence of photosynthesis. All of this highlights the profound cosmic role of green plants.

The overall reaction of photosynthesis can also be represented through redox reactions: one involving the photodissociation of water, and the other—the reduction of CO₂ to carbohydrates, which proceeds independently of solar quanta:

Indeed, it is now well established that photosynthesis occurs in two distinct stages: the light-dependent and light-independent phases.