Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Hydrogen donors vary among different photosynthetic organisms

Photosynthetic organisms can be divided into two classes: oxygen-evolving and non-oxygen-evolving. The green leaf Cells of higher plants belong to oxygen producers.

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Fig. 23-2. Photosynthetic planktonic organisms. A and B. Two typical freshwater species, Euglena and Chlamydomonas, which possess flagella and are capable of orienting toward a light source. C–E. Three species of diatoms. Diatoms are extremely abundant in marine plankton. Their cells are encased in a rigid shell composed of two halves; this shell features a highly intricate Structure characteristic of each diatom species. The shell consists primarily of silica (SiO₂). Microscopists once used empty diatom frustules (E) to determine the resolving power of lenses. E and F. Dinoflagellates are also typical photosynthetic organisms of marine plankton. Their cells possess flagella and often exhibit very bizarre shapes imparted by their outer cellulosic walls. Among them are species toxic to fish and humans. "Red tides" off the coast of the United States are caused by massive blooms of red dinoflagellates, which secrete a potent neurotoxin.

To reduce carbon dioxide, they use Water as a hydrogen donor, releasing molecular oxygen in the process, According to the equation

where n is generally considered to be 6, since the ultimate product of CO₂ reduction is glucose [(СН₂O)₆=С₆Н₁₂O₆].

Photosynthetic Bacteria do not evolve oxygen, with the exception of cyanobacteria (whose oxygen-evolving photosynthetic system is similar to that of green plants). Furthermore, many photosynthetic bacteria are obligate anaerobes, meaning they cannot tolerate oxygen at all. Some photosynthetic bacteria utilize Inorganic Compounds as hydrogen Donors. For green sulfur bacteria, for instance, hydrogen sulfide serves as the hydrogen donor according to the equation

Instead of molecular oxygen, these bacteria release elemental sulfur, which is the oxidation product of H₂S. Other photosynthetic bacteria use Organic compounds, such as lactate, as hydrogen donors

Fig. 23-3. Some hydrogen donors (electron donors) utilized by various photosynthetic organisms. The transferred hydrogen atoms are highlighted in red. H₂O serves as the electron donor for green plants, from which they release O₂.

Cornelis van Niel, a pioneer in comparative METABOLISM, became convinced that the photosynthetic processes in plants and bacteria are fundamentally identical, even though they employ different hydrogen donors. This similarity becomes apparent when the photosynthetic equation is written in a more general form:

where H₂D is the hydrogen donor and D is its oxidized form. Depending on the type of photosynthetic Organism, H₂D can be water, hydrogen sulfide, lactate, or other organic compounds (Fig. 23-3). Van Niel also hypothesized that the molecular oxygen generated during plant Photosynthesis originates entirely from water rather than carbon dioxide. Isotope-labeling experiments, in which either water or carbon dioxide was labeled with the ¹⁸O isotope, confirmed this hypothesis (Fig. 23-4). In this chapter, we will primarily examine oxygen-evolving photosynthesis in higher plants.

Fig. 23-4. The source of O₂ evolved during plant photosynthesis is H₂O.