Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Stoichiometry and Energetics of Metabolic Conversions
Photosynthesis and Its Relationship to the Primary Energy Source
Accumulation of Light Energy

In most types of Respiration, hydrogen atoms are continually transferred from fuel molecules to oxygen, a process accompanied by the release of energy. Photosynthesis is essentially the reverse process, capturing light energy used to convert carbon dioxide into glucose and its polymers. Photosynthesis serves as the primary energy supplier for the entire biosphere. It harnesses the energy of the sole significant energy source in the solar system—the Sun itself. Another vital function of photosynthesis lies in its role in the Carbon and Oxygen cycles; specifically, it drives the reduction of carbon that was oxidized during respiration:

Class="center">Photosynthesis: 6СO₂ + 6Н₂O + light → С₆Н₁₂O₆ + 6O₂      (5.26)

In prokaryotes (cyanobacteria, also called blue-green Algae; green sulfur Bacteria; purple sulfur bacteria), photosynthesis takes place in multilayered membranes, whereas in eukaryotes (algae, plants), the centers of photosynthesis are specialized Organelles called METABOLISM/14.html">Chloroplasts (Fig. 1.5). Both types of systems contain chlorophyll—a complex organic compound with strong Light absorption in the visible region of the spectrum (Fig. 5.9). The energy of a single photon, $E_p$, depends on its frequency (or wavelength) as follows:

where h is Planck's constant; v is the photon frequency; λ is the photon wavelength; and c is the speed of light in the medium. The two main peaks in the absorption spectrum of chlorophyll (Fig. 5.9) correspond to photon energies of 43.5 kcal/mol (650 nm) and approximately 67 kcal/mol (430 nm). Both absorption bands clearly correspond to Free energy significantly exceeding the minimum required for ADP phosphorylation.

Two distinct systems for light energy storage and associated Chemical Reactions are known, designated as Photosystems I and II. Photosystems I and II are activated by light with wavelengths greater than and less than 680 nm, respectively. Both systems are found in all oxygen-evolving photosynthetic organisms. Conversely, non-oxygen-evolving photosynthetic bacteria contain only Photosystem I.

FIG. 5.9. The maxima in the absorption spectrum of chlorophyll a (lower curve) correlate with the absorption maxima in THE SPECTRUM OF green leaves (upper curve) and the spectrum of photosynthetic activity of leaves (middle curve). The corresponding leaf pigments enhance the efficiency of light utilization in the 500–600 nm range compared to pure chlorophyll. (Lehninger A., Biochemistry, vols. 1–3.) 1—absorption spectrum; 2—photosynthetic activity spectrum; 3—chlorophyll a.