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Bioenergetic Aspects of Photosynthetic Gas
Exchange and Respiratory Processes in Algae
and Plants
Klaus Peter Bader
University of Bielefeld, Bielefeld, Germany
Refat Abdel-Basset
Assiut University, Assiut, Egypt
299I. INTRODUCTION
A. General Scientific Features
The nuclear reaction that takes place deep inside the sun consists of the fusion of hydrogen (H 2 ) to helium
(He). According to up-to-date estimates about 5 109 kg of molecular hydrogen is converted every sec-
ond with the difference in the masses of the involved molecules being emitted as radiation equivalent to
approximately 6000 K photons. The resulting energy can be calculated to a value of 1400 kW m^2. Per
year, this process supplies earth with the immense energy amount of 56 1023 J. Even under the as-
sumption that about 50% of the radiation might be reflected by clouds and gases of the atmosphere (thus
not reaching the earth’s surface) and that 50% of the passing radiation is in the long-wavelength region
with a low energy content (e.g., infrared), energy of about 14 1023 J is available. This gigantic amount,
however, is “mere” radiation energy, which cannot be readily used by (nearly) any biological organism.
Only one type of organism is capable of converting this abiotic energy into a biologically useful form, and
consequently these organisms are called photoautotrophs. Among these are cyanobacteria, green algae,
photosynthetic bacteria, and higher plants.
Similarly, rough estimates give a value of 3 1021 J for the overall biomass produced by autotrophic
organisms and 13 1018 J for the energy uptake by mankind. In any case and without overestimating the
correctness of the given values, it is clear that less than 1% of the available radiation energy is converted
into biomass! Of this value, less than 1% is actually taken up as nutrition (of any type) by mankind! Thus,
the significance and the importance of photosynthesis (and the organisms involved) can hardly be over-
valued.
In evolutionary terms, blue-green algae (cyanobacteria) must be mentioned in the first place because
they were the organisms that “invented” an “improved” and most important form of photosynthesis,
namely oxygenic photosynthesis, about 3–4 billion years ago. It must be emphasized that this process
took place in a reducing atmosphere that consisted of nitrogen and carbon dioxide (possibly some hydro-
gen) with virtually no oxygen present. This generally accepted view, however, does not completely hold,
as small but substantial amounts of oxygen must have been present at the time. Detailed analyses of the
biosynthesis of essential pigments such as chlorophyll have shown that at least one step in the biosynthe-
sis sequence requires molecular oxygen! The reaction from coproporphyrinogen III to protoporphyrino-
gen IX, i.e., the formation of the vinyl groups from the propionic acid side chains, is catalyzed by the co-