these microorganisms has been H₂S, which releases sulphur accumulated
in the environment after its participation in the reduction process. The green
sulphur bacteria also use H₂S according to the following equation:
Lehninger has also stated that photosynthesizing bacteria do not use
and do not release molecular oxygen. Instead of H₂O they use as donors of
electrons inorganic substances (hydrogen sulphide, thiosulphate, gaseous
hydrogen), as well as organic compounds (lactic acid, alcohols, etc.).
Organisms not releasing oxygen do not possess photosystem II.
Using more precise methods it was established that CO₂ is not always
the acceptor of electrons. In some cases (in higher plants) this role is
played by nitrates. In nitrogen fixing photosynthesizing organisms CO2 can
be the acceptor of electrons as well as molecular nitrogen (N₂), which is
thus oxidized to ammonia (NH₃). Hydrogen ions (H+) can also play such a
role resulting in the formation of molecular hydrogen (H₂).
In different organisms photosynthesis can take place with the
participation of different donors and acceptors of electrons. The process
can be summed up as follows:
where D.H₂ is the donor, and A — the acceptor of the electrons or the
hydrogen atoms.
It has been initially assumed that molecular oxygen released in
photosynthesis comes from CO₂. In 1931 van Niel has suggested for it to
originate from water molecules, which was later confirmed by a number of
experiments including such with labelled isotopes (¹⁸O). In order to
emphasize upon its origin the summed up equation of photosynthesis is
presented in the following manner:
The process of photosynthesis engages a large number of enzymes,
different pigments organized in cellular organelles (mainly chloroplasts) and
the macroergic bonds of ATP formed by the way of phosphorylation. Their
indetail account is quite intentionally skipped so that the development of
metabolism could be better outlined from an evolutionary standpoint.
As far as living entities are closely associated with metabolism, the
review in retrospect gives a certain idea of the initial stages of development of
life. At the very beginning nascent oxygen (O₂) was not present in earth
atmosphere, or it was but in minimal quantities. For that reason this chemical