The difference between the anaerobic degradation of sugars, i.e. the
fermentations (wittily called by Pasteur “life without oxygen”), and
respiration, which is only possible in the presence of oxygen shows that
oxygen has joined at a later stage when its content in the atmosphere has
risen. This is a clear and very convincing proof in support of the concept of
the earth origin and development of cells, since they have been in
dependence of its absence and later its presence in the earth atmosphere.
Figure 1–8. Scheme of anaerobic and aerobic degradation of sugars.
Due to the exhaustion of organic matter supplies (as are the conjectures
of some authors) or because of the increased metabolic capacities of the cells
as a result of the acquisition of new mechanisms and genetic changes, some
of the cells have reached up to photosynthesis. This has been a turning point
in the evolution of life on Earth and has unleashed a variety of forms of
unicellular and multicellular organisms. This process has become the main
source of organic substances both for the photosynthesizing organisms
themselves as well as for the non-photosynthesizing heterotrophs which have
adapted to their utilization as ready products.
The discovery of photosynthesis represents a fascinating story. It has
begun with the classical experiments of the English chemist J. Priestley
(1772—77) on the recovery of the air composition through the release of
oxygen by plants. Priestley has proved experimentally that a mouse if placed
under a glass lid can only survive in the presence of a green plant exposed to
light. At the same time Ingen-Housz (1779) has discovered that for the
accomplishment of this process the presence of sunlight is crucial and
Senebier (1782) has supplemented it with the participation of CO₂ from the air.
Further studies on photosynthesis have been carried out of a great number of
authors who cannot be mentioned in this short presentation of the problem for
brevity.
Undoubtedly photosynthesis is a biochemical mechanism that has
emerged later, by which organic compounds are synthesized from H₂O and
CO₂ at the expense of solar energy. Sun rays falling onto chlorophyll
molecules excite electrons which upon passing to a lower energy level release