CONCLUSION
The planet Earth represents a unique combination of favourable conditions
for the origination and development of life. As a result of the abiogenic
chemical evolution based on hydrocarbon compounds formed from the
methane (CH₄), ammonia (NH₃), molecular hydrogen (H₂) and water
vapours (H₂O) available in the primitive atmosphere on the Earth, under the
action of Sun’s ultraviolet irradiation, the various space and radioactive
emissions, the electric discharges, favourable temperature, etc., living
matter emerges. Cell structures have arisen from it with their immanent
properties to exchanges with the surroundings and to reproduce. Most
probably, the Earth is not the only planet in the Universe where life exists,
but up to now there are no concrete and reliable proof of that.
The cell as a generalized notion, becomes the basic structural and
functional biological system. Under the influence of environmental factors
and regular internal biochemical and genetic processes, it is subjected to a
permanent evolution. The metabolic changes lead to differences both in
structural and functional organization. Together with the primary autotrophic
cells, heterotrophic ones arise, satisfying their life needs by the use of
ready-made synthesized substances. Depending on the energy sourse
used in the biochemical processes, these are divided into phototrophic and
heterotrophic ones. If metabolic processes take part without oxygen
consumption the cells are anaerobic, while if they use oxygen — they are
aerobic. Some of them are obligatory anaerobic and aerobic respectively,
others are facultatively because of their ability to exist both in the absence
of oxygen as well as in the presence of it. All that is in favour of the concept
of earth origin of life, respectively the cell.
The emergence of photosynthesis lays the foundations not only of a
more effective biosynthesis, but also for the enlargement of the diversity of
living forms. It is the basic and most powerful mechanism for incorporating
the energy of the Sun in the synthesis of organic compounds and the
release of molecular oxygen (O₂) in the atmosphere, which introduces
radical changes in the metabolism of the living organisms. Besides
photosynthesis, hemosynthesis and nitrogen fixation take place in a much
more limited scale. These facts indicate the great biological capacities of
the cells and their adaptive ability to the conditions of existence.
In the course of evolution, except the mechanisms of metabolism, cells
have built many others — protection from the various harmful factors of the
environment, mutations and reparations, modes and types of cell division,
relationships and interactions with cell organelles, etc. The cell underlies as
well the two of the most important biological phenomena concerning
directly the human — senescence and neoplasms resulting from cell
specialization, differentiation and dedifferentiation.
The main compounds building materials of the cells are proteins
(polypeptides), nucleic acids (polynucleotides) and carbohydrates
(polysaccharides). Some authors are inclined to add lipids as well. The
problem of the sequence of their arising and the stages of their