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A CELL IS AN ELEMENTARY UNIT OF A LIVING BEING
A cell is considered an elementary structural, functional and informational
(genetic) unit of life on Earth. This means that the basic properties of living matter (such
as metabolism and energy conversion, reproduction, irritability, homeostasis, etc.) can
manifest themselves only at the cellular and higher levels of organization.
Viruses are often called a non-cellular form of life. However, the reproduction of
viruses, the synthesis of their constituent proteins and nucleic acids is possible only in
the cell they infect. Outside the host cell, viruses are not able to exhibit the properties
of a living one.
The first cells on Earth appeared about 3.5 billion years ago during chemical and
then prebiological evolution. Biogenesis is not the only hypothesis of the origin of life,
but only it is at least partially confirmed by laboratory experiments and has a scientific
justification.
Prokaryotic type cells were the first to appear. To date, they are represented by
bacteria and archaea. Prokaryotes are simpler (they do not have a cell nucleus and other
membrane organelles, significantly less genetic material), in the course of their
evolution they have not formed multicellular life forms. However, prokaryotes have
more diverse metabolic variants.
From prokaryotic cells, presumably by symbiogenesis, eukaryotic cells evolved. They
have a more complex structure and a large genome. Their heyday began only about 1
billion years ago, and during this time, in the course of their evolution, they formed
almost the entire diversity of life on Earth.
Eukaryotes include protozoa (unicellular eukaryotes), plants, animals and fungi.
While maintaining the general plan of structure and functionality, cells of different
groups have some differences among themselves. So animal cells lack a cell wall and
chloroplasts (the latter are not present in fungi).

The study of the structure of cells is engaged in the science of cytology. In
modern biology, the term "cytology" is often replaced by "cell biology".
Having appeared on Earth, the cell became the basis of the structure, vital activity
and development of all living organisms, both unicellular and multicellular. The cell is
the smallest isolated living structure in size, while it is characterized by a complex
structure. It contains mechanisms of metabolism, storage and use of biological
information, reproduction, properties of heredity and variability.
Science's understanding of the fundamental role of the cell in the organization of
living matter was reflected in the cell theory developed in the 30-50-ies of the XIX
century.

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