Irrespective of the very nature and complexity of an organism, the cell designates the fundamental
structural unit of life. In other words, all living cells are basically similar. The
‘cell theory’
concept of the cell as the structural unit of life, was duly put forward by Schleiden and Schwann.
In short, all biological systems essentially have the following characteristic features in common,
namely :
(1) ability for reproduction,
(2) ability to assimilate or ingest food substances, and subsequently metabolize them for energy
and growth,
(3) ability to excrete waste products,
(4) ability to react to unavoidable alterations in their environment — usually known as
irritability,
and
(5) susceptibility to mutation.
The
plants and the animals were the two preliminary kingdoms of living organisms duly recognized
and identified by the earlier
biologists. However, one may articulately distinguish these two
groups by means of a number of well-defined structural and functional characteristic features
Soon after the discovery of the
‘microbial world’ — the immotile multicellular and photosynthetic
algae were classified duly in the
plant kingdom; whereas — the microscopic motile forms of
algae were placed duly in the
animal kingdom. Hence, a close and careful examination revealed the
presence of both plant — and animal-like characteristic features in the
‘microorganisms’. Further,
supporting evidences and valuable informations strongly established that the
‘microorganisms’ could
not fit reasonably into the two aforesaid kingdoms, namely :
‘plants’ and ‘animals’. Therefore, Haeckel
(1866) legitimately and affirmatively proposed a
‘third kingdom’ termed as the ‘Protista’* to include
the
‘microorganisms’ exclusively.
Importantly,
Protista group usually comprises of both the photosynthetic and non-photosynthetic
microorganisms
, of course, with certain members sharing the characteristic features of both the
usual traditional plant and animal kingdoms. Nevertheless, the most prominent and predominant attribute
of this particular group being the comparatively much simpler biological organization. Most of
the representative members of this group are normally unicellular and undifferentiated unlike the animals
and the plants.
Noticeably, further categorization of this kingdom was exclusively dependent upon the extent of
complexity encountered by the cellular organization, substantial progress in microscopy, and the ‘biochemistry
of various microorganisms’ has ultimately paved the way towards a much advanced and
better understanding of the differences with regard to the
‘internal architectural design of the microbialcells’.