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CHAPTER 2 BIOLOGICAL CLASSIFICATION
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CHAPTER 2 BIOLOGICAL CLASSIFICATION
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16 BIOLOGYC HAPTER 2
B IOLOGICAL C LASSIFICATION
2.1 Kingdom Monera Since the dawn of civilisation, there have been many attempts to classify
living organisms. It was done instinctively not using criteria that were
2.2 Kingdom Protista
scientific but borne out of a need to use organisms for our own use – for
2.3 Kingdom Fungi food, shelter and clothing. Aristotle was the earliest to attempt a more
2.4 Kingdom Plantae scientific basis for classification. He used simple morphological characters
to classify plants into trees, shrubs and herbs. He also divided animals
2.5 Kingdom into two groups, those which had red blood and those that did not.
Animalia In Linnaeus' time a Two Kingdom system of classification with
2.6 Viruses, Viroids Plantae and Animalia kingdoms was developed that included all
and Lichens plants and animals respectively. This system did not distinguish between
the eukaryotes and prokaryotes, unicellular and multicellular organisms
and photosynthetic (green algae) and non-photosynthetic (fungi)
organisms. Classification of organisms into plants and animals was easily
done and was easy to understand, but, a large number of organisms
did not fall into either category. Hence the two kingdom classification
used for a long time was found inadequate. Besides, gross morphology
a need was also felt for including other characteristics like cell structure,
nature of wall, mode of nutrition, habitat, methods of reproduction,
evolutionary relationships, etc. Classification systems for the living
organisms have hence, undergone several changes over the time.
Though plant and animal kingdoms have been a constant under all
different systems, the understanding of what groups/organisms be
included under these kingdoms have been changing; the number and
nature of other kingdoms have also been understood differently by
different scientists over the time.
2021-22
, BIOLOGICAL CLASSIFICATION 17
TABLE 2.1 Characteristics of the Five Kingdoms
Five Kingdoms
Characters
Monera Protista Fungi Plantae Animalia
Cell type Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic
Cell wall Noncellulosic Present in Present Present
(Polysaccharide some with chitin (cellulose) Absent
+ amino acid)
Nuclear Absent Present Present Present Present
membrane
Body Cellular Cellular Multiceullar/ Tissue/ Tissue/organ/
organisation loose tissue organ organ system
Autotrophic
Autotrophic Heterotrophic Autotrophic Heterotrophic
(chemosyn-
(Photosyn- (Saprophytic/ (Photosyn- ( H o l o z o i c /
thetic and
Mode of thetic) and Parasitic) thetic) Saprophytic
photosynthetic)
nutrition and Hetero- Hetero- etc.)
trophic (sapro- trophic
phytic/para-
sitic)
R.H. Whittaker (1969) proposed a Five Kingdom Classification. The
kingdoms defined by him were named Monera, Protista, Fungi, Plantae
and Animalia. The main criteria for classification used by him include cell
structure, body organisation, mode of nutrition, reproduction and
phylogenetic relationships. Table 2.1 gives a comparative account of different
characteristics of the five kingdoms.
The three-domain system has also been proposed that divides the Kingdom
Monera into two domains, leaving the remaining eukaryotic kingdoms in the
third domain and thereby a six kingdom classification. You will learn about
this system in detail at higher classes.
Let us look at this five kingdom classification to understand the issues
and considerations that influenced the classification system. Earlier
classification systems included bacteria, blue green algae, fungi, mosses,
ferns, gymnosperms and the angiosperms under ‘Plants’. The character
that unified this whole kingdom was that all the organisms included had a
cell wall in their cells. This placed together groups which widely differed in
other characteristics. It brought together the prokaryotic bacteria and the
blue green algae (cyanobacteria) with other groups which were eukaryotic.
It also grouped together the unicellular organisms and the multicellular
ones, say, for example, Chlamydomonas and Spirogyra were placed together
under algae. The classification did not differentiate between the heterotrophic
group – fungi, and the autotrophic green plants, though they also showed
a characteristic difference in their walls composition – the fungi had chitin
2021-22
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