
BIOLOGICAL CLASSIFICATION
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SUMMARY
Biological classification of plants and animals was first proposed by Aristotle on the
basis of simple morphological characters. Linnaeus later classified all living organisms
into two kingdoms – Plantae and Animalia. Whittaker proposed an elaborate five
kingdom classification – Monera, Protista, Fungi, Plantae and Animalia. The main
criteria of the five kingdom classification were cell structure, body organisation,
mode of nutrition and reproduction, and phylogenetic relationships.
In the five kingdom classification, bacteria are included in Kingdom Monera.
Bacteria are cosmopolitan in distribution. These organisms show the most extensive
metabolic diversity. Bacteria may be autotrophic or heterotrophic in their mode of
nutrition. Kingdom Protista includes all single-celled eukaryotes such as
Chrysophytes, Dinoflagellates, Euglenoids, Slime-moulds and Protozoans. Protists
have defined nucleus and other membrane bound organelles. They reproduce
both asexually and sexually. Members of Kingdom Fungi show a great diversity
in structures and habitat. Most fungi are saprophytic in their mode of nutrition.
They show asexual and sexual reproduction. Phycomycetes, Ascomycetes,
Basidiomycetes and Deuteromycetes are the four classes under this kingdom.
The plantae includes all eukaryotic chlorophyll-containing organisms. Algae,
bryophytes, pteridophytes, gymnosperms and angiosperms are included in this
group. The life cycle of plants exhibit alternation of generations – gametophytic
and sporophytic generations. The heterotrophic eukaryotic, multicellular
organisms lacking a cell wall are included in the Kingdom Animalia. The mode of
nutrition of these organisms is holozoic. They reproduce mostly by the sexual
mode. Some acellular organisms like viruses and viroids as well as the lichens are
not included in the five kingdom system of classification.
Viroids : In 1971, T.O. Diener discovered a new infectious agent that
was smaller than viruses and caused potato spindle tuber disease. It was
found to be a free RNA; it lacked the protein coat that is found in viruses,
hence the name viroid. The RNA of the viroid was of low molecular weight.
Prions : In modern medicine certain infectious neurological diseases
were found to be transmitted by an agent consisting of abnormally folded
protein. The agent was similar in size to viruses. These agents were called
prions. The most notable diseases caused by prions are bovine spongiform
encephalopathy (BSE) commonly called mad cow disease in cattle and
its analogous variant Cr–Jacob disease (CJD) in humans.
Lichens : Lichens are symbiotic associations i.e. mutually useful
associations, between algae and fungi. The algal component is known as
phycobiont and fungal component as mycobiont, which are autotrophic
and heterotrophic, respectively. Algae prepare food for fungi and fungi
provide shelter and absorb mineral nutrients and water for its partner.
So close is their association that if one saw a lichen in nature one would
never imagine that they had two different organisms within them. Lichens
are very good pollution indicators – they do not grow in polluted areas.