Biological Classification Introduction: There have been many attempts to classify living organisms. It was done instinctively not using criteria that were scientific but borne out of a need to use organisms for our own use – for food, shelter and clothing. Aristotle was the earliest to attempt a more scientific basis for classification. He used simple morphological characters to classify plants into trees, shrubs and herbs & divided animals into two groups, those which had red blood and those that did not. During Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed that included all plants and animals respectively, which was used till very recently & 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, inspite, a large number of organisms did not fall into either category. Hence the two kingdom classification used for a long time was found inadequate. It was felt to include, other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships, etc. besides gross morphology, Hence classification systems for the living organisms have undergone several changes over time. Five Kingdom Classification: Whittaker (1969) proposed the Five Kingdom Classification. He named them Monera, Protista, Fungi, Plantae and Animalia where the main criteria for classification used by him include cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships.
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Biological Classification
Introduction:
There have been many attempts to classify living organisms. It was done instinctively not
using criteria that were scientific but borne out of a need to use organisms for our own use –
for food, shelter and clothing.
Aristotle was the earliest to attempt a more scientific basis for classification. He used simple
morphological characters to classify plants into trees, shrubs and herbs & divided animals
into two groups, those which had red blood and those that did not.
During Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia
kingdoms was developed that included all plants and animals respectively, which was used
till very recently & 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, inspite, a large number of organisms did not fall into either category. Hence the
two kingdom classification used for a long time was found inadequate.
It was felt to include, other characteristics like cell structure, nature of wall, mode of
nutrition, habitat, methods of reproduction, evolutionary relationships, etc. besides gross
morphology,
Hence classification systems for the living organisms have undergone several changes over
time.
Five Kingdom Classification:
Whittaker (1969) proposed the Five Kingdom Classification.
He named them Monera, Protista, Fungi, Plantae and Animalia where the main criteria for
classification used by him include cell structure, thallus organisation, mode of nutrition,
reproduction and phylogenetic relationships.
The classification of living organisms according to Whittaker into five kingdoms namely are as
follows:
KINGDOM: MONERA
Monera consists of primitive organisms. The organisms are very small and single celled.
They consists of prokarytotes which includes species like the bacteria, archaebacteria,
cyanobacteria and mycoplasma.
Example: bacteria, oscillatoria, nostoc, etc.
KINGDOM: PROTISTA
Protista are single-celled eukaryotes and are mainlky aquatic. It includes diatoms, golden
algae, euglena and protozoans like amoeba, paramoecium, plasmodium etc. They are mostly
marine and photosynthetic.
Example: paramecium
KINGDOM: MYCOTA
Kingdom Mycota or Kingdom Fungi consists of network of thread-like structures called as
mycelium. The bodies consists of long, thread-like structures which are called hyphae.
These organisms are mostly saprophytes or parasites and also symbionts. This kingdom of
fungi also includes lichens, mycorrhiza, etc.
Example: Aspergillus
KINGDOM: METAPHYTA
Kingdom Metaphyta or Kingdom Plantae are eukaryotic, mutlicellular plants, They contain
chlororphyll pigment, which helps them prepare their own food by the process of
photosynthesis. This kingdom includes all types of plants like herbs, shrubs, trees, flowering
and non flowering plants.
Example: rose plant, mango tree, algae, bryophytes, etc.
KINGDOM: METAZOA
Kingdom Animalia or Kingdom Metazoa are heterotrophic, eukaryotic, multicellular
organisms. They lack cell wall. This kingdom includes all types of animals.
Example: lion, peacock, etc.
Merits of Five Kingdom Classification
This system of classification is more scientific and natural.
It is the most accepted system of modern classification as the different groups of animals are
placed phylogenetically.
The prokaryotes are placed in a separate kingdom as they differ from all other organisms in
their organization.
As the unicellular organisms are placed under the kingdom protista, it has solved many
problem related to the position of organisms like euglena.
The fungi totally differ from other primitive eukarytotes hence, placing the group fungi in a
status of kingdom is justifiable.
The kingdom Plantae and Animalia shows the phylogeny of different life styles, in the five
kingdom classification, they are more homogeneous group than the two kingdom
classification.
This system of classification clearly indicates cellular organization and modes of nutrition,
the character which have appeared very early in the evolution of life.
Demerits of Five Kingdom Classification
This system of classification has drawbacks with reference to the lower forms of life.
The Kingdom of Monerans and the Protists include diverse, heterogenous forms of life. In
both the kingdoms there are autoptrophic and hetertrophic organisms. They also include
organisms which have cells with cell wall and cells without cell wall.
All the organisms of these three kingdoms do not originate from a single ancestor.
Multicellular organisms have originated from protists several times.
Organisms like the unicelluar green algae like volvox and chlamydomonas have not been
included under the Kingdom Protista because of their resemblance to other greeen algae.
The general organization of the slime moulds are completely different from the members of
protists.
In this system of classification viruses have not been given proper place.
Kingdom Monera:
All the organisms of this kingdom are prokaryotes. Complex structure was the basis of
classification of organisms, many centuries ago.
Monera are considered as the most primitive group of organisms. They include various types
of bacteria and blue-green algae.
Monerans are most abundant of all organisms, due to their versatility of their habitat. It is
estimated that a single drop of water contains 50 billion bacteria.
The kingdom is divided into two groups Archaebacteria and Eubacteria.
All the organisms of this kingdom are prokaryotes.
These cells do not have nuclear membrane, the chromosome is a single and circular, they
also lack membrane bound cellular organelles.
This kingdom includes bacteria, cyanobacteria, mycoplasma etc.
They are unicellular organisms and do not have specific mode of nutrition. They can be
either aerobic or anaerobic.
These organisms have cell wall which is made up of peptidoglycans. The cell organelles are
not membrane bound. Cell organelles like endoplasmic reticulum, mitochondria are absent.
Reproduction is by spore formation and binary fission.
Bacteria are grouped under four categories based on their shape: the spherical Coccus (pl.:
cocci), the rod-shaped Bacillus (pl.: bacilli), the comma-shaped Vibrium (pl.: vibrio) and the
spiral Spirillum (pl.: spirilla)
Though the bacterial structure is very simple, they are very complex in behaviour. Compared to
many other organisms, bacteria as a group show the most extensive metabolic diversity. Some of
the bacteria are autotrophic, i.e., they synthesise their own food from inorganic substrates. They
may be photosynthetic autotrophic or chemosynthetic autotrophic. The vast majority of bacteria are
heterotrophs, i.e., they do not synthesise their own food but depend on other organisms or on dead
organic matter for food.
Kingdom Monera has been classified into two groups - Archaebacteria and Eubacteria.
Archaebacteria are microbes that live in extreme and harsh conditions, they are known as
extremophiles. These bacteria lack cell wall, their cell membrane is made up of different lipids, and
their ribosomes are similar to that of eukaryotes. Archaebacteria are of three major groups of
bacteria based on their habitat i.e., thermophiles, halophiles and methanogen
Some extremophiles live in boiling water like geysers of Yellowstone National Park and and in
volcanoes. These are known as 'thermophiles'.
Some extremophiles live in extremely salty water, the salt loving bacteria are known as
'halophiles'.
Some bacteria are present in the guts of ruminants and are responsible for production of methane
gas from their dung. These bacteria are known as 'methanogens'.
Eubacteria are true bacteria. The characteristic feature is the presence of rigid cell wall and if
present a motile flagelllum that aids in locomotion. These organisms are characterized based on
their nutrition and their shapes.
Classification based on Shape
Bacteria can be classified in four groups based on shape : Spherical or round shaped bacteria
are called cocci, Rod-shaped are bacilli, Comma-shaped bacteria are vibrio and spiral
shaped bacteria are spirilla.
Classification Based on Mode of Nutrition:
Based on the mode of nutrition bacteria are broadly classified into Autotrophic and Heterotrophic.
Autotrophic bacteria - Bacteria which prepare their own food are autotrophic.
(Example Cyanobacteria)
Heterotrophic bacteria - Bacteria which are dependent on other organisms for their food are
heterotrophic.(Example Escherichia coli)
Autotrophic bacteria can be Chemosynthetic or Photosynthetic.
Chemosynthetic bacteria are those which prepare their food with the help of inorganic substrates.
Photosynthetic bacteria are autotrophic bacteria which prepare their own food by the process of
photosynthesis.
Eubacteria are characterised by the presence of a rigid cell wall, and if motile, a flagellum. The
cyanobacteria (also referred to as blue-green algae) have chlorophyll ‘a’ similar to green plants and
are photosynthetic autotrophs.
Cyanobacteria are unicellular, colonial or filamentous, marine or terrestrial algae.
The colonies are generally surrounded by gelatinous sheath. They often form blooms in polluted
water bodies.
Cyanobacteria multiply by fragmentation, hormogones and akinetes. Hormogones are small
trichome segments which separate naturally in filamentous forms due to death of an intervening cell
or presence of heterocyst. Akinetes are thick-walled resting spores.
Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g.,
Nostoc and Anabaena.
Cyanobacteria protect the soil from erosion. They are also used in reclamation of saline & alkaline
soil.
Chemosynthetic autotrophs:
Oxidize various inorganic substances like nitrates/nitrites, ammonia and use released energy for
their ATP production.
Heterotrophic bacteria:
Mostly decomposer
Helpful in making curd from milk
Produce antibiotics
Symbiotically associated with leguminous plant and fix nitrogen.
Some are pathogen causing diseases like cholera, typhoid, and tetanus.
Bacteria reproduce mainly by fission, also produce spore in unfavorable condition.
Reproduce sexually by transfer of DNA form one bacteria to other, the process called
conjugation.
Mycoplasma :
They are tiny organisms found in soil and sewage water. Some are parasites. They are the smallest
among living organisms (other than viruses) varying in size from 0-125 to 0-150mm in diameter.
They exhibit a great deal of variation in their shape and form.
They are unicellular, gram negative, non-motile prokaryotes which lack a distinct cell wall.
Some examples of mycoplasmas are pathogenic parasites in humans, animals and plants. The most
familiar example is Mycoplasma pneumoniae which causes the disease pneumonia.
Kingdom Protista:
All are unicellular and eukaryotic.
Mostly aquatic, can live in moist places.
Forms a link between plants, animals and fungi.
The cell contain nucleus and membrane bound organelles.
May be ‘a photosynthetic protistan’ to one biologist may be ‘a plant’ to another.
Being eukaryotes, the protistan cell body contains a well defined nucleus and other