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Biological classification



Since the dawn of civilisation, 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. He also divided animals
into two groups, those which had red blood and those that did not.
In Linnaeus’ time a Two Kingdom system of classification with
Plantae and Animalia kingdoms was developed that included all plants
and animals respectively. This system was used till very recently. 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,
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. A need was also felt for including, besides gross morphology,
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 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 time.


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, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships.  gives a
comparative account of different characteristics of the five kingdoms.
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 characterstics. It brought together the prokaryotic bacteria and
the blue green algae 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 in their walls while the green plants had a cellulosic cell wall. When such characterstics were considered, the fungi were placed in a separate kingdom – Kingdom Fungi. All prokaryotic organisms were
grouped together under Kingdom Monera and the unicellular eukaryotic
organisms were placed in Kingdom Protista. Kingdom Protista has brought together Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both having cell walls) with Paramoecium and Amoeba
(which were earlier placed in the animal kingdom) which lack it. It has
put together organisms which, in earlier classifications, were placed in
different kingdoms. This happened because the criteria for classification
changed. This kind of changes will take place in future too depending on
the improvement in our understanding of characteristics and evolutionary
relationships. Over time, an attempt has been made to evolve a classification system which reflects not only the morphological, physiological and reproductive similarities, but is also phylogenetic, i.e., is based on evolutionary relationships.


Bacteria are the sole members of the Kingdom Monera. They are the most
abundant micro-organisms. Bacteria occur almost everywhere. Hundreds
of bacteria are present in a handful of soil. They also live in extreme habitats
such as hot springs, deserts, snow and deep oceans where very few other
life forms can survive. Many of them live in or on other organisms as
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.


These bacteria are special since they live in some of the most harsh habitats
such as extreme salty areas (halophiles), hot springs (thermoacidophiles)
and marshy areas (methanogens). Archaebacteria differ from other bacteria
in having a different cell wall structure and this feature is responsible for
their survival in extreme conditions. Methanogens are present in the guts
of several ruminant animals such as cows and buffaloes and they are
responsible for the production of methane (biogas) from the dung of these


There are thousands of different eubacteria or ‘true bacteria’. They 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. The
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. Some of
these organisms can fix atmospheric nitrogen in
specialised cells called heterocysts, e.g., Nostoc and
Anabaena. Chemosynthetic autotrophic bacteria
oxidise various inorganic substances such as
nitrates, nitrites and ammonia and use the released
energy for their ATP production. They play a great
role in recycling nutrients like nitrogen,
phosphorous, iron and sulphur.
Heterotrophic bacteria are the most abundant
in nature. The majority are important decomposers.
Many of them have a significant impact on human affairs. They are helpful in making curd from milk, production of antibiotics, fixing nitrogen in legume roots, etc. Some are pathogens causing damageto human beings, crops, farm animals and pets.Cholera, typhoid, tetanus, citrus canker are wellknown diseases caused by different bacteria.
Bacteria reproduce mainly by fission Sometimes, under unfavourable conditions,they produce spores. They also reproduce by a
sort of sexual reproduction by adopting a
primitive type of DNA transfer from one bacteriumto the other.
The Mycoplasmas are organisms that completely lack a cell wall. They are the smallestliving cells known and can survive without oxygen. Many mycoplasma
are pathogenic in animals and plants.


All single-celled eukaryotes are placed under Protista, but the boundaries
of this kingdom are not well defined. What may be ‘a photosynthetic
protistan’ to one biologist may be ‘a plant’ to another. In this book we
include Chrysophytes, Dianoflagellates, Euglenoids, Slime moulds and
Protozoans under Protista. Members of Protista are primarily aquatic.
This kingdom forms a link with the others dealing with plants, animals
and fungi. Being eukaryotes, the protistan cell body contains a well defined
nucleus and other membrane-bound organelles. Some have flagella or
cilia. Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.


This group includes diatoms and golden algae (desmids). They are found
in fresh water as well as in marine environments. They are microscopic
and float passively in water currents (plankton). Most of them are photosynthetic. In diatoms the cell walls form two thin overlapping shells,
which fit together as in a soap box. The walls are embedded with silica
and thus the walls are indestructible. Thus, diatoms have left behind
large amount of cell wall deposits in their habitat; this accumulation over
billions of years is referred to as ‘diatomaceous earth’. Being gritty this soil is used in polishing, filtration of oils and syrups. Diatoms are the chief ‘producers’ in the oceans.


These organisms are mostly marine and photosynthetic.
They appear yellow, green, brown, blue or red depending on the main pigments present in their cells. The cell wall has stiff cellulose plates on the outer surface. Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates. Very often, red dianoflagellates (Example: Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides). Toxins released by such large numbers may even kill other marine animals such as fishes.


Majority of them are fresh water organisms found in stagnant water. Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible.
They have two flagella, a short and a long one. Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by
predating on other smaller organisms. Interestingly, the pigments of euglenoids are identical to those present in
higher plants. Example: Euglena


Slime moulds are saprophytic protists. The body moves along decaying twigs and leaves engulfing organic material. Under suitable conditions, they form an
aggregation called plasmodium which may grow and spread over several feet. During unfavourable conditions,
the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls.
They are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed
by air currents.


  • All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major groups of protozoans.
  • Amoeboid protozoans: These organisms live in fresh water, sea water or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba
    are parasites.
  • Flagellated protozoans: The members of this group are either free-living
    or parasitic. They have flagella. The parasitic forms cause diaseases such
    as sleeping sickness. Example: Trypanosoma.
  • Ciliated protozoans: These are aquatic, actively moving organisms because
    of the presence of thousands of cilia. They have a cavity (gullet) that opens
    to the outside of the cell surface. The coordinated movement of rows of
    cilia causes the water laden with food to be steered into the gullet. Example:
  • Sporozoans: This includes diverse organisms that have an infectious
    spore-like stage in their life cycle. The most notorious is Plasmodium
    (malarial parasite) which causes malaria which has a staggering effect on
    human population.


The fungi constitute a unique kingdom of heterotrophic organisms. They
show a great diversity in morphology and habitat. When your bread
develops a mould or your orange rots it is because of fungi. The common
mushroom you eat and toadstools are also fungi. White spots seen on
mustard leaves are due to a parasitic fungus. Some unicellular fungi,
e.g., yeast are used to make bread and beer. Other fungi cause diseases
in plants and animals; wheat rust-causing Puccinia is an important
example. Some are the source of antibiotics, e.g., Penicillium. Fungi are
cosmopolitan and occur in air, water, soil and on animals and plants.
They prefer to grow in warm and humid places. Have you ever wondered
why we keep food in the refrigerator ? Yes, it is to prevent food from going
bad due to bacterial or fungal infections.
With the exception of yeasts which are unicellular, fungi are
filamentous. Their bodies consist of long, slender thread-like structures
called hyphae. The network of hyphae is known as mycelium. Some hyphae
are continuous tubes filled with multinucleated cytoplasm – these are
called coenocytic hyphae. Others have septae or cross walls in their
hyphae. The cell walls of fungi are composed of chitin and polysaccharides.
Most fungi are heterotrophic and absorb soluble organic matter from
dead substrates and hence are called saprophytes. Those that depend
on living plants and animals are called parasites. They can also live as
symbionts – in association with algae as lichens and with roots of higher
plants as mycorrhiza.
Reproduction in fungi can take place by vegetative means –
fragmentation, fission and budding. Asexual reproduction is by spores.called conidia or sporangiospores or zoospores, and sexual reproduction
is by oospores, ascospores and basidiospores. The various spores are
produced in distinct structures called fruiting bodies. The sexual cycle
involves the following three steps:
(i) Fusion of protoplasms between two motile or non-motile gametes
called plasmogamy.
(ii) Fusion of two nuclei called karyogamy.
(iii) Meiosis in zygote resulting in haploid spores.
When a fungus reproduces sexually, two haploid hyphae of compatible mating types come together and fuse. In some fungi the fusion of two haploid cells immediately results in diploid cells (2n). However, in other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage (n + n i.e. two nuclei per cell) occurs; such a condition is called a dikaryon and the phase is called dikaryophase of fungus. Later, the parental nuclei fuse and the cells become diploid. The fungi form fruiting bodies in which reduction division occurs, leading to formation of haploid spores.
(The morphology of the mycelium, mode of spore formation and fruiting bodies form the basis for the division of the kingdom into various classes.)


Members of phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants. The mycelium is aseptate and coenocytic. Asexual reproduction takes place by zoospores (motile) or by aplanospores (non-motile). These spores are endogeneously produced in sporangium.
Zygospores are formed by fusion of two gametes. These gametes are similar in morphology (isogamous) or dissimilar (anisogamous or oogamous). Some common examples are Mucor , Rhizopus (the bread mould mentioned earlier) and Albugo (the parasitic fungi on mustard).


Commonly known as sac-fungi, the ascomycetes are unicellular, e.g., yeast (Sacharomyces) or multicellular,
e.g., Penicillium. They are saprophytic, decomposers, parasitic or coprophilous (growing on dung). Mycelium is branched and septate. The asexual spores are conidia produced
exogenously on the special mycelium called conidiophores. Conidia on
germination produce mycelium. Sexual spores are called ascospores
which are produced endogenously in sac like asci (singular ascus). These
asci are arranged in different types of fruiting bodies called ascocarps.
Some examples are Aspergillus Claviceps and Neurospora.
Neurospora is used extensively in biochemical and genetic work. Many
members like morels and buffles are edible and are considered delicacies.


Commonly known forms of basidiomycetes are mushrooms, bracket fungi
or puffballs. They grow in soil, on logs and tree stumps and in living
plant bodies as parasites, e.g., rusts and smuts. The mycelium is branched
and septate. The asexual spores are generally not found, but vegetative
reproduction by fragmentation is common. The sex organs are absent,
but plasmogamy is brought about by fusion of two vegetative or somatic
cells of different strains or genotypes. The resultant structure is dikaryotic
which ultimately gives rise to basidium. Karyogamy and meiosis take
place in the basidium producing four basidiospores. The basidiospores
are exogenously produced on the basidium (pl.: basidia). The basidia are arranged in fruiting bodies called basidiocarps. Some common members are Agaricus (mushroom) Ustilago (smut) and Puccinia (rust fungus).


Commonly known as imperfect fungi because only the asexual or
vegetative phases of these fungi are known. When the sexual forms of
these fungi were discovered they were moved into classes they rightly
belong to. It is also possible that the asexual and vegetative stage have
been given one name (and placed under deuteromycetes) and the sexual
stage another (and placed under another class). Later when the linkages
were established, the fungi were correctly identified and moved out of
deuteromycetes. Once perfect (sexual) stages of members of dueteromycetes were discovered they were often moved to ascomycetes and basidiomycetes. The deuteromycetes reproduce only by asexual spores known as conidia. The mycelium is septate and branched. Some members are saprophytes or parasites while a large number of them are decomposers of litter and help in mineral cycling. Some examples are Alternaria, Colletotrichum and Trichoderma.


Kingdom Plantae includes all eukaryotic chlorophyll-containing
organisms commonly called plants. A few members are partially heterotrophic such as the insectivorous plants or parasites. Bladderwort and Venus fly trap are examples of insectivorous plants and Cuscuta is a parasite. The plant cells have an eukaryotic structure with prominent chloroplasts and cell wall mainly made of cellulose. Life cycle of plants has two distinct phases – the diploid sporophytic and the haploid gametophytic – that alternate with each other. The lengths
of the haploid and diploid phases, and whether these phases are free– living or dependent on others, vary among different groups in plants.
This phenomenon is called alternation of generation.


This kingdom is characterised by heterotrophic eukaryotic organisms
that are multicellular and their cells lack cell walls. They directly or
indirectly depend on plants for food. They digest their food in an internal
cavity and store food reserves as glycogen or fat. Their mode of nutrition
is holozoic – by ingestion of food. They follow a definite growth pattern
and grow into adults that have a definite shape and size. Higher forms
show elaborate sensory and neuromotor mechanism. Most of them are
capable of locomotion.
(The sexual reproduction is by copulation of male and female followed
by embryological development.)


In the five kingdom classification of Whittaker there is no mention of some
acellular organisms like viruses and viroids, and lichens. These are briefly
introduced here.
All of us who have suffered the illeffects of common cold or ‘flu’ know
what effects viruses can have on us, even if we do not associate it with our
condition. Viruses did not find a place in classification since they are not
truly ‘living’, if we understand living as those organisms that have a cell
structure. The viruses are non-cellular organisms that are characterised
by having an inert crystalline structure outside the living cell. Once they infect a cell they take over the machinery of the host cell to replicate
themselves, killing the host.

  • Would you call viruses living or non-living?
    The name virus that means venom or poisonous fluid was given by
    Pasteur. D.J. Ivanowsky (1892) recognised certain microbes as causal
    organism of the mosaic disease of tobacco. These were found to be smaller
    than bacteria because they passed through bacteria-proof filters. M.W.
    Beijerinek (1898) demonstrated that the extract of the infected plants of
    tobacco could cause infection in healthy plants and called the fluid as
    Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935)
    showed that viruses could be crystallised and crystals consist largely of
    proteins. They are inert outside their specific host cell. Viruses are obligate
  • In addition to proteins viruses also contain genetic material, that could
    be either RNA or DNA. No virus contains both RNA and DNA. A virus is
    a nucleoprotein and the genetic material is infectious. In general, viruses
    that infect plants have single stranded RNA and viruses that infect animals
    have either single or double stranded RNA or double stranded DNA.
    Bacterial viruses or bacteriophages (viruses that infect the bacteria) are
    usually double stranded DNA viruses. The protein coat called capsid made
    of small subunits called capsomeres, protects the nucleic acid. These
    capsomeres are arranged in helical or polyhedral geometric forms. Viruses
    cause diseases like mumps, small pox, herpes and influenza. AIDS in
    humans is also caused by a virus. In plants, the symptoms can be mosaic
    formation, leaf rolling and curling, yellowing and vein clearing, dwarfing
    and stunted growth.
  • Viroids : In 1971 T.O. Diener discovered a new infectious agent that wassmaller 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.
  • 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.)
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