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Biological Classification
Artificial Classification System:
This system was proposed by Linneaus. It was based on vegetative characters and
androecium structure.
Natural Classification System: This system was used by George Bentham and Joseph
Dalton Hooker. This system was based on natural affinities among organisms. In this
system, both external and internal features were considered.
Two Kingdom Classification; as proposed by Linneaus:
Animal Kingdom
Plant Kingdom
Five Kingdom Classification; as proposed by R.H. Whittaker (1969):
This is the most accepted system of classification. The five kingdoms are as follows:
Monera
Protista
Fungi
Plantae and
Animalia
The main criteria for classification used by Whittaker:
Cell Structure
Thallus Organization
Mode of Nutrition
Reproduction and
Phylogenetic Relationships
KINGDOM MONERA
The bacteria are kept under the Kingdom Monera. They are prokaryotic and possess
cell wall. The cell wall is composed of polysaccharides and amino acids. Bacteria can
be autotrophic and heterotrophic. The autotrophic bacteria can be chemosynthetic or
photosynthetic. The heterotrophic bacteria can be saprophytic or parasitic.
Based on their shape, bacteria are classified into four types:
a.
b.
c.
d.
Spherical bacteria are called Coccus (pl.: cocci),
Rod-shaped bacteria are called Bacillus (pl.: bacilli),
Comma-shaped bacteria are called Vibrium (pl.: vibrio) and
Spiral shaped bacteria are called Spirillum (pl.: spirilla)
Archaebacteria: The archaebacteria live in some of the harshest habitats. The different
structure of their cell wall helps them in surviving in extreme conditions. Based on their
habitats, the archaebacteria are classified as follows:
a. Halophiles: They live in extremely salty areas.
b. Thermoacidophiles: They live in hot spring.
c. Methanogens: They live in marshy areas. They also live in the guts of the
ruminant animals. They are responsible for the production of methane from the
dung of these animals.
Eubacteria: They are also called the ‘true bacteria’. They possess a rigid cell wall, and a
flagellum (in motile bacteria). The cyanobacteria are also called ‘blue-green algae’
because they contain chlorophyll. The cyanobacteria can be unicellular or filamentous.
They can live solitary or in colonies. The colony of cyanobacteria is usually surrounded
by a gelatinous sheath. Some of the cyanobacteria are capable of nitrogen-fixation, e.g.
Nostoc and Anabaena.
Heterotrophic: These are the most abundant in nature. Most of them have economic
significance for human beings. While many of them are beneficial for humans, many
others are quite harmful.
Reproduction in Bacteria:
Binary fission is the usual mode of reproduction in bacteria. Under unfavourable
conditions, they reproduce by spore formation. They also reproduce by adopting a
primitive type of DNA transfer from one bacterium to another. This is similar to sexual
reproduction.
KINGDOM PROTISTA
They are eukaryotes and unicellular. Some of them have flagella or cilia for locomotion.
Reproduction is asexual and sexual. Sexual reproduction is by a process of cell fusion
and zygote formation. Kingdom Protista is divided into following groups:
Chrysophytes:
The diatoms and golden algae (desmids) are included in this group. They are found in
freshwater and marine habitats. They are microscopic and float passively in water
currents and hence are considered as planktons. Organisms which cannot swim against
the current are called planktons. Most of the chrysophytes are photosynthetic.
The cell walls in diatoms form two thin overlapping shells; which fit together as the two
parts of a soapbox. The cell walls are embedded with silica and hence are
indestructible. Due to this, the diatoms leave behind a large amount of cell wall deposits
in their habitat. The cell wall accumulation of diatoms; over billions of years is called
‘diatomaceous earth’. This soil is gritty and hence is used in polishing, filtration of oils
and syrups. Diatoms are the main ‘producers’ in the oceans.
Dianoflagellates:
They are usually marine and photosynthetic. Depending on the main pigments in their
cells; they appear yellow, green, brown, blue or red. Stiff cellulose plates are present on
the outer surface of cell wall in dianoflagellates. Two flagella are present in most of
them. One flagellum lies longitudinally and another transversely in a furrow between the
wall plates.
Euglenoids:
Most of them live in freshwater habitat in stagnant water. Cell wall is absent in them and
instead there is a protein rich layer; called pellicle. The pellicle makes their body flexible.
Two flagella; one short and another long; are present. They are photosynthetic; but
behave as heterotrophs in the absence of sunlight. Example: Euglena.
Slime Moulds:
These are saprophytic. They body moves along decaying twigs and leaves and feeds
on organic material. Under favourable conditions, they form an aggregation called
plasmodium. The plasmodium may grow and spread over several feet. Under
unfavourable conditions, the plasmodium differentiates and forms fruiting bodies. The
fruiting bodies bear spores at their tips. True walls are present on the spores. The
spores are extremely resistant and survive for many years. The spores are dispersed by
air currents.
Protozoans:
They are heterotrophs and live as predators or parasites. The protozoans are classified
into following four major groups:
a. Amoeboid protozoans: The amoeboid protozoans live in freshwater, sea water or
in moist soil. They produce pseudopodia for locomotion and for capturing food.
The marine forms have silica shells on their surface. Some of them are parasites,
e.g.
Entamoeba
histolytica.
b. Flagellated protozoans: They are either free-living or parasitic. Flagella is present
for locomotion. Many of them are parasites, e.g. Trypanosoma.
c. Ciliated protozoans: They are aquatic. Cilia are present for locomotion. A cavity
(gullet) is present which opens to the outside of the cell surface. The coordinated
movement of cilia facilitates the entry of food-laden water into the gullet.
Example:
Paramoecium.
d. Sporozoans: The sporozoans have an infectious spore-like stage in their life
cycle. Example: Plasmodium.
KINGDOM FUNGI:
The fungi are filamentous; except yeast (unicellular). The body consists of long, slender
thread-like structures; called hyphae. The network of hyphae is called mycelium. Some
hyphae are continuous tubes which are filled with multinucleated cytoplasm. Such
hyphae are called coenocytic hyphae. The other type of hyphae has septae or crosswalls. The cell wall of fungi is composed of chitin and polysachharides.
Most of the fungi are heterotrophic and are saprophytes. Some are parasites. Some of
the fungi also live as symbionts. Some of the symbiont fungi live as lichens; in
association with algae. Some of the symbiont fungi live as mycorrhiza; in association
with roots of higher plants.
Reproduction in Fungi:
Reproduction by vegetative means takes place by fragmentation, fission and budding.
Some fungi reproduce asexually by forming spores which are called conidia or
sporangiospores or zoospores. Sexual reproduction is by oospores, ascospores and
basidiospores. The spores are produced in distinct structures called fruiting bodies.
The sexual cycle involves three steps which are as follows:
a. Plasmogamy: This involves fusion of protoplasm between two motile or nonmotile gametes.
b. Karyogamy: This involves fusion of two nuclei.
c. Formation of haploid spores: This happens because of meiosis in zygote.
During sexual reproduction, two haploid hyphae of compatible mating types come
together and fuse. In some fungi, the fusion of two haploid cells immediately results in a
diploid cell (2n). In other fungi (ascomycetes and basidiomycetes), an intervening
dikaryotic stage occurs. In this stage, two nuclei are present in each cell. This condition
is called dikayron. The parental nuclei fuse at a later stage and the cells become diploid.
Reduction division in the fruiting bodies leads to the formation of haploid spores.
On the basis of morphology of mycelium, mode of spore formation and fruiting bodies;
Kingdom Fungi is divided into following classes:
Phycomycetes:
The members of phycomycetes are found in aquatic habitats and on decaying wood in
moist and damp places. They can also be found as obligate parasites on plants.
Mycelium: Aseptate and coenocytic.
Reproduction:
Asexual reproduction is by zoospores (motile) or by aplanospores (non-motile).
These spores are endogeneously produced in sporangium.
Zygospores are formed by fusion of two gametes. The gametes can be similar
(isogamous) or dissimilar (anisogamous).
Examples: Mucor, Rhizopus and Albugo (the parasitic fungi on mustard).
Ascomycetes:
They are commonly known as sac-fungi. They are unicellular or multicellular. They are
saprophytic, decomposers, parasitic or coprophilous. Those growing on dung are called
coprophilous.
Mycelium: Branched and septate.
Reproduction:
Asexual spores are exogenously produced on the special mycelium called
conidiophores. Sexual spores are called ascospores. They are produced endogenously
in sac like asci. These asci are arranged in various kinds of fruiting bodies called
ascocarps.
Examples: Aspergillus, Claviceps, Neurospora, yeast, penicillium, morels, baffles, etc.
Basidiomycetes:
They grow in soil, on logs and tree stumps. Some of them also grow in living plant
bodies as parasites. Mushrooms, bracket fungi or puffballs are the commonly known
forms.
Mycelium: Branched and septate.
Reproduction:
Asexual spores are usually absent. Vegetative reproduction by fragmentation is
common. Sex organs are absent. But plasmogamy is brought about by fusion of two
vegetative cells of different strains. The resultant structure is dikaryotic which finally
forms the basidium. Karyogamy and meiosis are responsible for formation of four
basidiospores in a basidium. The basiodiospores are exogenously produced. Basidia
are arranged in fruiting bodies called basidiocarps.
Examples: Agaricus (mushroom), Ustilago (smut) and Puccinia (rust).
Deuteromycetes:
They are usually known as imperfect fungi because only the asexual or vegetative
phase of them is known. A large number of deuteromycetes are decomposers, while
some members are parasites.
Mycelium: Branched and septate.
Examples: Alternaria, Colletotrichum and Trichoderma.
KINGDOM PLANTAE
All eukaryotes which have chloroplast are kept under this kingdom. Most of them are
autotrophic; but some are heterotrophic. Cell wall is mainly composed of cellulose.
The life cycle of plants has two distinct phases; the diploid saprophytic and the haploid
gametophytic phase. These phases alternate with each other. The lengths of the
haploid and diploid phases vary among different groups of plants. This phenomenon is
called Alternation of Generation.
KINGDOM ANIMALIA:
All multicellular eukaryotes which lack cell wall and are heterotrophs are kept under this
kingdom. The animals indirectly or directly depend for food on plants. They follow the
holozoic mode of nutrition. Holozoic nutrition involves ingestion of food and use of
internal cavity for digestion of food. Most of the animals are capable of locomotion.
Sexual reproduction is by copulation of male and female which is followed by
embryological development.
VIRUSES, VIROIDS AND LICHENS:
Virus is non-cellular organism. It is characterized by an inert crystalline structure outside
the living cell. Once a virus infects a living cell, it begins to replicate and kills the host in
the process. Virus is considered as a borderline case between living and non-living.
The name virus was given by Pasteur D. J. Ivanowsky (1892). M. W. Beijerinek (1898)
demonstrated that the extract of infected tobacco plants could cause infection in healthy
plants. He called the fluid from the infected plant as Contagium vivum fluidum
(infectious living fluid). W. M. Stanley (1935) demonstrated that viruses could be
crystallized and the crystals mainly consist of proteins.
Viruses also contain genetic material; which can be either RNA or DNA. Both RNA and
DNA cannot be present in the same virus. Plant infecting viruses usually have singlestranded RNA and animal infecting viruses usually have double-stranded RNA or
double-stranded DNA. Bacteriophages (bacterial viruses) usually have double-stranded
DNA.
The protein coat on the virus is called capsid. It is made up of small subunits called
capsomeres. The capsid protects the nucleic acid. The capsomeres are arranged in
helical or polyhedral geometric forms.
Mumps, small pox, herpes, influenza and AIDS are examples of viral diseases in
humans. In plants; viral infection leads to the symptoms; like mosaic formation, leaf
rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.
Question – 1- Discuss how classification systems have undergone several changes
over a period of time?
Answer: Scientific classification of living beings was first done by Aristotle. He used
morphological characters as the basis of classification. He classified the living beings
into plants and animals. He further classified the plant into trees, shrubs and herbs. He
further classified the animals on the basis of presence of absence of red blood.
After that, Linnaeus proposed two kingdoms, i.e. Plant Kingdom and Animal Kingdom.
But there were certain organisms which could be kept in both of the kingdoms or could
not be kept in either of the kingdoms. Hence, a need was felt for a better system of
classification. At present, the Five Kingdom Classification is the most accepted one.
This was proposed by Robert Whittaker in 1969. Whittaker used phylogenetic
relationship to classify the living beings.
Question – 2 - State two economically important uses of:
(a) Heterotrophic bacteria
Answer: Curd and antibiotic are made by using heterotrophic bacteria.
(b) Archaebacteria
Answer: Methanogens are responsible for production of biogas which can be used as
fuel. The archaebacteria which live in extreme conditions give us a clue about the
beginning of life on earth.
Question – 3 - What is the nature of cell-walls in diatoms?
Answer: The cell walls in diatoms form two thin overlapping shells; which fit together as
the two parts of a soapbox. The cell walls are embedded with silica and hence are
indestructible.
Question – 4 - Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.
Answer: A rapid increase in the population of microscopic algae in an aquatic habitat is
called algal bloom. The algal bloom involving the dianoflagellates is called the ‘red tide’
because of its red hue. Red tide can be harmful for other aquatic life forms.
Question – 5 - How are viroids different from viruses?
Answer: The free RNAs without the protein coat are called viroids, while virus have a
protein coat to protect the genetic material.
Question – 6 - Describe briefly the four major groups of Protozoa.
Answer: Four major groups of Protozoa is as follows:
a. Amoeboid protozoans: The amoeboid protozoans live in freshwater, sea water or
in moist soil. They produce pseudopodia for locomotion and for capturing food.
The marine forms have silica shells on their surface. Some of them are parasites,
e.g. Entamoeba histolytica.
b. Flagellated protozoans: They are either free-living or parasitic. Flagella is present
for locomotion. Many of them are parasites, e.g. Trypanosoma.
c. Ciliated protozoans: They are aquatic. Cilia are present for locomotion. A cavity
(gullet) is present which opens to the outside of the cell surface. The coordinated
movement of cilia facilitates the entry of food-laden water into the gullet.
Example: Paramoecium.
d. Sporozoans: The sporozoans have an infectious spore-like stage in their life
cycle. Example: Plasmodium.
Question – 7 - Plants are autotrophic. Can you think of some plants that are partially
heterotrophic?
Answer: Pitcher plant, Venus fly trap and bladderwort are examples of partially
heterotrophic plants.
Question – 8 - What do the terms phycobiont and mycobiont signify?
Answer: Lichens are the symbiotic association of fungi and algae. The algal part of
lichen is called phycobiont and the fungal part is called mycobiont. The mycobiont part
provides minerals and support, while the phycobiont part provides nutrition.
Question – 9 - Give a comparative account of the classes of Kingdom Fungi under the
following:
(a) Mode of nutrition
Answer: Phycomycetes are obligate parasites or saprophytes.
Ascomycetes are saprophytes or parasites or coprophilous.
Basidiomycetes are saprophytes or parasites.
Deuteromycetes are manly saprophytes, some are parasites.
(b) Mode of reproduction
Answer: In phycomycetes, asexual reproduction is by zoospores (motile) or by
aplanospores.
In ascomycetes, asexual spores (conidia) and sexual spores (ascospores) are
produced.
In basidiomycetes, vegetative reproduction takes place by fragmentation.
Plsamogamy is also seen.
In deuteromycetes, only vegetative reproduction is seen.
Question – 10 - What are the characteristic features of Euglenoids?
Answer: Most of them live in freshwater habitat in stagnant water. Cell wall is absent in
them and instead there is a protein rich layer; called pellicle. The pellicle makes their
body flexible. Two flagella; one short and another long; are present. They are
photosynthetic; but behave as heterotrophs in the absence of sunlight.
Question – 11 - Give a brief account of viruses with respect to their structure and nature
of genetic material. Also name four common viral diseases.
Answer: Virus contains genetic material surrounded by a protein capsule. The protein
coat on the virus is called capsid. It is made up of small subunits called capsomeres.
The capsid protects the nucleic acid. The capsomeres are arranged in helical or
polyhedral geometric forms.
The genetic material can be either RNA or DNA. Both RNA and DNA cannot be present
in the same virus. Plant infecting viruses usually have single-stranded RNA and animal
infecting viruses usually have double-stranded RNA or double-stranded DNA.
Bacteriophages (bacterial viruses) usually have double-stranded DNA.
Four common viral diseases are: Common cold, mumps, jaundice and influenza.