NCERT Textbook: Biological Classification | Biology Class 11 - NEET PDF Download

 Page 1


10 BIOLOGY
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 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.
BIOLOGICAL CLASSIFICATION
CHAPTER  2
2.1 Kingdom Monera
2.2 Kingdom Protista
2.3 Kingdom Fungi
2.4 Kingdom Plantae
2.5 Kingdom
Animalia
2.6 Viruses, Viroids
and Lichens
2024-25
Page 2


10 BIOLOGY
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 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.
BIOLOGICAL CLASSIFICATION
CHAPTER  2
2.1 Kingdom Monera
2.2 Kingdom Protista
2.3 Kingdom Fungi
2.4 Kingdom Plantae
2.5 Kingdom
Animalia
2.6 Viruses, Viroids
and Lichens
2024-25
BIOLOGICAL CLASSIFICATION 11
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
Five Kingdoms
Characters
Cell type
Cell wall
Nuclear
membrane
Body
organisation
Mode of
nutrition
Monera
Prokaryotic
Noncellulosic
(Polysaccharide
+ amino acid)
Absent
Cellular
Autotrophic
(chemosyn-
thetic and
photosynthetic)
and Hetero-
trophic (sapro-
phytic/para-
sitic)
Protista
Eukaryotic
Present in
some
Present
Cellular
Autotrophic
(Photosyn-
thetic) and
Hetero-
trophic
Fungi
Eukaryotic
Present
with chitin
Present
Multiceullar/
loose tissue
Heterotrophic
(Saprophytic/
Parasitic)
Plantae
Eukaryotic
Present
(cellulose)
Present
Tissue/
organ
Autotrophic
(Photosyn-
thetic)
Animalia
Eukaryotic
Absent
Present
Tissue/organ/
organ system
Heterotrophic
(Holoz o ic/
Saprophytic
etc.)
TABLE 2.1  Characteristics of the Five Kingdoms
2024-25
Page 3


10 BIOLOGY
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 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.
BIOLOGICAL CLASSIFICATION
CHAPTER  2
2.1 Kingdom Monera
2.2 Kingdom Protista
2.3 Kingdom Fungi
2.4 Kingdom Plantae
2.5 Kingdom
Animalia
2.6 Viruses, Viroids
and Lichens
2024-25
BIOLOGICAL CLASSIFICATION 11
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
Five Kingdoms
Characters
Cell type
Cell wall
Nuclear
membrane
Body
organisation
Mode of
nutrition
Monera
Prokaryotic
Noncellulosic
(Polysaccharide
+ amino acid)
Absent
Cellular
Autotrophic
(chemosyn-
thetic and
photosynthetic)
and Hetero-
trophic (sapro-
phytic/para-
sitic)
Protista
Eukaryotic
Present in
some
Present
Cellular
Autotrophic
(Photosyn-
thetic) and
Hetero-
trophic
Fungi
Eukaryotic
Present
with chitin
Present
Multiceullar/
loose tissue
Heterotrophic
(Saprophytic/
Parasitic)
Plantae
Eukaryotic
Present
(cellulose)
Present
Tissue/
organ
Autotrophic
(Photosyn-
thetic)
Animalia
Eukaryotic
Absent
Present
Tissue/organ/
organ system
Heterotrophic
(Holoz o ic/
Saprophytic
etc.)
TABLE 2.1  Characteristics of the Five Kingdoms
2024-25
12 BIOLOGY
in their walls while the green plants had a cellulosic cell wall.  When such
characteristics 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 cell wall). 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.
In this chapter we will study characteristics of Kingdoms Monera,
Protista and Fungi of the Whittaker system of classification.  The Kingdoms
Plantae and Animalia, commonly referred to as plant and animal
kingdoms, respectively, will be dealt separately in chapters 3 and 4.
Spore
Flagellum
Cocci
Bacilli
Spirilla
Vibrio
Figure 2.1 Bacteria of different shapes
2.1 KINGDOM MONERA
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
parasites.
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)
(Figure 2.1).
2024-25
Page 4


10 BIOLOGY
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 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.
BIOLOGICAL CLASSIFICATION
CHAPTER  2
2.1 Kingdom Monera
2.2 Kingdom Protista
2.3 Kingdom Fungi
2.4 Kingdom Plantae
2.5 Kingdom
Animalia
2.6 Viruses, Viroids
and Lichens
2024-25
BIOLOGICAL CLASSIFICATION 11
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
Five Kingdoms
Characters
Cell type
Cell wall
Nuclear
membrane
Body
organisation
Mode of
nutrition
Monera
Prokaryotic
Noncellulosic
(Polysaccharide
+ amino acid)
Absent
Cellular
Autotrophic
(chemosyn-
thetic and
photosynthetic)
and Hetero-
trophic (sapro-
phytic/para-
sitic)
Protista
Eukaryotic
Present in
some
Present
Cellular
Autotrophic
(Photosyn-
thetic) and
Hetero-
trophic
Fungi
Eukaryotic
Present
with chitin
Present
Multiceullar/
loose tissue
Heterotrophic
(Saprophytic/
Parasitic)
Plantae
Eukaryotic
Present
(cellulose)
Present
Tissue/
organ
Autotrophic
(Photosyn-
thetic)
Animalia
Eukaryotic
Absent
Present
Tissue/organ/
organ system
Heterotrophic
(Holoz o ic/
Saprophytic
etc.)
TABLE 2.1  Characteristics of the Five Kingdoms
2024-25
12 BIOLOGY
in their walls while the green plants had a cellulosic cell wall.  When such
characteristics 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 cell wall). 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.
In this chapter we will study characteristics of Kingdoms Monera,
Protista and Fungi of the Whittaker system of classification.  The Kingdoms
Plantae and Animalia, commonly referred to as plant and animal
kingdoms, respectively, will be dealt separately in chapters 3 and 4.
Spore
Flagellum
Cocci
Bacilli
Spirilla
Vibrio
Figure 2.1 Bacteria of different shapes
2.1 KINGDOM MONERA
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
parasites.
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)
(Figure 2.1).
2024-25
BIOLOGICAL CLASSIFICATION 13
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 depend on other
organisms or on dead organic matter for food.
2.1.1 Archaebacteria
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 gut
of several ruminant animals such as cows and buffaloes and they are
responsible for the production of methane (biogas) from the dung of these
animals.
Figure 2.2 A filamentous blue-green
algae – Nostoc
2.1.2 Eubacteria
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 (Figure 2.2). The
cyanobacteria are unicellular, colonial or filamentous,
freshwater/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 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
2024-25
Page 5


10 BIOLOGY
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 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.
BIOLOGICAL CLASSIFICATION
CHAPTER  2
2.1 Kingdom Monera
2.2 Kingdom Protista
2.3 Kingdom Fungi
2.4 Kingdom Plantae
2.5 Kingdom
Animalia
2.6 Viruses, Viroids
and Lichens
2024-25
BIOLOGICAL CLASSIFICATION 11
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
Five Kingdoms
Characters
Cell type
Cell wall
Nuclear
membrane
Body
organisation
Mode of
nutrition
Monera
Prokaryotic
Noncellulosic
(Polysaccharide
+ amino acid)
Absent
Cellular
Autotrophic
(chemosyn-
thetic and
photosynthetic)
and Hetero-
trophic (sapro-
phytic/para-
sitic)
Protista
Eukaryotic
Present in
some
Present
Cellular
Autotrophic
(Photosyn-
thetic) and
Hetero-
trophic
Fungi
Eukaryotic
Present
with chitin
Present
Multiceullar/
loose tissue
Heterotrophic
(Saprophytic/
Parasitic)
Plantae
Eukaryotic
Present
(cellulose)
Present
Tissue/
organ
Autotrophic
(Photosyn-
thetic)
Animalia
Eukaryotic
Absent
Present
Tissue/organ/
organ system
Heterotrophic
(Holoz o ic/
Saprophytic
etc.)
TABLE 2.1  Characteristics of the Five Kingdoms
2024-25
12 BIOLOGY
in their walls while the green plants had a cellulosic cell wall.  When such
characteristics 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 cell wall). 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.
In this chapter we will study characteristics of Kingdoms Monera,
Protista and Fungi of the Whittaker system of classification.  The Kingdoms
Plantae and Animalia, commonly referred to as plant and animal
kingdoms, respectively, will be dealt separately in chapters 3 and 4.
Spore
Flagellum
Cocci
Bacilli
Spirilla
Vibrio
Figure 2.1 Bacteria of different shapes
2.1 KINGDOM MONERA
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
parasites.
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)
(Figure 2.1).
2024-25
BIOLOGICAL CLASSIFICATION 13
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 depend on other
organisms or on dead organic matter for food.
2.1.1 Archaebacteria
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 gut
of several ruminant animals such as cows and buffaloes and they are
responsible for the production of methane (biogas) from the dung of these
animals.
Figure 2.2 A filamentous blue-green
algae – Nostoc
2.1.2 Eubacteria
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 (Figure 2.2). The
cyanobacteria are unicellular, colonial or filamentous,
freshwater/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 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
2024-25
14 BIOLOGY
roots, etc. Some are pathogens causing damage
to human beings, crops, farm animals and pets.
Cholera, typhoid, tetanus, citrus canker are well
known diseases  caused by different bacteria.
Bacteria reproduce mainly by fission (Figure
2.3). 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 bacterium
to the other.
The Mycoplasma are organisms that
completely lack a cell wall. They are the smallest
living cells known and can survive without oxygen. Many mycoplasma
are pathogenic in animals and plants.
2.2 KINGDOM PROTISTA
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, Dinoflagellates, 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.
2.2.1 Chrysophytes
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.
Figure 2.3 A dividing bacterium
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