Detailed NCERT Notes: Biological Classification Notes | Study Biology Class 11 - NEET

NEET: Detailed NCERT Notes: Biological Classification Notes | Study Biology Class 11 - NEET

The document Detailed NCERT Notes: Biological Classification Notes | Study Biology Class 11 - NEET is a part of the NEET Course Biology Class 11.
All you need of NEET at this link: NEET

Biological Classification

Biological classification is the scientific procedure of arranging organisms in a hierarchical series of groups and sub-groups on the basis of their similarities and dissimilarities.

Need of Classification

  • There have been many attempts to classify living organisms since ancient times. Aristotle was the earliest to attempt a scientific basis of classification. 
  • He used simple morphological characters to classify plants as trees, shrubs and herbs. A need for proper system of classification was always felt.

Living organisms need to be classified because of the following reasons

  • The study of one or two organisms is not sufficient to know the essential features of the group.
  • All kinds of organisms do not occur in one locality.
  • Classification helps in knowing the relationship among-est different groups of organisms.
  • It helps in knowing the evolutionary relationship between organisms.

Five Kingdom Classification System
This classification was proposed by RH Whittaker, in 1969. Before 1969, the classification systems for the living organisms have undergone several changes overtime.
He created fungi, as separate kingdom.
The main criteria for classification used by Whittaker

  1. Cell structure
  2. Modes of nutrition
  3. Thallus organisation
  4. Reproduction
  5. Phylogenetic relationships.

Merits of six Kingdom Classification System are

  1. Euglena and other transition types which had been included both amongst plants and animals are given proper place under kingdom—Protista.
  2. Fungi have their own biochemical, physiological and structural organisation. They have never been related to plants. In this system of classification fungi are separately placed.
  3. A separate kingdom of prokaryotes include Monera has been created. Monerans differ from all other organisms in their cellular, reproductive and physiological organisations.
  4. The five kingdom classification system is based on cellular organisation, the mode of nutrition and complexity of structure. These were the basic factors used in earliest two kingdom system of classification.
  5. The system shows the gradual evolution of early organisms into plants and animals.
  6. The plant and animal kingdoms are more homogenous than, they were in the two kingdom system of classification.

Demerits of Five Kingdom Classification System are

  1. Animal protozoans have been included in kingdom—Protista, which also includes unicellular plants. They show different modes of nutrition.
  2. Yeasts are though, unicellular eukaryotes, do not belong to kingdom—Protista.
  3. Chlorella and Chlamydomonas, though unicellular included under the kingdom-Plantae. They should be kept in Protista.
  4. Euglena like organisms and slime moulds with flexible life style may need the creation of an intermediate kingdom of Protista.
  5. Viruses and viroids are not kept in proper place in this system.

Monerans and Protists
As we have already read in the previous about the topic most widely accepted five kingdom classification given by Whittaker in general. Now, we will study in details about the monerans and protists before the other three kingdoms. This is because, monerans are thought to have given rise to the protistans from which the remaining three has been evolved along the separate lines.

Kingdom Monera
The kingdom-Monera includes all prokaryotes such as bacteria, mycoplasma, Actinomycetes and cyanobacteria (blue-green algae).

The characteristic features of kingdom-Monera are given below

  • They are simplest or most primitive, unicellular prokaryotes.
  • The cell wall contains peptidoglycan or murein (no cellulose) and the membrane bound cell organelles are not present.
  • They have various types of nutrition like saprophytic, parasitic, chemoautotrophic, photoautotrophic and symbiotic.
  • DNA is naked. It lies inside the cytoplasm in coiled form. This is called nucleoid.
  • The flagella, if present are single-stranded instead of being 11 stranded as in eukaryotes. These contain a protein called flagellin.
  • Reproduction is by asexual method. Gametes are not present.
  • Mitotic spindle is absent.
  • Some of the monerans have the ability to fix-nitrogen into useful nitrates.


The term Bacteria was proposed by Ehrenberg in 1829. They have widespread distribution be it air, water or soil. They can survive in extreme range of temperatures like up to 78°C and -190°C.

Important characteristics of bacteria are

  • Bacteria are found in all kinds of habitats.
  • They are prokaryotic microorganisms.
  • They are unicellular.
  • Cell wall contains peptidoglycan.
  • An organised nucleus in absent.
  • Extrachromosomal self replicating DNA segments called plasmids occur in most of the bacteria.
  • Mitochondria, plastids, Golgi apparatus,endoplasmic reticulum and other membrane covered cell organelles are absent.


  • The size of bacterial cell ranges from 1-10 Jim in length and from 0.7-1.5 flm in width.

The bacteria possess the following forms

  • Coccus (PI. cocci) bacteria are oval or spherical cells without flagella. The spheres occur as single cells (Monococcus), a pair of cells (Diplococcus), in groups of four cells (Tetracoccus), as chain of cells (Streptococcus) or in sheets (Staphylococcus).
    A few cocci may also occur in cube-like arrangements of 8 or more cells (Sarcina).
  • Bacillus (PI. bacilli) bacteria are rod-shaped cells which many occur singly (Monobacillus), in pairs (Diplobacillus), in chains (Streptobacillus) or as a layer (suck) with many cells called Palisade bacillus.
  • Spirillum (PI. spirilla) bacteria are cells, which are twisted, like a screw. They occur as free single cells, e.g., Spirillum, Spirochaete, etc.
  • Vibrio are cells which are curved, C-shaped or comma-shaped, e.g., Vibrio cholerae.
    Apart from these some other shapes of bacteria are also found
    Detailed NCERT Notes: Biological Classification Notes | Study Biology Class 11 - NEET


Archaebacteria (Archae-ancient; 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).
The characteristics of this domain are

  • They are most primitive prokaryotes.
  • They are found in stressed environment, such as high salt content (Great salt lake, the dead sea), edge of the ocean, hot sulphur springs, volcanic walls, etc.
  • Their cell walls lack peptidoglycan. In most cases, the wall composed of non-cellulosic polysaccharides and some proteins. In some members, there is no cell wall. This feature of having different cell walls is responsible for their survival in extreme condition.
  • Most of the archaebacteria are chemoautotrophs.

Types of Archaebacteria
Archaebacteria are of following three types

  1. Methanogens
    These are stricdy anaerobes. They live anaerobically in gut of several ruminants such as cows, buffaloes, goat, etc. These bacteria help in fermentation of cellulose. They produce almost 65% of atmospheric methane.
    Example Methanobacterium, Methanobacillus, Methanosarcina and Methanococcus.
  2. Halophiles
    These are found in extreme saline environments like salt lakes, salt marshes, salt pans, salt solutions, etc. They are mosdy anaerobes. They contain a chemical called halorhodopsin to pump in chlorides into the cell to prevent cellular dehydration. Halobacterium develops purple membrane having photoreceptor pigment bacteriorhodopsin. In light, it acts as a proton pump and helps in synthesise of ATP. The formation of ATP is a survival mechanism under anaerobic condition.
    Examples Halobacterium and Halococcus.
  3. Thermoacidophiles
    These archaebacteria can live in both extreme heat and acidic pH (around 2) environment. Under anaerobic conditions, these organisms oxidise sulphur to sulphuric acid.
    2S + 2H2O + 3O2 → 2H2SO4 + Energy
    Thermoacidophiles can survive in high temperature and low pH conditions because of
    (i) Special branched chain lipids in cell membranes that reduce cell fluidity.
    (ii) Enzymes can work at low pH.
    (iii) Enzymes are resistant to high temperature coagulation. Examples Sulfobolus, Thermoplasma and Thermoproteus.


  • They are called ‘true bacteria’ and are characterised by the presence of a rigid cell walls, and if motile, have flagellum.


  • Cyanobacteria, member of this group (blue-green algae) have many characters similar to bacteria. The examples of cyanobacteria are Nostoc, Oscillatoria, Spirulina, Rivularia, Anabaena, etc. They can survive in a wide variety of habitats, such as hot springs, sea water, polluted water, etc.

Detailed NCERT Notes: Biological Classification Notes | Study Biology Class 11 - NEET

Cyanobacteria have following three forms

  1. Unicellular as in Chroococcus.
  2. Colonial as in Microcystis and Gloeocapsa.
  3. Filamentous as in Nostoc, Oscillatoria and Anabaena.

Uses of Cyanobacteria
Some uses of cyanobacteria are

  • Some cyanobacteria have the ability to fix atmospheric nitrogen. The green manuring by farmers is done on this basis to enrich the soil with nitrogenous fertilisers.
  • Cyanobacteria like Anabaena, Tolypothrix, etc., help in prevention of soil erosion and its conservation.
  • Spirulina is a protein rich supplement for humans. It is a fast growing cyanobacteria. It is also known as Single Cell Protein (SCP).
  • Cyanobacteria like Anabaena and Aulosira prevent mosquito larvae to grow in surroundings.

Harmful Effects of Cyanobacteria
Some harmful effects of cyanobacteria are

  • Cyanobacteria discolour the walls and roofs of buildings, movements and statues.
  • Oscillatoria causes asthma and gastrointestinal problems by releasing its toxins.
  • Growth of Oscillatoria in water bodies shows pollution by organic matter.
  • Excessive growth of cyanobacteria form water blooms, which decreases oxygen level in water causing death of aquatic animals.

Mycoplasma are organisms that completely lack a cell wall.
They were discovered by Roux (1898) in pleural fluid of cattle suffering from pleuropneumonia. The organisms are often called MLOs (Mycoplasma Like Organisms) or PPLOs (Pleuropneumonia Like Organisms).
The characteristic features of mycoplasma are

  • Their size ranges from 0.1-0.5 pm and have organised nucleus, plastids, mitochondria and other organelles are absent.
  • DNA is naked (because of absence of histones) and ribosomes (of 70S type).
  • Mycoplasma possess heterotrophic nutrition. Examples Mycoplasma gallisepticum, M. laidlawii. They cause pleuropneumonia in domestic animals, mycoplasmal urethritis in humans.

Kingdom—Protista includes all single-celled eukaryotes but, the boundaries of this kingdom are not well defined. It was first proposed by Ernst Haeckel (1866). Physiologically kingdom-Protista acts as a connecting link between the kingdom-Monera and the complex multicellular kingdom-Fungi, Plantae and Animalia. Kingdom-protista includes the following categories such as dinoflagellates, chrysophytes, euglenoids, slime moulds and protozoans.

The general characteristic features of kingdom-Protista are given below

  • These are mostly aquatic organisms. Some protists also live in the bodies of animals as parasites.
  • The cells are eukaryotic. These contain membrane bound cell organelles like mitochondria, Golgi complex, endoplasmic reticulum, 80S ribosomes, etc.
  • Locomotion may either occur by Pseudopodia (Amoeba, Euglypha), Cilia (Parameciuni),
    Wriggling (sporozoans, non-flagellates) and Mucilage propulsion (some protists like diatoms).
  • Diatoms do not have any organelles for locomotion.
  • Protists shows various modes of nutrition such as
    (i) Photosynthetic (holophytic) Dinoflagella- tes, diatoms and euglenoids.
    (ii) Holozoic (zootrophic) Protozoans like Amoeba and Paramecium.
    (iii) Saprobic (saprotrophic) In slime moulds.
    (iv) Parasitic Trypanosoma, Giardia, Plasmodium, Entamoeba.
    (v) Mixotrophic InEuglena.
    (vi) Symbiotic In zooflagellates like Trichonympha and Lophomonas.
    (vii) Pinocytosis In Amoeba to absorb soluble organic substances.
  • Most of the protists are aerobic. However, some protists that live at the bottom of aquatic habitats can respire anaerobically.
  • Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.
  • Kingdom Protista includes Chrysophytes, Dinoflagellates, Eugleoids, slime mould and Protozoans.


This group includes diatoms and golden algae (desmids).
1. Diatoms

  • Diatoms occur in all aquatic and moist terrestrial habitats and are also known as chief producer in the ocean.
  • They pile up at the bottom of water reservoirs and form big heaps called diatomaceous earth.
  • They are microscopic unicellular organisms of different shapes, such as circles, semicircles, triangular, spindle-shaped, boat-shaped, etc.
  • The body wall of the diatoms is made up of cellulose impregnated with glass like silica. The cell wall has two overlapping halves like a sapbox called shell or pustule, i.e., a lid and a lower half fitted together.
  • Diatoms are variously coloured, do not possess flagella except in the reproductive state.
  • Each cell has a large central vacuole.
  • Chloroplast are yellowish brown to greenish brown. They contain chlorophyll-^ and c. They contain fucoxanthin that provides brownish ting.
  • Food is reserved in the form of oils and leucosin (polysaccharide).
  • The diatoms mostly reproduce asexually by binary fission. Sexual reproduction varies from isogamy to oogamy. Examples Navicula, Amphipleura , Triceratium and Cymbella.

Economic Importance of Diatoms

  • Diatoms are economically important in the following ways –
  • Diatoms are very important photosynthesizers.
  • Diatomite deposits are often accompanied by petroleum fields.
  • These are used as a cleaning agent in tooth pastes and metal polishes and are used in filtration of oil and syrups.
  • Diatoms are used as insulation material in refrigerators boilers and furnaces. These are also used to make sound-proof rooms.
  • Diatoms are also very good pollution indicators.

2. Golden Algae (Desmids)

These are unicellular green algae. Their cell walls have distinct halves. Sexual reproduction occurs by ‘conjugation’ (similar to Spirogyra). They are usually found in freshwater and acts as an indicators of polluted water.

These are mainly marine and photosynthetic organism. There are about 1,000 species of photosynthetic protists.
The general characteristic features of dinoflagellates are listed below

  • These are important phytoplanktons. Most of them are marine but some occur in freshwater.
  • They appear yellow, green, brown, blue or red depending on the main pigments present in their cells.
  • The cell wall in dinoflagellates, if present is composed of number of plates made up of cellulose.
    Some dinoflagellates like Gonyautax and Gymnodinium grow in large number in sea and make the water look red and form ‘red tide’.
    Toxins released by such large numbers may even kill other aquatic animals.
  • The cells usually possess two flagella which are of different types (heterokont). One flagellum is transverse arising from the anterior part. The other flagellum arises in the vertical furrow. Both these flagella beat in different directions.
  • The nucleus is bigger in size, named as mesokaryon. Chromosomes do not have histone and RNA.
  • The cells possess an osmoregulatory organelle called pusule, which superficially looks like contractile vacuole.
  • Dinoflagellates reproduce asexually through cell division or by the formation of zoospores and cysts.
  • Varieties of eye spots’ occur in dinoflagellates. Some of them are like ocelli.
  • Reserve food is stored in the form of starch and oils, e.g., Gonyaulax, Ceratium, Noctiluca, Peridinium and Gymnodinium, etc.

Euglenoids live in fresh aquatic habitats and damp soils.
The characteristic features of euglenoids are described below

  • They are unicellular flagellate protists.
  • Body is covered by thin and flexible pellicle. It lacks ceflulosic cell wall.
  • Euglenoids have two flagella, usually one long and one short.
  • They perform creeping movements by expansion and contraction of their body. This phenomenon is called metaboly.
  • Nutrition is holophytic, saprobic or holozoic. This mode of nutrition is called mixotrophic.
  • The photosynthetic pigments include chlorophyll-^ and b.
  • Reserve food is carbohydrate in the form of paramylon or paramylum bodies.
  • Euglenoids reproduce by longitudinal binary fission under favourable conditions. The palmella stage is found during unfavourable conditions. Examples Euglena, Perenema, Eutreptia, Phacus, etc.
    Detailed NCERT Notes: Biological Classification Notes | Study Biology Class 11 - NEET
  • Euglena is considered as plant as well as animal. It is also called as plant animal. Plant and animal features are 
  • Plant Features Chloroplasts and chlorophyll are present has holophytic nutrition.
  • Animal Features Presence of pellicle which is not made of cellulose. Contractile vacuole is present. Longitudinal binary fission.


  • Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs and pathogenic parasites.
  • The main feature that distinguishes protists in this clade is the presence of a spiral or crystalline rod of unknown function inside the flagella.

Slime Moulds

Slime moulds are saprophytic protists. Anton De Bary (1887) related them to animals and called them as Mycetozoa. These are also named as fungus animals because they share the common characters of both animals and are known as protistian jungi, and due to their protistian nature.
The generalfeatures of slime moulds are discussed here

  • Slime moulds are acellular and cellular types, about 600 species of slime moulds are reported by biologists out of which 27 species are known from India.
  • They are found in moist terrestrial places rich in decaying organic food.
  • The body of slime moulds is covered with mucilage having gelatinous consistency, they do not have chlorophyll.
  • They are surrounded by plasma membrane. However, the spores have the ceflulosic cell walls.
  • They show phagotropic or saprotrophic nutrition.
  • Both sexual and asexual modes of reproduction occur.
  • They are like Protozoa in their amoeboid plasmodial stage and similar to true fungi in spore formation.
  • Acellular slime moulds (plasmodial slime moulds) are commonly found on dead and decaying plant matter. The cellular slime moulds occur in all humus-containing upper layer of damp soil. When the food supply is shorter or conditions are not favourable, the amoeboid cells form aggregate without any fusion.
    This aggregated mass is called pseudoplasmodium. The examples of cellular slime moulds are dictyostelium and polysphondylium.
  • Plasmodium is the free-living thalloid body of the acellular slime moulds. It is wall-less mass of multinucleate protoplasm covered by slime layer. During unfavourable conditions, the Plasmodium differentiates and forms fruiting bodies bearing spores at their tips. WTiile during favourable conditions, Plasmodium can spread over several feet.
  • Slime moulds are beneficial as they cause the decomposition of organic matter in the soil.

Protozoan Protists

  • Include unicellular protists with animal like behaviour.
  • They were first studied by Leeuwenhoek (1677).
    Protozoan protists may be aquatic, terrestrial or parasites.
    They can cause several diseases in humans and animals.
  • General characteristics of protozoans are described below
  • They are microscopic small unicellular and colourless organism with different shapes.
  • Locomotion occurs with the help of finger-like pseudopodia, flagella or hairy cilia.
  • All protozoans are heterotrophs and live as predators or parasites.
  • Respiration occurs through the general surface of the body.
  • Reproduction occurs by binary fission, multiple fission or budding. Sexual reproduction occurs by syngamy and conjugation.

There are four major groups of protozoans

Amoeboid Protozoans

  • These organisms live in freshwater, seawater or moist soil.
    Examples Amoeba, Entamoeba, Radiolarians, Pelomyxa, Foraminiferans and Heliozoans.

General features of this group are following

  • They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba (as mouth is absent).
  • The body is without periplast. It may be naked or have a calcareous shell.
  • Flagella are present in some developmental stages. They also develop when food become scarce.
  • Nutrition is holozoic.
  • Asexual reproduction occurs by binary fission, multiple fission, spores and budding and sexual reproduction occurs by syngamy.

Flagellated Protozoans
The members of this group are either free-living or parasitic. Examples Giardia, Trypanosoma, Leishmania, Trichonympha and Trichomonas.
General features of this group are following

  • They have flagella for locomotion as their name suggests.
  • They may be aquatic, free-living, parasitic, commensals or symbiotic.
  • The body is enclosed by a firm pellicle.
  • Nutrition is holozoic, saprobic and parasitic.
  • Asexual reproduction is by binary fission.
  • Sexual reproduction is observed in some forms only.
  • Various species of these protozoans causes diseases in humans. For examples,
  • Trypanosoma (sleeping sickness)
  • Leishmania (kala-azar, dum-dum fever)
  • Giardia (giardiasis)
  • Trichomonas (leucorrhoea).

Ciliated Protozoans

These are aquatic, actively moving organisms because of the presence of thousands of cilia.

Examples Paramecium, Opalina, Vorticella, Podophyra, Balantidium, etc.

Generalfeatures of this group are following

  • Many ciliates live as free-living individual in fresh % and marine water (Paramecium).
  • A large number of cilia present on whole body surface.Cilia are used to capture food and for locomotion.
  • Nutrition is holozoic except in some parasitic forms.
  • The body is covered with flexible pellicle.
  • There are definite regions for ingestion and egestion.
  • Ciliates have a larger macronucleus and smaller micronudeus.
  • They have small ejectable trichocysts for defense.
  • Osmoregulation occurs by contractile vacuoles.
  • Asexual reproduction occurs by transverse binary fission or budding. Cyst formation also occurs during unfavourable condition.
  • Sexual reproduction by means of conjugation.

Detailed NCERT Notes: Biological Classification Notes | Study Biology Class 11 - NEET

Sporozoan Protozoans

This group includes organisms that have an infections spore-like stage in their life cycle.

Examples Plasmodium, Monocystis, Eimeria.

General features of this group are following

  • All sporozoans are endoparasites and pathogenic.
  • Locomotory organs are absent.
  • Nutrition is parasitic (absorptive).
  • Body is covered with an elastic pellicle or cuticle and contractile vacuoles are absent.
  • A sexual reproduction occurs through multiple fission and sexual reproduction by syngamy.
  • Life cycle may include two different hosts, e.g., Plasmodium requires two hosts (digenetic), female Anopheles mosquito and human beings. It is responsible for causing malaria, in humans.

Classes of Fungi

1. Phycomycetes: 

  • grow on decoying wood or as obligate parasitos on plants
  • Mycolium aseptate and coonocytic
  • Spores produced ondogenously in sporangium
  • Asaxual ropdouction by Zoosporos or Aplanosporos
  • Zygosporos are formed by the fusion of gamotes
    Example: Rhizopus Alougo, Mucor

2. Ascomycetes:

  • also known as sac fungi.
  • Are saprophytic, docomposers, parasitic or coprophilous (growing on dung).
  • Mycolium branched and soptate
  • Asexual spores are called conidia produced exogonously on the conidiophores. -Sexual spores are called ascosporas produced endogenously in ascus, produced insido fruiting body called Ascocarp.
    Example: Aspergillus, Nourospora, Sacharomyces (Unicallular fungi), Clavicops, morels, trufles.

3. Basidiomycetes:

  • Mycelium saptate and branched.
  • Genarally asexual sprors are not found.
  • Vegetative reproduction by fragmentation.
  • Sexual reproduction by fusion of vogotative or somatic cells to form basidium produced in basidiocarp.
  • Basidium produces four basidiosporos exogenously after moiosis. Example: Agaricus, Ustilago, Puccinia.

4. Deuteromycetes:

  • Callod as Fungi Imperfecti as sexual form (porfect stage) is not known for thom
  • Onco sexual form is discovered the mombor is moved to Ascomycetes or Basidiomycotas
  • Mycelium is septate and branched
  • They are saprophytic parasitic or decomposers.
    Example: Altorriaria, Colletotrichum, Trichoderma.

Virus, Viroids and Lichens

Five kingdom system of classification do not includes Virus, Viroids and Lichens.

  • Viruses are non-cellular organisms having inert crystalline structure outside the living. When they enter the living cell, they take over the machinery of living cell to replicate themselves.
  • D.J.Ivanowsky recognized certain microbes as causal organism of mosaic disease of tobacco.
  • In addition to proteins, viruses also contain genetic material that could be DNA or RNA. In general, virus that infect plants have single stranded RNA and virus that infect animals have double stranded DNA.
  • Some common diseases caused by virus are common cold, influenza, AIDS, small pox, leaf rolling and curling.
  • Bacteria feeding virus are called Bacteriophage.They are usually double stranded DNA viruses.
  • The protein coat called capsid is made of small subunits called capsomeres, protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.
  • Viroids are discovered by T.O.Diener as new infectious agent smaller than virus causing potato spindle tuber disease. They are free RNA without protein coat.
  • Lichens are symbiotic association between algae and fungi. The algal part is called Phycobiont and fungal parts are called Mycobiont. They are good pollution indicator as they do not grow in polluted area.
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