Class 11 Biology chapter 2 Biological Classification notes


Biological classification is the scientific procedure of arranging organisms into groups and subgroups on the basis of their similarities and dissimilarities and the placing of group in a hierarchy of categories. Class 11 Biology chapter 2 Biological Classification notes

Importance of classification:-

  • It is not possible to of study one or two organisms of a group give a sufficient information about the essential features of the group.
  • classification helps in identification of new organisms.
  • The organism of past cannot be identified without a proper system of classification.

Classification are of three types:-

  1. Artificial system of classification:-

Only one or two morphological characters are used in grouping of organisms. Flowering and non flowering plants, enaima and anaima. Aristotle classification


2. Natural system of classification:-

Takes into consideration comparable study of a number of characters as to bring out the natural similarities and dissimilarities and hence natural relationships among the organisms. Bentham and Hooker classification.

3. Phylogenetic system of classification:-


Based on the evolutionary relationships of organisms. In this system the organisms are classified on the basis of their evolution on earth from primitive to highly evolved. Engler and Prattle classification and Hutchinson.

Organisms are classified into following groups:-

  1. Two kingdom of classification system:

Linnaeus (Father of taxonomy system) divided all the living organisms into two kingdom in 1758. These are Plantae and Animalia.

Objectives Against two kingdom classification system:


The two kingdom system of classification was accepted for a long time. However, some difficulties arises form this classification as several new organisms have been discovered. Some of these difficulties are mentioned below

  • The first formed organisms were neither plants nor animals.
  • Fungi do not show similarity with structure, physiology and reproduction system of plants.
  • It is not easy to recognize the lower organisms as the plants nor animals. For example, Euglena has mixotrophic (dual) mode of nutrition, while sponges are fixed, branched and irregular creature like plants.
  • Slime moulds a group of fungi, are wall less in vegetative phase. They develop cell wall in the reproductive phase. Slime moulds can neither be placed in fungi, nor plants.
  • Lichens are formed by the symbiotic association of an alga and a fungus. They neither resemble plants nor animals.
  • Prokaryotes do not have an organized nucleus. They have single envelope organization, absence of spindle apparatus, meiosis and sexual reproduction.
  • Eukaryotes have a well defined nucleus, a double envelope organization, spindle apparatus, meiosis and sexual reproduction.

Five kingdom classification system:-

This classification system was prepared by RH Whittaker, in 1969. Before 1969, the classification system 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. Thallus organization
  3. Phylogenetic relationship
  4. Mode of nutrition
  5. Reproduction

Kingdom Monera:-

Bacteria are the sole members of the kingdom Monera. Bacteria are grouped under four categories based on their shape.

  1. Spherical coccus.
  2. Rod shaped bacillus.
  3. comma shaped Vibrium.
  4. Spiral spirillum.

Bacteria are divided into two major groups.

  1. Archibacteria:

They live in some of the most harsh habitats such as extreme salty areas called as halophils, hot springs called thermoacidophils, marshy areas called methanogens.

Archibacteria can survive in extreme habitats due to a different cell wall structure.

Methanogens are present in the gut of several ruminant animals like cow etc. Responsible for the production of methane. (bio gas).


2. Eubacteria:

They are known as true bacteria, and are characterized by presence of rigid cell wall and If motile, a flagellum.

The cyanobacteria or blue green algae have chlorophyll a similar to plants and are photosynthetic autotrophs.

The cyanobacteria are unicellular, colonial or filamentous, marine or terrestrial alga, and the colonies are generally surrounded by a gelatinous sheath. Some of the cyanobacteria can fix atmospheric nitrogen in specialized cells called heterocyst. for ex- nostac and anabaena.

Chemosynthetic bacteria are oxidize various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production.

Heterotrophic bacteria are important decomposers. They helpful in making curd from milk, antibiotics, nitrogen fixing in legum roots.

Some are pathogens causing damage to human, crops and causes are cholera, typhoid, tetanus to human being.

Bacteria reproduce mainly by fission or under unfavorable conditions produce spores or by a sort of sexual reproduction by transferring DNA from one bacterium to other.

The Mycoplasm are organisms that completely lack of cell wall, and they are the smallest living cell known, which can survive without oxygen.

Kingdom Protista:-

  • All single cell eukaryotes are placed under Protista.
  • Chrsophytes, Dinoflagelletes, Euglenoid, Slime moulds and Protozoans are under Protista.
  • They reproduce asexually and sexually by a process involving cell fusion and zygote formation.
  • The protistan cell body contain a well defined nucleus and membrane bound organelles.


This group include diatoms and golden algae (desmids), found in fresh as well as marine environment. In diatoms the cell wall form two thin overlapping shells embedded in silica.

Diatoms have left behind large amount of cell wall deposit in their habitat, which accumulation over billions of years is reffered to as ‘diatomaceous earth’.

Most of them are photosynthetic and are the chief producers of ocean.


The cell wall has stiff cellulose plates on the outer surface, found in marine and photosynthetic.

Yellow, green, brown, blue or red depending on the pigment in their cell. Most of them have two flagella: one lies longitudinally and the other transversely in a furrow between the wall plates.

Red dinoflagellates undergo such rapid multiplication that they make the sea appear red (red tides) ex- gonyaulax.

Slime moulds

Slime moulds are saprophytic Protists. under suitable condition they form an aggregation called plasmodium, and during unfavorable conditions, the plasmodium differentiates and form fruiting bodies bearing spores at their tips.

The spores posses true cell wall, and they are extremely resistant and survive for many years.


All the protozoans are heterotrophic and live as predators or parasites. There are four major groups of protozoans.

  1. Amoeboid protozoans:

These organisms live in fresh water, sea water or moist soil. They show Amoeba like moments and move and capture their pray with the help of pseudopodia. Example- Endameba, a parasite.

2. Flagellated protozoans:

The members of this group are either free living or parasitic, and have flagella. The parasitic forms causes disease such as sleeping sickness. Ex- Trypanosoma

3. Cilliated 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. ex- Paramecium

4. Sporozoans :

Sporozoans include diverse organisms that have an infectious spore like stage in their life cycle.

The most notorious is plasmodium (malarial parasite) which causes malaria.

Class 11 Biology chapter 2 Biological Classification notes

Kingdom Fungi:-

They are achlorophyllous, heterotrophic spore forming, non vascular eukaryotic organisms.

Cell wall is made up of chitin. Reserved food is glycogen.

Mode of nutrition is saprophytic, parasitic or symbiotic.

Reproduction may be vegetative (fragmentation, fission or budding), asexual (conidia, sporangiospores or zoospores) or sexual reproduction by oospores, ascospore and basidiospores.

Sexual cycle involves the following steps :-

  1. Plasmogamy, fussion of male and felame gametes.
  2. Karyogamy, fusion of two nuclei.
  3. Meiosis in zygote to produce haploid spores.
Class 11 Biology chapter 2 Biological Classification notes
Class 11 Biology chapter 2 Biological Classification notes


They are found in aquatic habitat, and on decaying wood in moist and damp places or as obligate parasites in plants.

Mycelium is asepetate and coenocytic. Asexual reproduction by zoospores or aplanospores. ex- Mucor, Rhizopus, Albugo etc.


They are saprophytic, decomposers, parasitic or coprophilous (growing on dung).

Mycelium is branched and septate and asexual spores are condia.

Sexual spores are called ascospores produced inside the fruiting body called ascocarps. ex- Nurospora, Aspergillus, claviceps.

Class 11 Biology chapter 2 Biological Classification notes

Basidiomycetes (the club fungi)

The mycelium is branched and septate. Vegetative reproduction is by frgmentation. Asexual spores are not found. Sexual reproduction is by two vegetative or somatic cells forming basidium.

Basidiospores are produced in basidium by developing a fruiting body called basidiocarps. Ex- Agaricus, Ustilago, Puccinia.

Deuteromycetes (The imperfect fungi)

Only vegetative and asexual phase is known. Mycelium is septate and branched some members are saprophytic or parasitic. ex- Alternaria, Trichoderma, colletotrichu.

Kingdom Plantae:-

  • Eukaryotic, chlorophyll bearing organisms.
  • Life cycle is divided into diploid saprophytic and haploid gametophytic which alternate with each other.
  • Kingdom Plantae includes algae, Bryophytae, Pteridophytae, Gymnosperms and Angiosperms.

Kingdom Animalia:-

  • Heterotrophic, eukaryotic organisms that are multicellular and cell wall is absent in the cell.
  • Mode of nutrition is holozoic and reserve food is glycogen or fats.
  • Sexual reproduction is by copulation between male and female followed by embryological development.

Class 11 Biology


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