Biological Classification Class 11 Notes

Biological Classification class 11 notes

This article is on the Biological Classification Class 11 Notes of Biology. The notes on biological classification of class 11 biology have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the biological classification chapter of Class 11 for the students so that they can give a quick glance of the chapter.

The chapter has been divided into two articles. This article (Part 1) includes the system of classification, Kingdom system of classification and at last the Kingdom Monera. The second article (Part 2) includes the Kingdom Protista, Kingdom Fungi, Viruses, Viroids and Lichens.

Biological Classification Part 1

Systems of Classification

Earlier at the dawn of the classification systems, the basis for classification was only the habitat and external morphological characters of the organisms. However later with development in science, taxonomist used various another basis of classification like natural affinity, and even phylogeny (evolutionary tendencies).

The systems were divided into three major categories. They are as follows:

Artificial System of Classification

The classification system used one / few external or morphological characters of the organisms for differentiating with few exceptions in classification by using even habitats of the organisms. Artificial classification system use habitat as the basis e.g., Theophrastus system for classification. Mechanical systems for classification are dependent on selected characters that are morphological type e.g. Linnaeus system.

Practical classification is the ones involving both plants and animals on the basis of their importance and uses in man’s life.

Natural Systems for Classification

These classification systems were based on the taxonomic characters and on natural affinities of the organisms. The usual characters used for classification of organisms are reproductive, morphological and anatomical in nature. The branch dealing with the classification based on the biochemicals in the cell of an organism for taxonomy is called as chemotaxonomy. This system reveals the origin of the organism and their evolutionary relationship.

The artificial and natural system of classification was dependent on a fixed number of organisms and no new species were included. Thus, they are the static classification systems with no change in them.

Phylogenetic Systems for Classification

The system classifies organisms on the evolutionary and genetic relationship between them. The system here changes with time and thus, is called a dynamic system for classification.

Endlicher and Eichler were botanists who first proposed a phylogenetic classification system. Engler and Prantl modified their work and published in a book which was first fully developed phylogenetic system. The phylogeny system is the most important one for classification that is used by many scientists.

Numerical Taxonomy or Phonetics

The numerical technique for evolution that included both similarities and differences between the species is the basis. Calculators and computers are used for this technique. All possible characters that are known are used for comparison. All characters have equal importance and weightage for the analysis purpose. Here data for many characteristics are defined statistically and objectively, then taken into consideration. Then codes are assigned to each data on computers, then denoted with a plus (+) and minus (-) or data not available (θ).


It provides cytological information of the cell, the chromosome number, the structure etc. It also involves the behaviour and expression of chromosomes during classification.

Very few species have a constant number of chromosomes. A man has the diploid 46 chromosome number and potato has 48 chromosomes. Size of the different chromosome in different herbaceous plants has been found to be larger than woody plants. The relationship among different species has become clear with the help of the method of the pairing of chromosomes during meiosis.


This type of taxonomy utilizes the chemical constituents of plants. Such characters like fragrance and taste are stable not change easily. Other characters of taxonomic value that have importance are the presence of calcium oxalate crystals (raphides) and sulphur containing compounds in organisms of family Cruciferae. Many similarities and relationships among organisms have been put forth by studies and research on DNA sequencing and the chemical nature of proteins. Study of chemical characters has greatly further helped the taxonomists in gathering possible relationships and statistical evolution of taxonomic value.

Kingdom Systems of Classification

Two Kingdom Classification

There was a huge list of organisms present without any grouping. Linnaeus was the first naturalist to classify organisms on the basis of their cell structure. The major difference in the organisms was the presence and absence of cell wall in the cell. Thus all the organisms of the world were divided into two main kingdoms-the animal kingdom (Animalia) and the plant kingdom (Plantae). Other basis for classification were the presence or absence of locomotion, mode of nutrition, response to external stimuli etc. in the organisms.

Biological Classification class 11 notes

Drawbacks: This was the first classification that gave many naturalists a hint for further classification of organisms. The problems associated with the two-kingdom system of classification were that:

The two-kingdom system of classification failed to distinguish between the eukaryotes and prokaryotes, unicellular and multicellular organisms and photosynthetic and non-photosynthetic organisms which made them all come into one single kingdom. There are few microorganisms like Chlamydomonas, Euglena and the slime moulds which belong to both zoology and botany world (organisms which share characteristics of both animals and plants). There are few sets of organisms that are neither plants nor animals, which made scientist to propose a new kingdom that will include such organisms and their characters.

Three Kingdom Classification

To overcome the two kingdom classification problems, Haeckel, a German zoologist (1866), suggested that a third kingdom Protista should be created. This kingdom will include all unicellular microorganisms separating multicellular and unicellular organisms.

Three Kingdom Classification

Drawbacks: The problems faced by the three kingdom classification were:

The organisms with both the characters of animals and plants were in the same kingdom. The organisms with prokaryotic and eukaryotic nature were in the same kingdom. Then also the organisms that were plants and animals but had a few different characteristics from the traditional characteristics of plants and animals were placed in the same kingdom.

Four Kingdom Classification

Copeland (1956) gave four kingdom of classification to answer all the questions raised from the three kingdom. He included Monera as the fourth kingdom that included all the prokaryotes. The kingdom was originally named ‘Mychota’ then called ‘Monera’ by Daugherty and Allen.

Four Kingdom Classification

Problems faced by Copeland after publishing his classification were: Though he distinguished the cells on their nature type, there were organisms still as exceptions to the usual organism characters in that particular kingdom. The organisms thus had to be given a separate kingdom.

Five Kingdom Classification

A new concept for classification proposed by R. H. Whittaker (1969), the organisms were divided on the basis of several distinguishing characters. The organisms were divided into five kingdoms namely Monera, Protista, Fungi, Plantae, and Animalia.

The basis of classification for the organisms as per Whittaker are:

  • Complexity and organisation of cell structure: prokaryotic and eukaryotic cells.
  • The complexity of the body organisation of the organism: unicellular and multicellular – simple multicellular forms and complex multicellular forms.
  • Mode of nutrition for all the organisms: Autotrophic and heterotrophic (parasitic or saprobic or ingestive organisms). It was the major criteria of this classification system.
  • Reproduction type: asexual and sexual.
  • Phylogenetic or evolutionary interrelations between the organisms.

Five Kingdom Classification

Drawbacks: This system of classification had overcome all the problems faced by the two, three and four classification systems.

The only problem that exists is the bacteria species in the Monera kingdom. The bacteria have various types and classes different from each other that were grouped in a single kingdom. Hence microbiologist insisted to place them separately. There was no kingdom or even class for viruses in the system. Algae was separated into Plantae, Protista and Monera kingdoms.

Kingdom: Monera

The Kingdom Monera includes all prokaryotic organisms. Monerans as a whole represent the most primitive forms of life on earth during evolution. These organisms originated from more ancient living stock or a unit called as progenote present in the oceans. The kingdom Monera includes eubacteria and archaebacteria. Eubacteria includes Cyanobacteria, Actinomycetes, Mycoplasma, Rickettsiae, Chlamydiae and Spirochaetes etc.

Kingdom Monera has the Following Special Factors

It is unicellular, colony forming, multicellular prokaryotic organisms lacking a eukaryotic nucleus and having nucleoid instead. Nucleoid or genophore or incipient nucleus or prochromosome is composed of naked DNA, RNA and non-histone proteins. Cell wall consists of peptidoglycan (exceptions are Archaebacteria and Mycoplasma). It lacks membrane-bound organelles. Cyclosis is absent in the cells and ribosomes have 70 S molecular structure. Respiratory enzymes are present in the cell in association with the plasma membrane. Mode of reproduction is asexual. Cell division is not by mitosis and has no spindle fibres formed.


Bacteria are omnipresent that is found almost in every habitat whether it is living or dead organic matter.

Shapes of Bacteria

As the types of bacteria changes so does its shape changes. Bacteria are found in four basic forms or shapes. They are – spherical (Cocci), rod (Bacilli), Vibrio (comma) and Spiral (spring). Though most bacterial species are made of shapes that are constant and characteristic feature of that particular cell, some species have pleomorphic cell type (i.e., these can exhibit a variety of shapes), e.g., Rhizobium leguminosarum.

Shapes of Bacteria

(a) Autotrophic Bacteria

Photoautotrophic or autotrophic bacteria: are capable of entrapping solar energy and utilizing it for the synthesis of complex food materials due to the presence of pigments like bacteriochlorophyll (bacteriopurpurin) and bacterioviridin.

(b) Chemoautotrophic Bacteria

Bacteria belonging to this category obtain energy for the synthesis of food oxidising certain inorganic substances like ammonia, nitrates, ferrous ions etc. Thus, they do not utilise light as the energy source. The chemical energy thus obtained, is trapped in ATP molecules. This energy is then used in carbon assimilation with the help of hydrogen from some source other than water, e.g., hydrogen bacteria, nitrifying bacteria, sulphur bacteria, etc. They play a great role in recycling nutrients like hydrogen, phosphorus, iron, sulphur.

(c) Heterotrophic bacteria

Biological Classification class 11 notes


Cyanobacteria are the only moneran Gram-negative photosynthetic bacteria that have a prokaryotic cell structure. These are the most primitive organisms that have the ability of oxygenic photosynthesis. They played a major role in adding oxygen to the atmosphere, which made the existence of aerobic forms of living organisms. They are also called as BGA (Blue-green algae) and are classified as Cyanophyceae or Myxophyceae.

Cyanobacteria are mainly freshwater forms, and a few are marine habitat. Red sea has an abundant population of a cyanobacterium Trichodesemium erythodesmium, which gives a red colour to the water. They occur in symbiosis with several groups of eukaryotes i.e. green algae, fungi, bryophytes like mosses and Anthoceros, ferns, gymnosperms, angiosperms, sponge, shrimps, mammals etc. Anabaena azollae is associated with Azolla, which is an aquatic fern. Anabaena cycadeae is associated with coralloid roots of Cycas. In many lichens which are the symbiotic association of algae and fungi, wherein the algae may be a cyanobacterium. When they live endozoically in protozoans they are called cyanelle.

Reproduction in Bacteria

Bacteria as a whole reproduce by asexual method while some bacteria also show sexual reproduction (True sexual reproduction is absent).

(i) Asexual Reproduction

Bacteria reproduce with the help of several types of asexual spores such as sporangiospores, oidia, conidia and endospores. The most common mode of asexual reproduction is binary fission.

(a) Binary Fission: This process of cell division is amitotic type i.e., not involving the spindle formation. Since at one stage, the replicating chromosome appears like the Greek letter θ, this mode of replication is called the theta model.

Binary Fission

(b) Endospores: Cells of certain bacteria, e.g., Bacillus, Clostridium etc. from thick-walled, highly resistant bodies within the cell, called endospores. One bacterial cell normally produces only a single endospore. The endospores may be spherical or oval in shape and are terminal or central in position. Anticoagulant nature of endospore is due to the presence of Ca-dipicolinic acid in the cortex layer of the wall.

(ii) Sexual Recombination (Genetic Recombination):

The bacteria exhibit a primitive form of sexual reproduction which differs from eukaryotic sexual reproduction because there is no gamete formation and fusion. However, the essential feature of sexual reproduction, i.e., the exchange of genetic material does take place and is called genetic recombination. Three methods are known by which genetic recombination is achieved by bacteria. In the order of their bacteria, these are transformation, conjugation and transduction.


This article has tried to highlight the important points of Biological Classification in the form of short notes for class 11 students in order to understand the basic concepts of the chapter. The notes on biological classification have not only been prepared for class 11 but also for the different competitive exams such as neet, etc.

Check Part 2 of the Chapter Here: BIOLOGICAL CLASSIFICATION PART 2

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