Biological Classification | Chapter 2 Notes

Biological classification CBSE, class 11 Biology notes. This CBSE Biology class 11 note is having a brief explanation of every topic that the NCERT Biology syllabus has. You will also get ncert solutions, CBSE class 11 Biology sample paper, CBSE Biology class 11 previous year paper.

Biological Classification

Biological classification is the scientific procedure of grouping or arranging organisms into groups and subgroups according to their similarities, dissimilarities and placing the group in a hierarchy of categories.

There have been various attempts to classify organisms. 

Aristotle (Greek philosopher), who classified plants into herbs, shrubs and trees and animals into two groups, based on the presence of red blood cells (vertebrates) and absence of red blood cell (invertebrates)

Carl Linnaeus (Swedish botanist) gave the Two Kingdom system of classification and divided living organisms into Plantae and Animalia.

R.H. Whittaker(American ecologist) gave the Five Kingdom system of classification based on cellular structure, complexity, mode of nutrition, phylogenetic relationship and ecological role performed by them.

Whittaker divided organisms into Monera, Protista, Fungi, Plantae and Animalia

Importance of classification

  • Study of one or two organisms of a group gives sufficient information about the essential features of the whole group.
  • Classification helps in identification of new organisms.
  • It helps in knowing the relationship amongst different groups of organisms.
  • The past organism cannot be studied without a proper system of classification.


  • Artificial system of classification
  • Natural system of classification
  • Phylogenetic system of classification

Artificial system of classification– Only one or two morphological characters for grouping of organisms is used. Aristotle classification.

The natural system of classification– Comparative study of a number of characters so as to bring out natural similarities and dissimilarities. Bentham and Hooker classification

Phylogenetic System of Classification– Organisms are classified on the basis of their evolution on earth from primitive to highly evolved. Engler and Prantl classification and Hutchinson classification.

Five Kingdom Systems

R.H.Whittaker divided all the organisms into five kingdoms in order to develop phylogenetic classification.

Kingdom Monera

Kingdom Monera belongs to the prokaryote family. It is the oldest known microorganism on earth that doesn’t contain cell structures including the nucleus and other cell organelles and contain naked DNA.

Monerans are the most abundant of all. It generally comprises unicellular organisms with a prokaryotic cell (mycoplasma, bacteria, actinomycetes and cyanobacteria), the organization found mostly in a moist environment e.g. hot springs, snow, deep oceans, or as parasites in other organisms.

Features of Monera Kingdom 

  • They are simple, unicellular, eukaryotic organisms.
  • Protist include Chrysophytes, Dinoflagellates, Euglenoids, slime mould and Protozoans.
  • Most of them live in water, some in moist soil or even the body of humans and plants.
  • Protists have a membrane-bound nucleus, endomembrane systems, mitochondria for cellular respiration and some have chloroplasts for photosynthesis.
  • Nuclei contain multiple DNA molecules 
  • Cellular respiration is the primarily aerobic process, some are facultative anaerobes.
  • Movement is often by flagella or cilia.
  • Nutrition may include both heterotrophic and autotrophic.
  • Reproduction is sexual and asexual both. Some protists are pathogens of both plants and animals, e.g. Plasmodium falciparum causes malaria in humans.

Kingdom Fungi

Fungi are a group of organisms that are found everywhere from air, water, land to the soil including plants and animals.

Some fungi are microscopic and others are gargantuan. Fungi appear like plants, they are closely related to animals.

Fungi have great economic value. It shows a great diversity in morphology and habitat. More than 70,000 species of fungi have been identified, including mushrooms, smuts, yeasts, puffballs, rusts, smuts,  truffles, morels, and moulds.

Features of Kingdom Fungi 

  • Fungi are eukaryotic, non-vascular and non-motile organisms. They are achlorophyllous, heterotrophic, spore forming, non-vesicular eukaryotic organisms.
  • The growth rate of fungi is slower than 
  • bacteria. It grows best in an acidic environment. 
  • Fungi consist of both unicellular (e.g. Yeast, Molds) and multicellular (e.g. mushrooms) organisms.
  • Like plant cells, fungi also have cell walls made up of complex sugar molecules called chitin. But they do not undergo photosynthesis.
  • The body of the fungi may be unicellular or composed of microscopic threads called hyphae.
  • Fungi have a heterotrophic mode of nutrition. Few species are saprophytes i.e., they feed on dead and decaying organic matters.
  • Some fungi are parasitic while some are symbionts. They can live in relationship with algae, like blue-green algae. These are called lichens.
  • Reproduction is both by sexual and asexual means. Asexual reproduction takes place by means of spores and sexual reproduction takes place by means of gametic copulation, somatic copulation, and Spermatization.
  • Sexual cycles involve Plasmogamy, fusion of male and female gametes.
  • Fungi reserves food is glycogen.
  • Mode of nutrition is saprophytic, parasitic or symbiotic.
  • Fungi are found in aquatic habitats and on decaying wood in moist and damp places.

Kingdom Plantae 

Kingdom Plantae includes all the plants on the earth. They are eukaryotic, multicellular and autotrophic organisms. It includes green, brown and red algae, liverworts, mosses, ferns and seed plants with or without flowers

The plant cell contains a rigid cell wall. They have chloroplast and chlorophyll pigment, which is required for photosynthesis.

The plant kingdom is further classified into five subgroups Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms

Features of Kingdom Plantae

  • They are non-motile and live anchored to a substrate.
  • They are autotrophs and make their own food 
  • Plant contain photosynthetic pigment called chlorophyll in plastids. 
  • The principal mode of nutrition is photosynthesis.
  • Reproduction is primarily asexually by vegetative propagation or sexually.
  • The plant cell contains the outer cell wall and a large central vacuole.
  • Plants form a multicellular embryo during development from the zygote.
  • The life cycle of plants consists of alternating haploid gametophyte and diploid sporophyte generation. This phenomenon is called the alternation of generation.                     

Kingdom Animalia

Kingdom Animalia constitutes all animals of the earth. Animal Kingdom is the largest kingdom amongst the five kingdoms

Features of kingdom Animalia

  • Animals are multicellular eukaryotes and multicellular
  • Animal kingdom exhibits a heterotrophic mode of nutrition. They has been classified into ten different subphyla based on their body design 
  • Animals are heterotrophic, feeding on organic material and digesting it internally 
  • They respire aerobically. 
  • All animals are motile (able to spontaneously move their bodies)
  • Some animals, such as sponges, corals, mussels, and barnacles, later become sessile. 
  • The different phylum of the animal kingdom are Porifera, Coelenterata (Cnidaria), Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata

Virus, Viroids and Lichens

Five kingdom systems of classification do not include viruses, Viroids and Lichens.


Viruses are non-cellular, microscopic infectious agents that can only reproduce inside a host cell. Viruses cannot be classified as living organisms or nonliving. Because they possess certain defining characteristic features of living and nonliving organisms.

The virus is made up of genetic material and protein that can replicate only within the living cells of bacteria, plants and animals.

It enters the host and attaches itself to a specific host cell, injects its genetic material, reproduces by using the host genetic material and finally, the host cell splits open, releasing the new viruses.

Viruses can also be crystallized. Other than proteins, viruses also contain genetic material that possesses nucleic acids (DNA or RNA) which are encased within a  protective protein coat. 

In general, viruses that infect plants have single-stranded RNA and viruses that infect animals have double-stranded DNA.

Bacteria feeding viruses are called Bacteriophages. They are usually double-stranded DNA viruses.

Viruses are inactive when they are outside of host cells, but become active within host cells. They cause several infections and reproduce by using the enzymes and raw materials

These entities are able to infect all forms of life, ranging from bacteria to humans, and consequently, they bring about a multitude of diseases in their host.


Viroids are new infectious agents smaller than viruses, causing potato spindle tuber disease. They are free RNA without a protein coat.

Viroids are plant pathogens that are infectious and only affect plants. Viroids possess circular strands of ribonucleic acids (RNA’s) which hijack the cellular machinery present in plant cells to reproduce new copies of themselves. It primarily affects all forms of higher plants.

The plants that are infected by viroids that infect other plants and responsible for the crop failures 

Viroids were discovered by T.O. Diener in the year 1971. Viroids are the plant parasites like transcriptional machinery of the cell organelles.

Viroids are classified into two families namely Pospiviroidae- nuclear viroids and Avsunviroidae- chloroplastic viroids.

They move in an intracellular manner, cell to cell through the plasmodesmata, and a long distance through the phloem.


A lichen is not a single organism but a symbiotic association between algae and fungi. The algae part is called Phycobiont and the fungi parts are called Mycobiont.

Ascomycetes or Basidiomycetes are the most common fungi in lichens. The common algal partners are either green algae Chlorophyta or Cyanophyceae family of blue-green bacteria.

Most lichens are smaller than a few centimetres, the largest lichen can be up to 3ft long. They are colourful, ranging from yellow to greens and black.

Generally, lichens grow slowly. The one in which the phycobiont is a blue-green bacterium that has the ability to convert nitrogen gas into ammonia. 

Also Explore Chapter 1 The Living World

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