Biological classification Notes

Table of content

Introduction to Biological Classification

Biological classification is the scientific method of arrangement of organisms on the basis of resemblance and differences into the hierarchy of categories.

Two Kingdom system:

Carolus Linnaeus classified all the living organism into two large systems-
Kingdom Plantae: Chlorophyll containing green plants, (for example; mosses, fungi, mould, lichens), bacteria, many cellulosic and colored unicellular organisms and multicellular seaweeds.
Kingdom Animalia: for example- unicellular Protozoa and multicellular organisms.

Objection: The place and distinctiveness of a bacteria, cyanobacteria and higher fungi are not clear.

Three Kingdom system:

It was given by Ernest Haeckel, identified the Kingdom Protista in which unicellular animals, algae and fungi were placed on the basic absence of tissue differentiation.

Four Kingdom system:

It was given by Copeland.
  1. Monera
  2. Protista
  3. Metaphyta (Plantae)
  4. Metazoa (Animalia)

Five kingdom system:

It was given by R. H. Whittaker.
Classification based on following characters:
  • Complexity of cell structures
  • Complexity of organism’s body
  • Mode of obtaining nutrition
  • Phylogenetic relationship
Following are the five kingdoms:
  1. Monera
  2. Protista
  3. Fungi
  4. Plantae
  5. Animalia

Monera (Archaebacteria, Cyanobacteria, Eubacteria, Mycoplasma):

It includes unicellular prokaryotic and shows following characteristics:
  • They lack nuclear membrane.
  • They are devoid of plastids, mitochondria and other cell organelles.
  • These organisms bear rigid cell wall.
  • The predominant mode of nutrition is absorptive but some groups are photosynthetic and chemosynthetic.
  • Organisms have both autotrophic nutrition and heterotrophic nutrition.
  • Reproduction is primarily asexual by fission or budding.
  • They are found at most extreme habits such as salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).
  • They have different cell wall structure when compared with other bacteria, as a result they are able to tolerate extreme conditions.
  • Methanogens are present in the gut of several ruminant animals such as cow and buffalo.
Cyanobacteria (Blue Green Algae):
  • Gram Negative and photosynthetic bacteria.
  • Some blue green algae live in symbiotic relationships with other organisms such as lichens.
  • The cell wall is two layered and possesses an outer sheath which is Jelly like, slimy and mucilaginous and the inner cell wall is made up of peptidoglycan.
  • The cell content is divided into regions; outer chromatoplasm (containing photosynthetic pigments) and inner colourless (centroplasm).
  • True nucleus is absent, and its chromosomes are bacterial chromosome.
  • The colour changes from green to deep purple and then blue green. These colours are produced by different proportions of several pigments like chlorophyll, carotenoids, xanthophyll (brown), phycocyanin (blue) and phycoerythrin (red).
  • Flagella are absent in vegetative phase as well as in the reproductive phase.
  • Reproduction is very simple and takes place by vegetative means and binary fission.
  • Gas vacuoles are formed in this.
  • Photosynthetic bacteria.
  • Like many bacteria, several forms of blue green algae have the capacity to fix atmospheric nitrogen into nitrogen compounds.
Examples: Nostoc, Anabaena fix nitrogen by some specialized cell structure called heterocyst.

Bacteria(Eubacteria and Heterotrophic bacteria):
  • They are called eubacteria and they are mostly single cell form colonies.
  • They possess a rigid cell wall and if motile they have flagella.
  • They also they have four basic shapes of cells namely:
    • Coccus (spherical)
    • Bacillus (rod shaped)
    • Spiral (spiral shaped)
    • Vibrio (comma shaped)
  • Bacteria are helpful in making curd from milk, production of antibiotics and fixing nitrogen in legume roots etc.
  • Some are pathogens and cause disease like Cholera, tetanus, typhoid etc. in humans and citrus canker in plants.
  • Bacteria reproduce mainly by fission but under unfavorable conditions, they reproduce by spores. They also reproduce sexually by adopting primitive types of DNA transfer from one bacterium to another.

A dividing bacterium

MYCOPLASMAS : [smaller than bacteria]
  • These are the organisms which completely lack cell wall & can survive without O2. Many mycoplasmas are pathogenic in animals and plants. For Example PPLO (Pleuropneumonia Like Organism).


(coloured protozoan, Dinoflagellates, Euglenoids, Slime moulds, chrysophytes)
  • Protists include solitary (alone) unicellular, colonial Eukaryotic organisms which doesn’t form tissues.
  • They possess nuclear membrane & mitochondria.
  • In many forms, plastids, flagella (9+2) and other organelles are present.
  • The mode of nutrition in these organisms include photosynthesis, absorption, ingestion & combination of these.
  • Unicellular coloured algal & diatoms are photosynthetic autotroph & protozoans are predatory or holozoic type.
  • Their reproduction cycles typically includes asexual division of haploids forms & true sexual process with karyogmy & meiosis.

CHRYSOPHYTES (diatoms, golden algae-desmids)
  • Diatoms are microscopic single- celled forms, which have cell wall containing silica constructed into two overlapping halves which fit together as in soap box.
  • Their cell is called a frustal or shell & they float passively in water current. (Planktons)
  • Most of them are photosynthetic.
  • Due to their silica, impregnation (composition) the wall of diatoms is indestructible as a result diatomaceous earth formation has occurred.
  • Found in fresh as well as marine water.

DINOFLAGELLATES :- (coloured organisms)
  • These organisms are mostly marine & photosynthetic.
  • They are single- celled & the cell wall has stiff cellulose plates on the outer surface.
  • Most of them have 2 flagella, one lies longitudinally and the other transversally in a furrow b/w the wall plates.
  • Occasionally members like Gonyaulax accumulate in large numbers in some parts of seas, colouring the water red & responsible for red tides.
  • Method of reproduction is only asexual.
  • Toxin released by these organisms may even kill over marine animals like fishes.

  • Euglena is unicellular & its relatives have both animals & plants characteristics.
  • Found in fresh & stagnant water.
  • Instead of cell wall they have a protein rich layer called ‘pellicle’ which makes their body flexible.
  • They have two flagella (short & long).
  • They are photosynthetic but in absence of sunlight they are heterotrophic.
  • Reproduction is generally asexual. Ex. Euglena, Astasia.

  • These are widely distributive organisms growing in damp & shady places. And they are saprophytic protists.
  • Under suitable conditions they form an aggregation called plasmodium which may grow & spread over several feet.
  • The plasmodium shows amoeboid movement by producing pseudopodia.
  • During unfavorable conditions the plasmodium differentiates & forms fruiting bodies bearing spores at their tips, which possess true cell wall.

  • All protozoans’ are heterotrophs & live as predators & parasites.
  • There are four major groups of protozoans:-
  1. Amoeboid Protozoans
    Ex. Amoeba
  2. Flagellated Protozoans
    Ex. Trypanosoma, Leishmania
  3. Ciliated Protozoans:
    Ex. Paramoecium (cavity known as gullet).
  4. Sporozoans:
    Ex. Plasmodium



  1. A fungi constitutes a unique kingdom of heterotrophic organisms.
  2. They become either parasites or saprophytes.
  3. Cell wall is made up of fungal cellulose.
  4. They can also live as symbionts in association with algae as Lichens & with roots of higher plants as mycorrhiza.
  5. Reproduction in fungi can take place by vegetative means- fragmentation, fission & budding.
  6. Asexual reproduction is by spores called ‘Conidia’, ‘Sporangiospores’ or ‘zoo spores’. Sexual reproduction is by ‘Oospores, Ascospores & basidiospores. The various spores are produced in a distinct structure called fruiting bodies.


The sexual reproduction involves following steps:-
  1. Fusion of protoplasm b/w two motile or non- motile gametes called plasmogamy.
  2. Fusion of two nuclei called karyogamy.
  3. Mitosis in zygote results in haploid (single set of chromosomes) spores.
    • In some fungi fusion of two haploid cells immediately results in diploid cell (2n).
    • However, in some other fungi (Ascomycetes & basidiomycetes). Dikaryotic stage occur in which two nuclei per cell is present; such a condition is called dikaryon & the phase is called dikaryophase of fungi. Later the parental nuclei fuse & cell become diploid.
    • The fungi forms fruiting body in which reduction division occurs leading to formation of haploid spores.


The morphology of the mycelium, mode of spores formation & fruiting bodies form the basis for the division of kingdom into various classes:-
  1. Phycomycetes (Algal fungi)
  2. Ascomycetes (Sac fungi)
  3. Basidiomycetes (Puffballs/Bracket fungi)
  4. Deuteromycetes (Incomplete fungi)

PHYCOMYCETES (algal fungi):-
  1. These are also called Algal fungi because of aquatic habitat & form of thallus. They are found on decaying wood and in moist, damp places or as obligate parasite on plants.
  2. The mycelium is aseptate or Coenocytic.
  3. Asexual reproduction takes place by motile zoospores or by non- motile aplanospores.
  4. Example- Mucor, Rhizopus, Albugo

  1. Unicellular (yeast) or multicellular (penicillium)
  2. They are saprophytic, decomposer, parasitic or coprophilous (growing on dung)
  3. Mycelium is branched & septate.
  4. Reproduction occurred asexually by budding (yeast) or by spores called conidia.
  5. Sexual reproduction occurs by spores called ascospores which are present in ‘sac’ like asci which arrange in d/f types of fruiting bodies called Ascocarp. Ex. Aspergillus, Claviceps, Neurospora.

BASIDIOMYCETES(puff balls/ bracket fungi)
  1. Commonly known basidiomycetes are mushrooms. They grow in soils or logs of trees & in living plant bodies as parasites. For example- Rust & smuts.
  2. The mycelium is branched & septate.
  3. The asexual spores generally not formed but vegetative reproduction by fragmentation is common. They reproduce sexually by basidiospores.
  4. The sexual process is represented by plasmogamy & karyogamy. Example- Agaricus (mushroom), Puccinia (Rust fungus).

DEUTEROMYCETES(imperfect fungi)
  1. In these only the asexual vegetation phases are known & they are reproduced by asexual spores known as ‘conidia’.
  2. Mycelium is septate & branched.
  3. Mode of nutrition is parasitic or saprophytic.
  4. They help in mineral cycling.
  5. They have members belonging to both Ascomycetes & basidiomycetes.
  6. Sexual reproduction is absent. Ex. Trichoderma, Alternaria

Fungi: (a) Mucor (b) Aspergillus (c) Agaricus


  • Lichens are a symbiotic association that is a mutually useful association b/w blue green algae & fungus.
  • The algal component is known as phycobiont & the fungal component is known as mycobiont which are autotrophic & heterotrophic respectively.
  • An alga prepares food for fungi & fungi provide shelter & absorb mineral nutrients & water for its panther.
  • Lichens are a very good pollution indicator. They don’t grow in polluted areas.


It is a root- fungus association.
The mycorrhizal roots are usually covered with fungal woolly out growth. For ex- Pinus. The association may be of 2 types:-
  1. Ectotrophic:- When the hyphae do not penetrate deep into the root & remains superficial.
  2. Endotrophic:- When Hyphae grow within the root surface.


  1. The viruses are non- cellular organisms that are characterized by having an inert (inactive) crystalline structure outside the living cell.
  2. The name virus that means Venom (poisonous fluid) was given by Pasture D.I. Ivanowsky (1892), recognized certain microbes as causative organisms of the mosaic disease of tobacco.
  3. These were found to be smaller than bacteria because they pass through bacteria proof filters.
  4. They are inert outside their specific host cell. Viruses are obligate (dependent) parasites.
  5. In addition to proteins, viruses also contain genetic material that could be either DNA or RNA. No viruses contain both DNA & RNA.
  6. A virus is a nucleoprotein & the genetic material is infectious.
  7. In general, viruses that infect plants have single- stranded RNA & viruses that infect animals have either single- stranded or double- stranded DNA.
  8. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.
Viruses cause diseases like Mumps, small- pox, influenza, AIDs etc.
In plants the symptoms include mosaic formation, leaf rolling & curling, yellowing, drawfing & stunted growth.

(a) Tobacco Mosaic Virus (TMV) (b) Bacteriophage

  • It is a type of retro- virus (RNA virus) linked with Acquired ImmunoDeficiency Syndrome (AIDS) & the virus is known as Human ImmunoDeficiency Virus (HIV).
  • The protein core encloses a central core of viral RNA.
  • There are 2 molecules of RNA associated with the molecule of enzyme Reverse- transcriptase.
  • The virus attacks T- helper cells of the host which have an important role in regulating many processes of the immune system.
The AIDs virus could be transmitted by sexual contact, contaminated blood or blood products sharing, infected mother to the child before, during or shortly on birth.


In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses & cause potato spindle tuber disease. It was found to be a free RNA. It lacked a protein coat that is found in viruses; hence the name viroids. The DNA of the viroids was of low molecular weight.

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