Biological classification-Five Kingdom System

Biological Classification:

First of all, Aristotle attempts a scientific basis for classification. Aristotle used simple morphological characters to classify plants into trees, shrubs, and herbs. He also divided animals on the basis of red blood (enaima) and one which did not have red blood (anaima).

Two kingdom classification

Two kingdoms classification was introduced by Carolus Linnaeus in the 20th century. Linnaeus introduced the Two Kingdom classification system as Plantae and Animalia, it just divides plants, and animals and does not distinguish between eukaryotes, prokaryotes, unicellular, or multicellular. The two-kingdom system was inadequate as many organisms did not fit into either category.

Three Kingdom System

Three Kingdom classification was given by Ernst Haeckel- Protista, Animals, and Plants.

Four Kingdom System

The system of the Four Kingdom Classification was given by Copeland in 1956, the system was based on Monera, Protista, Animals, and Plants.

Five kingdom classification

The five-kingdom classification was introduced In 1969 by R.H. Whittaker. The kingdoms were as follows: Monera, Protista, Fungi, Plantae, and Animalia. 

Six Kingdom Classification

The most recent system, given by Carl Woese 1990– Archebacteria, Eubacteria, Protista, Fungi, Animals, and Plants.

Five kingdom classification

In 1969 R.H. Whittaker proposed a Five Kingdom Classification. These kingdoms defined by him were named Monera, Protista, Fungi, Plantae, and Animalia.

The main criteria for classification used by R.H. Whittakar were: cell structure, body organization, mode of nutrition, reproduction, and phylogenetic relationships.

Kingdom Monera

The main members of the Kingdom Monera are bacteria.

  • Since only bacteria are prokaryotic and lack a true nucleus, i.e. without a nuclear membrane, Monera is the only prokaryotic kingdom.
  • Bacteria were the first cellular organisms to evolve on the planet Earth after life originated around 3.5 billion years ago and were the only cellular organisms on Earth for almost the next two billion years.
  • Monerans are also the most numerous organisms than other cellular organisms.
  • Monerans occur almost everywhere as they can survive in the worst conditions.

Types of bacteria on the basis of shape

Bacteria are grouped under four categories based on their shape:

  1. the spherical Coccus (plural: cocci),
  2. the rod-shaped Bacillus (plural: bacilli),
  3. the comma-shaped Vibrium (plural: vibrio) and
  4. the spiral Spirillum (plural: spirilla)
classification of bacteria- Biological classification-Five Kingdom

Classification of bacteria on the basis of shapes

Structure of a bacterial cell

The structure of bacteria is very simple. The single-celled bacteria contain the following:

  • The cell wall is made up of a chemical, peptidoglycan, lipids, polysaccharides, and some proteins.
  • Pili – These are short and thin thread-like tubular structures projecting out from the cell wall found in some bacteria.
  • Some bacteria move with the help of one or more flagella.
  • Prokaryotes have no nuclear membrane around genetic material and no membrane-bound cell organelles except mesosomes. They have only the ’70s ribosomes


Shape of bacterial cell

Fig showing: typical bacterial cell

Mode of Nutrition in Monera:

  • Autotrophic mode – synthesize their own organic food.
  • Saprotrophic mode – These feed on dead organic matter.
  • Symbionts –use food from other living organisms with which they are associated for mutual benefit.
  • Parasites –This type of nutrition absorbs food from living organisms and causes harm to them.


The Archaebacteria are very special, 16S RNA is present in them which is unique, and can survive in any harsh situation such as extremely salty areas (halophiles), can live in hot springs (thermoacidophiles), and marshy areas (methanogens).

  • Archaebacteria are different from other bacteria as they consist of a different cell wall structure and this feature is responsible for their survival in the worst and most extreme conditions.
  • Archaebacteria are also known as living fossils.
    • Methanogens– are found in the gut of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals. These are Obligate anaerobic. eg. methanobacterium, methanococcus.
    • Halophiles– Anaerobic bacteria, found in strong Salty conditions. eg. Halobacterium (it also forms ATP from solar energy).
    • Thermoacidophiles– Aerobic bacteria, Found in acidic sulfur springs. eg-Sulfolbus, Thermoproteus. (They oxidize Sulphur to sulphuric acid.
    • Importance of Archaebacteria-
  • They are mostly used in biotechnology, the Generation of biogas, Thermophilic enzymes, Biosensors, etc.


Thousands of different eubacteria or ‘true bacteria’ are found. Their characteristic feature is a rigid cell wall, and if they are motile, consist of a flagellum.


The respiration in bacteria is of the following types:

  1. Obligate Aerobes- bacillus subtitles
  2. Facultative anaerobes- Clostridium tetani
  3. Obligate anaerobes- Clostridium botulinum
  4. Facultative aerobes- Rhodopseudomonas

Reproduction in bacteria:

  • Asexual method -By Fission, budding, and spore formation.
  • Sexual method- Transformation, conjugation, Transduction.
Biological classification-Five Kingdom

Binary fission in bacteria

  • The cyanobacteria (also called blue-green algae) have ‘chlorophyll a’ similar to green plants and these are photosynthetic autotrophs.
  • cyanobacteria are unicellular, colonial, filamentous, freshwater/marine, or terrestrial algae.
  • They form blooms in polluted water bodies.
  • Bacteria like Nostoc and Anabaena can fix atmospheric nitrogen in specialized cells called heterocysts.
  • Some are chemosynthetic autotrophic bacteria that oxidize various inorganic substances like nitrates, nitrites, and ammonia and use the released energy for ATP production.
  • Heterotrophic bacteria are most abundant in nature, as decomposers.
  • They play an important role in human life by helping in making curd from milk (lactobacillus bacteria), production of antibiotics (Streptomyces), and fixing nitrogen in legume roots (Rhizobia).
  • Some are pathogens, which cause damage to human beings, crops, farm animals, and pets.
  • Cholera, typhoid, tetanus, and citrus canker are well-known diseases caused by different bacteria.
  • Mycoplasma is an organism in which the cell wall is completely absent. This is the smallest living cell that can survive without oxygen.       


The Protists are unicellular eukaryotes chrysophytes (unicellular algae) diatoms and Protozoa are included in it. The term ‘Protista’ was Given by German biologist Ernst Haeckel in 1866.

  • These are primarily aquatic.
  • The Protistan cell body consists of a well-defined nucleus and other membrane-bound organelles.
  • Some protist consists of flagella or cilia for movement.
  • Reproduction by both processes- asexually and sexually by a process involving cell fusion and zygote formation.

Chrysophytes(unicellular algae)

In this group, diatoms and golden algae (desmids) are placed.

  • They are aquatic and found in freshwater as well as in marine environments.
  • These are microscopic organisms and float passively in water currents (plankton). Mostly these are photosynthetic.
  • In Diatoms shell is formed by the cell wall, embedded with silica. Diatom’s cell walls cannot be destroyed and they accumulated over the years. So ‘diatomaceous earth’ is termed as after billions of years a large amount of cell walls will be deposited.
  • They are good indicators of pollution.
  • Diatoms are referred to as chief ‘producers’ in the oceans.


These are mostly marine and photosynthetic, found in various colors because of the pigment they contain. They may appear yellow, green, brown, blue, or red in color.

  • The cell was made up of cellulose.
  • Mostly they consist of two flagella; one lies longitudinally and the other transversely in between the wall plates
  • Some undergo rapid multiplication, for example- red dinoflagellates (Example: Gonyaulax) undergo rapid multiplication and it makes these appear red (red tides). These algae release toxins which are a disaster for other marine organisms, even they can kill other marine animals like fish.


These are freshwater organisms found in stagnant water. They consist of a protein-rich layer called pellicle which makes their body flexible.

  • They consist of two flagella, one short and another long.
  • They are autotropic as well as heterotrophic, i.e. in the presence of sunlight they are photosynthetic and in the absence of sunlight, they behave like heterotrophs and predate on other smaller organisms.
  • They contain pigment as Chlorophyll. 
  • Euglena is regarded as a connecting link between animals and plants.
  • Example: Euglena, Phacus, paranema.


Slime Moulds

These are saprophytic protists. Grows to survive on dead and decaying organic materials.

They can live as a single cell as well in colonies. Their aggregation is called Plasmodium. Differentiation of plasmodium in unfavorable conditions. They can survive in any condition as it is extremely resistant and can survive for many years. They are of two types- acellular or plasmodial (Fuligo and physarum) and cellular (Dictyostelium and polysphondylium).


They are heterotrophs and live as predators or parasites on other organisms. These are called primitive relatives of animals. Four major groups of protozoans-

  • Amoeboid protozoans: These organisms can live in freshwater, seawater, or moist soil. consisting of pseudopodia (false feet) for movement and predation. Ex-Amoeba.

The marine protozoans have silica shells on their surface.

  • Flagellated protozoans: These may be free-living or parasites. Flagella is found in them.
    •  Diseases caused by parasitic forms such as sleeping sickness. Example: Trypanosoma.
  • Ciliated protozoans: These are aquatic organisms. Active movement is found in organisms because of the presence of thousands of cilia.
  • The body consists of a cavity (gullet) that opens outside the cell surface.
  • Coordination in the movement of rows of cilia causes the water laden with food to be steered into the gullet. Example: Paramoecium
  • Sporozoans: These organisms have an infectious spore-like stage in their life cycle.
  • Plasmodium- a malarial parasite, causes malaria, a disease that has a hazardous effect on the human population.


Fungi are eukaryotic organism which is either unicellular or multicellular saprotrophs having filaments that grow through the soil, wood, and other substrates.

  • This is a unique kingdom of heterotrophic organisms. Fungi exist as slender thread-like filaments called hyphae.
  • Hypha may be one-celled or multicelled and has one or more nuclei. Yeast, however, is single-celled, and uninucleate.
  • Their cell walls are made of chitin and polysaccharides. A group of hyphae forming a network is called mycelium (mycetos meaning fungus).
  • Aquatic fungi have flagellate gametes or flagellate spores
  • Higher fungi do not have flagellum at any stage of the life cycle.
  • Reproduction in fungi is both asexual by means of flagellate or non-flagellate spores and sexual through conjugation

Classification of fungi

Fungi are divided into various classes on the basis of morphology.

  • Phycomycetes
  • Ascomycetes
  • Basidiomycetes
  • Deuteromycetes

Mucor- five kingdom classification


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

  • The mycelium (consists of a mass of branching, thread-like hyphae) is aseptate and coenocytic( multinucleate cell ).
  • Asexual reproduction is through zoospores (motile) or by aplanospores (non-motile). A zygospore is formed by the fusion of two gametes.
  • These gametes may be similar in morphology (isogamous) or dissimilar (anisogamous or oogamous).
  • examples are Mucor, Rhizopus(the bread mould mentioned earlier), and Albugo (the parasitic fungi on mustard).

Ascomycetes- also called sac-fungi.

  • They are mostly multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast (Saccharomyces).
  • They may be saprophytic, decomposers, parasitic or coprophilous (growing on dung).
  • In ascomycetes, the mycelium is found branched and septate.
  • The asexual spores called conidia are produced exogenously on the special mycelium called conidiophores.
  • Conidia produces mycelium during germination.
  • The Ascospores are sexual spores produced endogenously in sac-like asci (singular ascus). These asci get arranged in different types called ascocarps.
  • examples- Aspergillus, Claviceps, and Neurospora.
  • Neurospora is used extensively in biochemical and genetic work. The members like morels and truffles are edible and are considered delicacies.


mushrooms, bracket fungi, or puffballs are commonly known as Basidiomycetes.

Their habitat is soil, on logs and tree stumps, and in living plant bodies as parasites, e.g., rusts and smuts.

  • Branched and septate mycelium are found.
  • Asexual spores are generally not seen, otherwise, vegetative reproduction is commonly found by fragmentation.
  • In basidiomycetes sex organs are absent. The fusion of two vegetative or somatic cells of different strains or genotypes(Plasmogamy), by this dikaryotic (dikaryotic* characterized by the presence of two nuclei in each cell) structure, is formed as a basidium.
  • In basidium, Karyogamy and meiosis take place as a result of four basidiospores produced.
  • The basidia (sing. basidium) are arranged in fruiting bodies called basidiocarps.
  • Ex. Agaricus (mushroom) (Figure 2.5c), Ustilago (smut), and Puccinia (rust
  • fungus)

Deuteromycetes- (also called imperfect fungi)

The Deuteomycetes are commonly called imperfect fungi because only asexual or vegetative phases are known of these fungi.

  • In Deuteromycetes, only asexual reproduction is found through spores known as conidia.
  • The mycelium is septate and branched.
  • Some members of this group are saprophytes or parasites while a large number of them are decomposers and help in mineral cycling.
  • Examples are Alternaria, Colletotrichum, and Trichoderma.


  • Kingdom Plantae includes all eukaryotic organisms containing chlorophyll commonly called plants. Some of them are partially heterotrophic like insectivorous plants or parasites.
  • For example, Bladderwort and Venus flytrap are insectivorous plants. Cuscuta is a parasitic plant.
  • The plant cells are eukaryotic in a structure having chloroplast. The cell wall is made up of cellulose.
  • (Please go through the blog Structure and Function of the Cell for brief knowledge of eukaryotic cells)
  • The kingdom Plantae is a major kingdom including algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
  • Plants have two distinct phases in their life cycle- the diploid sporophytic and the haploid gametophytic –which are alternatives to each other.
  • Alternation of generation is the phenomenon of the cycle of the sexual stage and asexual stage.


    • The members of Kingdom Animalia are heterotrophic eukaryotic organisms that are multicellular.
    • In these organisms cell wall is absent.
    • For food, They directly or indirectly depend on plants.
    • They are having an organ system for digesting food consisting of cavities and storing food reserves such as glycogen or fat.
    • These organisms’ mode of nutrition is holozoic – by ingestion of food. (For the different modes of nutrition please visit the given site   Life Processes A definite growth pattern is followed by them and grow into adults that have a definite shape and size.
    • Higher forms of these organisms show elaborate sensory and neuromotor mechanisms.
    • Mostly these are capable of locomotion.
    • Reproduction is sexual by the population of males and females followed by embryological development. 


    In the five-kingdom system of classification of Whittaker, there is not any information about lichens and some acellular organisms like viruses, viroids, and prions, but these also affect lives and show their existence.


    • The name virus which means venom or poisonous fluid was given by Dmitri Ivanowsky (1892).
    • He recognized certain microbes as organisms causing the mosaic disease of tobacco.
    • These were smaller than bacteria because they passed through bacteria-proof filters.
    • M.W. Beijerinek (1898) demonstrated that the infected plant of tobacco extract can infect a healthy plant. So he called the fluid Contagium vivum fluidumi.e. infectious living fluid.
    • W.M. Stanley (1935) showed these viruses could be crystallized and crystals consist of proteins.
    • StructureViruses have a simple structure consisting of a core and a cover. The core particle is the genetic material, either DNA or RNA. The cover is a protein coat called a capsid
    • They do not show living characters outside their specific host cell. They don’t consist of their own enzyme. Viruses are obligate parasites. Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.
    • Viruses contain proteins and genetic material, that could be either RNA or DNA. The genetic material is infectious.
    • Generally, the viruses that infect plants have single-stranded RNA.
    • Viruses that infect animals have either single or double-stranded RNA or double-stranded DNA
    • Viruses cause diseases like mumps, smallpox, herpes, and influenza. AIDS in humans is also caused by the virus HIV.
    • In plants, mosaic formation, leaf rolling and curling, yellowing, and vein clearing, dwarfing, and stunted growth can be caused by viruses.
    Structure of Bacteriophage


    • In 1971, T.O. Diener discovered an infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was without protein Coat (found in viruses) hence termed viroid.
    • Viroids are circular RNA molecules, consisting of several hundred nucleotides.
    • Plants get infected by them. In plants, they use the enzymes of the plant cells to replicate as the viruses do. RNA molecules are of low molecular weight.


    • It consists of abnormally folded protein and is similar in size to viruses.
    • It causes infectious neurological diseases.
    • The most common diseases caused by prions are bovine spongiform encephalopathy (BSE) commonly called mad cow disease in cattle and its analogous variant Cr–Jacob disease (CJD) in humans.


    • These are symbiotic associations i.e. mutually useful associations, between algae and fungi.
    • The algal component in lichens is known as phycobiont which is autotrophic.
    • The fungal component is heterotrophic called mycobiont.
    • Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for algae.
    • Lichens are excellent pollution indicators and they do not grow in polluted areas.

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    sushmabio · October 30, 2020 at 9:44 pm

    For details of bacterial cell structure, you can visit prokaryotic cell

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