Biological Classification

Biological Classification is a process of grouping all the living organisms on earth into different categories based on some easily observable characters, such as their morphological, physiological and reproductive similarities.

  • All these ‘Dogs’, ‘Cats’, ‘Mammals’, ‘Wheat’, ‘Rice’, ‘Birds’, ‘Animals’, etc., are convenient categories we use to study organisms.
  • The scientific term for these categories is taxa (sing. texon).

In biological classification, we mainly study the characteristics of certain groups/categories/taxa of organisms. This helps us to identify and distinguish different organisms.

A taxon can indicate any category or group of categories (texa) at different levels. i.e., the ‘Plants’ category is also a taxon. ‘Wheat’ is also a taxon. Similarly, ‘animals’, ‘mammals’, ‘dogs’ categories are also taxa.

Therefore, animals, plants, mammals, and dogs, … represent taxa at different levels.

Taxonomy

Hence, based on characteristics, all living organisms can be classified into different taxa. This process of classification is taxonomy.

Need for Biological Classification

Biological classification of organisms:

  • Helps in shorting out the diverse organisms
  • Tells which organisms resemble each other
  • Helps in identifying and distinguishing different organism’s species
  • Helps in studying the origin and evolution of organisms
  • Makes the study of organisms convenient

Five (5) Kingdom Classification

R.H. Whittaker (1969) proposed a Five Kingdom Classification. You can think of these Kingdoms as the largest categories for grouping all the living organisms on earth into five groups. The kingdoms defined by him were named as:

  1. Monera
  2. Protista
  3. Fungi
  4. Plantae
  5. Animalia
Biological Classification System Five Kingdom
Five Kingdom Classification of Living Organisms

As mentioned above all the organisms on earth are categorized into five Kingdoms based on the organism’s certain characters like cell structure, body organisation, mode of nutrition, reproduction and evolutionary relationships (in the past, if that species shared common ancestors).

1. Kingdom Monera

All the organisms on earth that are unicellular and have prokaryotic cell structures are placed in the Kingdom Monera.

  • Prokaryotic organisms are single-celled, microscopic organisms that lack a membrane-enclosed nucleus and membrane-bound cell organelles.
  • Prokaryotic cells are single-celled microorganisms known to be the earliest and oldest organisms on earth.
  • Bacteria are the sole members of the Kingdom Monera. Bacteria are the most abundant micro-organisms, they occur almost everywhere on earth.
  • Bacteria can live in extreme habitats such as hot springs, deserts, snow and deep oceans where very few other life forms can survive.

Examples of Bacteria

  1. Salmonella typhi
  2. Lactobacillus acidophilus
  3. Neisseria gonorrhoeae
  4. Haemophilus influenzae
  5. Clostridium tetani
  6. Bacillus thuringiensis
  7. Helicobacter pylori

2. Kingdom Protista

Protists are all the organisms on earth that have eukaryotic cell structures except animals, plants, or fungi are placed in the Kingdom Protista.

  • Eukaryotes are any cell or organism that has a clearly defined nucleus in cells. And the nucleus is surrounded by a membrane, and also other cell organelles are surrounded by membranes.
  • Some protists have flagella or cilia for locomotion.
  • Kingdom Protista includes the following group of organisms
    • Protozoans such as Amoeba, Paramecium, Plasmodium, Trypanosoma. They may be aquatic, terrestrial, free-living or parasitic.
    • Euglenoids such as Euglena, Phacus, Eutreptia. They are mixotrophic i.e., photosynthetic in sunlight and heterotrophic in absence of sunlight.
    • Slime molds such as Physarum, Dictydium, Dictyostelium. They are saprophytic widely distributed in damp shady places.
    • Chrysophytes such as diatoms and golden algae (desmids). Most of Them are photosynthetic.
    • Dinoflagellates such as Pyrodinium, Gonyaulax, Pyrocystis. Mostly photosynthetic, and marine, some shows bioluminescence (they emit light).

Example of organisms from Kingdom Protista

  1. Entamoeba Histolytica (Protozoa)
  2. Paramecium Aurelia (Paramecium)
  3. Euglena gracilis (Euglena)
  4. Gonyaulax catenella (Dinoflagellates)
  5. Noctiluca scintillans (Dinoflagellates)
  6. Prymnesium parvum [Golden algae (Chrysophytes)]
  7. Vaucheria sessilis [Yellow-green algae (Chrysophytes)]

3. Kingdom Fungi

Kingdom fungi include all organisms on earth that are eukaryotic, achlorophyllous, and their cell wall is made up of chitin, which include microorganisms such as yeasts and moulds and the more familiar mushrooms.

A characteristic that places fungi in a different kingdom is that the cell wall of all fungus is made up of chitin, also called fungal cellulose.

  • The cells of most fungi grow as tubular, elongated, and thread-like (filamentous) structures called hyphae. The network of hyphae is known as mycelium.
  • There are also single-celled fungi called yeasts that do not form hyphae.
  • Fungi reproduce by three methods:
    • Vegetative reproduction (by fragmentation, budding, fission, etc.)
    • Asexual reproduction (by Zoospores, Conidia, Sporangiospores etc.)
    • Sexual reproduction (by Oospores, ascospores, Basidiospores.)
  • Mycorrhiza: Some fungus can also live as symbionts in association with algae that are known as Lichens.

Examples of organisms from Kingdom Fungi

  1. Ascomycota
    • Candida albicans (yeast that causes candidiasis)
    • Saccharomyces cerevisiae (yeast used in baking bread)
    • Penicillium (penicillin and various cheeses)
  2. Basidiomycota
    • Agaricus bisporus (edible mushrooms)
    • Lycoperdon perlatum (puffball mushrooms)
  3. Zygomycota
    • Rhizopus stolonifer (black bread)
  4. Chytridiomycota
    • Polyphagous euglena (algae parasite)
  5. Glomeromycota
    • Gigaspora margarita (a fungus that lives on plant roots without destroying the plant)

4. Kingdom Plantae

Kingdom Plantae includes all the organisms on earth that are eukaryotic, chlorophyllous, photosynthetic, and multicellular. These organisms are commonly called plants.

  • A few members are partially heterotrophic such as the insectivorous plants or parasites.
    • Bladderwort and Venus flytrap are examples of insectivorous plants and
    • Cuscuta is a parasite (have no roots, grows on other plants).
  • The plant cells have a eukaryotic structure with prominent chloroplasts and cell wall mainly made of cellulose.
  • The life cycle consists of a dominant sporophyte and a highly reduced gametophyte showing alternation of generation.
  • Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
  • They are very important for the existence of organisms of other kingdoms as they produce food and oxygen, which are crucial for living organisms.

5. Kingdom Animalia

Kingdom Animalia includes all the organisms on earth that are heterotrophic, have eukaryotic cell structure, are multicellular, and cells lack cell walls.

  • They directly or indirectly depend on plants for food. Their mode of nutrition is holozoic (by ingestion of food).
  • They digest their food in an internal cavity and store food reserves as glycogen or fat.
  • They follow a definite growth pattern and grow into adults that have a definite shape and size.
  • Higher forms show elaborate sensory and neuromotor mechanism.
  • Most of them are capable of locomotion.
  • Sexual reproduction is by the copulation of male and female followed by embryological development.