Organisms and their Classification

Organisms can be defined as a living thing, a unique, complex organization exhibiting the following significant characteristics:

  • Has cellular structure: Each living organism is a complex entity made up of protoplasm called living matter; contains biomolecules such as protein, nucleic acid, lipids, carbohydrates, etc.
  • Can react to stimuli: Organisms can sense the external factors and respond accordingly.
  • Can grow: Organisms can increase the mass of an individual.
  • Reproduce: Organisms can form new individuals similar to the pre-existing life.
  • Adapt to the environment: Useful heritable changes in function, and behavior can help the organism to adapt to changing environmental conditions. Organisms also undergo sudden genetic changes called mutation, mostly favoring them to adjust to the new environment. Unwanted mutation can also occur sometimes.
Organisms and their Classification
Organisms and their Classification

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Diversity in the living world

A wide range of living forms exist in the environment. Organisms can be of different types such as animals, fungi, plants, protists, and bacteria.

These organisms can be classified in various ways based on their distinguishing characteristics

They are broadly classified on the following basis:

Complexity of cell structure: prokaryotic and eukaryotic organisms

Prokaryotes: They are single-celled microorganisms, the most primitive on earth. They lack true nuclei and other membrane-bounded cell organelles and contain 70s ribosomes in their cell. E.g. Bacteria, cyanobacteria, rickettsia, etc.

Read Also: Prokaryotes vs. Eukaryotes (47 Key Differences)

Eukaryotes: They may be unicellular or multicellular having nucleoplasm enclosed in a nuclear membrane (true nucleus) along with the presence of membrane-bound cell organelles. They contain both 70S ribosomes (in mitochondria and plastid) and 80s ribosomes in their cells. All the plant and animal cells are eukaryotic cells.

Complexity of body structure: unicellular or multicellular organisms

Unicellular organisms: These organisms are made up of a single cell. They are usually microscopic, too small to be observed with the naked eye. They usually exhibit a simpler structure and a single cell is responsible for carrying out all the metabolic functions in the body. The injury to the cell can lead to the death of the organism. These reproduce asexually by binary fission, budding, etc. E.g. Amoeba, Paramecium

Multicellular organisms: These types of organisms are made up of more than one cell ranging from two to a million cells forming a single body. The structural organization is complex and different cells are responsible for carrying out different functions in the body. They comprise tissue, organs, and organ systems to carry out various metabolic and regulatory functions in the body. The death of a few cells does not always lead to the death of the organisms. They can reproduce both sexually and asexually. Asexual reproduction takes place through fragmentation, spore formation, etc. whereas sexual reproduction takes place through fusion of gametes. E.g. Fish, Lizard, Moss, Mustard, Animals

Mode of nutrition: autotrophic and heterotrophic organisms

Autotrophic organisms: These organisms are capable of synthesizing organic compounds solely from inorganic carbon. It includes green plants and blue-green algae.

Heterotrophic organisms: These organisms are not able to synthesize organic compounds on their own. Hence, they are dependent on autotrophic organisms for the source of carbon and ATP. It includes bacteria and animals.

Heterotrophic organisms can be further divided as

Saprophytic organisms: They feed on dead and decaying matter by absorption. Their enzymatic secretion. It includes Fungi and yeast.

Parasitic organisms: They obtain nutrients from the host organisms. They are found residing in the host organisms as both ectoparasite (living outside the body) or endoparasite (living inside the body) and drive nutrients and shelter form them. It includes Cuscuta, Plasmodium, and Ascehelminthes.

Holozoic organisms: These organisms feed by ingesting solid organic matter which is then further digested with the aid of digestive enzymes It includes animals such as Man, and Dog and unicellular organisms such as Amoeba, and Euglena.

Five Kingdom Classification

Among different systems of classification, the Five Kingdom classification proposed by R.H. Whittaker (1969) is a widely accepted classification system.

Five Kingdom Classification
Five Kingdom Classification

It classifies organisms based on:

  • Cellular structure
  • Body organisation
  • Mode of Nutrition
  • Reproduction
  • Ecological role
  • Phylogenetic relationships

The five kingdoms include:

Monera: unicellular, prokaryotic organisms, important forms bacteria (eubacteria), archaebacteria, cyanobacteria (blue-green algae), actinomycetes

Protista: unicellular, or colonial, eukaryotic, mode of nutrition- photosynthetic or by ingestion, important forms- unicellular algae, dinoflagellates, diatoms, slime molds, protozoans

Fungi: multicellular, eukaryotic, by absorption of food, i.e. heterotrophic (saprophytic or parasitic), important forms: Phycomycetes, ascomycetes, basidiomycetes

Plantae: multicellular, eukaryotic, photosynthetic, includes all multicellular algae

Animalia: multicellular, eukaryotic, lack cell wall, holozoic nutrition, includes invertebrates and vertebrates (metazoan)

Kingdom Monera

  • Smallest organisms are present in this kingdom.
  • Unicellular and colonial
  • Most abundant organisms. Found thriving in extreme environmental conditions such as high temperature, pH, hot springs, snow, etc. It mainly includes Archaebacteria.
  • They are microscopic lacking true nucleus and membrane-bounded cell organelles.
  • They exhibit both modes of nutrition: autotrophic (phototrophs or chemotrophs) and heterotrophic (free-living, symbiotic, parasitic).
  • The mode of reproduction is primarily through binary fission or budding.
  • They are mostly decomposers; cyanobacteria are producers.

Kingdom Monera includes three major divisions:


  • Thrive in harsh environmental conditions such as  salty areas(halophiles), hot springs (thermoacidophiles), marshy areas(methanogens).
  • Differ from other bacteria in having a false peptidoglycan layer.
  • Eg. Methanobacterium, Methanococcus.


  • Contains chlorophyll similar to green plants; are photosynthetic autotrophs. Some can fix atmospheric nitrogen in structures known as heterocysts. E.g. Nostoc, Anaebena.

Eubacteria (true bacteria):

  • consists of a peptidoglycan layer in the cell wall, both beneficial and pathogenic E.g. E. coli, Salmonella, Lactobacillus.
  • Note: Mycoplasma are organisms lacking cell walls and are the smallest living cells

Kingdom Protista

  • This Kingdom includes single-celled (unicellular) eukaryotes.
  • It includes diverse forms of organisms ranging from minute flagellated cells to macroscopic kelp.
  • All the members of protists are aquatic organisms. Mostly plankton.
  • They exhibit diverse modes of nutrition i.e. photosynthetic, saprophytic, parasitic, and holozoic.
  • These organisms are frequently motile with 9+2 cilia and flagella.
  • They reproduce by both sexual and asexual methods with cell fusion and zygote formation without the formation of the embryo.
  • The cell contains a well-defined nucleus and membrane-bound cell organelles.

Protists include:


  • It includes diatoms and golden algae (desmids).
  • They are found floating passively in water currents (plankton). They are mostly photosynthetic.
  • In diatoms, cell walls form two thin overlapping shells fitting together as in a soap box.
  • Diatoms comprise chief contributors to the ocean.


  • Mostly marine and photosynthetic.
  • Appear red, yellow, and green based on the pigments present. They appear yellow, green, brown, blue, or red depending on the pigments present. 
  • The cell wall contains stiff cellulose plates on the outer surface. 
  • Red dinoflagellates (Gonyaulax) make the sea appear red due to their rapid multiplication (red tides). Their toxins may kill other marine forms.


  • Freshwater organisms are found in stagnant water.
  • They have pellicles, a protein-rich layer instead of cell walls providing them flexibility.
  • Are photosynthetic in the presence of sunlight but can act as heterotrophs in light deprivation.
  • E.g. Euglena.

Slime molds:

  • They are saprophytic protists.
  • Their fruiting bodies bearing spores provide them with resistance to extreme environments.
  • Fungi-like feature: forms fruiting bodies.
  • Plant-like features: Cell wall around spores.
  • Animal-like features: Plasmodium without cell wall.


  • It includes heterotrophs acting as predators or parasites.

Four major groups: 

  • Amoeboid protozoans: Locomotion through pseudopodia. Eg. Entamoeba
  • Flagellated protozoans: Locomotion through flagella. Eg. Trypanosoma
  • Ciliated protozoans: Locomotion through cilia. E.g. Paramecium
  • Sporozoans: Locomotion through pseudopodia. E.g. Plasmodium

Kingdom Fungi

  • Eukaryotic and heterotrophic organisms, mostly observed on moist bread and rotten fruits. They absorb soluble organic matter from dead substrates, hence called saprophytes.
  • They are saprophytic in nutrition.
  • The cell wall consists of chitin. 
  • Fungi include yeast (widely used to make beer and bread), mushrooms, rusts, and smuts. 
  • Some are sources of antibiotics. E.g. Penicillium.
  • They consist of long, slender thread-like structures known as hyphae. The network of hyphae is called mycelium.
  • Reproduction takes place by fragmentation, fission, budding, and spore formation. In spore formation, asexual reproduction takes place through the formation of conidiospores, sporangiospores, arthrospores, and chlamydospores, whereas sexual reproduction takes place through oospores, ascospores, zygospores, and basidiospores.

Based on the morphology of the mycelium, the mode of spore formation, and fruiting bodies form the basis for the division of the kingdom into various classes.


  • They are found in aquatic habitats; also on decaying wood, moist or damp places.
  • Mycelium: aseptate and coenocytic.
  • Reproduction: Asexual- zoospores (motile) or by aplanospores (non-motile)
  • Sexual: By zygospores by fusion of two gametes. Isoagamous( if similar in morphology), anisogamous( if dissimilar in morphology).
  • E.g. Mucor, Rhizopus (bread mold)

Ascomycetes (Sac-fungi)

  • Mostly multicellular (Penicillium are unicellular)
  • They are saprophytic feeding on dead and decaying material. Some are coprophilous (growing on dung).
  • Asexual spores: Conidia produced exogenously on mycelium called conidiospores 
  • Sexual spores: Ascospores produced endogenously in ascus
  • E.g. Aspergillus, Claviceps, Neurospora.


  • Commonly called mushrooms, bracket fungi, or puffballs.
  • They are found growing on tree stumps, and logs.
  • Mycelium is branched and septate.
  • Asexual spores: Not common but vegetative reproduction occurs by fragmentation. 
  • The basidiospores are exogenously produced on the basidium. The basidia are arranged in basidiocarps (fruiting bodies) 
  • E.g. Agaricus, Ustilago, Puccinia.


  • It is called fungi imperfect.
  • Only asexual spores (conidia) or vegetative phases are known, called fungi imperfecti.
  • Mycelium is septate and branched.
  • Major organisms are decomposers. Some are saprophytic and parasitic.
  • E.g. Alternaria, Colletotrichum, Trichoderma.

Kingdom Plantae

  • They are multicellular, eukaryotic, and photosynthetic containing chlorophyll.
  • They are mostly autotrophic, and there are few parasites (insectivorous plants such as Cuscuta, Venus fly trap) and symbionts.
  • Includes both vascular and non-vascular plants. Non-vascular plants include mosses and liverworts.
  • Seed-bearing vascular plants include spermatophytes.
  • The cell wall is composed mainly of cellulose and hemicellulose.
  • Plant kingdoms are the chief producers in the ecosystem.
  • They store food in the form of starch.

Kingdom Plantae includes:

  • Algae
  • Bryophytes
  • Pteridophytes
  • Angiosperm
  • Gymnosperm

Their life-cycle consists of two phases: diploid sporophyte and haploid gametophyte.

Kingdom Animalia

  • They are multicellular, eukaryotes, heterotrophic, without a cell wall.
  • They have the largest degree of development compared to other classes.
  • Organisms belonging to the animal kingdom lack cell walls, central vacuoles, and plastids.
  • They exhibit a holozoic mode of nutrition (by ingestion of food), however, some are also parasitic or saprozoic.
  • They digest their food in an internal cavity and store food reserves as glycogen or fat. 
  • The higher forms of organisms belonging to the Animal kingdom show developed sensory and neuromotor mechanisms.
  • Sexual reproduction is the fusion of gametes with the development of the embryo.
  • Animal kingdom broadly includes:

In R.H Whittaker’s five-kingdom system of classification, acellular organisms such as viruses, viriods, prions lichens have not been placed in any of the five kingdoms due to their varying characters making them unfit to be placed specifically in a single class. Hence, these are studied as a separate group of organisms.


  • Viruses: venom or poisonous fluid
  • It is an obligatory parasite.
  • Smaller than bacteria passing through a bacteria-proof filter. Ultramicroscopic can only be observed under the electron microscope.
  • Recognized by Dmitri Ivanovsky in mosaic disease in tobacco.
  • Non-cellular organisms. They lack cellular machinery.
  • They are inner crystalline structures when outside the living cell. W.N. Stanley found out the crystallizing property.
  • Viruses contain genetic material (either DNA or RNA), an outer protective protein coat made up of monomers called capsomeres to form capsid. These capsomeres are arranged in helical or polyhedral geometric forms. 
  • Are infectious and can cause disease inside the specific host.
  • Diseases caused by viruses in animals: smallpox, HIV, COVID, influenza, herpes.
  • Disease caused by viruses in plants:  mosaic, leaf rolling, curling, stunted growth

Viruses contain the following major structures

  • Sheath
  • Head
  • Tail fibers
  • Collar


  • Another acellular entity.
  • They are pathogenic.
  • They contain genetic material. They have small single-stranded RNA that are infectious. They lack protein coating as in viruses.
  • Diseases caused by viroids are citrus exocortis, chlorosis of leaves, etc.


  • Transmissible and infectious protein.
  • They can induce abnormal folding of normally functioning proteins in the brain.
  • They are rapidly progressive and always fatal.


  • It is a composite organism.
  • They are associations between algae (phycobiont) and fungi (mycobiont).
  • Algae and fungi maintain a mutual relationship where one is benefitted from the other. Hence the association is termed as mutualism/symbiosis.


  1. Mehta, K. R. (2008). A complete textbook of biology. Satyal Publication Pvt. Ltd.
  2. Adhikari, K. R. Smooth Biology. Readers’ Point.
  3. Tyagi and Goyal. (2022). Objective Biology for NEET. A Trueman Publication.
  4. Organism. Biology Online. Retrieved from
  5. Accessed on 13th February 2024.
  6. Prion disease. Centers for Disease Control and Prevention. Retrieved from Accessed on 17th February 2024.
  7. Prokaryote. (2024). Britannica. Retrieved from Accessed on 15th February 2024.

About Author

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Agrani Paudel

Agrani Paudel is doing her B.Sc. in Microbiology from St. Xavier’s College, Kathmandu, Nepal. She is also working as a President of Alumni Club, Department of Microbiology, SXC Alumni Forum, St. Xavier’s College. She did her mini thesis on the topic, "Isolation and Identification of Multi-Drug Resistant E. coli and Salmonella in Feral Pigeons’ Droppings". She also published her review on the topics, "Pre-leukemic Cell Detection and Leukemic Transformation of a Normal Marrow Cell: A Mini-Review".

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