BIOLOGICAL CLASSIFICATION » Organism and Kingdom Binomial nomenclature

Biological Classification

The biological classification of plants and animals was first proposed by Aristotle on the basis of simple morphological characters. Linnaeus later classified all living organisms into two kingdoms – Plantae and Animalia. Whittaker proposed an elaborate five-kingdom classification – Monera, Protista, Fungi, Plantae, and Animalia. The main criteria of the five kingdom classifications were cell structure, body organization, mode of nutrition and reproduction, and phylogenetic relationships.

In the five kingdom classifications, bacteria are included in Kingdom Monera. Bacteria are cosmopolitan in distribution. These organisms show the most extensive metabolic diversity. Bacteria may be autotrophic or heterotrophic in their mode of nutrition. Kingdom Protista includes all single-celled eukaryotes such as Chrysophytes, Dinoflagellates, Euglenoids, Slime-moulds, and Protozoans. Protists have defined nuclei and other membrane-bound organelles. 

They reproduce both asexually and sexually. Members of Kingdom Fungi show a great diversity in structures and habitat. Most fungi are saprophytic in their mode of nutrition. They show asexual and sexual reproduction. Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes are the four classes under this kingdom. The plantae includes all eukaryotic chlorophyll-containing organisms. Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms are included in this group.

The life cycle of plants exhibit an alternation of generations – gametophytic and sporophytic generations. The heterotrophic eukaryotic, multicellular organisms lacking a cell wall are included in the Kingdom Animalia. The mode of nutrition of these organisms is the holozoic. They reproduce mostly in sexual mode. Some acellular organisms like viruses and viroids as well as lichens are not included in the five-kingdom system of classification.

Since the dawn of civilisation, there have been many attempts to classify living organisms. It was done instinctively not using criteria that were scientific but borne out of a need to use organisms for our own use – for food, shelter, and clothing. Aristotle was the earliest to attempt a more scientific basis for classification.

He used simple morphological characters to classify plants into trees, shrubs, and herbs. He also divided animals into two groups, those which had red blood and those that did not. In Linnaeus' time, a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed that included all plants and animals respectively. This system was used till very recently. This system did not distinguish between the eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic (green algae) and non-photosynthetic (fungi) organisms.

Classification of organisms into plants and animals was easily done and was easy to understand, but, a large number of organisms did not fall into either category. Hence the two-kingdom classification used for a long time was found inadequate. A need was also felt for including, besides gross morphology, other characteristics like cell structure, nature of the wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships, etc.

Classification systems for living organisms have hence, undergone several changes over time. Though plant and animal kingdoms have been a constant under all different systems, the understanding of what groups/organisms be included under these kingdoms has been changing; the number and nature of other kingdoms have also been understood differently by different scientists over time.

Spore Flagellum Cocci Bacilli Spirilla Vibrio Bacteria of different shapes Bacteria are the sole members of the Kingdom Monera. They are the most abundant microorganisms. Bacteria occur almost everywhere. Hundreds of bacteria are present in a handful of soil. They also live in extreme habitats such as hot springs, deserts, snow, and deep oceans where very few other life forms can survive. Many of them live in or on other organisms as parasites. 

Though the bacterial structure is very simple, they are very complex in behavior. Compared to many other organisms, bacteria as a group show the most extensive metabolic diversity. Some of the bacteria are autotrophic, i.e., they synthesize their own food from inorganic substrates. They may be photosynthetic autotrophic or chemosynthetic autotrophic. The vast majority of bacteria are heterotrophs, i.e., they do not synthesize their own food but depend on other organisms or on dead organic matter for food.

1. Archaebacteria

These bacteria are special since they live in some of the harshest habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens). Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions. Methanogens are present 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.

2. Eubacteria

There are thousands of different eubacteria or ‘true bacteria’. They are characterized by the presence of a rigid cell wall, and if motile, a flagellum. The cyanobacteria (also referred to as blue-green algae) have chlorophyll similar to green plants and are photosynthetic autotrophs. The cyanobacteria are unicellular, colonial, or filamentous, and freshwater/marine or terrestrial algae. The colonies are generally surrounded by a gelatinous sheath. They often form blooms in polluted water bodies. Some of these organisms can fix atmospheric nitrogen in specialized cells called heterocysts, e.g., Nostoc and Anabaena. Chemosynthetic autotrophic bacteria oxidize various inorganic substances such as nitrates, nitrites, and ammonia and use the released energy for ATP production. They play a great role in recycling nutrients like nitrogen, phosphorous, iron, and sulfur. 


All single-celled eukaryotes are placed under Protista, but the boundaries of this kingdom are not well-defined. What may be a photosynthetic protistan’ to one biologist may be ‘a plant’ to another. In this book, we include Chrysophytes, Dinoflagellates, Euglenoids, Slime molds, and Protozoans under Protista. Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals, and fungi. Being eukaryotes, the protistan cell body contains a well-defined nucleus and other membrane-bound organelles. Some have flagella or cilia. Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation. 

1. Chrysophyte

This group includes diatoms and golden algae (desmids). They are found in freshwater as well as in marine environments. They are microscopic and float passively in water currents (plankton). Most of them are photosynthetic. In diatoms, the cell walls form two thin overlapping shells, which fit together as in a soap box. The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind a large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as ‘diatomaceous earth. Being gritty this soil is used in the polishing and filtration of oils and syrups. Diatoms are the chief ‘producers’ in the oceans.

2 Dinoflagellates

These organisms are mostly marine and photosynthetic. They appear yellow, green, brown, blue, or red depending on the main pigments present in their cells. The cell wall has stiff cellulose plates on the outer surface. Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates. Very often, red dinoflagellates (Example: Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides). Toxins released in such large numbers may even kill other marine animals such as fish.

3 Euglenoids

The majority of them are freshwater organisms found in stagnant water. Instead of a cell wall, they have a protein-rich layer called a pellicle which makes their body flexible. They have two flagella, a short and a long one. Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms. Interestingly, the pigments of euglenoids are identical to those present in higher plants. Example: Euglena.

4 Slime Moulds

Slime molds are saprophytic protists. The body moves along decaying twigs and leaves engulfing organic material. Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet. During unfavorable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls. They are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed by air currents.

5. Protozoans

All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major groups of protozoans. Amoeboid protozoans: These organisms live in freshwater, seawater, or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites. Flagellated protozoans: The members of this group are either free-living or parasitic. They have flagella. The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma.


The fungi constitute a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat. When your bread develops a mold or your orange rots it is because of fungi. The common mushroom you eat and toadstools are also fungi.

White spots seen on mustard leaves are due to a parasitic fungus. Some unicellular fungi, e.g., yeast are used to make bread and beer. Other fungi cause diseases in plants and animals; wheat rust-causing Puccinia is an important example. Some are the source of antibiotics, e.g., Penicillium. Fungi are cosmopolitan and occur in air, water, soil and on animals and plants. They prefer to grow in warm and humid places. Have you ever wondered why we keep food in the refrigerator?

Yes, it is to prevent food from going bad due to bacterial or fungal infections. With the exception of yeasts which are unicellular, fungi are filamentous. Their bodies consist of long, slender thread-like structures called hyphae. The network of hyphae is known as mycelium. Some hyphae are continuous tubes filled with multinucleated cytoplasm – these are called coenocytic hyphae. Others have septae or cross walls in their hyphae. The cell walls of fungi are composed of chitin and polysaccharides. Most fungi are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes. Those that depend on living plants and animals are called parasites. They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza.


Kingdom Plantae includes all eukaryotic chlorophyll-containing organisms commonly called plants. A few members are partially heterotrophic such as insectivorous plants or parasites. Bladderwort and Venus fly trap are examples of insectivorous plants and Cuscuta is a parasite. The plant cells have a eukaryotic structure with prominent chloroplasts and cell walls mainly made of cellulose. You will study the eukaryotic cell structure in detail in Chapter 8. Plantae includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms. The life cycle of plants has two distinct phases – the diploid sporophytic and the haploid gametophytic – that alternate with each other. The lengths of the haploid and diploid phases, and whether these phases are free-living or dependent on others, vary among different groups in plants. This phenomenon is called the alternation of generation. 


This kingdom is characterized by heterotrophic eukaryotic organisms that are multicellular and whose cells lack cell walls. They directly or indirectly depend on plants for food. They digest their food in an internal cavity and store food reserves as glycogen or fat. Their mode of nutrition is the holozoic – by ingestion of food. They follow a definite growth pattern and grow into adults that have a definite shape and size. Higher forms show elaborate sensory and neuromotor mechanisms. Most of them are capable of locomotion.


In the five kingdom classification of Whittaker, there is no mention of some acellular organisms like viruses and viroids and lichens. These are briefly introduced here. All of us who have suffered the ill effects of the common cold or ‘flu’ know what effects viruses can have on us, even if we do not associate it with our condition.

Viruses did not find a place in classification since they are not truly ‘living’ if we understand living as those organisms that have a cell structure. Viruses are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell. Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host. Would you call viruses living or non-living?

The name virus which means venom or poisonous fluid was given by Pasteur. D.J. Ivanowsky (1892) recognized certain microbes as causal organisms of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters. M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed that viruses could be crystallized and crystals consist largely of proteins.

They are inert outside their specific host cell. Viruses are obligate parasites. In addition to proteins, viruses also contain genetic material, which could be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious. In general, viruses that infect plants have single-stranded RNA, and viruses that infect animals have either single or double-stranded RNA or double-stranded DNA.

Bacterial viruses or bacteriophages (viruses that infect bacteria) are usually double-stranded DNA viruses. The protein coat called capsid made of small subunits called capsomeres protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms. Viruses cause diseases like mumps, smallpox, herpes, and influenza. AIDS in humans is also caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing, and stunted growth.

Binomial nomenclature is the biological system of naming the organisms in which the name is composed of two terms, where, the first term indicates the genus and the second term indicates the species of the organism.”

Question on Biological Classification

Discuss how classification systems have undergone several changes over a period of time?

The classification systems have undergone several changes with time. The first attempt at classification was made by Aristotle. He classified plants as herbs, shrubs, and trees. Animals, on the other hand, were classified on the basis of the presence or absence of red blood cells. New organisms are constantly being discovered and for classifying them, a new system needs to be used. This is why the new classification system is constantly developed to classify organisms based on the similarity in characters.

Organize a discussion in your class on the topic – Are viruses living or nonliving?

Viruses are microscopic organisms that have characteristics of both living and non-living. A virus consists of a strand of DNA or RNA covered by a protein coat called the capsid. The presence of nucleic acid (DNA or RNA) suggests that viruses are alive.

What are the characteristic features of Euglenoids?

Euglenoids have certain features like-
  • They are unicellular protists.
  • They are found in freshwater.
  • They have a pellicle present- which is a protein-rich membrane.
  • They lack a cell wall.
  • They have two flagella on the anterior of the body.
  • They can prepare their food with photosynthetic pigments called chloroplasts.
  • In the absence of light, they acquire food as heterotrophs. They will then feed on other organisms for feed.
  • They are the connecting bridge between plants and animals as they have featured both plants and animals.
  • They have a contractile vacuole to regulate the fluid in the cytoplasm.
  • They are usually red due to the presence of carotenoids.
  • They attack their food and eat via phagocytosis in which the entrapment of food takes place by an outer membrane.
  • They reproduce by longitudinal cell division, a form of asexual reproduction mode.
  • They generate cysts that can withstand the drying period.

Give a brief account of viruses with respect to their structure and nature of genetic material. Also, name four common viral diseases

Viruses are sub-microscopic, infectious, nucleoprotein particles that can infect all living organisms. A virus consists of genetic material either in the form of RNA or DNA surrounded by a protein coat. Most of the viruses, infecting plants, have single-stranded RNA as genetic material. On the other hand, the viruses infecting animals have single or double-stranded RNA or double-stranded DNA. Bacteriophages or viruses infecting bacteria mostly have double-stranded DNA. Their protein coat called capsid is made up of capsomere subunits. These capsomeres are arranged in helical or polyhedral geometric forms. AIDS, smallpox, mumps, and influenza are some common examples of viral diseases.

What do the terms phycobiont and mycobiont signify?

They both are components of a lichen. Phycobiont signifies the algal component of lichen. On the other hand, mycobiont signifies the fungal constituent of a lichen. Lichens are symbioses between fungi (mycobionts) and photoautotrophic green algae or cyanobacteria (photobionts). Many lichens occupy large distributional ranges covering several climatic zones.

Plants are autotrophic. Can you think of some plants that are partially heterotrophic?

However, some insectivorous plants are partially heterotrophic. They have various means of capturing insects so as to supplement their diet with the required nutrients derived from insects, causing the proliferation of growth. Examples include pitcher plants (Nepenthes), Venus fly trap, bladderwort, and sundew plant.

Describe briefly the four major groups of Protozoa.

Protozoa are unicellular, heterotrophic, eukaryotic organisms comprising four organization types: amebae, flagellates, ciliates, and parasitic sporozoans. Protozoa can be divided into four phyla based on their locomotion: Mastigophora, Sarcodina, Ciliophora, and Sporozoa. Members of the phylum Mastigophora move about by using one or more whiplike flagella. The genus Euglena contains flagellated species.

For our purposes, there are only 4 groups of protozoa that will be covered here: these groups are separated by motility and cell structure.
  • Amebas (representative: Ameba proteus)
  • Flagellates (representative: Trypanosoma, Euglena)
  • Ciliates (representative: Paramecium)
  • Apicomplexa (representative: Plasmodium)

What is the nature of cell walls in diatoms?

Diatoms are algae that dwell in glass houses. They are the only organisms with cell walls made of transparent, opaline silica.

Classification of Organism

1. There are millions of organisms. It is impossible to study each individual separately. Classification means to categories organisms into different groups. The study of an individual in a group gives us an idea of the rest of the members of that group.

2. Linnaeus divides all organisms into two kingdoms - Planate and Animalia in his book “System of Nature”. The foundation of the modem classification system was laid in the line of classification system started by Linnaeus. Therefore Linnaeus is called the ‘Father of Taxonomy’. Due to the disputed positions of organisms like bacteria, viruses, fungi, and euglena, there is a need for reconsideration of the system of classification.

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