Five Kingdom System of Biological Classification

Classification 

The method of classifying organisms based on common characteristics is known as biological classification. Aristotle was the first to attempt to provide a scientific basis for biological classifications. He classified plants into trees, shrubs, and herbs using simple physical features. He also classified the animals into two groups: those with red blood and those without red blood. Linnaeus devised a Two Kingdom categorisation system with Plantae and Animalia kingdoms that contained all plants and animals, respectively. Although classification biology, i.e. classifying creatures into plants and animals, was straightforward, many species did not fit into either group. As a result, the long-used two-kingdom categorisation was determined to be insufficient.

Five Kingdom System of Biological Classification 

A Five Kingdom Classification was suggested by R.H. Whittaker, in 1969. He called the kingdoms Monera, Protista, Fungi, Plantae, and Animalia. He used his primary categorisation criteria to utilise cellular structure, body architecture, mode of feeding, reproducing, and evolutionary linkages.

Monera

The characteristics of the Monera kingdom are as follows: 

• Monera consists solely of bacteria. It is composed of unicellular prokaryotes that lack organelles and a distinct nucleus. 
• Bacteria of various forms are found, including spherical (cocci), comma (vibrio), rod-shaped (bacillus), and spiral (spirilla).
• They reproduce primarily by fission, spore production under unfavourable circumstances, and DNA transfer from one bacterial cell to another.
• They are further classified as archaebacteria and eubacteria.

Archaebacteria 

These bacteria may be found in the toughest environments, such as hot springs and marshy and salty surroundings. 

Bacteria found in: 

• Hot springs: Thermoacidophiles
• Salty environment: Halophiles
• Marshy environment: Methanogens. Ruminant guts contain methanogens, which create biogas. 

Eubacteria

These are also called ‘true bacteria’. They consist of a rigid cell wall. If the bacteria are motile, they have flagella. Ideally, eubacteria can be divided into three types: heterotrophs, autotrophs and chemotrophs. 

1. Photosynthetic Autotrophs – Autotrophs can make their food by photosynthesis. They include Cyanobacteria (Blue-green Algae). They colonise lakes and thrive in moist environments. 
2. Chemotrophs – They acquire nutrition by breaking down inorganic substances like sulphur and nitrogen. They are found mainly in the soil and are important for maintaining plant health. 
3. Heterotrophs – They function as decomposers. They are employed for various applications, including nitrogen fixation, curd formation, and antibiotic manufacturing. Many bacteria are pathogens that cause different plant and animal illnesses, such as citrus canker, cholera, and typhoid.

Protista

The characteristics of this group are as follows: 

• Protista majorly consists of single-celled eukaryotic organisms.
• They are usually aquatic. 
• Protista can reproduce by both asexual and sexual means. 
• They contain a well-differentiated nucleus and attain mobility through cilia and flagella. 

The Protista kingdom includes euglenoids, slime moulds, protozoans, dinoflagellates and chrysophytes. 

Euglenoids

• Euglenoids are versatile organisms and can function as both heterotrophic and autotrophic organisms. 
• Euglenoids can be found in stagnant water. 
• A distinguishing trait is the existence of a ‘pellicle’. This allows them to retain their shape and gives them flexibility. 

Slime moulds

• They are saprophytic protists that feed on decomposing organic components (leaves and twigs).
• Plasmodium is a slime mould clump that forms under ideal circumstances.
• Fruiting units form at the tip of plasmodium under adverse circumstances.
• These spores have real walls and may live in harsh circumstances for long periods.

Protozoans

This category contains organisms that are unicellular heterotrophs, i.e., are not capable of preparing their food and instead depend on other organisms. They can be predators or parasites. 

These are classified into four primary categories:

• Amoeboids – They are distinguished by the presence of pseudopodia, which are employed for locomotion and prey capture, such as Amoeba.
• Ciliated – They have cilia covering their body. The synchronised movement of cilia helps in movement and nourishment. Example: Paramoecium.
• Flagellated – The appearance of flagella marks these organisms. Some of them are parasitic that cause ailments like sleeping sickness caused by Trypanosoma.
• Sporozoans, such as plasmodium, are distinguished by the development of spores. They can cause infections such as malaria. 

Dinoflagellates

• Dinoflagellates prepare their food with the help of photosynthesis. 
• They are found in water bodies and present in various colours, such as yellow, blue and green depending on the pigmentation. 
• The cell wall is made of cellulose. 

Chrysophytes

• Chrysophytes consist of diatoms and desmids (golden algae). They are marine creatures.
• They can produce their food through photosynthesis. Chrysophytes also have an unbreakable cell wall attributed to silica.
• Diatomaceous earth is a cell wall residue that accumulates over time. It is employed in filtering and cleaning.

 Fungi

• Fungi are inherently heterotrophic. 
• Many obtain their nutrition from dead organic materials and are referred to as saprophytes, while others may feed on living creatures and are referred to as parasites. 
• Lichens are fungi that live in reciprocal interactions with other species, such as algae. These fungi are known as symbionts, and the process is called symbiosis. Another example of a symbiotic association involves fungi and the roots of plants called mycorrhiza.
• Fungi reproduce both sexually and asexually. Asexual reproduction spores are called zoospores. Sexual reproduction produces spores called oospores and basidiospores. 

 The kingdom of fungi is divided into four different classes, as explained in the table below: 

	Fungi
Class	Distinguishing characteristics	Asexual Reproduction	Sexual Reproduction	Examples
Zygomycetes	Found in moist areas. Multicellular, coenocytic mycelia.	Asexual spores formed on sporangium.	Sexual spores called zygospores.	Rhizopus (black bread mold)
Basidiomycetes	Can be detected in soil and in plant bodies where it produces rusts and smuts. Multicellular uninucleated mycelia.	Absent.	By basidiospores.	Agaricus (mushrooms)
Ascomycetes	Can be both unicellular and multicellular. Mycelia are septate.	Budding by conidia.	Ascospores formed in ascii.	Aspergillus, Penicillium
Deuteromycetes	Parasitic with many being human pathogens. Septate mycelia.	Budding	Absent	Alternaria, Trichoderma

Some Important Fungi:

• Yeast – Utilised in the fermentation of products like cheese and bread. 
• Penicillium – Used to produce antibiotics.
• Symbionts – Lichens (fungi that live in symbiotic relationships with algae), Mycorrhiza (fungi in symbiotic relationships with roots of green plants)
• Rhizopus – This mould is found on bread, grown under favourable conditions. 

 Kingdom Plantae

• This biological classification includes plants with well-defined nuclei, i.e., eukaryotic, photosynthetic plants.
• Chlorophyll is the most abundant pigment that aids in the production of food.
• These plants have a cell wall composed of cellulose that is rigid. 
• They can reproduce by both sexual and asexual means. 
• Kingdom Plantae comprises angiosperms, gymnosperms, pteridophytes, bryophytes and algae. 

 Kingdom Animalia

Kingdom Animalia is another one of the biological classifications. The following are some of the features of Animalia members:

• Kingdom Animalia consists of organisms with well-defined nuclei (eukaryotic), multicellular and heterotrophic. 
• The organisms can be of various sizes. 
• They have highly developed and specialised organ systems. Some of the systems present in them are the skeletal system, respiratory system, circulatory system, etc.
• The respiratory system has well-developed lungs, and breathing occurs through the skin, lungs, gills, etc.
• They have highly specialised organs for locomotion as well. 
• The cell membrane binds the cell organelles, and a nuclear membrane surrounds the nuclei. 
• The creation of haploid gametes is how reproduction occurs. The gametes fuse to generate a diploid zygote, which differentiates to create a new diploid species.
• These organisms are bilaterally symmetrical, where the left and right sides of the body are mirror images of each other in the sagittal plane. 

 Viruses

Since viruses are acellular entities, they do not fit within Whittaker’s five-kingdom biological classifications. They are made up of nucleic acid (either DNA or RNA) wrapped by a protein sheath. These viruses can only develop and increase within the confines of a host cell. Viruses occur in the form of crystals outside of the host cell. Variola, for example, produces smallpox, and HIV (human immunodeficiency virus) is the causative virus for AIDS.

 Viroids

Viroids differ from viruses in many ways. They lack a protein coat and consist of RNA molecules only. Viroids are much smaller than viruses. Viroids exclusively infect plants, but viruses infect all species.

 Lichens

They are known to be symbiotic relationships between algae and fungus. Autotrophic algal components synthesise and give nourishment. The fungal component provides shelter and protection. Because lichens do not develop in contaminated regions, they are an excellent pollution indicator.

 Conclusion 

The method by which scientists organise living species is known as classification biology. Aristotle postulated it initially based on simple physical traits. Eventually, Linnaeus divided all living creatures into two kingdoms: Plantae and Animalia. Whittaker suggested a five-kingdom categorisation divided into Monera, Protista, Fungi, Plantae, and Animalia. The five-kingdom categorisation was based on cell structure, body structure, mechanism of nourishment and procreation, and evolutionary links. Traditionally, the similarity between organisms was established by evaluating an organism’s physical traits; however, contemporary categorisation employs a range of approaches, particularly genetic research.

A system of seven levels is used to classify organisms:

1. Kingdom
2. Phylum
3. Class
4. Order
5. Family
6. Genus
7. Species

Frequently Asked Questions about Classifications

1. What is taxonomy? 

A. Taxonomy studies the ideas and techniques involved in organism categorisation, naming, and identification. This strategy represents the organism’s most important characteristics and interactions. The primary goal of classification biology is to classify various plant and animal species.

2. What exactly is diatomaceous earth? Why are diatoms described as “pearls of the ocean”?

A. The buildup of enormous diatom deposition over many years produces a silica coating that extends over a hundred metres. Diatoms are the most important producers in the marine ecosystem. They provide nutrition both for other sea creatures as well as for themselves. 

 3. Briefly explain what insectivorous plants are?

A. Insectivorous plants or carnivorous plants get nourishment from catching and eating insects and other protozoans. Insectivorous plants are more common in acidic bogs.