Biological classification is the scientific approach of categorising organisms and subgroups according to their similarities and differences and arranging the group in a hierarchical order of classifications.

Significance of Classification System

1. It is not easy to study each organism; analysing one or two group organisms will help attain adequate information regarding the significant features of the considered group.
2. Biological classification will help in recognition of a new organism.
3. It aids in understanding the relationships that exist between various groups of species. 

Types of Classification

• Artificial system 
• Natural system 
• Phylogenetic system 

Artificial System

In an artificial categorisation system, just one or two morphological characteristics are used to categorise an organism into different groups. Enaima, flowering and non-flowering plants, and Aristotle’s taxonomy are only a few of the examples provided.

A natural System 

With this method, the number of characteristics is compared to determine the similarities and differences between them, which aids in the understanding of the natural interactions that exist between organisms in the wild. Biological classifications such as those developed by Hooker and Bentham are two examples.

Phylogenetic System

It is based on the complex relationship of the creature. It categorises species according to their evolution on the planet, from the most primitive to the most advanced form of evolution. Hutchinson classification, Engler classification, and Prantl classification are only a few examples.

Kingdom system of classification

Two kingdoms: Plantae, Animalia

In this system, the organisms are classified according to the availability and absence of the cell wall. 

Three kingdoms: Plantae, Protista, Animalia

Haeckel separated the unicellular animals, fungi, and algae, based on the absence of tissue differentiation and formed an innovative kingdom, Protista.

Five kingdoms: Monera, Protista, Fungi, Plantae, Animalia

In this kingdom, R.H Whittaker has classified all the organisms into five kingdoms for developing the Phylogenetic classification.

Monera

This kingdom comprises entire prokaryotes, i.e., bacteria, mycoplasma, actinomycetes and cyanobacteria. Bacteria are the most abundant microorganism capable of surviving in all climate types.

Eubacteria 

If flagella are present, it is considered to be true bacteria. Gram-positive photosynthetic bacteria are known as blue-green algae. They may be marine, freshwater, or terrestrial (land), depending on their habitat. Some of them include the specialised heterocyst cell (Nostoc and Anabaena) to aid nitrogen-fixing.

Chemosynthesis bacteria are capable of oxidising inorganic compounds like ammonia to make energy and recycle sulphur and phosphorus. Curd and antibiotic synthesis are aided by heterotrophic bacteria, which break down and fix nitrogen in leguminous plants. But a few of them have been linked to illnesses like typhoid, tetanus, and citrus canker, so it is best to avoid them at all costs.

Mycoplasma 

It is generally agreed that Mycoplasma or Pleuropneumonia-like organisms are the most primitive free-living prokaryotes. They do not have a cell wall and can live without oxygen; yet, they appear to be a source of illness in both plants and animals.

Protista

This kingdom comprises Euglenoids, Chrysophytes, Dinoflagellates, Protozoans and slime mould. Some of the salient features of the Protista kingdom are listed below:

• It is made up of eukaryotes that are either unicellular or colonial.
• The majority of the creatures are aquatic and contribute to the formation of plankton.
• Genetic material comprises two or more DNA molecules, depending on the species.
• The mode of nutrition can be saprophytic, photosynthetic, or holozoic.
• The predominant colour is golden brown; however, green, yellow, and blue exist.
• When sunlight is available, they appear to be photosynthetic, and when sunlight is not available, they appear to operate as predators.

Example-Euglena, Peranema.

Slime Moulds

• They are saprophytic and feed on decaying leaves and twigs.
• The cell wall of spores comprises cellulose.
• Considering the favourable condition, they create an aggregation known as the plasmodium that produces fruiting bodies bearing spores.

Example-Physarum, Fuligo.

Protozoans

• They are identified as the primitive relatives of animals.
• The ciliated protozoans are aquatic and comprise cilia throughout the body for movement.
• The entire protozoans are heterotrophs and act as parasites or predators.
• The flagellated protozoans are known to be free-living or parasitic and include flagella.

Kingdom Fungi

• They are known as spore-forming heterotrophic, achlorophyllous, non-vesicular eukaryotic organisms.
• The cell wall of the organism is made of fungal cellulose or chitin.
• Reproduction of the organism can be vegetative, asexual, or sexual reproduction through ascospores, oospores, and basidiospores.

The sexual cycle comprises the following steps:

• Plasmogamy
• Karyogamy
• Meiosis in zygote

Phycomycetes

• They are present in the water and old wood in damp and moist places.
• Asexual reproduction by aplanospores or zoospores
• The mycelium is coenocytic and septate.

Example-Mucus, Rhizopus, Albugo, etc.

Ascomycetes

• They are also known as sac fungi and are saprophytic, coprophilous, or parasitic.
• The sexual spores are known as the ascospores and are produced inside the fruiting part, ascoscraps.

Example-Neurospora, Aspergillus, Claviceps, etc.

Basidiomycetes 

• It is sometimes referred to as club fungi, and its mycelium is branching and septate in appearance.
• There is no evidence of asexual spores in this species, which means that vegetative reproduction is accomplished by fragmentation.
• A fruiting body, referred to as a basidiocarp, is created in basidium, responsible for producing basidiospores.

Example-Agaricus, Ustilago, Puccinia.

Deuteromycetes

• It is referred to as fungi imperfect, and only the vegetative and asexual phases have been observed.
• Mycelium is unattached and branching in appearance.

Example- Alternaria, Trichoderma, Colletotrichu.

Kingdom Plantae

• A chlorophyll-producing eukaryotic creature whose life cycle is separated into two parts: diploid saprophytic and haploid gametophyte, which rotate with each other.

• The Kingdom Plantae is made up of the Bryophytes, Algae, Pteridophytes, Angiosperms, and Gymnosperms, among other groups.

Virus, Viroids and Lichens

The classification system does not comprise viruses, Lichens, and Viroids.

• When viruses penetrate live cells, they take over the cell’s machinery to multiply themselves. Viruses are characterised as non-cellular organisms with a crystalline structure external to living cells.
• Some illnesses caused by viruses are common cold, AIDS, influenza, smallpox, curling, and leaf rolling.
• The bacteria feeding virus is Bacteriophage, which is usually a double-stranded DNA virus.
• The nucleic acid is shielded by a protein coat known as a capsid, composed of tiny subunits known as capsomeres.
• T.O Diener has found viroids as a new infectious agent, which is comparably smaller than the virus causing potato spindle tuber disease.

Conclusion

We have provided biological classification study material to understand how organisms are classified into different groups and subgroups based on similarities and differences and then placed the group in a hierarchy of categories. The biological categorisation is the technique by which scientists organise biological organisms. Organisms are classified depending on how related they are. Historically, the similarity was established by evaluating the physical traits of an organism, but current categorisation includes a range of approaches, including genetic research.