Biological Classification is the grouping of living organisms based on similarities and dissimilarities in characters such as structure and evolutionary relationships. Due to which organism can easily organise, identify, and understand their relationship.
History of Biological Classification
Biological classification has evolved from the Two Kingdom system to the Six Kingdom classification. An overview of these systems and their proposers is given below.
System | Proposed by | Kingdoms included |
|---|---|---|
Two Kingdom Classification | Carolus Linnaeus | Plantae and Animalia |
Three Kingdom Classification | Ernst Haeckel | Protista, Plantae and Animalia |
Four Kingdom Classification | Copeland | Monera, Protista, Plantae and Animalia |
Five Kingdom Classification | R.H. Whittaker | Fungi, Monera, Protista, Plantae and Animalia |
Six Kingdom Classification | Carl Woese | Archaea, Eubacteria, Fungi, Protista, Plantae and Animalia |
The Two Kingdom Classification
The two-kingdom classification was given by a biologist, Carolus Linnaeus. He classified organisms into two kingdoms, i.e. Plantae (included all plants) and Animalia (included all animals).
Disadvantages of Two Kingdom Classification
This system failed to clearly distinguish between:
- Eukaryotes and prokaryotes
- Unicellular and multicellular organisms
- Photosynthetic (green algae) and non-photosynthetic (fungi) organisms.
Five Kingdom Classification
In 1969, R.H. Whittaker proposed the five-kingdom classification. He classified those five kingdoms as Monera, Protista, Fungi, Plantae, and Animalia. He primarily used the following criteria for classification:
- Cell structure
- Body organisation
- Mode of nourishment
- Reproduction
- Phylogenetic linkages or relationships
1. Kingdom Monera
Bacteria are the main members of this kingdom and do not contain a true nucleus or membrane-bound organelles. It contains unicellular organisms and found everywhere easily.
Based on their shape, the bacteria are divided into four groups:
- Coccus (spherical shape)
- Bacillus (rod-shaped)
- Spirillum (spiral shape)
- Vibrium (comma-shaped)
Kingdom Monera is further divided into:
1. Archaebacteria
They are special bacteria as they can withstand extreme environmental conditions because of their different cell wall structure. They can be:
- Thermoacidophiles: They are found in the hot springs
- Halophiles: They are found in the salty areas
- Methanogens: They are found in the marshy areas/ gut of ruminant animals (production of biogas)
2. Eubacteria or True Bacteria
They have rigid cell walls and a flagellum (locomotion), if motile. They can be photosynthetic autotrophs, chemosynthetic autotrophs and heterotrophs.
- Photosynthetic Autotrophs: Cyanobacteria (blue-green algae, which have chlorophyll a), Nostoc and Anabaena are common examples. They are surrounded by a gelatinous sheath or mucilaginous covering, which protects them from wetting. They fix atmospheric nitrogen in specialised cells called heterocysts (significance)
- Chemosynthetic Autotrophs: These are the bacteria which oxidise inorganic substances, e.g. nitrates, nitrites and ammonia and use the released energy for ATP production. They recycle nutrients, e.g. nitrogen, phosphorus, iron and sulphur (significance).
- Heterotrophs: They are decomposers. Some of them are pathogens, and some are beneficial as they help make curd from milk, produce antibiotics, and fixing atmospheric nitrogen in leguminous plants (significance).
Reproduction in Bacteria
They reproduce by asexual mode- binary fission, sexual mode- transfer of DNA and spore formation in unfavourable conditions.
Mycoplasma
It is an independently living prokaryotic organism and belongs to the kingdom Monera, and its characteristics are:
- smallest organism
- lacks cell walls
- survive in the absence of oxygen
- pathogenic in nature
2. Kingdom Protista
They are single-celled eukaryotes. They contain a well-defined nucleus and other membrane-bound organelles. They include:
- Chrysophytes
- Dinoflagellates
- Euglenoids
- Slime moulds
- Protozoans
The following table shows the characteristic features of different groups under Kingdom Protista:
| Classification of Protista | Characteristic Features | Examples |
|---|---|---|
| Chrysophytes (chief producers in oceans) | Their cell walls form two overlapping shells, which fit together and are embedded with silica, which makes them indestructible. So, cell wall deposits and their accumulation lead to ‘diatomaceous earth’. This soil can be used to polish things and filter oils and syrups. | Diatoms and golden algae (desmids) |
| Dinoflagellates | They show rapid multiplication and make the appearance of sea red (bioluminescence). Toxins released by them can kill other aquatic animals. | Red dinoflagellates (Example: Gonyaulax) |
| Euglenoids | They have a pellicle protein-rich layer, which keeps them flexible | Euglena |
| Slime Moulds | During suitable conditions, they form plasmodium and during unfavourable conditions, Plasmodium differentiates and forms fruiting bodies (spores inside) | Acrasia, Plasmodiophorina |
| Protozoans | They are heterotrophs and live as predators or parasites. They are classified into four types. | Plasmodium |
Types of Protozoans and their Features/ Diseases Caused
The following table shows the types of protozoans along with their features and the diseases they cause:
| Protozoans | Features/Diseases Caused | Examples |
|---|---|---|
| Amoeboid Protozoans | They form pseudopodia to capture their prey. Some of them are parasites | Amoeba, Entamoeba |
| Flagellated Protozoans | Sleeping sickness, a disease caused by the parasitic forms | Trypanosoma |
| Ciliated Protozoans | Cilia (locomotion) and a gullet, a cavity is present on the body | Paramoecium |
| Sporozoans | Some species cause malaria | Plasmodium |
3. Kingdom Fungi
Fungi are multicellular and the how heterotrophic mode of nutrition (saprophytes/parasites/symbiotic- mycorrhiza). Some fungi are unicellular, e.g. yeast.
Some Useful Fungi
Mushrooms and yeast are the most valuable fungi. Mushrooms are edible and are a good source of protein. Yeast is used to make bread and cheese. Penicillium fungi are used to produce antibiotics.
Some Harmful Fungi
Some fungi cause diseases in both plants and animals, e.g. wheat rust disease is caused by the Puccinia fungus.
Reproduction in Fungi
There are three modes of reproduction in fungi, i.e. vegetative, asexual and sexual.
- Vegetative Propagation: It takes place by fragmentation, fission and budding.
- Asexual Reproduction: It takes place by conidia or sporangiospores, or zoospores.
- Sexual Reproduction: It takes place by oospores, ascospores and basidiospores.
Stages of Sexual Reproduction in Fungi
- Plasmogamy: It is the fusion of protoplasts between two motile or non-motile gametes.
- Karyogamy: It is the fusion of two nuclei.
- Meiosis in the zygote gives rise to haploid spores.
In ascomycetes and basidiomycetes, the dikaryotic stage (n + n, i.e., two nuclei per cell) is formed, known as a dikaryon, and the phase is dikaryophase.
Four Major Groups of Fungi
Fungi are broadly classified into four major groups based on their structure and mode of reproduction.
| Classification of Fungi | Characteristic Features | Examples |
| Phycomycetes | They grow on decaying wood in moist sites and are obligate parasites on plant bodies | Mucor, Rhizopus (bread mould fungi) and Albugo (parasitic fungi on mustard) |
| Ascomycetes (sac-fungi) | Neurospora is used in biochemical and genetic work. Some are edible, e.g. morels and truffles | Penicillium, yeast, Aspergillus, Claviceps and Neurospora |
| Basidiomycetes | Some are edible, e.g. mushrooms. Mushrooms are rich in protein | Agaricus (mushroom), Ustilago (smut) and Puccinia (rust fungus), Mushrooms, bracket fungi, puffballs |
| Deuteromycetes | They are known as 'Imperfect Fungi' because in this group, only asexual or vegetative phases are seen. Some fungi of this group are saprophytes or parasites, while the majority are decomposers of litter, which aid in mineral cycling. | Alternaria, Colletotrichum and Trichoderma |
4. Kingdom Plantae
All eukaryotic organisms that contain chlorophyll, usually known as plants, are classified as Plantae. A few species, like parasites and plants that feed on insects, are partially heterotrophic. Insectivorous plants include bladderwort and Venus flytraps, and parasites like Cuscuta feed on them. The eukaryotic structure of plant cells has large chloroplasts and a cell wall comprised primarily of cellulose. Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms are all part of the plant kingdom.
Alternation of Generation
The haploid gametophytic and the diploid sporophytic phases of a plant's life cycle alternate with one another. Various plant families have different haploid and diploid phase lengths and whether they are independent or reliant on others.
5. Kingdom Animalia
They are multicellular and heterotrophic (show the holozoic mode of nutrition) eukaryotes. They lack cell walls. Almost all the animals show locomotion. Sexual reproduction occurs by the fusion of male and female gametes, which give rise to an embryo followed by repeated cell divisions.
Viruses, Viroids, Prions And Lichens
The differences between viruses, viroids and prions are given below:
| Viruses | Viroids | Prions |
|---|---|---|
| They are oblique intracellular agents | They are oblique intracellular agents | They are an abnormal form of a cellular protein |
| They have either DNA or RNA, which is surrounded by a protein coat | They consist of only RNA. The protein coat is absent | They don't possess DNA or RNA. Only the protein coat is present |
Bacteriophage
Bacteriophages are also known as phages. These are the viruses that infect and replicate in the bacterial cells.
Tobacco Mosaic Virus
The tobacco mosaic virus (TMV) consists of single-stranded RNA. It infects tobacco plants and members of the family Solanaceae. The infection can cause some patterns, like a mosaic, which shows mottling and discolouration on the surface of the leaves.
Lichens
The close association of fungus and algae form lichens. They are found in a pollution-free environment. Lichens are used in deodorants, pH papers, incense sticks, toothpaste and perfumes. The fungal component is known as the mycobiont, and the algal component is known as the phycobiont.
