The attempts to classify living organisms began centuries ago. The first classifications were made to identify organisms that were useful to man for food, shelter, and clothing. It was Aristotle who devised the first scientific method of classification. Based on morphological characters, he classified plants into trees, shrubs, and herbs. and animals into two groups, those that had red blood and those that didn't.
Later, Carl Linnaeus came up with the two kingdom system of classification, Kingdom Plantae, that included all the plants, and Kingdom Animalia, that included all the animals. This classification system was reasonable for that time. because it was easy to distinguish between immobile plants and mobile animals.
However, this system did not distinguish between unicellular and multicellular organisms, prokaryotes and eukaryotes, and photosynthetic and non-photosynthetic organisms. There were also a large number of living organisms, that did not fall into any category under the system. New studies about various organisms also revealed enormous variations of characteristics within the organisms of the same group, like cell structure, mode of nutrition, methods of reproduction, and evolutionary relationships. Therefore, though the two kingdom classification system was used for a long time, It proved to be an inadequate system recently. In 1969, R.H.
Whittaker proposed a five kingdom classification system, where three more kingdoms, Monera, Protista, and Fungi, were added to the existing kingdoms of Plantae and Animalia. The main criteria that Whittaker used for this classification system, were cell structure, phallus organization, mode of nutrition, methods of reproduction, and phylogenetic relationships. Phylogenetics is the study of evolutionary relatedness among various groups of organisms.
The earlier classification systems included all bacteria, algae, fungi, mosses, ferns, gymnosperms, and angiosperms under the plants category, based on the common premise that each one had a cell wall in their cells. The five kingdom classification remedied this by placing organisms in different kingdoms based on the differences in their characteristics. Based on the differences in cell wall composition, heterotrophic fungi differ from autotrophic plants and were grouped under kingdom fungi. All prokaryotic organisms were grouped together under Kingdom Monera, and unicellular eukaryotic organisms were placed in Kingdom Protista.
Previously, unicellular Chlamydominus and multicellular Spirogyra were placed together under Algae in Kingdom Plantae. However, in the new classification system, Chlamydominus moved to Kingdom Protista, while Spirogyra remained in Kingdom Plante. The five kingdom classification system not only classified organisms into different groups, but also put together organisms that were earlier placed in different kingdoms. For example, Kingdom Protista brought together both Chlamydominus and Chlorella.
with other eukaryotes like amoeba and paramecium, which were initially placed in kingdom animalia. Therefore, the five kingdom classification system was evolved with an advancement in our understanding of the characteristics of organisms and their phylogenetic relationships. Depending on more such discoveries, the classification system may further need to be evolved in future. Bacteria are the sole members of Kingdom Monera. Bacteria can be found anywhere, including extreme habitats like hot springs, deserts, and deep oceans.
Different bacteria synthesize their food in different ways. Some of them are autotrophs. That is, they synthesize their own food from inorganic substrates.
However, a majority of them are heterotrophs. That is, instead of synthesizing their own food, They depend on other organisms or dead organic matter for food. Based on their shape, bacteria can be classified into four types.
The spherical Coccus, the rod-shaped Bacillus, the Vibrium, which is in the shape of a comma, and the Spiral Spiralum. 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. Cyanobacteria commonly known as Blue-Green Algae are example of Eubacteria. They are unicellular, filamentous algae that live in colonies.
Their colonies are usually surrounded by gelatinous sheaths. Bacteria can be divided into autotrophs as well as heterotrophs. Autotrophs can further classified into photosynthetic autotrophs and chemosynthetic autotrophs. Like plants, cyanobacteria have chlorophyll A. and they are like them.
Photosynthetic autotrophs, that is, they synthesize their food by using light energy. Did you know cyanobacteria like Nostoc and Anabena can also fix atmospheric nitrogen in their specialized cells called heterocysts? Chemosynthetic autotrophs oxidize various inorganic substances such as nitrates or ammonia and use the energy released for ATP production. They help in recycling nutrients like nitrogen, sulfur, and phosphorus.
On the other hand, heterotrophic bacteria are decomposers. They help us make curd, fix nitrogen in legumes, and produce antibiotics. They can also cause harm by damaging crops. And some diseases like typhoid, tetanus, and cholera are known to be caused by bacteria.
Bacteria reproduce by fission. They can also reproduce sexually. by a primitive DNA transfer from one bacterium to another through the pilus. Under unfavorable conditions, bacteria produce spores. Apart from Eubacteria, there are special bacteria called Archaebacteria that live in extreme conditions.
For example, Halophiles, live in extreme salty areas. Thermo acidophiles live in hot springs, while methanogens live in marshy areas. Methanosians are also present in the gut of ruminant animals as cows and buffaloes.
These bacteria are responsible for the production of methane, also referred to as biogas, from the dung of these cattle. All Archae Bacteria have a different cell wall structure than other bacteria, which helps them to survive in extreme conditions. Unlike Eubacteria and Archae Bacteria, Mycoplasmas are the bacteria that do not have a cell wall.
They are also smallest living beings ever known who can survive without oxygen. They are pathogenic in animals and plants. Kingdom Monera therefore, consists of Eubacteria, Archae bacteria, and, Mycoplasmas. All unicellular Eukaryotes, come under Kingdom Protista. Members of the Kingdom Protista, are primarily Aquatic, Chrysophytes, Dianoflagellates, Euglenoids, Slime Molds, and Protozones, are few categories of Protists.
Since they are all eukaryotes, they contain a well-defined nucleus and other membranous organelles. Some protists also have either a flagellum or a coelium. They reproduce both asexually through cell fusion and sexually via zygote formation. Crucifites include diatoms and golden algae or desmids.
They are microscopic and float in great numbers in both fresh water and saline water. Also, they are mostly photosynthetic. Diatoms are known as chief producers in the oceans.
Diatom cell walls are embedded with silica, making them indestructible. Diatoms therefore manage to leave behind a large amount of their cell wall deposits in their habitats. These fossilized remains of diatoms, form, diatomaceous earth. It can be put to various uses like, polishing, and filtration of oils and syrups.
Dianoflagellates are colorful protists found both in marine and freshwater. Depending on the main pigments in their cells, they are yellow, green, brown, blue, or red. Sometimes, when a red dinoflagellate multiplies rapidly in the sea, it makes the sea appear red.
This red-tite phenomenon is known to be harmful, as the toxins so released can kill other marine animals, such as fish. Dinoflagellates usually have two flagella, longitudinal flagella and transverse flagella. Their names indicate how both the flagella are positioned.
Euglenoids too, like dinoflagellates, have two flagella, one short and the other a long one. Instead of a cell wall, euglenoids have a protein-rich layer called pellicle, that makes their bodies flexible. Euglenoids are phototrophic autotrophs, in the presence of sunlight, since they also have chloroplasts, like plants.
However, they behave as heterotrophs, in the absence of sunlight. Slime molds, are saprophytic protists, that is, they live on dead organic matter. They form a huge aggregation called plasmodium. However, during unfavorable conditions, this plasmodium differentiates and forms spores that possess true walls.
These spores are so resistant that they can survive for years under adverse conditions. Protozoans are protists. that are believed to be primitive relatives of animals.
Did you know that the word protozoans means the first animals? All protozoans are heterotrophs and live as predators or parasites. They can be classified into four types. Amoeboid protozoans, flagellated protozoans, ciliated protozoans and sporozoans. Out of these, amoeboid protozoans are found in fresh water, marine water, as well as moist soils.
They use false feet called pseudopodia to move and capture their prey. Some amoeboids like Entamoeba are parasites. Flagellated protozoans, as the name suggests, have flagella.
They can be either free living or parasitic. Some parasites like Trypanosoma can also cause diseases like sleeping sickness. Ciliated protozoans like Paramecium can be found at almost all places where there is water.
They are very active since they possess thousands of cilia. It is the coordinated movement of these cilia. that steers the water, containing their food, into their gullet. The gullet is a cavity that opens to the outside of the cell surface. Lastly, sporozoans are organisms that have an infectious spore-like stage in their life cycle.
Plasmodium is the most famous sporozoan. It causes malaria in humans, and is also known as the malaria parasite. Common mushrooms, toadstools, and the mold on leftover bread are all examples of fungi.
Fungi can be found everywhere in water, air, soil, plants, and animals. While there are some fungi like that can cause diseases like wheat rust in plants. There are also some like penicillium that provide us with antibiotics.
With the exception of yeasts, which are unicellular, fungi are usually filamentous. Fungi consist of long, slender, thread-like structures called hyphae. A network of hyphae is known as mycelium.
There are two types of hyphae. The first type is made up of continuous tubes filled with multinucleated cytoplasm and is called synocytic hyphae. The second type of hyphae has septae or cross walls. Apart from hyphae, fungi are distinguished by their cell walls.
which are made of chitin and polysaccharides. Fungi are heterotrophic. Fungi that absorb organic matter from dead substrates are called saprophytes.
Fungi which depend on living plants and animals are called parasites. Some fungi are associated with other organisms in mutually beneficial relationships. For example, fungi associate with algae to form lichens. Other fungi associate with the roots of plants as mycorrhizae.
In both lichens and mycorrhizae, the relationship shared by fungi with the algae and the plant is called symbiosis. Fungi can reproduce vegetatively, asexually, as well as sexually. Fungi reproduce vegetatively by fragmentation, fission, and budding. They can reproduce asexually by either one of the following spores, conidia, zoospores, or Sporangia spores. They also reproduce sexually by either Oospores, Ascospores or Basidiospores.
All these spores are produced in structures called fruiting bodies. There are three steps in the sexual cycle Plasmogamy, Karyogamy, and meiosis. The first stage involves the fusion of protoplasms between two motile or non-motile gametes. This is called plasmogamy. Plasmogamy occurs when two haploid hyphae of compatible mating types fuse.
After plasmogamy, the two nuclei fuse together in the second stage, known as whereas in some fungi, the fusion of two haploid cells, immediately results in the formation of diploid cells, or 2N. In other fungi, like, Ascomycetes, and, Basidiomycetes, an intervening, dicaryotic stage occurs. This condition of two nuclei per cell is called Dicarion and the phase is called Dicariophase. Later, the parental nuclei fuse to make a diploid cell.
The third stage involves reduction division, that is, meiosis, to form haploid spores. Meiosis occurs in the fruiting bodies of fungi. Together, the fruiting bodies, the mode of spore formation, and the morphology of the mycelium, form the basis of the division of the kingdom fungi, into various classes. There are four main classes of fungi.
Phycomycetes Ascomycetes, Basidiomycetes, and Deuteromycetes. Phycomycetes are found in aquatic habitat, decaying wood in moist places, and as obligate parasites on plants. Muca, Rhizopus, and albuco are examples of phycomycetes. Phycomycetes have aseptate and synocytic mycelium. They reproduce asexually by zoospores in the case of motile gametes and by aplanospores in the case of non-motile gametes.
On the other hand, sexual reproduction leads to the formation of zygospores After the fusion of gametes, the gametes may be either similar or dissimilar. Members of the Ascomycetes class are commonly known as sac fungi. For example, Aspergillus, Claviceps, and Neurospora. They can be unicellular, like yeast. or multicellular, like, Penicillium.
They are saprophytic, decomposers, parasitic, or, coprophilous. That is, they grow on dung. Ascomycetes, have branched and septate mycelium. They reproduce asexually, by spores called Conidia. that are produced exogenously on the special mycelium called conidiophores.
Sexually, they reproduce through spores called ascospores that are produced endogenously in sac-like ascii. The ascii in turn are arranged in fruiting bodies called ascocarps. Some members of Ascomycetes like morels, are edible, while others like Neurospora, are used in biochemical studies. Some common examples of Basidiomycetes, are mushrooms, bracket fungi, and puffballs.
They grow in soil, on logs, and inside plant bodies as parasites such as rust and smut. Like Ascomycetes, Basidiomycetes too have branched and septate mycelium. Vegetative reproduction by fragmentation is a common form of reproduction among Basidiomycetes. Sexual reproduction in Basidiomycetes takes place even in the absence of sex organs. In the plasmogamy stage, two somatic cells of different genotypes fuse to form a dicaryote.
The dicaryotic mycelium gives rise to the fruiting body called the basidiocarp, on which basidia arise and produce basidiospores exogenously as a result of meiosis. The last class of fungi Deuteromycetes is known as imperfect fungi because we do not know enough about their life cycles. Only their asexual or vegetative phases are known. Deuteromycetes reproduce only through asexual spores known as conidia.
They too have a septate and branched mycelium. They are saprophytes or parasites, while a large number of them are decomposers, which help in mineral cycling. Alternaria, Coletotrichum, and Trichoderma, are examples of deuteromycetes.
Therefore, Members of Kingdom Fungi display a diverse set of characteristics. All eukaryotic, chlorophyll containing organisms, that is, plants, fall under Kingdom Plante. Plant cells are characterized by the presence of a cell wall, which is made up of cellulose. While most plants are autotrophs, some plants such as bladderwort and venus flytrap are also insectivorous. Some of them, such as cascuta, are also parasites.
Kingdom plantae also includes algae, bryophytes, pterodophytes, gymnosperms, and angiosperms. Most plants have two alternate phases in their life cycle, the diploid sporophytic and the haploid gametophytic phases. The length of these haploid and diploid phases and whether they are free living or dependent varies between groups of plants.
This phenomenon of alternation of sexual and asexual forms in the life cycle, is referred to as, alternation of generation. It can be noticed in plants such as, liverworts and mosses. Unlike kingdom plantae, kingdom animalia, comprises only heterotrophic, multicellular, eukaryotic organisms. Also, members of kingdom Animalia lack cell walls in their cells.
Most of them depend on plants for food directly or indirectly. They digest their food in the internal cavity, which means that they have extracellular digestion. However, They can store food reserves as glycogen or fat.
Animals have a holozoic mode of nutrition. That is, they ingest food. In animals, ingestion generally means they take in food through the mouth and pass it through the gastrointestinal tract.
Animals reproduce sexually. After a male and a female copulate, which is followed by embryological development, they have a fixed growth pattern to adulthood, till they develop into a definite shape and size. Higher forms of animals, such as a monkey or a human, also have sensory and neuromotor mechanisms.
Also, most animals are motile. That is, they can move independently and are capable of locomotion. R.
H. Whittaker's Five Kingdom System of Classification did not take viruses, viroids and lichens into account. Viruses and viroids are acellular organisms. Since only organisms with cell structures were regarded as living, they were not included in the classification because they were not considered true living beings.
Viruses are non-cellular organisms that possess an inert, crystalline structure outside their living cell. The name virus was coined by Louis Pasteur. The word virus means venom.
The name is apt because when a virus infects a cell, it takes over the host cell's machinery to replicate itself. In this process, the host cell is killed. Though a virus remains inert outside its host cell, it cannot live independently without its host. Thus, viruses are known as, obligate parasites.
It was D.J. Ivanowski, who in 1882, discovered the tobacco mosaic virus. or TMV, the microbe that causes mosaic disease in tobacco.
This virus was smaller than a bacteria. Later in 1898, M.W.B. Geronik demonstrated that extracts of infected tobacco plants could also cause infection in other healthy plants. He referred to the virus as, Contagium vivum fluidum, meaning, infectious living fluid. W.M.
Stanley, in 1935, further proved that, viruses could be crystallized. This meant fluid could be converted into crystals. These viral crystals are mainly protein crystals.
In addition to proteins, viruses have a genetic makeup, comprising either RNA or DNA. Therefore, viruses are nucleoproteins, and their genetic material is infectious. Usually, Viruses that infect plants have single-stranded RNA, while those that infect animals have double-stranded RNA or DNA. Bacteriophages, the viruses that infect bacteria, usually have double-stranded DNA.
Typically, bacteriophages have a protein coat called a capsid that protects the nucleic acids. A capsid is made up of small subunits called capsomeres that are arranged either helically or in a polyhedral manner. Viruses infect plants causing symptoms such as mosaic formation, leaf rolling and yellowing, dwarfing and stunted growth. Humans too can be affected by viruses.
Diseases such as the common cold, mumps, and smallpox, are known to be caused by viruses. Even AIDS is caused by a virus known as HIV. Like viruses, viroids are also infectious.
only they are smaller. They were first discovered by Teodina in 1971. Viroids lack the protein coat that viruses have hence the name viroid. Also they have free RNA of low molecular weight. They are known to cause potato spindle tuber disease. Lichens, though they may look like single organisms, actually have two organisms inside them, algae and fungi.
Both algae and fungi together have a mutually beneficial association called symbiosis. The algae component, known as phycobiont, is autotrophic, while the fungal component, called mycobiont, is heterotrophic. So, algae prepare food for fungi, and fungi in turn, provide shelter.
and absorb water and nutrients for algae. Therefore, viruses, viroids and lichens were not placed anywhere in the classification system.