BIOLOGICAL CLASSIFICATION

Biological classification is the scientific procedure of arranging organisms into groups and subgroups on the basis of their similarities and dissimilarities, and placing the groups in a hierarchy of categories. Classifying things in day-to-day life makes life so much easier. Similarly, classifying organisms is important because of the following aspects:

(1) It makes identification, study and research easier for diverse life forms.

(2) It gives a glance at all the organisms in one picture.

(3) To make us understand and study the characteristics, similarities and differences among organisms so that grouping can be easy.

(4) To trace the ancestry Le. evolutionary relationships among them, understand missing links and connecting links.

(5) To understand the exact position of an organism in classification.

(6) To evolve a truly natural or phylogenetic system which should indicate the origin and evolution of the species.

KINGDOM SYSTEM OF CLASSIFICATION AND ITS CHARACTERISTICS

Aristotle done the earliest attempt to classify plants into trees, shrubs and herbs. He also divided animals into two groups on the basis of the presence or absence of red blood cells in them:

(1) Enaima (animals with red blood cells in them)

(2) Anaima (animals without red blood cells in them) Carolus Linnaeus gave two-kingdom systems of classification i.e. Plantae and Animalia which segregated plants and animals into two different groups Drawback of this system was that it does not segregate between eukaryotes and prokaryotes, unicellular and multicellular, photosynthetic and non-photosynthetic organisms.

RH Whittaker (1969), proposed the most widely accepted Five-Kingdom Classification of organisms. The kingdoms were named Monera, Protista, Fungi, Plantae, and Animalia. Main Criteria for Classification

(1) Cell structure

(2) Body organisation

(3) Mode of nutrition

(4) Reproduction

(5) Phylogenetic relationships

FIVE KINGDOMS

Kingdom Monera

Monera includes the most primitive form of life. They are the smallest, simplest form of an organism. Monerans are adapted to all kinds of habitats. They are the most abundant of all organisms as they are omnipresent, in oceans, dry deserts, hot springs, icebergs, and inside and outside the body of other organisms as parasites. They can tolerate extreme environmental conditions.

Important

Milk is converted into curd by Lactobacillus. It is rod-shaped bacteria, that enhances the nutritional quality of curd as it enriches it with vitamin B12.

Bacteria are simple, microscopic prokaryotic organisms. Although they are unicellular, in bacteria. all the metabolic functions are performed in the ell itself.

They have various modes of nutrition some are autotrophic, which make their own food from inorganic substances, they may be photoautotrophic or chemoautotrophic, while others are heterotrophic, which depend on other organisms to obtain their nutrition, they may be saprophytic (obtaining nutrition from dead and decaying organic matter) or parasitic (obtain nutrition from other organisms by living on or inside their body).

Bacteria can be found in different forms and shapes. On the basis of their shape they are of the following types- Coccus (Spherical), Bacillus (rod-shaped), Vibrio (comma-shaped). Spirillum (spiral-shaped).

Important

Bacteria was discovered by Anton van Leeuwenhoek (1632-1723). He observed bacteria in 1675. Louis Pasteur laid the foundation of Bacteriology by developing culture techniques.

As Monera is the only kingdom that consists of prokaryotic organisms i.e., bacteria (sole member of Kingdom Monera). This kingdom consists of two major groups- Archaebacteria (ancient bacteria) and Eubacteria (true bacteria).

Archaebacteria

Plasmid Archaebacteria are a unique type of bacteria which can survive in extreme conditions of the environment such as:

(1) Extreme salt-rich conditions (Halophiles).

(2) They have the ability to tolerate high temperature as well as high acid, often living in hot sulphur springs (Thermoacidophiles).

(3) They are also found in marshy areas (Methanogens).

Archaebacteria are characterised by the presence of non-cellulosic polysaccharides and protein cell wall instead of peptidoglycan wall, which help them to survive in extreme conditions. Methanogens live as symbionts in the gut of other ruminant animals such as cows and buffaloes and they are responsible for the production of the methane (biogas) from the dung of these cattles.

Eubacteria

There is a vast variety of eubacteria or true bacteria. They can be Identified by the presence of a rigid cell wall and if they are motile, a flagella can be seen as Locomotory organ.

Cyanobacteria: Cyanobacteria or blue-green algae (BGA) are gram positive photosynthetic prokaryotic organism which perform oxygenic photosynthesis. Cyanobacteria are one of the most successful autotrophic organisms on the earth. They have photosynthetic pigments like chlorophyll a, carotenoids and phycobilins for photosynthesis.

Cyanobacteria may be unicellular, colonial or filamentous. Some Cyanobacteria of filamentous forms possess special large cells called heterocysts which have the ability to fix atmospheric nitrogen E.g. Nostoс and Anabaena.

Some bacteria are chemosynthetic autotrophs that oxidise various inorganic substances like nitrates, nitrites and ammonia and use the released energy to produce AТР. They play a significant role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.

Important

Mycoplasma is the simplest and the smallest free-living organism present on earth, which lacks a cell wall and thus, it can change its shape. Mycoplasma is heterotrophic in nature Some live as saprophytes but mostly are parasitic in nature. They can survive in the absence of oxygen as well.

Kingdom Protista

Protista includes all the unicellular, microscopic and eukaryotic organisms. They are mostly aquatic. They have well-developed cell structures with a defined nucleus and other membrane-bound organelles. They possess special features for locomotion like cilia and flagella. They can reproduce by both asexual (Binary, multiple fission and spore formation) and sexual methods (fusion of gametes and cell fusion). Protists can be considered as a connecting link between Monera and multicellular Fungi, Plants and Animals.

Chrysophytes, Dinoflagellates, Slime moulds and Euglenoids groups come under the Kingdom Protista.

Chrysophytes

Chrysophytes include diatoms and golden algae (desmids). Diatoms occur in all aquatic and moist terrestrial habitats.

They may be free-floating or bottom dwellers. They constitute the major part of the phytoplankton of oceans.

They are microscopic, unicellular, photosynthetic organisms. The body wall of Diatoms is composed of cellulose and covered by a siliceous shell.

Cell wall is also composed of two overlapping halves which fit together like a soap box. These siliceous walls do not decay easily and were collected over millions of years on the seafloor and formed a big heap called diatomaceous earth. This diatomaceous earth is mined and used in industrial filtration and in metal polishing.

Diatoms are the main source of food for aquatic animals. Example: Triceratium.

Dinoflagellates

Dinoflagellates are unicellular photosynthetic organisms found in both marine as well as freshwater. Some Dinoflagellates (Gonyaulax) grow in large numbers on the sea surface and make the sea red, also called red tides.

They are yellow-brown to dark brown due to the presence of pigments. Most of them are motile and flagellated.

The cell wall of Dinoflagellates is composed of plates made up of cellulose.

The cell possesses two flagella if present- one is transverse and the other is longitudinal, lying in a furrow between the plates.

Some Dinoflagellates are poisonous to vertebrates; they release a toxin which kills the fish.

Euglenoids

Euglenoids usually occur in freshwater and some forms can be found in damp soil.

Euglenoids are unicellular flagellates and they lack a cell wall, their body is covered with a thin, flexible layer called a pellicle.

The number of flagella in Euglenoids can be one or two. Out of the two, usually, one is short and the other is long.

The mode of nutrition in Euglenoids is either photoautotrophic or heterotrophic in the absence of sunlight Photoautotrophs possess chloroplast and photosynthetic pigments which are similar to pigments present in higher plants which is why euglenoid's some members are considered as connecting links between plants and animals. Example: Euglena.

Slime Moulds

Slime moulds are generally found on dead, decaying leaves, twigs and other decaying matter. Their mode of nutrition is saprotrophic. The vegetative part or somatic phase does not possess a cell wall and forms aggregation called Plasmodium.

All the nuclei in the Plasmodium divide simultaneously and spread over several feet. However, the reproductive structure (spores) is produced when Plasmodium reaches maturity, differentiates and forms fruiting bodies (sporangia) under unfavourable conditions. These sporangia undergo meiotic division and become spores which have a true cell wall. They are dispersed by air. They can even survive in adverse conditions for many years.

Protozoans

Protozoans may be aquatic, terrestrial, free-living or parasitic. Their mode of nutrition is mainly heterotrophic (holozoic). On the basis of locomotory organelles, the protozoans are divided into four major groups:

(1) Amoeboid protozoans: They are mostly found in freshwater and free-floating in seawater and also live on damp or moist soil. They are parasitic in nature; they develop temporary outgrowth of protoplasm called pseudopodia for locomotion and capturing of food. Reproduction can be asexual or sexual type. For example, Amoeba

(2) Flagellated protozoans: Flagellated protozoans are free-living aquatic organisms. They are parasitic, some of them are human parasites which cause diseases like sleeping sickness. They possess flagella for locomotion. For example, Trypanosoma.

(3) Ciliated protozoans: They live as free-living organisms in fresh and marine water. The most unique feature of these organisms is the presence of a number of cilia on their whole body surface for locomotion attachment and food capture. They also have a cavity (gullet) that opens to the outside of the cell surface and the food is carried into it. For example, Paramecium.

(4)Sporozoans: All sporozoans are generally endoparasitic in nature and cause diseases. Their Life cycle may include two different hosts like Plasmodium which causes malaria, and requires two hosts. The infective stage of plasmodium is the formation of sporozoites present. In salivary glands of mosquito. which are then injected into humans. For example, Plasmodium and Monocystics.

Kingdom Fungi

The Kingdom Fungi is unique kingdom that have a great diversity in habitat and morphology. All the members of this kingdom show a heterotrophic mode of nutrition. They are mostly found in warm and humid areas.

Fungi are diverse organisms ranging from unicellular yeasts to highly complex multicellular organisms. Their body is typically thalloid. In most fungi, the body consists of long, tubular branched filaments called hyphae. The network of this hyphae is called mycelium. The hyphae can be of two types aseptate or septate.

In aseptate, there are no cross walls or septa in their hyphae which makes hyphae multinucleated during nuclear division and if mycelium is without septa it is called coenocytic hyphae. On the other hand, in hyphae having septae or cross walls, cell may have one, two or more nuclei in a single chamber or cell. The cell wall contains chitin along with other polysaccharides.

The mode of nutrition in fungi is heterotrophic as they lack chlorophyll so they obtain their food from other organisms either dead, decaying or living. If they obtain their food from non-living decaying organic matter, they are called saprophytes. If they depend on other living plants or animals, they are called parasites. Some fungi grow in symbiotic association with algae to form lichens, and some form associated with the roots of higher plants and form mycorrhiza.

The fungi reproduce by vegetative, asexual and sexual methods. Vegetatively it reproduces by Fragmentation, Budding, and Fission. Asexual means of reproduction occur by special reproductive structures - Zoospores, conidia, sporangiospores and sexual method takes place through oospores, ascospores and basidiospores. The sexual reproduction in fungi completes in three steps:

(1) Plasmogamy: In this, the protoplasms of two motile or non-motile gametes fuse with each other. The nuclei of two sex cells do not fuse. Thus, the cell becomes a binucleate or dikaryon. and the phase is known as dikaryophase

(2) Karyogamy: It is the fusion of two nuclei.

(3) Meiosis: After the fusion of nuclei, the diploid nucleus divides by meiosis and leads to formation of haploid nuclei or haploid spores.

Kingdom Fungi is further divided into various classes on the basis of morphology of mycelium, mode of spore formation and fruiting bodies.

Phycomycetes

The important characteristic feature of this class is mycelia, in which the hyphae are aseptate and multinucleate (coenocytic). Asexual reproduction takes place by formation of spores produced inside the sporangium - aplanospores (non-motile), and zoospores (motile). Sexual reproduction occurs through fusion of two gametes. It may be isogamous (morphologically similar) and anisogamous or oogamous (morphologically dissimilar). They are aquatic organisms found in damp and moist areas, on decaying matter and as parasites on plants. Examples: Mucor, Rhizopus (bread mould) and Albugo (parasitic).

Ascomycetes

Ascomycetes is a class of diverse fungi, mostly terrestrial and they are multicellular except yeast, which is basically unicellular. They may include morels and truffles which are edible. They are saprophytic, decomposers, parasitic or coprophilous (growing on dung). The main characteristic feature of this class is that mycelium consists of septate and branched hyphae. Mostly ascomycetes reproduce asexually by formation of conidia which are non-motile and are produced exogenously from hyphae. The sexual reproduction takes place by fusion of gametes and formation of haploid ascospores which remain inside the sac-like structure called asci and these asci are produced inside the complex fruiting bodies called ascocarps whereas ascospores are liberated from asci and germinate to produce new mycelia.

Examples: Aspergillus, Claviceps and Neurospora (used in biochemical and genetic work).

Basidiomycetes

They are a diverse type of fungi- some are parasitic which cause diseases such as smuts and rusts in crops, some are edible like mushrooms, and some are decomposers which decompose wood material. The characteristic feature of this class is their mycelium composed of branched, filamentous and septate hyphae. The asexual spores are absent but they reproduce asexually by the process of fragmentation. In sexual reproduction, the sex organs are absent so plasmogamy is achieved by somatogamy in which fusion of two vegetative cells takes place and results in the formation of dikaryon which gives rise to basidium. Each basidium forms four haploid basidiospores which are inside the basidium. These basidia produce fruiting bodies called basidiocarps.

Examples: Agaricus (mushroom), Ustilago (smut) and Puccinia (rust).

Deuteromycetes

They are saprotrophs, decomposers and parasitic in nature. The characteristic feature is that the mycelium is septate and branched. Reproduction only takes place by asexual method by formation of conidia. It is also known as imperfect fungi as these fungi are those in which the sexual stage is not known.

When the sexual forms of these fungi were discovered they were moved into classes they rightly belong to. The members of this class help in mineral cycling.

Examples: Altemaria, Colletotrichum and Trichoderma.

Important In the year 1973. Gaspard Bauhin gave the term fungus. The plural form of fungus was coined by Carolus Linnaeus He named its fungi.

Kingdom Plantae

This kingdom contains all photosynthetic organisms and their non-photosynthetic relatives.

The Kingdom Plantae is eukaryotic, multicellular with photosynthetic pigment called chlorophyll. The mode of nutrition is mainly autotrophic using chlorophyll, whereas some are partial heterotrophs - parasitic (Cuscuta) or insectivorous plants (Venus Flytrap and bladderwort). The plant cell contains a rigid cell wall made up of cellulose, large vacuole and prominent chloroplasts. Life cycle consists of alternating haploid gametophyte and diploid sporophyte generation and this phenomenon is known as alternation of generation. The Kingdom Plantae is further classified into Algae, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.

Kingdom Animalia

This kingdom has maximum number and most diverse types of organisms. They are multicellular, eukaryotic and heterotrophic organisms. Their cells lack a cell wall and plastids and they have smaller vacuoles. The mode of nutrition is holozoic and digestion takes place within the cavity and stores food as glycogen or fat. Reproduction is generally sexual. Zygote is formed by fusion of male and female gamete and then develops into embryo takes place. They follow a definite pattern of growth.

ACELLULAR ORGANISMS

Viruses

When you hear the phrase Virus, what comes to your mind? Perhaps an ordinary cold, cold sore, or perhaps a global pandemic, like the one we were experienced, in the year 2020. caused by a coronavirus that began in bats.

Viruses, in reality, are ubiquitous in the living world. infecting, altering, and interacting with all organisms, from the smallest to the largest, and may be found in every ecosystem on the planet. They are as much a part of our daily lives. But what are they, exactly? Is it true that they are alive? What exactly do they do?

Important →

Whittaker did not included viruses and lichens in his five kingdom classification as viruses are a group of noncellular, ultramicroscopic and highly infectious agents that multiply inside the living host. Outside the host they are non-living. They are sub-microscopic entities capable of being introduced into specific living cells and reproducing inside the host cells only.

Viruses are a group of non-cellular/acellular, infectious organisms. They require the host body to multiply or replicate. Outside the host body, they act as an inert particles. They are the connecting link between living and non-living organisms. Dmitri Ivanovsky gave the name virus which means poisonous fluid while extracting an infectious agent causing mosaic disease in the tobacco plants and MW. Beijerinck named the fluid Contagium vivum fluidum (infectious living fluid) which is extracted from infected plants. W.M Stanley stated that viruses are obligate parasites and exist in crystalline form outside the host body. Viruses have proteins and contain genetic material that is either RNA or DNA as their genetic material DNA- containing viruses are known as deoxyviruses while RNA-containing viruses are termed riboviruses. Each of them has two subtypes, double-stranded or single-strand. The viruses which infect bacteria (bacteriophages) have double-stranded DNA.

It has an outer coat of protein called capsid and capsid is made up of several subunits of capsomeres and is closely packed and arranged in helical forms.

Human diseases caused by viruses are mumps, smallpox, herpes, Influenza and AIDS, etc.

In plants, the viruses cause various diseases and their symptoms are mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

Viroids

Viroids are the smallest self-replicating particles which were discovered by a scientist named T.О. Diener in year 1971. Viroids are infectious agents composed of free RNA with low molecular weight and lack capsid or protein coat. They are responsible for a plant disease called potato spindle tuber.

Prions

They are misfolded proteins which cause infectious diseases. It can be transmitted from one organism to another. They are similar in size to viruses. The diseases caused by prions are Bovine Spongiform Encephalopathy (BSE) i.e., Mad cow disease in cattle and Cr-Jakob disease (CJD) in humans.

Lichens

Lichens are symbiotic organisms which are formed by a permanent association of a fungus and an alga. The fungal partner of a lichen is called mycobiont whereas the algal partner is called phycobiont. The fungus provides shelter water, minerals and protects the alga from unfavourable conditions and the alga prepares food by the process of photosynthesis. Both are associated in such a manner that if someone actually saw a lichen in nature would never imagine that it is made up of two different organisms. Lichens are very good pollution indicators as they are sensitive to sulphur dioxide. So, they do not grow in polluted areas.

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