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Biological Classification
Since the dawn of civilization, humans have attempted to classify living organisms, initially out of necessity for food, shelter,
and clothing. Aristotle was the first to attempt a more scientific classification, using morphological characters to classify
plants into trees, shrubs, and herbs, and animals based on the presence or absence of red blood.
In Linnaeus's time, a Two Kingdom system was developed, comprising Plantae and Animalia. This system did not differentiate
between:
Eukaryotes and prokaryotes
Unicellular and multicellular organisms
Photosynthetic and non-photosynthetic organisms
This two-kingdom classification was eventually deemed inadequate, leading to the incorporation of characteristics like:
Cell structure
Nature of the cell wall
Mode of nutrition
Habitat
Methods of reproduction
Evolutionary relationships
Five Kingdom Classification
R.H. Whittaker proposed the Five Kingdom Classification in 1969. The kingdoms include:
Monera
Protista
Fungi
Plantae
Animalia
The main criteria for classification included:
Cell structure
Body organization
Mode of nutrition
Reproduction
Phylogenetic relationships
The three-domain system further divides Kingdom Monera into two domains, placing the remaining eukaryotic kingdoms in
the third domain, resulting in a six-kingdom classification.
Early classification systems grouped bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under
'Plants' because they all had a cell wall. However, this system grouped together organisms with vastly different
characteristics, such as prokaryotic bacteria and eukaryotic organisms. It also grouped unicellular and multicellular organisms
together.
When characteristics like cell structure and mode of nutrition were considered, fungi were placed in a separate kingdom –
Kingdom Fungi. All prokaryotic organisms were grouped under Kingdom Monera, and unicellular eukaryotic organisms were
placed in Kingdom Protista.
Phylogenetic: based on evolutionary relationships.
Classification systems evolve as our understanding of characteristics and evolutionary relationships improves.
Kingdom Monera
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Bacteria are the sole members of the Kingdom Monera and are the most abundant microorganisms. They are found
everywhere, even in extreme habitats like hot springs, deserts, snow, and deep oceans.
Bacteria are grouped into four categories based on their shape:
Coccus (spherical)
Bacillus (rod-shaped)
Vibrium (comma-shaped)
Spirillum (spiral)
Though simple in structure, bacteria exhibit extensive metabolic diversity. Some are autotrophic, synthesizing their own food
from inorganic substrates. Most, however, are heterotrophs, depending on other organisms or dead organic matter for food.
Archaebacteria
These bacteria live in harsh environments like salty areas (halophiles), hot springs (thermoacidophiles), and marshy areas
(methanogens). Their unique cell wall structure allows them to survive in these extreme conditions. Methanogens in the gut
of ruminant animals produce methane from dung.
Eubacteria
Also known as "true bacteria," eubacteria possess a rigid cell wall and, if motile, a flagellum. Cyanobacteria (blue-green
algae) have chlorophyll similar to green plants and are photosynthetic autotrophs. They can be unicellular, colonial, or
filamentous, and are found in freshwater, marine, or terrestrial environments. Some can fix atmospheric nitrogen in specialized
cells called heterocysts.
Chemosynthetic autotrophic bacteria oxidize inorganic substances like nitrates, nitrites, and ammonia, using the released
energy for ATP production and playing a vital role in nutrient recycling.
Heterotrophic bacteria are abundant and act as decomposers. They are used in making curd from milk, producing antibiotics,
and fixing nitrogen in legume roots. Some are pathogens, causing diseases like cholera, typhoid, tetanus, and citrus canker.
Bacteria reproduce mainly by fission. Under unfavorable conditions, they produce spores. They also exhibit a primitive form of
sexual reproduction through DNA transfer.
Fission: asexual reproduction by a separation of the body into two new bodies.
Mycoplasma lack a cell wall, are the smallest living cells, and can survive without oxygen. Many are pathogenic in animals
and plants.
Kingdom Protista
Kingdom Protista includes all single-celled eukaryotes. Its boundaries are not well-defined. Protists are primarily aquatic and
form a link between plants, animals, and fungi. Protistan cells contain a well-defined nucleus and other membrane-bound
organelles. Some have flagella or cilia. They reproduce asexually and sexually through cell fusion and zygote formation.
Chrysophytes
This group includes diatoms and golden algae (desmids). They are found in freshwater and marine environments, are
microscopic, and float passively in water currents (plankton). Most are photosynthetic. Diatom cell walls form overlapping
shells embedded with silica, which accumulate over billions of years to form diatomaceous earth. This gritty soil is used in
polishing and filtration. Diatoms are chief producers in the oceans.
Dinoflagellates
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These organisms are mostly marine and photosynthetic, appearing yellow, green, brown, blue, or red based on their pigments.
Their cell wall has stiff cellulose plates. Most have two flagella. Red dinoflagellates (e.g., Gonyaulax) can undergo rapid
multiplication, causing red tides. Toxins released during red tides can kill marine animals.
Euglenoids
Most euglenoids are freshwater organisms found in stagnant water. They have a protein-rich layer called a pellicle instead of
a cell wall, making their body flexible. They have two flagella. While photosynthetic in sunlight, they behave as heterotrophs
when deprived of light. Their pigments are identical to those in higher plants. Example: Euglena.
Slime Moulds
Slime molds are saprophytic protists that engulf organic material. Under suitable conditions, they form an aggregation called
plasmodium, which can spread over several feet. During unfavorable conditions, the plasmodium differentiates and forms
fruiting bodies bearing resistant spores, which are dispersed by air currents.
Protozoans
All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There
are four major groups:
Amoeboid protozoans: live in fresh water, sea water, or moist soil, and move and capture prey using pseudopodia.
Some marine forms have silica shells. Some, like Entamoeba, are parasites.
Flagellated protozoans: are either free-living or parasitic and have flagella. Parasitic forms cause diseases like sleeping
sickness (e.g., Trypanosoma).
Ciliated protozoans: are aquatic and actively move using thousands of cilia. They have a gullet to steer food into the cell
(e.g., Paramoecium).
Sporozoans: have an infectious spore-like stage in their life cycle. Plasmodium (malarial parasite) causes malaria.
Kingdom Fungi
Fungi are heterotrophic organisms with great diversity in morphology and habitat. They are found on moist bread, rotten fruits,
and as mushrooms and toadstools. Some unicellular fungi, like yeast, are used to make bread and beer. Others cause diseases
in plants and animals, such as wheat rust caused by Puccinia. Some are a source of antibiotics (e.g., Penicillium).
Fungi are cosmopolitan, growing in air, water, soil, and on animals and plants, preferring warm and humid places.
With the exception of unicellular yeasts, fungi are filamentous. Their bodies consist of long, slender threads called hyphae.
The network of hyphae is called mycelium. Some hyphae are continuous tubes filled with multinucleated cytoplasm
(coenocytic hyphae), while others have septa (cross walls). The cell walls of fungi are composed of chitin and
polysaccharides.
Most fungi are heterotrophic, absorbing soluble organic matter from dead substrates (saprophytes). Those that depend on
living plants and animals are called parasites. They can also live as symbionts in association with algae as lichens and with
roots of higher plants as mycorrhiza.
Reproduction in fungi occurs through:
Vegetative means: fragmentation, fission, and budding
Asexual reproduction: spores called conidia, sporangiospores, or zoospores
Sexual reproduction: oospores, ascospores, and basidiospores
Sexual reproduction involves three steps:
1. Plasmogamy: Fusion of protoplasms between two gametes
2. Karyogamy: Fusion of two nuclei
3. Meiosis in zygote resulting in haploid spores
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In some fungi (ascomycetes and basidiomycetes), a dikaryotic stage (n + n, i.e., two nuclei per cell) occurs between
plasmogamy and karyogamy. The fungi form fruiting bodies in which reduction division occurs, leading to the formation of
haploid spores.
The morphology of the mycelium, mode of spore formation, and fruiting bodies are used to divide the kingdom into various
classes.
Phycomycetes
Members are found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants. The
mycelium is aseptate and coenocytic. Asexual reproduction occurs by zoospores (motile) or aplanospores (non-motile), which
are produced endogenously in a sporangium. A zygospore is formed by the fusion of two gametes, which can be similar
(isogamous) or dissimilar (anisogamous or oogamous). Examples include Mucor, Rhizopus (bread mold), and Albugo (parasitic
fungi on mustard).
Ascomycetes
Commonly known as sac-fungi, ascomycetes are mostly multicellular (e.g., Penicillium) or rarely unicellular (e.g., yeast
(Saccharomyces)). They are saprophytic, decomposers, parasitic, or coprophilous (growing on dung). The mycelium is branched
and septate. Asexual spores (conidia) are produced exogenously on conidiophores. Sexual spores (ascospores) are produced
endogenously in sac-like asci, which are arranged in fruiting bodies called ascocarps. Examples include Aspergillus, Claviceps,
and Neurospora. Neurospora is used extensively in biochemical and genetic work. Some members like morels and truffles are
edible.
Basidiomycetes
Commonly known as mushrooms, bracket fungi, or puffballs. They grow in soil, on logs and tree stumps, and in living plant
bodies as parasites (e.g., rusts and smuts). The mycelium is branched and septate. Asexual spores are generally not found, but
vegetative reproduction by fragmentation is common. Plasmogamy occurs by the fusion of two vegetative or somatic cells of
different strains. The resultant dikaryotic structure gives rise to a basidium, where karyogamy and meiosis take place,
producing four basidiospores exogenously on the basidium. The basidia are arranged in fruiting bodies called basidiocarps.
Examples include Agaricus (mushroom), Ustilago (smut), and Puccinia (rust fungus).
Deuteromycetes
Commonly known as imperfect fungi because only the asexual or vegetative phases are known. They reproduce only by
asexual spores (conidia). The mycelium is septate and branched. Some members are saprophytes or parasites, while many are
decomposers that aid in mineral cycling. Examples include Alternaria, Colletotrichum, and Trichoderma.
Kingdom Plantae
Kingdom Plantae includes all eukaryotic chlorophyll-containing organisms commonly called plants. A few members are
partially heterotrophic, such as insectivorous plants (e.g., bladderwort and Venus fly trap) or parasites (e.g., Cuscuta). Plant
cells have a eukaryotic structure with prominent chloroplasts and a cell wall made of cellulose. Plantae includes algae,
bryophytes, pteridophytes, gymnosperms, and angiosperms.
The life cycle of plants has two distinct phases: the diploid sporophytic and the haploid gametophytic, which alternate with
each other. This phenomenon is called alternation of generations.
Kingdom Animalia
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This kingdom is characterized by heterotrophic eukaryotic organisms that are multicellular and lack cell walls. They directly or
indirectly depend on plants for food, digesting it in an internal cavity and storing food reserves as glycogen or fat. Their mode
of nutrition is holozoic – by ingestion of food. They have a definite growth pattern and grow into adults with a definite shape
and size. Higher forms show elaborate sensory and neuromotor mechanisms. Most are capable of locomotion. Sexual
reproduction involves copulation followed by embryological development.
Viruses, Viroids, Prions, and Lichens
These entities are not included in Whittaker's five-kingdom classification.
Viruses are non-cellular organisms characterized by an inert crystalline structure outside the living cell. Once they infect a
cell, they take over the host cell's machinery to replicate themselves, killing the host. Viruses contain genetic material (RNA or
DNA) and a protein coat called a capsid, made of subunits called capsomeres. Viruses cause diseases like mumps, smallpox,
herpes, and influenza. AIDS in humans is also caused by a virus.
Capsid: protein shell of a virus
Viroids are infectious agents smaller than viruses, consisting of free RNA without a protein coat.
Prions are infectious agents consisting of abnormally folded proteins. They cause neurological diseases like bovine spongiform
encephalopathy (mad cow disease) and Creutzfeldt-Jacob disease (CJD).
Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont). Algae prepare food for fungi, and fungi
provide shelter and absorb mineral nutrients and water for the algae. Lichens are good pollution indicators.
Characteristic Monera Protista Fungi Plantae Animalia
Cell Type Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic
Cell Wall Non-cellulosic Present in some Present (chitin) Present (cellulose) Absent
Nuclear
Absent Present Present Present Present
Membrane
Body Multicellular/loose Tissue/organ/organ Tissue/organ/organ
Cellular Cellular
Organization tissue system system
Mode of
Autotrophic/Heterotrophic Autotrophic/Heterotrophic Heterotrophic Autotrophic Heterotrophic
Nutrition
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