Biological Classification: History and Systems

• Since the dawn of civilization, attempts to classify living organisms emerged, initially driven by practical needs rather than scientific criteria.
• Aristotle pioneered a more scientific classification using simple morphological traits, dividing plants into trees, shrubs, and herbs, and animals into those with and without red blood.
• Linnaeus introduced a Two Kingdom system, Plantae and Animalia, but it failed to differentiate between eukaryotes/prokaryotes and unicellular/multicellular organisms, or photosynthetic and non-photosynthetic organisms.
• The two kingdom system proved inadequate, spurring the inclusion of cell structure, nutrition mode, habitat, reproduction methods, and evolutionary relationships in classification.
• Plant and animal kingdoms remained constant across systems but the understanding of included organisms and other kingdoms evolved with scientific advancements.

Five Kingdom Classification

• R.H. Whittaker proposed it in 1969, it included Monera, Protista, Fungi, Plantae, and Animalia, based on cell structure, body organization, nutrition mode, reproduction, and phylogenetic relationships.
• A three-domain system divides Monera into two domains, placing eukaryotic kingdoms in the third.
• Earlier systems grouped bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under 'Plants' due to the presence of cell walls unified this kingdom.
• Fungi were separated into their own kingdom due to differences in cell wall composition, where fungi have chitin.
• Kingdom Protista grouped unicellular eukaryotic organisms like Chlamydomonas and Paramoecium.
• Classifications now consider morphological, physiological, reproductive similarities, and phylogenetic relationships.

Kingdom Monera

• Bacteria make up this kingdom and they are the most abundant micro-organisms, thriving in diverse habitats from soil to extreme conditions, also living as parasites.
• Bacteria are categorized by shape: spherical Coccus, rod-shaped Bacillus, comma-shaped Vibrium, and spiral Spirillum.
• Despite simple structure, bacteria exhibit complex behavior and vast metabolic diversity, including autotrophic (photosynthetic or chemosynthetic) and heterotrophic modes of nutrition.

Archaebacteria

• These bacteria inhabit harsh environments like salty areas, hot springs, and marshy areas, and they differ from other bacteria in cell wall structure, aiding survival in extreme conditions.
• Methanogens, found in ruminant animal guts, produce methane from dung.

Eubacteria

• These bacteria are characterized by a rigid cell wall and flagellum, if motile examples include cyanobacteria.
• Cyanobacteria, or blue-green algae, contain chlorophyll a, perform photosynthesis, and can be unicellular, colonial, or filamentous in various environments.
• Some cyanobacteria fix atmospheric nitrogen using heterocysts, like Nostoc and Anabaena.
• Chemosynthetic autotrophic bacteria recycle nutrients by oxidizing inorganic substances.
• Heterotrophic bacteria, abundant as decomposers, play roles in making curd from milk, producing antibiotics, and fixing nitrogen in legumes.

Reproduction in Bacteria

• Bacteria mainly reproduce by fission, forming spores under unfavorable conditions, and they also exhibit a primitive form of sexual reproduction through DNA transfer.
• Mycoplasmas lack a cell wall, are the smallest living cells, and can survive without oxygen.

Kingdom Protista

• This kingdom includes single-celled eukaryotes, such as Chrysophytes, Dinoflagellates, Euglenoids, Slime molds, and Protozoans and they are primarily aquatic.
• Protista links plants, animals, and fungi and they contain a defined nucleus, membrane-bound organelles, and reproduce asexually and sexually.

Chrysophytes

• This kingdom includes diatoms and golden algae (desmids) that can be found in marine and fresh water environments.
• Diatoms have indestructible silica-embedded cell walls forming overlapping shells, accumulating as 'diatomaceous earth', which is used in polishing and filtration.

Dinoflagellates

• This kingdom is mostly marine and photosynthetic, appearing in various colors based on pigments in their cells and their cell walls have stiff cellulose plates with two flagella.
• Red dinoflagellates, like Gonyaulax, cause red tides and may release toxins harmful to marine life.

Euglenoids

• Most of them are freshwater organisms with a protein-rich pellicle instead of a cell wall, having two flagella.
• Euglenoids are photosynthetic in sunlight but behave as heterotrophs when deprived of it.

Slime Moulds

• These are saprophytic protists that engulf organic material, forming a plasmodium under suitable conditions, and they differentiate into fruiting bodies with spores under unfavorable conditions which can be dispersed by air currents.

Protozoans

• All protozoans are heterotrophs, living as predators or parasites which can be divided into four groups based on structure
• Amoeboid protozoans use pseudopodia to capture prey, with some marine forms having silica shells.
• Flagellated protozoans are free-living or parasitic, causing diseases like sleeping sickness (Trypanosoma).
• Ciliated protozoans are aquatic organisms with cilia that help steer food into their gullet, such as Paramoecium.
• Sporozoans have an infectious spore-like stage, including Plasmodium, the malarial parasite.

Kingdom Fungi

• Fungi are heterotrophic organisms with diverse morphology and habitat, including mushrooms, toadstools, and parasitic forms.
• Unicellular fungi, like yeast, are used in bread and beer making, while others cause plant and animal diseases or are sources of antibiotics.
• Most fungi are filamentous, with hyphae forming a network called mycelium, and cell walls of fungi are composed of chitin and polysaccharides.
• Most fungi are saprophytes, absorbing nutrients from dead substrates, while others are parasites or symbionts like lichens and mycorrhiza.
• Fungi reproduce vegetatively (fragmentation, fission, budding), asexually (spores), and sexually (oospores, ascospores, basidiospores) produced in fruiting bodies.
• Sexual reproduction involves plasmogamy, karyogamy, and meiosis.

Phycomycetes

• Their members live in aquatic habitats, on decaying wood, or as plant parasites, and they have aseptate and coenocytic mycelium.
• Asexual reproduction occurs through zoospores or aplanospores produced in sporangium, while a zygospore forms from gamete fusion which can be isogamous or anisogamous.

Ascomycetes

• Commonly known as sac-fungi, ascomycetes are mostly multicellular (Penicillium) or unicellular (yeast), and can be saprophytic, decomposers, parasitic, or coprophilous.
• Conidia are produced exogenously, while ascospores are produced endogenously in sac-like asci within fruiting bodies (ascocarps).

Basidiomycetes

• The most known forms include mushrooms, bracket fungi, or puffballs that grow in various habitats and the mycelium is branched and septate.
• Asexual spores are generally absent, and plasmogamy leads to a dikaryotic structure that develops into a basidium.
• Karyogamy and meiosis occurs in the basidium, producing basidiospores which are arranged in basidiocarps.

Deuteromycetes

• These fungi are known as imperfect fungi because they are missing one of the phases, and they reproduce asexually via conidia, and the mycelium is branched and septate.
• They can be saprophytes, parasites, or decomposers.

Kingdom Plantae

• Kingdom Plantae includes all eukaryotic chlorophyll-containing organisms commonly known as plants.
• A few members are partially heterotrophic such as the insectivorous plants or parasites.
• The plant cells have an eukaryotic structure with prominent chloroplasts and cell wall mainly made of cellulose.
• Life cycle of plants has two distinct phases – the diploid sporophytic and the haploid gametophytic – that alternate with each other.

KINGDOM ANIMALIA

• This kingdom is characterized by heterotrophic eukaryotic organisms that are multicellular and their cells lack cell walls.
• They directly or indirectly depend on plants for food.

Viruses, Viroids, Prions and Lichens

• Viruses, viroids and prions were not a part of the five kingdom classification.
• Viruses did not find a place in classification since they are not considered truly 'living', if we understand living as those organisms that have a cell structure.
• Viruses are non-cellular organisms characterized by an inert crystalline structure outside the living cell.
• They take over the machinery of the host cell to replicate themselves, killing the host.
• In addition to proteins, viruses also contain genetic material, that could be either RNA or DNA.
• Viroids are infectious agents smaller than viruses, consisting of free RNA without a protein coat.
• Certain infectious neurological diseases are because of abnormally folded protein called prions.
• Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi.
• Lichens are very good pollution indicators.