Biological Classifications

Biological Classification

Systems of Classification

• Aristotle's Classification: Earliest scientific approach. Grouped plants by morphology into trees, shrubs, and herbs. Divided animals into Enaima (with red blood) and Anaima (without red blood).
• Two Kingdom Classification (Linnaeus): Classified organisms into Plantae and Animalia strictly based on the presence/absence of a cell wall.
  • Crucial detail: Demerits included grouping prokaryotes with eukaryotes, unicellular with multicellular, and photosynthetic (green algae) with non-photosynthetic (fungi) organisms.
• Five Kingdom Classification (R.H. Whittaker, 1969): Created Monera, Protista, Fungi, Plantae, and Animalia.
  • Core Criteria: Cell structure, thallus organization (body complexity), mode of nutrition, reproduction, and phylogenetic relationships AIPMT 2014.
  • Exception/Crucial detail: This system is not based on the presence/absence of a well-defined nucleus AIPMT 2014.
  • Crucial grouping: Chlamydomonas and Chlorella (formerly algae) and Amoeba and Paramecium (formerly animalia) were brought together under Kingdom Protista AIPMT 2012 Mains.
• Three Domain System (Carl Woese): Based on 16S rRNA sequences. Divides Monera into two domains (Archaea and Bacteria) and groups all eukaryotes into the third domain (Eukarya), establishing a six-kingdom classification.

Kingdom Monera

General Characteristics

• Contains all prokaryotes. True nucleus is completely absent AIPMT 2015.
• Bacteria are the sole members. They exhibit the maximum nutritional diversity of any kingdom AIPMT 2012 Pre.
• Genetic material is a naked, double-stranded circular DNA (nucleoid/genophore) lacking histones AIPMT 1997, AIPMT 1993. Extrachromosomal circular DNA is called a plasmid.
• Monerans lack an alternation of generation because there is neither syngamy nor reduction division AIPMT 1992, AIPMT 1991.

Archaebacteria (Extremophiles)

Known as the "oldest living fossils."

• Crucial detail: Archaebacteria differ from Eubacteria by having a different cell wall composition (lacking peptidoglycan) and a highly complex cell membrane structure (branched lipid chain) that enables survival in extreme, harsh habitats AIPMT 2014, AIPMT 2008, AIPMT 2001.
• Halophiles: Thrive in extreme saline environments NEET 2017, AIPMT 2016. Use bacteriorhodopsin to harness solar energy for ATP.
• Thermoacidophiles: Flourish in hot sulphur springs; can survive extreme temperatures of 104^°C to 106^°C and high acidity AIPMT 2006.
• Methanogens: Strict anaerobes. Found in marshy areas and the gut of ruminant animals (cow/buffalo rumen) AIPMT 2015. Responsible for the production of methane (biogas) from dung AIPMT 2016, NEET 2022. Highly abundant in cattle yards AIPMT 2012 Pre.

Eubacteria (True Bacteria)

Characterized by a rigid peptidoglycan cell wall. If motile, possess a flagellum. The plasma membrane is the primary cellular component resembling a eukaryotic cell structurally AIPMT 2011 Pre.

• Cyanobacteria (Blue-Green Algae): Unicellular, colonial, or filamentous autotrophs AIPMT 2016, AIPMT 2012 Pre, AIPMT 1998.
  • Possess chlorophyll 'a' and perform oxygenic photosynthesis similar to green plants NEET 2020.
  • Frequently form water blooms in polluted water bodies NEET 2020.
  • Specialized thick-walled cells called Heterocysts (e.g., Nostoc, Anabaena) are responsible for fixing atmospheric nitrogen.
• Crucial detail: Green sulphur bacteria (e.g., Chlorobium, Chromatium) are anoxygenic; oxygen is NOT produced during their photosynthesis NEET 2018.
• Chemosynthetic Autotrophs: Oxidize inorganic substances (nitrates, nitrites, ammonia) to obtain energy for ATP production AIPMT 2002, AIPMT 2001. Play a major role in recycling nutrients (N, P, Fe, S).
• Heterotrophic Bacteria: The most abundant prokaryotes in nature AIPMT 2012 Pre. Majority are vital decomposers. Highly useful in making curd (Lactic Acid Bacteria) and antibiotics.
  • Pathogenic examples: Cholera (Vibrio), Tetanus (Clostridium), Typhoid (Salmonella), and Citrus canker (Xanthomonas).
  • Crucial detail: Clostridium botulinum causes severe food poisoning (botulism) and is an obligate anaerobe AIPMT 2006.
• Reproduction: Primarily via binary fission. Form endospores under adverse, unfavourable conditions. Can perform primitive genetic transfer (genetic recombination).
  • Transduction: DNA transfer from one bacterium to another mediated by a bacteriophage AIPMT 2002, AIPMT 1998.

Mycoplasma

• The smallest known living cells. They completely lack a cell wall NEET 2017, AIPMT 2015 Re.
• Can survive entirely without oxygen NEET 2017.
• Highly pleomorphic ("Jokers of the plant kingdom"). Pathogenic to animals and plants.
• Crucial detail: They are completely insensitive to penicillin (due to lack of a cell wall) but sensitive to tetracycline AIPMT 2007.

Kingdom Protista

General Features & Chrysophytes

Comprises all single-celled eukaryotes. Primarily aquatic. Have a true nucleus and membrane-bound organelles.

• Chrysophytes (Diatoms & Golden Algae/Desmids):
  • Planktonic organisms that float passively in water currents AIPMT 2016.
  • Crucial detail: The cell wall forms two thin, overlapping shells fitting together like a soap box AIPMT 2015 Re.
  • Walls are heavily embedded with silica, making them indestructible AIPMT 2016. Over billions of years, their dead accumulation forms Diatomaceous earth.
  • Diatoms are recognized as the chief producers in the oceans NEET 2018, AIPMT 2016.

Dinoflagellates & Euglenoids

• Dinoflagellates: Mostly marine and photosynthetic. Cell wall bears stiff cellulose plates on the outer surface. Possess two flagella (one longitudinal, one transverse).
  • Rapid multiplication of red dinoflagellates (e.g., Gonyaulax) causes Red Tides. They release potent toxins capable of paralyzing and killing marine life.
• Euglenoids: Found mostly in stagnant fresh water.
  • Lack a cell wall; instead, have a flexible protein-rich layer called a pellicle.
  • Mixotrophic Nutrition: Behave as photoautotrophs in the presence of sunlight, but switch to heterotrophic predators when deprived of light NEET 2024 Re.

Slime Moulds & Protozoans

• Slime Moulds: Saprophytic protists. The body moves along decaying organic matter engulfing nutrients.
  • Under favourable conditions, they form a multinucleate, thalloid aggregation called Plasmodium AIPMT 2007, AIPMT 2006.
  • Under unfavourable conditions, the Plasmodium differentiates to form fruiting bodies bearing spores at their tips. These spores possess true walls, are highly resistant, and disperse via air currents.
• Protozoans: Purely heterotrophic (predators/parasites).
  • Amoeboid: Catch prey via pseudopodia. Marine forms possess silica shells. (e.g., Entamoeba).
  • Flagellated: Possess flagella. Cause diseases like sleeping sickness (e.g., Trypanosoma).
  • Ciliated: Aquatic, move actively via thousands of cilia. Have a gullet that opens outward. Contractile vacuoles handle osmoregulation AIPMT 1995.
    • Crucial detail: Ciliates uniquely differ from all other protozoans by possessing two types of nuclei (macronucleus and micronucleus) NEET 2018. E.g., Paramecium.
  • Sporozoans: Have an infectious spore-like stage in their life cycle. E.g., Plasmodium (malarial parasite, discovered in RBCs by Laveran AIPMT 1991).

Kingdom Fungi

General Features

• Unique kingdom of achlorophyllous AIPMT 1992, heterotrophic (saprophytic/parasitic) organisms.
• Cell walls are strictly composed of chitin and polysaccharides.
• The body consists of long, slender, thread-like structures called hyphae. The network of hyphae forms a mycelium.
• Exception: Yeast (e.g., Saccharomyces) is a rare unicellular fungus NEET 2024, NEET 2021.
• Sexual Cycle: Completes in three steps:
  • Plasmogamy: Fusion of protoplasms of motile or non-motile gametes.
  • Karyogamy: Fusion of two nuclei.
  • Meiosis: Occurs in the zygote, yielding haploid spores.
• Crucial exception: In Ascomycetes and Basidiomycetes, plasmogamy does not immediately lead to karyogamy. An intervening dikaryotic stage (n + n) occurs, where the haploid cells do not fuse completely right away NEET 2024, NEET 2023.

Fungal Classification & High-Yield Trends

Class	Mycelium	Asexual Reproduction	Sexual Reproduction	Key Examples (PYQs)
Phycomycetes (Algal Fungi)	Aseptate, coenocytic	Zoospores (motile) or Aplanospores (non-motile) produced endogenously in sporangium.	Zygospores formed by gametic fusion. Can be isogamous or oogamous.	Rhizopus (bread mould), Mucor, Albugo (parasitic fungus causing white rust on mustard AIPMT).
Ascomycetes (Sac Fungi)	Septate, branched	Conidia produced exogenously on special hyphae (conidiophores) AIPMT.	Ascospores produced endogenously in sac-like asci, organized in ascocarps.	Penicillium, Yeast, Aspergillus, Claviceps (ergot/LSD), Neurospora (extensive use in genetics). Edible: Morels, Truffles.
Basidiomycetes (Club Fungi)	Septate, branched	Generally absent. Vegetative reproduction by fragmentation is most common.	Sex organs absent. Plasmogamy via somatic cell fusion. Dikaryon forms a basidium. Meiosis yields 4 exogenous basidiospores.	Agaricus (mushroom), Ustilago (smut), Puccinia (rust).
Deuteromycetes (Fungi Imperfecti)	Septate, branched	Conidia produced exogenously.	Sexual stage is entirely absent or undiscovered NEET 2024.	Alternaria, Colletotrichum, Trichoderma. Primarily decomposers of litter involved in mineral cycling.

Viruses, Viroids, Prions & Lichens

Viruses

• Not classified in the five-kingdom system as they are non-cellular, obligate intracellular parasites forming inert crystalline structures outside a host.
• History: Pasteur coined the term "virus" (venom/poison). D.J. Ivanowsky (1892) recognized TMV as smaller than bacteria. M.W. Beijerinck (1898) called the infectious sap Contagium vivum fluidum. W.M. Stanley (1935) proved viruses could be crystallized.
• Structure: A nucleic acid core (DNA or RNA, never both) protected by a protein coat called a capsid. The capsid is made of capsomeres arranged in helical or polyhedral geometric forms.
• Plant viruses generally have ssRNA. Animal viruses have ssRNA, dsRNA, or dsDNA. Bacteriophages (infecting bacteria) usually have dsDNA.

Viroids & Prions

• Viroids: Discovered by T.O. Diener (1971). Smaller than viruses. Consist solely of free infectious RNA of low molecular weight. They strictly lack a protein coat. Cause Potato spindle tuber disease.
• Prions: Consist of abnormally folded, infectious proteins. Similar in size to viruses. Cause devastating neurological diseases such as Bovine Spongiform Encephalopathy (BSE / Mad Cow Disease) in cattle and its human variant, Cr-Jacob disease (CJD).

Lichens

• Symbiotic, mutually beneficial associations between algae (phycobiont - autotrophic, synthesizes food) and fungi (mycobiont - heterotrophic, provides shelter, absorbs minerals/water).
• Highly sensitive to SO₂ pollution; they act as excellent pollution indicators because they do not grow in polluted areas.

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