Biological Classification

CLASSIFICATION OF LIVING ORGANISMS

Historical Background of Biological Classification

• Instinctive Classification:

  • Since the dawn of civilization, humans instinctively classified organisms based on utility (food, shelter, clothing).

• Aristotle's Contribution:

  • Earliest attempt at scientific classification.

  • Classified plants into trees, shrubs, and herbs.

  • Divided animals into two groups:

  • Those with red blood.

  • Those without red blood.

Development of Classification Systems

• Two Kingdom System (Linnaeus' Time):

  • Established kingdoms Plantae and Animalia.

  • Included all plants and animals, respectively.

  • Limitations:

  • Did not differentiate between eukaryotes and prokaryotes.

  • Did not distinguish unicellular and multicellular organisms.

  • Failed to classify photosynthetic (green algae) and non-photosynthetic (fungi) organisms.

• Problems Identified:

  • Inadequate as many organisms did not fit easily into the two categories.

  • Need for other criteria:

  • Cell structure.

  • Nature of walls.

  • Mode of nutrition.

  • Habitat.

  • Methods of reproduction.

  • Evolutionary relationships.

Modern Classification Systems

• Five Kingdom Classification (R.H. Whittaker, 1969):

  • Proposed five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.

  • Main criteria for classification:

  • Cell structure.

  • Body organization.

  • Mode of nutrition.

  • Reproduction.

  • Phylogenetic relationships.

  • Comparison of characteristics of five kingdoms is shown in Table 2.1.

• Three-Domain System:

  • Divides Kingdom Monera into two domains, leading to a six-kingdom classification.

  • More details to be covered in higher studies.

Issues with Earlier Classifications

• Earlier systems included bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms under ‘Plants.’

  • A characteristic they shared was the presence of a cell wall.

• Grouped together organisms with vastly different attributes:

  • Prokaryotic bacteria and blue-green algae together with eukaryotic organisms.

  • Fostered misclassifications, such as placing fungi and green plants under the same category despite cell wall composition differences (fungi - chitin; plants - cellulose).

Kingdoms Overview in Five Kingdom Classification

2.1 KINGDOM MONERA

• Definition:

  • Comprises the most abundant microorganisms, primarily bacteria.

• Habitat Distribution:

  • Present in various environments including extreme habitats (hot springs, deserts, oceans).

• Classification by Shape:

  • Coccus (pl.: cocci): spherical.

  • Bacillus (pl.: bacilli): rod-shaped.

  • Vibrio (pl.: vibrio): comma-shaped.

  • Spirillum (pl.: spirilla): spiral-shaped.

• Metabolic Diversity:

  • Bacteria exhibit extensive metabolic diversity; can be:

  • Autotrophic (self-feeding):

    • Photosynthetic or chemosynthetic.

  • Heterotrophic (depend on others):

    • Saprozoic (decomposing) or parasitic.

2.1.1 Archaebacteria

• Characteristics:

  • Live in hostile environments (e.g., saline, hot springs).

  • Different cell wall structure, allowing survival in extreme conditions.

  • Example: Methanogens produce methane in the guts of ruminants.

2.1.2 Eubacteria

• Characteristics:

  • Possess rigid cell walls; motile forms have flagella.

  • Include cyanobacteria (blue-green algae) which possess chlorophyll and are photosynthetic.

  • Can fix atmospheric nitrogen (e.g., Nostoc, Anabaena).

  • Most significant decomposers in ecosystems.

2.2 KINGDOM PROTISTA

• Definition:

  • Composed of all single-celled eukaryotes with poorly defined boundaries.

  • Includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans.

• Characteristics:

  • Eukaryotic cells with defined nuclei and organelles, some possess flagella/cilia.

  • Reproduce asexually and sexually.

2.2.1 Chrysophytes

• Definition:

  • Includes diatoms and golden algae found in aquatic environments.

• Significance:

  • Most are photosynthetic.

  • Diatoms have silica-embedded cell walls; accumulate as diatomaceous earth used in various industries.

2.2.2 Dinoflagellates

• Characteristics:

  • Marine, photosynthetic; pigments might lead to varied cell colors.

  • Equipped with stiff cellulose plates and two flagella, causing phenomena such as red tides.

2.2.3 Euglenoids

• Distinguishing Feature:

  • No cell wall; flexible pellicle instead.

• Adaptation:

  • Can switch modalities between photosynthesis and heterotrophy depending on light availability.

2.2.4 Slime Moulds

• Functionality:

  • Saprophytic protists engaging in movement to engulf organic materials.

• Reproduction:

  • Form plasmodium and advance to fruiting bodies in adverse conditions.

2.2.5 Protozoans

• Overview:

  • Heterotrophs and believed to be primitive relatives of animals.

  • Sub-divided into major groups:

  • Amoeboid protozoans: e.g., Amoeba use pseudopodia.

  • Flagellated protozoans: e.g., Trypanosoma causes diseases.

  • Ciliated protozoans: e.g., Paramecium filter water using cilia.

  • Sporozoans: e.g., Plasmodium, known for malaria transmission.

2.3 KINGDOM FUNGI

• Definition:

  • Heterotrophic organisms with diverse morphology and habitat.

• Common Examples:

  • Mushrooms, yeast, and parasitic fungi like Puccinia.

• Structure:

  • Comprised of hyphae; network known as mycelium.

  • Cell walls made of chitin.

• Nutritional Mode:

  • Most are saprophytes, with some functioning as parasites or symbionts.

• Reproduction involves both vegetative processes and spore formation:

  • Asexual reproduction: via conidia, sporangiospores.

  • Sexual reproduction: via various spore types (e.g., oospores).

2.3.1 Phycomycetes

• Habitat:

  • Aquatic environments and decaying organic matter.

• Life Cycle:

  • Asexual reproduction through zoospores.

2.3.2 Ascomycetes

• Common Name:

  • Sac-fungi, includes species like Penicillium (important in medicine).

• Reproduction:

  • Forms ascospores in sac-like asci.

2.3.3 Basidiomycetes

• Examples:

  • Mushrooms, bracket fungi.

• Lifecycle:

  • Plasmogamy leads to dikaryotic basidia, producing basidiospores.

2.3.4 Deuteromycetes

• Common Name:

  • Imperfect fungi, lacking known sexual phases initially.

• Reproduction:

  • Only asexual spores (conidia) were known prior to classification revisions.

2.4 KINGDOM PLANTAE

• Definition:

  • Eukaryotic organisms capable of photosynthesis; primarily autotrophic yet includes partially heterotrophic species.

• Notable Examples:

  • Bladderwort, Venus flytrap, Cuscuta (a parasite).

• Structure:

  • Cells contain chloroplasts and cell walls made of cellulose.

2.5 KINGDOM ANIMALIA

• Overview:

  • Heterotrophic, eukaryotic, multicellular organisms lacking cell walls.

• Nutritional Mode:

  • Holozoic (ingestion).

2.6 VIRUSES, VIROIDS, PRIONS, AND LICHENS

• Viruses:

  • Non-cellular, infect host cells to replicate.

  • Comprised of proteins and either RNA or DNA.

  • Cause diverse diseases such as AIDS, influenza, etc.

• Viroids:

  • Infectious agents with only RNA, causing diseases like potato spindle tuber disease.

• Prions:

  • Composed of abnormally folded proteins causing neurodegenerative diseases (e.g., BSE, CJD).

• Lichens:

  • Symbiotic relationships between fungi (mycobiont) and algae (phycobiont), both benefiting each other for nutrition and habitat.

SUMMARY OF CLASSIFICATION

• Various systems of classification have evolved from Aristotle's morphologically based classification to the intricate models proposed by Linnaeus and Whittaker’s five kingdom classification.

• Bacteria are classified under Kingdom Monera; Protista consists of single-celled eukaryotes; Fungi are diverse and generally saprophytic; Plantae encompasses all chlorophyll-containing organisms; and Animalia includes multicellular, eukaryotic forms.

• Additional organisms like viruses, viroids, prions, and lichens exist outside traditional kingdoms.