Classification of Living Organisms

Classification of Living Organisms

• Early classification relied on instinct and practical uses (food, shelter)
• Aristotle employed morphological characteristics (trees, shrubs, herbs) and blood presence
• Linnaeus's two-kingdom system (Plantae and Animalia) wasn't sufficient, encompassing eukaryotes and prokaryotes, unicellular/multicellular, photosynthetic/non-photosynthetic organisms.
• Insufficient coverage led to inadequacies and a need for further classification details like cell structure, wall composition, mode of nutrition, habitats and reproduction.
• Many organisms didn't fit into either category.
• Classification systems have evolved over time.
• The understanding of organisms within kingdoms has changed.

Five Kingdom Classification

• Proposed by R.H. Whittaker (1969)
• Kingdoms: Monera, Protista, Fungi, Plantae, Animalia
• Criteria for classification: Cell structure, body organization, mode of nutrition, reproduction, phylogenetic relationships

Kingdom Monera

• Sole members: Bacteria
• Diverse habitats; present in soil, extreme environments (hot springs, deserts, oceans)
• Also live as parasites on other organisms.
• Categorized by shape (cocci, bacilli, vibrio, spirilla)
• Show vast metabolic diversity (autotrophic, heterotrophic)
• Autotrophic bacteria are either photosyntheic (e.g., Cyanobacteria) or chemosynthetic.
• Heterotrophic bacteria are important decomposers.
• Helpful in food production (curd), antibiotic production, and nitrogen fixation.

Archaebacteria

• Special group within bacteria
• Thrive in extreme habitats (salty areas, hot springs, marshy areas)
• Different cell wall structure compared to other bacteria.
• Methanogens: found in ruminant animal guts, produce methane from dung

Eubacteria

• "True bacteria" characterized by a rigid cell wall and flagella (if motile)
• Cyanobacteria (blue-green algae): photosynthetic autotrophs with chlorophyll a.
• Exist as unicellular, colonial or filamentous forms.
• Often form blooms in polluted water bodies.
• Some fix atmospheric nitrogen in specialized cells (heterocysts) .
• Chemosynthetic autotrophs: oxidise inorganic substances (nitrates, nitrites, ammonia) to produce ATP.
• Play vital roles in nutrient recycling (nitrogen, phosphorus, iron, sulfur)

Further Classification Considerations

• Early classifications grouped diverse organisms based on limited criteria (e.g., cell wall).
• Kingdoms Monera and Protista grouped organisms that had previously been in different kingdoms, showing a change in classification emphasis to evolutionary relationships

Three-Domain System

• A proposed more recent system.
• Further divides the Kingdom Monera into two domains.
• The remaining eukaryotic kingdoms are grouped in the third domain.