Need for Classification
There are millions of living organisms on Earth. Classification helps in:
- Systematic study of organisms.
- Identifying relationships between organisms.
- Understanding evolution.
Binomial Nomenclature (Carolus Linnaeus)
Each organism has a two-part scientific name:
- Genus (capitalised) + species (lowercase).
- Both in italics (or underlined when handwritten).
- Example: Homo sapiens, Mangifera indica.
Rules of Nomenclature
- Names are in Latin or Latinised.
- Genus starts with capital letter, species with small letter.
- When printed, names are in italics.
- Author's name is abbreviated after species name.
Five-Kingdom Classification (Whittaker, 1969)
| Kingdom | Cell Type | Organisation | Nutrition | Examples |
|---|---|---|---|---|
| Monera | Prokaryotic | Unicellular | Autotrophic/Heterotrophic | Bacteria, Cyanobacteria |
| Protista | Eukaryotic | Unicellular | Autotrophic/Heterotrophic | Amoeba, Paramecium, Euglena |
| Fungi | Eukaryotic | Multicellular (except yeast) | Heterotrophic (saprophytic) | Mushroom, Yeast, Penicillium |
| Plantae | Eukaryotic | Multicellular | Autotrophic | Moss, Fern, Pine, Mango |
| Animalia | Eukaryotic | Multicellular | Heterotrophic | Sponge, Insect, Fish, Human |
Three-Domain System (Woese, 1990)
An alternative system: Bacteria, Archaea, and Eukarya.
Kingdom Monera
- Prokaryotic (no membrane-bound organelles).
- Single-celled.
- Cell wall present (peptidoglycan).
- Bacteria classified by shape: cocci (spherical), bacilli (rod), spirilla (spiral), vibrio (comma).
Archaebacteria
- Live in extreme environments (halophiles, thermoacidophiles, methanogens).
Eubacteria (True Bacteria)
- Includes cyanobacteria (blue-green algae), nitrogen-fixing bacteria, pathogens.
Kingdom Protista
- Unicellular eukaryotes.
- Includes: Amoeba (phagocytosis), Paramecium (cilia), Euglena (flagellum, photosynthesis).
- Diatoms: Cell walls of silica, chief producers in oceans.
Kingdom Fungi
- Eukaryotic, heterotrophic (absorption).
- Cell wall of chitin.
- Body: mycelium of hyphae.
- Nutrition: saprophytic, parasitic, symbiotic (lichens, mycorrhiza).
Major Classes
- Phycomycetes: Rhizopus (bread mould).
- Ascomycetes: Yeast, Penicillium, Aspergillus.
- Basidiomycetes: Mushrooms, bracket fungi.
- Deuteromycetes: Fungi imperfecti (asexual reproduction only).
Kingdom Plantae
Major Divisions (Bold System)
Thallophyta: Simple body (thallus), no differentiation.
- Algae (Spirogyra, Ulva, Chlamydomonas).
Bryophyta: Amphibians of plant kingdom.
- No vascular tissue. Gametophyte dominant.
- Examples: Moss (Funaria), Liverwort (Marchantia).
Pteridophyta: First vascular plants.
- Sporophyte dominant. Reproduce by spores.
- Examples: Fern (Pteris), Selaginella, Equisetum.
Gymnosperms: Naked seeds.
- No fruit formation. Conifers.
- Examples: Pinus, Cycas, Ginkgo.
Angiosperms: Flowering plants, seeds enclosed in fruits.
- Monocots (one cotyledon, parallel venation) and Dicots (two cotyledons, reticulate venation).
Kingdom Animalia
Major Phyla
Porifera: Pore-bearing, aquatic, sessile. Examples: Sponge (Sycon, Spongilla).
Cnidaria (Coelenterata): Nematocysts, radial symmetry. Examples: Hydra, Jellyfish, Coral.
Platyhelminthes: Flatworms, bilateral symmetry, acoelomate. Examples: Tapeworm, Liver fluke.
Aschelminthes (Nematoda): Roundworms, pseudocoelomate. Examples: Ascaris, Wuchereria.
Annelida: Segmented worms, closed circulatory system. Examples: Earthworm, Leech, Nereis.
Arthropoda: Jointed appendages, exoskeleton of chitin. Largest phylum. Examples: Insects, Spiders, Crabs.
Mollusca: Soft-bodied, muscular foot. Examples: Snail, Octopus, Pearl oyster.
Echinodermata: Spiny-skinned, water vascular system, radial symmetry. Examples: Starfish, Sea urchin.
Chordata: Notochord, dorsal hollow nerve cord, pharyngeal gill slits, post-anal tail.
Viruses
- Acellular (no cell structure).
- Obligate intracellular parasites.
- Have either DNA or RNA (never both).
- Protein coat (capsid) surrounding nucleic acid.
- Discovered by Dmitri Ivanovsky (Tobacco Mosaic Virus).
- Not included in five-kingdom classification.
Viroids
- Infectious RNA particles without protein coat (discovered by T.O. Diener).
Prions
- Infectious protein particles (cause mad cow disease, Creutzfeldt-Jakob disease).
Worked Examples
Example 1: Classify a mushroom in the five-kingdom system. Solution: Kingdom Fungi, Division Basidiomycetes. Eukaryotic, multicellular, heterotrophic saprophyte with chitin cell wall.
Example 2: Differentiate between archaebacteria and eubacteria. Solution: Archaebacteria live in extreme environments, have different cell wall composition. Eubacteria are true bacteria found in normal environments.
Common Mistakes
- Monera vs Protista: Monera are prokaryotic (no nucleus). Protista are eukaryotic (have nucleus).
- Fungi vs Plantae: Fungi have chitin cell wall (not cellulose) and are heterotrophic (not autotrophic).
- Viruses are not cells: They are considered as 'biological entities' but not truly living.
- Binomial nomenclature format: Genus capitalised, species lowercase, always italicised.
ISC Exam Focus
- Theory (70%): Classification systems, kingdom characteristics, binomial nomenclature, virus structure.
- Application (30%): Identifying kingdom/phylum, scientific names of common organisms.
- ISC frequently asks: "Distinguish between ..." and "Give the characteristic features of ...".
Self-Test Questions
Q1: Who proposed the five-kingdom classification and what are the kingdoms? Answer: R.H. Whittaker (1969). Monera, Protista, Fungi, Plantae, Animalia.
Q2: State the rules of binomial nomenclature. Answer: Genus capitalised, species lowercase, Latinised, italicised in print.
Q3: Differentiate between prokaryotic and eukaryotic cells. Answer: Prokaryotic: no nucleus, no membrane-bound organelles. Eukaryotic: nucleus present, membrane-bound organelles.
Q4: Give two examples each of Porifera and Arthropoda. Answer: Porifera: Sycon, Spongilla. Arthropoda: Cockroach, Butterfly, Crab.
Q5: Write the scientific name of humans. Answer: Homo sapiens.
Q6: List the characteristic features of viruses and explain why they are not included in the five-kingdom classification. Answer: Acellular, obligate parasites, DNA or RNA. Not living (no metabolism outside host), so excluded.
