A complete visual guide — why we classify, the basis of classification (cells, nutrition, symmetry, germ layers, coelom), the taxonomic hierarchy and the three domains, the five kingdoms, the plant divisions and animal phyla, the vertebrate classes, binomial nomenclature and viruses.
📌 Chapter 4
Basis of Classification
Five Kingdoms
Plant Divisions
Animal Phyla
Nomenclature
150 Practice Qs
Section 4.1 – 4.2
Biodiversity & the Basis of Classification
Earth has millions of organisms with unique features — this variety is biodiversity (term coined by Walter G. Rosen, 1986). Classification groups organisms by their similarities and differences so they can be studied systematically.
📜 Early attempts
Aristotle classified animals by habitat (land, water, air); Theophrastus classified plants by form (trees, shrubs, herbs). These were artificial classifications — based on a few easily-seen features, not on fundamental body characteristics, so they grouped very different organisms together.
Modern classification uses many fundamental characteristics:
Number of cells — unicellular or multicellular.
Mode of nutrition — autotrophs (make food) or heterotrophs (depend on others).
Cell type — prokaryotic (no true nucleus) or eukaryotic (true nucleus).
Level of organisation — cellular (sponges), tissue (Hydra), organ (flatworms) or organ-system (earthworm, humans).
Body symmetry: asymmetrical (no plane), radial (many planes through a centre) and bilateral (one plane, left & right mirror)
Germ layers
Formed in the early embryo. Diploblastic = two layers (Hydra); triploblastic = three layers (earthworm, humans).
Coelom (body cavity)
The cavity between the body wall and gut. Acoelomate = none (flatworms); pseudocoelomate = false cavity (roundworms); coelomate = true cavity (earthworm, humans).
Coelom (purple/blue ring): absent in acoelomates, a false cavity in pseudocoelomates, a true lined cavity in coelomates
Section 4.3 – 4.5
Classification Systems & the Taxonomic Hierarchy
Classification is linked to evolution: simpler body designs are usually older, complex ones evolved later (Darwin, "Origin of Species", 1859). Over time the systems have grown more refined.
System
Proposed by
Groups
Two kingdoms
Linnaeus (1735)
Plantae, Animalia
Three kingdoms
Haeckel (1866)
+ Protista
Five kingdoms
Whittaker (1969)
Monera, Protista, Fungi, Plantae, Animalia
Three domains
Woese et al. (1990)
Bacteria, Archaea, Eukarya (from rRNA)
Linnaeus's seven ranks — from the broad Kingdom down to the specific Species (in plants, "Division" replaces "Phylum")
❖ Species
The basic unit of classification — a group of individuals that closely resemble one another and can interbreed to produce fertile offspring. As we move up from species to kingdom, organisms share fewer characteristics and the groups get broader.
💡 Did You Know?
The three-domain system uses ribosomal RNA, which changes slowly and is found in all cells, making it a reliable molecular clock. Archaea are prokaryotes whose genes resemble eukaryotes and that survive extreme conditions.
Section 4.6 – 4.8
The Five Kingdoms (Monera, Protista, Fungi)
Whittaker's five kingdoms are distinguished by cell type, organisation and nutrition.
Kingdom
Cells
Nutrition
Example
Monera
Prokaryotic, unicellular
Auto- & heterotrophic
Bacteria, cyanobacteria
Protista
Eukaryotic, unicellular
Auto- & heterotrophic
Amoeba, Paramecium, Euglena
Fungi
Eukaryotic, mostly multicellular
Heterotrophic (saprophytic/parasitic)
Rhizopus, yeast, mushroom
Plantae
Eukaryotic, multicellular
Autotrophic
Mosses, ferns, trees
Animalia
Eukaryotic, multicellular
Heterotrophic
Insects, fish, humans
Monera
Prokaryotic, no true nucleus or membrane-bound organelles. Two sub-kingdoms: Archaebacteria (extreme habitats — methanogens, halophiles, thermoacidophiles) and Eubacteria (true bacteria, with peptidoglycan walls; bacilli, cocci, spirilla, cyanobacteria).
Protista
Unicellular eukaryotes: protozoa, unicellular algae and diatoms. Move by pseudopodia (Amoeba), cilia (Paramecium) or flagella (Euglena). Auto- or heterotrophic.
🍄 Fungi
Non-green eukaryotes with heterotrophic nutrition — saprophytic (Rhizopus, on dead matter), parasitic (Puccinia) or symbiotic. Made of thread-like hyphae forming a mycelium (yeast is unicellular); cell wall of chitin; reserve food is glycogen.
💡 Did You Know?
A lichen is a symbiosis of a fungus and an alga: the fungus gives shelter, water and minerals; the alga makes food. Lichens grow on bare rock, start soil formation and are sensitive indicators of air pollution.
Section 4.9
Kingdom Plantae
Multicellular, autotrophic eukaryotes with a cellulose cell wall and plastids — the primary producers. Classified by body differentiation, vascular tissue and seeds.
Plant classification: thallophyta (algae) → bryophyta (no vascular tissue) → pteridophyta (vascular, no seed) → gymnosperms (naked seeds) → angiosperms (seeds in fruit)
Division
Key features
Examples
Thallophyta (algae)
No true root/stem/leaf (thallus); no vascular tissue
Spirogyra, Ulothrix, Chara
Bryophyta
"Amphibians of the plant kingdom"; rhizoids; no vascular tissue; need water for fertilisation
Riccia, Marchantia, Funaria (moss)
Pteridophyta
True root, stem & leaf; vascular tissue; spores (no seeds)
Fern (Dryopteris), Equisetum
Gymnosperms
Naked seeds (not in fruit); woody, perennial
Cycas, Pinus, Cedrus
Angiosperms
Seeds enclosed in fruit; flowers; monocots & dicots
Wheat (mono), mango (dicot)
Monocots
Seed with one cotyledon, parallel leaf venation, fibrous roots, flower parts in threes. E.g. wheat, rice, maize, onion.
Dicots
Seed with two cotyledons, reticulate venation, tap root, flower parts in fives. E.g. pea, mango, mustard, sunflower.
💡 Did You Know?
Cryptogamae (thallophytes, bryophytes, pteridophytes) are the seedless "lower plants"; Phanerogamae (gymnosperms, angiosperms) are the seed-bearing "higher plants". Ginkgo biloba is a "living fossil".
Section 4.10
Kingdom Animalia — The Phyla
Multicellular, heterotrophic eukaryotes with no cell wall, capable of locomotion. Divided into ten phyla by body plan, symmetry, germ layers and coelom.
Phylum
Key features
Examples
Porifera
Pores; cellular level; sponges; non-motile
Sycon, Spongilla
Cnidaria (Coelenterata)
Hollow body, tentacles with stinging cnidoblasts; radial; diploblastic
Hydra, jellyfish, corals
Platyhelminthes
Flatworms; bilateral; triploblastic; acoelomate
Planaria, liver fluke, tapeworm
Nematoda
Roundworms; pseudocoelomate; complete gut
Ascaris, Wuchereria
Annelida
Segmented; true coelom; closed circulation
Earthworm, leech
Arthropoda
Jointed legs; exoskeleton; largest phylum; open circulation
Prawn, cockroach, spider
Mollusca
Soft body, muscular foot, mantle, often a shell
Snail, Pila, Octopus
Echinodermata
Spiny skin; tube feet; marine; radial adult
Starfish, sea urchin
Hemichordata
Worm-like; proboscis, collar, trunk; gill slits
Balanoglossus
Chordata
Notochord, dorsal nerve cord, gill slits
Fish, frog, birds, humans
Animal phyla arranged by rising complexity — from sponges (cellular) through worms and arthropods to chordates (notochord/backbone)
💡 Did You Know?
Animals from Porifera to Echinodermata have no backbone — they are invertebrates. Only the Chordata (vertebrates) have a backbone. A spider has 8 legs and 2 body parts, so it is an arthropod, not an insect (insects have 6 legs, 3 body parts).
Section 4.10 (Chordata)
The Vertebrate Classes
Chordates with a true backbone are vertebrates, grouped into five classes by their skin, breathing, heart and reproduction.
Class
Skin / cover
Heart
Breathing
Reproduction
Pisces (fish)
Scales, fins
2-chambered
Gills
Eggs, external; cold-blooded
Amphibia
Moist, slimy
3-chambered
Gills/lungs/skin
Eggs in water; cold-blooded
Reptilia
Dry, scaly
3-chambered (4 in crocodile)
Lungs
Eggs on land; cold-blooded
Aves (birds)
Feathers, wings
4-chambered
Lungs
Eggs; warm-blooded
Mammalia
Hair, mammary glands
4-chambered
Lungs
Mostly live young; warm-blooded
The five vertebrate classes — fishes and amphibians and reptiles are cold-blooded; birds and mammals are warm-blooded
Pisces — cartilaginous fish (shark) vs bony fish (rohu, with an air bladder).
Amphibia — first tetrapods on land; lay eggs in water (frog, toad).
Reptilia — dry scaly skin; lay eggs on land (snake, lizard, crocodile).
Aves — feathers, hollow bones for flight, beak with no teeth (pigeon, ostrich).
Common names vary by place and language, causing confusion. A scientific name is unique and accepted worldwide. The two-word system was introduced by Carolus Linnaeus.
Two-word naming
Mangifera indica
Genus (capital first letter) + species (small letter). Written in italics (or underlined separately by hand). E.g. human = Homo sapiens, peacock = Pavo cristatus.
Genus name starts with a capital letter; species name with a small letter.
Printed in italics; handwritten with each word underlined separately.
The same name is used in every country — it is universal.
Prevents confusion from local names and shows relationships (same genus = closely related).
Section 4.12
Acellular Entities: Viruses
Viruses are acellular — not made of cells, with no cytoplasm, nucleus, membrane or organelles — so they are not true living organisms and are kept outside the five kingdoms.
A virus is just genetic material (DNA or RNA) inside a protein coat (capsid), sometimes with an outer envelope
Living & non-living nature
Outside a host, viruses are inactive and can even be crystallised (non-living). Inside a living host cell, they reproduce using the host's machinery (living). Examples: influenza virus, polio virus, HIV, coronavirus.
Glossary
Key Terms
Quick definitions for fast revision.
Biodiversity
The variety of living organisms in an area.
Classification
Grouping organisms by similarities and differences.
Artificial classification
Grouping using a few easily-seen features (habitat, size).
Hierarchical classification
Arranging organisms from larger to smaller groups.
Autotroph / Heterotroph
Makes its own food / depends on others for food.
Prokaryotic / Eukaryotic
Cell without a true nucleus / cell with a true nucleus.
Symmetry
Arrangement of body parts (asymmetrical, radial, bilateral).
Germ layers
Embryonic layers; diploblastic (two) or triploblastic (three).
Coelom
Body cavity between body wall and gut (acoelomate/pseudo/coelomate).
Notochord
A rod-like support structure in chordates.
Vertebrate / Invertebrate
Animal with a backbone / without a backbone.
Thallus
A simple plant body with no true root, stem or leaf.
Taxonomy
Identification, naming and classification of organisms.
Species
The basic unit; organisms that interbreed to give fertile offspring.
Binomial nomenclature
Two-word scientific naming (Genus + species).
Virus
An acellular entity of genetic material in a protein coat.
Exam Strategy
Top Exam Tips
High-yield reminders that catch most students out.
1
Hierarchy order
Kingdom → Phylum/Division → Class → Order → Family → Genus → Species. Up = broader, fewer shared traits.