8.1 Classification System and Naming of Organisms
Biodiversity represents the immense variety of living organisms interacting on Earth. To study, organise, and discuss this vast diversity globally, scientists use taxonomy—the biological branch dedicated to classification, identification, and naming.
Learning Objectives
- State the meaning of taxonomy and explain the necessity of classifying and naming organisms scientifically.
- Identify the six kingdoms of organisms and describe their main characteristics based on cell type, number of cells, and mode of nutrition.
- List the levels of the Linnaeus taxonomic hierarchy in the correct sequence from domain down to species.
- State the rules of the Linnaeus binomial nomenclature system and correctly write scientific names.
- Construct and use dichotomous keys (couplets and spider keys) to identify organisms based on observable features.
The Linnaeus Taxonomic Hierarchy and Binomial Naming Format
A step-down ranking from Domain to Species, with the correct printed and handwritten binomial nomenclature styles.
Eight Ranks in Order
- 1 Domain
- 2 Kingdom
- 3 Phylum
- 4 Class
- 5 Order
- 6 Family
- 7 Genus
- 8 Species
Binomial Nomenclature Format
Printed: Alcedo atthis
Handwritten: Alcedo atthis
Genus name is capitalised; species name is lowercase. Printed names are italicised; handwritten names are underlined separately.
Short Explanation
What is Taxonomy?
Taxonomy is a field in biology involving the classification, identification, and naming of organisms in an organised manner. All organisms must be scientifically classified based on defining features systematically to facilitate international studies and discussions among scientists. Carolus Linnaeus is known as the "father of taxonomy" who created a classification and naming system still in use today.
Three Main Pillars of Taxonomy
1. Classification: Categorising organisms based on physical features in a taxonomic hierarchy system.
2. Identification: Identifying organisms using dichotomous keys.
3. Naming: Naming organisms using a binomial nomenclature system.
Basis of the Six-Kingdom System
Carl Woese (1990) classified organisms into six kingdoms based on:
• Types of cells (prokaryote vs eukaryote)
• Number of cells (unicellular vs multicellular)
• Types of nutrition (autotroph vs heterotroph)
Summary of the Six Kingdoms
| Kingdom | Cell Type | Cell Number | Nutrition | Key Features | Examples |
|---|---|---|---|---|---|
| Archaebacteria | Prokaryote | Unicellular | Autotroph / Heterotroph | No peptidoglycan in cell wall; live in extreme environments; methanogens, halophiles, thermophiles | Sulfolobus sp., Halobacterium salinarum |
| Eubacteria | Prokaryote | Unicellular | Autotroph / Heterotroph | Cell wall made of peptidoglycan (murein); no membrane-enclosed organelles; classified by shape | Streptococcus pneumoniae, Vibrio cholerae, Cyanobacteria |
| Protista | Eukaryote | Unicellular / Multicellular | Autotroph / Heterotroph / Both | Simple cell organisation without specialised tissue; protozoa, algae, slime moulds | Euglena sp., Amoeba sp., Paramecium sp. |
| Fungi | Eukaryote | Unicellular / Multicellular | Heterotroph (saprophyte / parasite) | Cell wall made of chitin; body of hyphae called mycelium; no chlorophyll | Saccharomyces cerevisiae, Agaricus sp. |
| Plantae | Eukaryote | Multicellular | Photoautotroph | Contain chlorophyll; undergo sexual or asexual reproduction; seedless plants and plants with seeds | Bougainvillea sp., Coleus blumei |
| Animalia | Eukaryote | Multicellular | Heterotroph | Most can move; most reproduce sexually; invertebrates and vertebrates | Asterias sp., Alcedo atthis, Elephas maximus |
Taxonomic Hierarchy
The Linnaean hierarchy classifies organisms from the broadest to the most specific level:
Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species
Moving downward, the number of organisms decreases and the genetic link becomes closer. Moving upward, the number of organisms increases.
Binomial Nomenclature Rules
1. Each name has two words: genus name and species name.
2. Genus name is capitalised; species name is lowercase.
3. Printed names are italicised (Alcedo atthis).
4. Handwritten names are underlined separately (Alcedo atthis).
Dichotomous Key
A tool used to identify organisms based on observable similarities and differences. It is constructed using a series of couplets of contrasting statements. Features must be obvious and observable; overlapping traits must be avoided.
Forms of Dichotomous Keys
• Textual sequential list (couplet format)
• Branching diagram (spider dichotomous key)
Try to Answer First
Answer in your mind, then press "Check Answer".
What is the main structural difference between the cell walls of Archaebacteria and Eubacteria?
Arrange the taxonomic hierarchy in descending order from the highest rank to the lowest rank.
If a student writes the scientific name of the paddy plant by hand as "Oryza sativa" with one continuous underline, is it correct? Explain why.
Common Mistakes
- Writing handwritten scientific names with a single continuous underline across both words instead of two separate underlines.
- Capitalising the first letter of the species name (e.g., writing Alcedo Atthis instead of Alcedo atthis).
- Forgetting to italicise scientific names when typing or printing them.
- Reversing the order of the taxonomic hierarchy (e.g., putting family before order, or phylum before kingdom).
- Using ambiguous, subjective, or overlapping traits (such as "large" vs "small") in couplets when constructing dichotomous keys.
Concept Misunderstandings
Archaebacteria and Eubacteria belong to the same kingdom because both are prokaryotes.
Although both are unicellular prokaryotes, molecular studies revealed significant RNA and cell wall differences (Archaebacteria lack peptidoglycan; Eubacteria contain peptidoglycan), leading them to be separated into two distinct kingdoms.
Fungi are non-flowering plants because they are rooted in the ground and have cell walls.
Fungi are heterotrophic organisms with cell walls made of chitin and have no chlorophyll, meaning they cannot undergo photosynthesis like plants.
An organism's scientific name can just be its species name alone.
A scientific name is binomial, meaning it must always consist of two words: the genus name followed by the species name.
Summary
- Taxonomy is the biological field encompassing classification, identification, and naming.
- Carolus Linnaeus is the father of taxonomy; he introduced binomial nomenclature and hierarchical classification.
- Organisms are classified into six kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae, and Animalia.
- Archaebacteria lack peptidoglycan in cell walls and survive extreme environments (methanogens, halophiles, thermophiles).
- Eubacteria are true bacteria with peptidoglycan (murein) cell walls and lack membrane-enclosed organelles.
- Protista are simple eukaryotes without specialised tissues, divided into protozoa, algae, and slime moulds.
- Fungi are eukaryotic heterotrophs with chitin cell walls and hyphal mycelium, lacking chlorophyll.
- Plantae are multicellular photoautotrophs with chlorophyll; Animalia are multicellular heterotrophs that generally move.
- Taxonomic hierarchy: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species. Downward, organism count decreases and genetic similarity increases.
- Binomial rules: First word = Genus (capitalised), second word = species (lowercase). Printed in italics; handwritten with separate underlines.
- Dichotomous keys identify organisms through numbered couplets of non-overlapping, observable features.
Short Activity
Objective: test your understanding of the six kingdoms, binomial nomenclature, and taxonomic hierarchy.
A. Objective Quiz
An unknown unicellular prokaryote is discovered flourishing in an acidic hot spring at 70 °C. Laboratory testing shows that its cell wall lacks peptidoglycan. In which kingdom should this organism be classified?
Which of the following shows the correct way to write the scientific name of the common sunflower in print?
B. Fill in the Blanks
In the taxonomy hierarchy, the smallest classification rank consisting of organisms capable of interbreeding to produce viable fertile offspring is called the .
The cell walls of true bacteria belonging to kingdom Eubacteria are made up of a polymer of sugars and amino acids called .
C. Matching / Drag and Drop
Drag each kingdom to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Kingdom to Its Description
Write the correct kingdom next to each description.
| Description | Kingdom |
|---|---|
| 1. Unicellular prokaryotes without peptidoglycan that include methanogens and halophiles | |
| 2. Unicellular true bacteria with peptidoglycan cell walls, such as Vibrio cholerae | |
| 3. Simple eukaryotes without specialised tissues, including protozoa, algae, and slime moulds | |
| 4. Heterotrophs with cell walls made of chitin and bodies composed of hyphal mycelium |
Answer choices: Archaebacteria • Eubacteria • Protista • Fungi
