System of Classification and Naming of Organisms

Why organisms are classified

Classification is the process of grouping organisms based on similarities and differences in their characteristics. It allows scientists to organise the huge diversity of living things into a manageable system, identify and name newly discovered organisms consistently, and study the evolutionary relationships between groups.

The classification hierarchy

1. Kingdom, the largest and broadest group.
2. Phylum, organisms sharing a basic body plan.
3. Class, narrower groups within a phylum.
4. Order, narrower groups within a class.
5. Family, closely related genera.
6. Genus, closely related species.
7. Species, the smallest unit; organisms that can interbreed to produce fertile offspring.

The five kingdoms

• Monera, single-celled organisms without a true nucleus, such as bacteria.
• Protista, mostly single-celled organisms with a true nucleus, such as Amoeba and algae.
• Fungi, organisms that absorb nutrients from their surroundings, such as mushrooms and moulds.
• Plantae, multicellular organisms that make their own food by photosynthesis.
• Animalia, multicellular organisms that obtain food by ingestion.

Binomial nomenclature and how this is examined

Each species is given a unique two-part scientific name consisting of the genus name followed by the species name, for example Homo sapiens for humans. The genus name is capitalised, the species name is not, and both are written in italics (or underlined when handwritten).

This system, developed by Carl Linnaeus, gives every organism one internationally recognised name regardless of the local language used.

SPM questions commonly ask you to arrange the classification levels in the correct order, name the five kingdoms with an example each, or apply the rules of binomial nomenclature to identify a correctly written scientific name.

Worked exam-style question

Question. The table describes four organisms. Organism A: unicellular, no true nucleus, cell wall made of peptidoglycan.

Organism B: multicellular, has chlorophyll, makes its own food. Organism C: unicellular, has a true nucleus, some members are photosynthetic.

Organism D: multicellular, obtains food by ingestion, has no cell wall. (a) Name the kingdom each organism most likely belongs to.

(b) State the classification level at which two organisms would be grouped together if they could interbreed naturally to produce fertile offspring. (c) Write the correctly formatted binomial name for the lion, given the genus Panthera and species leo.

Model answer. (a) A: Monera (no true nucleus, peptidoglycan cell wall, a bacterium); B: Plantae (chlorophyll, photosynthesis); C: Protista (true nucleus, unicellular, some photosynthetic); D: Animalia (multicellular, ingests food, no cell wall). (b) Species, organisms that can interbreed naturally to produce fertile offspring belong to the same species.

(c) Panthera leo (genus capitalised, species not capitalised, both italicised).

Practice question

Try this. A newly discovered organism is multicellular, absorbs nutrients from decaying wood rather than making its own food or ingesting solid particles, and has no chlorophyll. (a) Name the kingdom this organism belongs to.

(b) State one feature from the description that supports your answer. (c) A scientist writes the organism's scientific name as 'agaricus Bisporus'.

Identify one error in this name and correct it.

Exam tip

Key terms

• Biodiversity, the variety of living organisms in an area, often measured at the species, genetic and habitat levels, which classification helps document.
• Species, a group of organisms that can interbreed naturally to produce fertile offspring; the smallest unit in the classification hierarchy.
• Classification, the process of grouping organisms by shared characteristics into a hierarchy from kingdom down to species.
• Binomial nomenclature, the two-part naming system, genus followed by species, that gives every organism one internationally recognised scientific name.

Source:SRC-DSKP-EN

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