Cambridge O Level Biology 5090: Classification (5090)

This guide teaches Topic 2, Classification, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 2.1 Concept and use of a classification system and 2.2 Features of organisms. The 5090 syllabus is not tiered, so everything here is examinable for every candidate. Subject content like this is assessed on Paper 1 (Multiple Choice, 1 hour) and Paper 2 (Theory, 1 hour 45 minutes); Papers 3 and 4 assess experimental skills.

Use this page with the classification revision notes and the classification practice questions. The course hub is at /boards/cambridge/o-level/biology/ and the printable topic checklist is at /checklists/cambridge/o-level/biology/.

What this topic covers

Syllabus ref.	What you must be able to do
2.1.1	Understand that organisms can be classified into groups by the features they share
2.1.2	Describe a species as a group of organisms that can reproduce to produce fertile offspring
2.1.3	Describe the binomial system: an internationally agreed two-part scientific name showing genus and species
2.1.4	Construct and use dichotomous keys based on identifiable features
2.2.1	State the main features used to place organisms into the five kingdoms: Animal, Plant, Fungus, Prokaryote, Protoctist
2.2.2	State the main features of the vertebrate groups (mammals, birds, reptiles, amphibians, fish) and arthropod groups (myriapods, insects, arachnids, crustaceans)
2.2.3	State the main features of ferns and flowering plants (dicotyledons and monocotyledons)
2.2.4	Classify organisms using the features in 2.2.1 to 2.2.3
2.2.5	State the main features of viruses, limited to protein coat and genetic material
2.2.6	Understand that viruses can only replicate in living cells

This topic builds on cell structure and organisation, because several kingdom features (nucleus, cell wall, chloroplasts) are cell structures you learned there.

2.1 Concept and use of a classification system

Grouping by shared features (2.1.1)

There are far too many kinds of organism to study one at a time. Biologists sort them into groups, and each group is defined by features its members share. Organisms that share many features are placed in the same small group. Organisms that share only a few features end up in the same large group but different small groups.

The features used must be ones that every member of the group has. “Lives in water” is a poor basis for a group: whales, sharks and water lilies all live in water but have very different body structures. “Has feathers” is a good basis, because every bird has feathers and no other animal does.

Species (2.1.2)

A species is a group of organisms that can reproduce to produce fertile offspring. The word “fertile” is the part examiners look for. Offspring that are healthy but cannot themselves reproduce do not show that their parents belong to one species.

Worked example. A horse and a donkey can mate and produce a mule. Mules are almost always infertile. Are horses and donkeys the same species?

• Step 1: check whether they can produce offspring. They can (the mule).
• Step 2: check whether the offspring are fertile. They are not.
• Step 3: conclude. Horses and donkeys are different species, because the definition needs fertile offspring.

By contrast, two very different-looking dog breeds produce puppies that grow up able to breed, so all domestic dogs belong to one species.

The binomial system (2.1.3)

The binomial system is an internationally agreed way of naming species. Every species has a scientific name made of two parts:

1. the genus name, written first, with a capital letter
2. the species name, written second, all in lower case.

The whole name is printed in italics. When you handwrite it, underline the two words instead. After the first use, the genus can be shortened to its initial, for example P. leo.

Because the system is agreed worldwide, one organism has one scientific name, whatever the local name is in Urdu, Arabic or English. Common names can be confusing: different places use the same common name for different animals.

Worked example. The lion is Panthera leo, the tiger is Panthera tigris and the domestic cat is Felis catus. Which two are most closely related?

• Step 1: compare the first word (genus). Lion and tiger share the genus Panthera; the cat is in Felis.
• Step 2: organisms in the same genus share more features than organisms in different genera.
• Step 3: conclude: the lion and tiger are most closely related.

Dichotomous keys (2.1.4)

A dichotomous key identifies an organism by a series of steps. “Dichotomous” means dividing into two. Each step (a couplet) gives two contrasting statements about one feature. You pick the statement that fits your specimen, and it either names the organism or sends you to another numbered step.

Rules for constructing a key

• Use features you can see on the specimen: number of legs, presence of wings, leaf shape, veins. Avoid size and colour, which vary between individuals of the same species, and avoid features you cannot check, such as behaviour.
• Make each pair a true opposite about the same feature (“three pairs of legs” / “four pairs of legs”), not two unrelated features.
• Every organism must end at its own name, and no organism may appear twice.
• A key for n organisms needs n − 1 couplets.

Worked example: constructing a key. Make a key to separate a housefly, a spider, a crab, a centipede, a sparrow and a frog.

Six organisms need 6 − 1 = 5 couplets. Start with the feature that splits the group most evenly: four of the six have an exoskeleton and jointed legs.

1 a  Jointed legs and a hard exoskeleton ............. go to 2
  b  No exoskeleton .................................. go to 5
2 a  More than four pairs of legs .................... go to 3
  b  Four pairs of legs or fewer ..................... go to 4
3 a  Two pairs of antennae ........................... crab
  b  One pair of antennae ............................ centipede
4 a  Three pairs of legs ............................. housefly
  b  Four pairs of legs .............................. spider
5 a  Body covered in feathers ........................ sparrow
  b  No feathers ..................................... frog

Using it. A specimen has jointed legs, ten legs and two pairs of antennae. Step 1: exoskeleton, go to 2. Step 2: five pairs is more than four, go to 3. Step 3: two pairs of antennae, so it is the crab. Always write down the route (1a → 2a → 3a) in an exam answer if the question asks how you reached the name.

2.2 Features of organisms

The five kingdoms (2.2.1)

Kingdom	Main features	Examples
Animal	Multicellular; cells have a nucleus but no cell wall and no chloroplasts; feed on organic substances made by other organisms; usually move from place to place	Insects, fish, humans
Plant	Multicellular; cells have a nucleus and a cellulose cell wall; many cells have chloroplasts, so they make their own food by photosynthesis	Ferns, maize, mango trees
Fungus	Cells have a nucleus and a cell wall not made of cellulose; no chloroplasts; usually made of thread-like hyphae; feed by secreting enzymes onto dead or living material and absorbing the products	Mushrooms, moulds; yeast is single-celled
Prokaryote	Single-celled; no nucleus: circular DNA lies free in the cytoplasm, often with plasmids; cell wall, cell membrane, cytoplasm and ribosomes	Bacteria
Protoctist	Mostly single-celled, with a nucleus; some have chloroplasts and are plant-like, others are animal-like	Amoeba (animal-like), Euglena (has chloroplasts)

The key splits to remember: nucleus or no nucleus separates Prokaryote from the rest; cell wall and chloroplasts separate Plant from Animal; non-cellulose wall, no chloroplasts, hyphae mark out Fungus. The prokaryote structures are the same ones you met for the bacterial cell in cell structure and organisation.

Groups within the animal kingdom (2.2.2)

Vertebrates have a backbone. The syllabus lists five groups.

Group	Main features
Mammals	Hair or fur on the skin; external ears (pinnae); mammary glands that produce milk for the young; most give birth to live young
Birds	Feathers; a beak; front limbs are wings; lay eggs with hard shells
Reptiles	Dry skin covered in scales; lay eggs with rubbery or leathery shells on land
Amphibians	Moist skin with no scales; lay soft eggs without shells in water; larvae (tadpoles) live in water and use gills; adults usually live on land and use lungs
Fish	Body covered in wet scales; fins; gills for gas exchange; a lateral line along each side

Arthropods have no backbone. All arthropods have jointed legs, a segmented body and a hard exoskeleton. The four groups are separated mainly by counting legs, antennae and body sections.

Group	Body sections	Legs	Antennae	Other
Insects	3 (head, thorax, abdomen)	3 pairs	1 pair	Usually 1 or 2 pairs of wings
Arachnids	2 (cephalothorax, abdomen)	4 pairs	None	Several simple eyes
Crustaceans	Often 2 (cephalothorax, abdomen)	More than 4 pairs	2 pairs	Many live in water; exoskeleton often hardened
Myriapods	Many similar segments	1 or 2 pairs on each segment	1 pair	Centipedes and millipedes

Groups within the plant kingdom (2.2.3)

Ferns have roots, stems and leaves called fronds, but no flowers. They reproduce by spores, which form in small brown patches on the underside of the fronds.

Flowering plants produce flowers and reproduce by seeds, which form inside the ovary (later the fruit). They are split into two groups.

Feature	Monocotyledons	Dicotyledons
Cotyledons (seed leaves) in the seed	One	Two
Leaf veins	Parallel	Branching network
Flower parts	Usually in threes or multiples of three	Usually in fours or fives, or multiples of these
Examples	Maize, rice, grasses, onion	Bean, sunflower, mango

Classifying organisms (2.2.4)

To classify an organism, work from the largest group down and justify each step with a feature.

Worked example. An organism has jointed legs, a body in two sections, four pairs of legs and no antennae. Classify it.

• Kingdom: multicellular, moves, has legs, so Animal.
• Large group: jointed legs and exoskeleton, so arthropod.
• Arthropod group: two body sections, four pairs of legs, no antennae, so arachnid.

Each feature you quote should be one the syllabus lists for that group. “It has eight legs” earns more credit than “it looks like a spider”.

Viruses (2.2.5 and 2.2.6)

A virus is not a cell. The syllabus limits its features to two:

• a protein coat
• genetic material inside it (DNA or RNA).

A virus has no cytoplasm and none of the structures that a cell uses to make proteins. So a virus can only replicate inside a living cell. It enters a host cell and the host cell’s materials are used to make new copies of the virus. Outside a living cell, a virus does not reproduce. This is why viruses are not placed in any of the five kingdoms.

Common errors

• Defining a species as “organisms that can reproduce together” and leaving out fertile offspring.
• Writing a binomial name with two capital letters (Panthera Leo) or without underlining when handwritten.
• Using colour or size in a key, or putting two different features in one couplet.
• Saying prokaryotes have “no DNA”. They have DNA; they have no nucleus.
• Calling a fungus a plant. Fungi have no chloroplasts and their cell walls are not cellulose.
• Giving “eight legs” for an insect, or “antennae” for an arachnid.
• Saying amphibians have scales, or that reptile eggs are laid in water.
• Describing a virus as a bacterium or a prokaryote. A virus has no cell structure at all.

Next steps

Test your recall with the classification revision notes, then try the classification practice questions. For the cell structures behind the kingdom features, revisit cell structure and organisation. You can check your overall readiness with our free diagnostics.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, version 4, published by Cambridge International (Cambridge Assessment International Education). Topic 2: Classification, sections 2.1 and 2.2.