Cambridge O Level Biology 5090: Classification (5090) – Revision Notes

These are condensed notes for the final weeks. For full explanations and worked examples, use the classification study guide.

They cover Topic 2, Classification, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4): sections 2.1 Concept and use of a classification system and 2.2 Features of organisms. The syllabus is not tiered, so all of this is examinable for every candidate. When you are ready, test yourself with the classification practice questions. Course hub: /boards/cambridge/o-level/biology/. Topic checklist: /checklists/cambridge/o-level/biology/.

2.1 Classification systems

Definitions to learn word for word

Term	Definition
Classification	Sorting organisms into groups by the features they share
Species	A group of organisms that can reproduce to produce fertile offspring
Binomial system	An internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species
Dichotomous key	A key in which each step splits the organisms into two groups using one identifiable feature

Binomial names: four rules

1. Genus first, with a capital letter: Panthera.
2. Species second, all lower case: leo.
3. Printed in italics; underlined when handwritten.
4. Same genus means more closely related than different genera.

Worked reminder: Oryza sativa (Asian rice) and Oryza glaberrima (African rice) share the genus Oryza, so they are more closely related to each other than either is to maize, Zea mays.

Method in steps: constructing a dichotomous key

1. List the organisms and the visible features of each.
2. Choose ONE feature that splits the list into two groups.
3. Write two opposite statements about it (1a / 1b).
4. Repeat for each group until every organism stands alone.
5. Check: n organisms -> n - 1 couplets; each name appears once.
6. Test the key by running every organism through it.

Good features: number of legs, wings present or absent, leaf divided or not, veins parallel or branching, shell present or absent. Poor features: colour, size, “lives in ponds”, “fast-moving”.

Method in steps: using a key

1. Start at couplet 1. Read BOTH statements before choosing.
2. Follow the one that fits the specimen to the next number.
3. Write down the route (e.g. 1b -> 5a) if asked.
4. Stop when you reach a name.

Small worked reminder: a four-animal key

Four vertebrates: a pigeon, a shark, a toad and a lizard. Four organisms need 4 − 1 = 3 couplets.

1 a  Has feathers ............................ pigeon
  b  No feathers ............................. go to 2
2 a  Has fins ................................ shark
  b  No fins ................................. go to 3
3 a  Dry skin covered in scales .............. lizard
  b  Moist skin with no scales ............... toad

Check it: each couplet asks about one feature, and each animal appears once. Notice that the key uses features straight from the vertebrate table below. That is a quick way to build a key in the exam: pick features the syllabus already lists for each group.

2.2 Features of organisms

Classifying in steps (2.2.4)

1. Kingdom first: nucleus? cell wall? chloroplasts? hyphae?
2. If Animal: backbone (vertebrate) or jointed legs + exoskeleton (arthropod)?
3. Then the group: use the tables below.
4. Quote one listed feature for EACH step you name.

Worked reminder: an organism has a nucleus, a cellulose cell wall, chloroplasts, flowers, and leaves with a branching network of veins. Plant kingdom (cellulose wall, chloroplasts), flowering plant (flowers), dicotyledon (network veins).

Five kingdoms at a glance

Kingdom	Nucleus?	Cell wall?	Chloroplasts?	Cells	Other main features
Animal	Yes	No	No	Many	Feed on organic substances made by other organisms
Plant	Yes	Yes, cellulose	Yes (in many cells)	Many	Photosynthesis
Fungus	Yes	Yes, not cellulose	No	Usually many (yeast: one)	Hyphae; secrete enzymes and absorb food
Prokaryote	No	Yes	No	One	Circular DNA free in cytoplasm; plasmids; ribosomes
Protoctist	Yes	Some	Some	Mostly one	A mixed group: some plant-like, some animal-like

Vertebrates

Group	Skin covering	Eggs / young	Other
Mammals	Hair or fur	Most give birth to live young	External ears (pinnae); mammary glands
Birds	Feathers	Hard-shelled eggs	Beak; wings
Reptiles	Dry scales	Rubbery or leathery-shelled eggs, laid on land
Amphibians	Moist, no scales	Soft eggs without shells, laid in water	Larvae (tadpoles) with gills; adults with lungs
Fish	Wet scales		Fins; gills; lateral line

Arthropods

All arthropods: jointed legs, segmented body, exoskeleton.

Group	Body sections	Pairs of legs	Pairs of antennae
Insects	3	3	1
Arachnids	2	4	0
Crustaceans	Often 2	More than 4	2
Myriapods	Many segments	1 or 2 per segment	1

Memory hook: count the legs first, then the antennae. Three pairs means insect; four pairs means arachnid.

Worked reminder: a small animal has jointed legs, a body in two sections, five pairs of legs and two pairs of antennae. Jointed legs and an exoskeleton make it an arthropod. More than four pairs of legs and two pairs of antennae make it a crustacean. Watch whether a question gives legs or pairs of legs: ten legs is five pairs, not ten pairs.

Plants

	Ferns	Flowering plants
Flowers	No	Yes
Reproduce by	Spores (under the fronds)	Seeds
Leaves	Fronds	Varied

	Monocotyledons	Dicotyledons
Cotyledons	1	2
Leaf veins	Parallel	Branching network
Flower parts	Threes	Fours or fives

Viruses

• Features (syllabus limit): protein coat and genetic material.
• Not cells; not in any of the five kingdoms.
• Can only replicate inside living cells, using the host cell.

Must-know distinctions

• Prokaryote vs other kingdoms. Prokaryotes have DNA but no nucleus. All the other kingdoms have a nucleus.
• Plant vs Fungus. Both have cell walls; only plants have cellulose walls and chloroplasts.
• Reptile vs amphibian. Dry scales and shelled eggs on land, against moist scaleless skin and soft eggs in water.
• Insect vs arachnid. Three pairs of legs and antennae, against four pairs of legs and no antennae.
• Crustacean vs myriapod. Two pairs of antennae, against one pair with legs on every segment.
• Fern vs flowering plant. Spores against seeds; no flowers against flowers.
• Virus vs prokaryote. A prokaryote is a cell with a membrane, cytoplasm and ribosomes. A virus is a protein coat around genetic material.

Quick self-test

1. Give the definition of a species.
2. Write the binomial name of the lion correctly as you would by hand.
3. Two organisms are Musca domestica and Musca autumnalis. What does the shared first word tell you?
4. How many couplets does a dichotomous key need to separate nine organisms?
5. Why should colour not be used in a key?
6. Which kingdom has cells with no nucleus?
7. State two features that place a mushroom in the Fungus kingdom.
8. An animal has moist skin with no scales and lays eggs in water. Which vertebrate group?
9. An arthropod has three body sections and one pair of antennae. How many pairs of legs would you expect?
10. A leaf has parallel veins. Monocotyledon or dicotyledon?
11. How do ferns reproduce?
12. Why can a virus not be grown in a flask of nutrients with no cells in it?

Answers

1. A group of organisms that can reproduce to produce fertile offspring.
2. Panthera leo, with each word underlined, capital P, lower-case l.
3. They are in the same genus (Musca), so they are closely related but different species.
4. 9 − 1 = 8 couplets.
5. Colour varies between individuals of the same species, so it is not reliable.
6. Prokaryote.
7. Any two: cell wall not made of cellulose; no chloroplasts; body of hyphae; secretes enzymes and absorbs the products.
8. Amphibians.
9. Three pairs (it is an insect).
10. Monocotyledon.
11. By spores, formed on the underside of the fronds.
12. A virus can only replicate inside a living cell; it has only a protein coat and genetic material, so it cannot copy itself on its own.

Where marks are usually lost

• Writing “can reproduce” for a species without “to produce fertile offspring”: the key word earns the mark.
• Capitalising the species name or leaving a handwritten binomial name without underlining.
• Constructing a key where one couplet compares two different features, such as “has wings” against “has six legs”.
• Using colour, size or habitat in a key, or leaving one organism unidentified at the end.
• Saying prokaryotes “have no DNA” or “no genetic material” instead of “no nucleus”.
• Stating that fungi photosynthesise, or that fungal cell walls are made of cellulose.
• Mixing up arthropod counts: giving antennae to arachnids, or four pairs of legs to insects.
• Giving vague features such as “it has skin” or “it has legs”, which apply to many groups and earn no credit.
• Calling a virus a cell or putting it in the Prokaryote kingdom.

Related pages

• Classification study guide
• Classification practice questions
• Cell structure and organisation, for the bacterial cell and the plant and animal cell structures used to define kingdoms
• 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.