Characteristics & Classification of Living Organisms
The seven characteristics of life, the binomial system, dichotomous keys, animal & plant kingdom features, the five kingdoms, and viruses — with worked exam-style questions and high-frequency mistakes for both Core and Extended candidates.
Characteristics of Living Organisms
COREAll living organisms share seven characteristics. The syllabus requires you to describe each one using the precise wording below — examiner reports note that vague answers (e.g. "things move" instead of using the formal definition) lose marks even when the underlying idea is correct.
Movement · Respiration · Sensitivity · Growth · Reproduction · Excretion · Nutrition
The Seven Characteristics — Syllabus Definitions
An action by an organism or part of an organism causing a change of position or place.
Example: A cheetah running; a plant shoot bending toward light.
The chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
Example: Glucose + oxygen → carbon dioxide + water + energy (in mitochondria).
The ability to detect and respond to changes in the internal or external environment.
Example: Pupil constricts in bright light; Venus flytrap closes when an insect touches it.
A permanent increase in size and dry mass.
Example: A seedling becoming a sapling. Note "permanent" — water uptake alone doesn't count.
The processes that make more of the same kind of organism.
Example: A bacterium dividing in two; a flowering plant producing seeds.
The removal of the waste products of metabolism and substances in excess of requirements.
Example: Carbon dioxide from respiration leaves the lungs; urea leaves in urine.
The taking in of materials for energy, growth and development.
Example: A plant absorbing CO₂ and water for photosynthesis; an animal eating food.
Easily-Confused Pairs
| Often confused | What's the difference? |
|---|---|
| Respiration vs. breathing | Respiration is the chemical reaction in cells that releases energy. Breathing (ventilation) is the physical movement of air in and out of the lungs. Plants respire too — they don't breathe. |
| Excretion vs. egestion | Excretion = removing waste products of metabolism (CO₂, urea). Egestion = removing undigested food as faeces. Faeces is NOT excretion — it has never been part of the body's metabolism. |
| Movement vs. growth | Movement is a change of position or place. Growth is a permanent increase in size and dry mass. A growing root tip moves and grows — both characteristics apply. |
| Nutrition vs. ingestion | Nutrition is the broader process — taking in materials for energy, growth, development. Ingestion is just one step (taking food into the alimentary canal). Plants have nutrition without ingestion. |
Examiners frequently give a scenario describing a living thing or non-living object and ask students to identify which characteristic is being demonstrated — or to explain why something is alive/not alive. To answer fully, you must address several characteristics, not just one.
Common scenarios: "A car uses fuel and moves — is it alive? Justify." → Answer: a car shows movement and uses energy, but does not reproduce, grow, respond to stimuli, or excrete metabolic waste. It fails most of the seven criteria, so it is not alive.
Which of the following processes is the best example of excretion rather than egestion?
- A. The removal of undigested fibre as faeces
- B. The release of carbon dioxide from the lungs after exhalation
- C. Vomiting after eating contaminated food
- D. Removing seeds from a fruit before eating it
A student finds a small object on the ground that appears to grow over a few days. Describe three further observations the student would need to make to confirm the object is a living organism. [3 marks]
- Test whether the object responds to stimuli (e.g. touch, light) — sensitivity
- Check whether it carries out respiration (e.g. evidence of gas exchange / heat production)
- Observe whether it reproduces or produces offspring
- Check whether it carries out nutrition (taking in materials)
- Check whether it excretes waste products of metabolism
- Observe whether it moves (whole body or parts)
Examiner note: Growth alone is not sufficient — crystals also grow when added to a saturated solution, but they are not alive. Students must mention multiple characteristics to demonstrate they understand that all seven are required.
❌ Saying "respiration = breathing" → Wrong. Respiration is a chemical reaction in cells; breathing is ventilation of lungs.
❌ Calling faeces "excretion" → Wrong. Faeces is undigested food (egestion). Urine and exhaled CO₂ are excretion.
❌ Defining growth as "getting bigger" → Imprecise. The syllabus definition requires "permanent increase in size and dry mass" — water uptake alone (which inflates a balloon, for example) is not growth.
❌ Forgetting that plants show all seven characteristics → Plants respire (always, not just at night), respond to stimuli (gravitropism, phototropism), excrete (oxygen is technically a waste of photosynthesis), and move (slow growth movements).
❌ Confusing characteristic of life with feature of an organism → "Has fur" is a feature; "growth" is a characteristic of life. Don't mix them up in answers.
Concept and Uses of Classification Systems
CORE EXTENDEDClassification is the grouping of organisms by shared features. The system most widely used today is based on structural and biochemical similarities; for Extended candidates, it is also linked to evolutionary relationships revealed by DNA.
The Concept of a Species
A species is a group of organisms that can reproduce to produce fertile offspring. The "fertile offspring" requirement is essential — without it, a horse and donkey would count as the same species.
Horse (Equus caballus) × Donkey (Equus asinus) → Mule. Mules are infertile — they cannot produce offspring of their own. Therefore horse and donkey remain two separate species, and the mule is a hybrid, not a new species.
A Great Dane and a Chihuahua look very different but can (in principle) interbreed and produce fertile offspring. They are both Canis familiaris — the same species, just different breeds.
The Binomial System
The binomial system is an internationally agreed naming system in which the scientific name of an organism is made up of two parts showing the genus and the species.
| Convention | Rule | Correct | Incorrect |
|---|---|---|---|
| Capitalisation | Genus capitalised; species lowercase | Homo sapiens | Homo Sapiens / homo sapiens |
| Italicisation | Both words italicised (or underlined if handwritten) | Panthera leo | Panthera leo |
| Order | Genus first, then species | Quercus robur | robur Quercus |
| Abbreviation | Genus may be abbreviated after first use | "E. coli" (after introducing Escherichia coli) | "e. Coli" |
Common names vary by country and language — a "robin" in the UK is a different species from a "robin" in North America. The binomial system gives every species one universally agreed name, allowing scientists worldwide to communicate without confusion. This is the answer examiners expect when asked "Why use the binomial system?".
Dichotomous Keys
A dichotomous key is a tool for identifying organisms. At each step it offers two contrasting choices based on observable, identifiable features. Each choice either reveals the answer or directs you to the next step.
Worked Example — A 4-step key for arthropods
1a. Has wings ……… go to 2
1b. No wings ……… go to 3
2a. Wings covered by hard wing case ……… Beetle
2b. Wings membranous and visible at rest ……… Butterfly
3a. 8 legs ……… Spider
3b. 6 legs ……… Ant
How to use it: at every step, choose the option that matches the organism. Each choice eliminates half the possibilities. After at most a few steps, you reach an identification.
① Choose observable, unambiguous features — colour, number of legs, presence/absence of wings, body shape. Avoid features that vary between individuals (e.g. "small" vs "large").
② Each pair of choices must be mutually exclusive — every organism in the group must fit exactly one option, never both.
③ Use contrasting wording: "has wings" vs "no wings" (not "has wings" vs "has 6 legs" — those aren't opposites).
④ Keep working through pairs until each organism in your group has its own end-point (terminal identification).
Classification Reflects Evolutionary Relationships
Modern classification systems aim to reflect evolutionary relationships. Organisms placed in the same group share a common ancestor; the more closely related two organisms are (the more recent their common ancestor), the more features they share — both physical features and, more reliably, DNA.
DNA Base Sequences as a Classification Tool
The sequences of bases in DNA are used as a means of classification. The principle is straightforward:
- When organisms reproduce, DNA is copied. Occasional mutations (random base changes) accumulate over generations.
- Two species that diverged from a common ancestor recently have had less time for mutations to accumulate, so their DNA sequences remain very similar.
- Two species that diverged long ago have accumulated many independent mutations, so their DNA sequences differ more.
Conclusion: Groups of organisms which share a more recent ancestor (are more closely related) have base sequences in DNA that are more similar than those that share only a distant ancestor.
If species A and B share 98% of their DNA, and species A and C share 72% — A and B share a more recent common ancestor than A and C. Less time has passed since A and B diverged, so fewer mutations have accumulated.
Three species share the following percentages of identical DNA bases: A & B = 88%; A & C = 96%; B & C = 89%. Which pair shares the most recent common ancestor?
- A. Species A and B
- B. Species A and C
- C. Species B and C
- D. All three pairs share equally recent ancestors
A horse (Equus caballus) and a donkey (Equus asinus) can mate to produce a mule. Mules are infertile. The horse and donkey are best described as:
- A. The same species, because they can mate
- B. Different species, because their offspring are not fertile
- C. Different genera, because they have different scientific names
- D. The same species, because their genus name is the same
A student is given five small invertebrates labelled W, X, Y, Z, V with the following features:
W: 6 legs, has wings · X: 8 legs, no wings · Y: 6 legs, no wings · Z: 8 legs, has pincers · V: more than 8 legs, no wings
Construct a dichotomous key that would identify each of the five organisms. [4 marks]
- 1a. Has wings → W
- 1b. No wings → go to 2
- 2a. 6 legs → Y
- 2b. More than 6 legs → go to 3
- 3a. More than 8 legs → V
- 3b. Exactly 8 legs → go to 4
- 4a. Has pincers → Z
- 4b. No pincers → X
Mark scheme principles: Each pair must use mutually exclusive, observable features (1 mark); the key must lead to a unique identification for each of the 5 organisms (1 mark for reaching 3+ correctly, additional marks for full coverage). Other valid keys are accepted as long as they correctly separate all five organisms.
❌ "Same genus = same species" → Wrong. Lions (Panthera leo) and tigers (Panthera tigris) share a genus but are different species.
❌ Capitalising the species name → Wrong. Homo Sapiens is incorrect; only the genus is capitalised. Always: Homo sapiens.
❌ Forgetting to italicise (or underline) binomial names → On exams, both italics or underlining count. Plain print does not.
❌ Using subjective features in a dichotomous key → "Pretty" or "small" are not acceptable — features must be objective and measurable (number of legs, presence of wings, body symmetry).
❌ Ext: Mistaking similarity in appearance for evolutionary closeness → Two organisms can look alike due to convergent evolution (e.g. dolphins and sharks). DNA evidence is more reliable than appearance for determining evolutionary relationships.
Features of Organisms
CORE EXTENDEDThis sub-section is about identifying which group an organism belongs to from its features. Examiner reports note that classification questions — especially distinguishing arthropod groups — are a common weak area, particularly in Extended multiple-choice papers.
Animal Kingdom vs Plant Kingdom — Core Features
| Feature | Animals | Plants |
|---|---|---|
| Cell wall | Absent | Present (made of cellulose) |
| Chloroplasts | Absent | Present in many cells |
| Nutrition | Heterotrophic — feed on other organisms | Autotrophic — make food by photosynthesis |
| Movement | Most are mobile (whole body movement) | Fixed in position; only growth movements |
| Storage carbohydrate | Glycogen | Starch |
Vertebrate Groups (Animal Kingdom) — Core
| Group | Body cover | Reproduction | Body temperature | Other key features |
|---|---|---|---|---|
| Mammals | Hair / fur | Most give birth to live young; produce milk | Maintains constant warm body temperature | Mammary glands, external ears (most) |
| Birds | Feathers | Lay eggs with hard shells | Maintains constant warm body temperature | Beaks (no teeth), wings, hollow bones |
| Reptiles | Dry, scaly skin | Lay eggs with leathery shells (on land) | Body temperature varies with surroundings | Mostly four limbs (snakes are an exception) |
| Amphibians | Moist, permeable skin | Lay jelly-coated eggs (in water); larva → adult | Body temperature varies with surroundings | Live in water as larvae, on land as adults |
| Fish | Scales (often slimy) | Most lay eggs in water; external fertilisation | Body temperature varies with surroundings | Gills for gas exchange; fins |
Arthropod Groups (Animal Kingdom) — Core
Arthropods are invertebrates with jointed legs and an external skeleton (exoskeleton). They are split into four groups by body segmentation, number of legs and antennae.
| Group | Body segments | Legs | Antennae | Examples |
|---|---|---|---|---|
| Myriapods | Many similar segments | 1 or 2 pairs per body segment | 1 pair | Centipedes, millipedes |
| Insects | 3 (head · thorax · abdomen) | 3 pairs (6 legs) | 1 pair | Bees, butterflies, beetles, ants |
| Arachnids | 2 (cephalothorax · abdomen) | 4 pairs (8 legs) | None | Spiders, scorpions, ticks, mites |
| Crustaceans | Often 2 main regions (cephalothorax · abdomen) | 5 or more pairs (varies) | 2 pairs | Crabs, lobsters, shrimps, woodlice |
① Count the legs first. 6 legs → insect. 8 legs → arachnid. Many legs (a pair per segment) → myriapod. Five or more pairs, often with claws → crustacean.
② Then check antennae if needed. Two pairs → crustacean (this is the most reliable way to distinguish a crustacean from an insect when leg count is unclear). None → arachnid.
③ Body region count is the slowest indicator but settles ambiguous cases. Three regions (head/thorax/abdomen) is unique to insects.
A small invertebrate has 8 legs, no antennae, and a body divided into 2 regions. To which group does it belong?
- A. Insect
- B. Crustacean
- C. Arachnid
- D. Myriapod
The Five Kingdoms — Extended
All living organisms are placed into one of five kingdoms based on cell type, cell wall composition, and nutrition. Viruses are not included in the five-kingdom system because they are not made of cells.
| Kingdom | Cell type | Cell wall? | Number of cells | Nutrition | Examples |
|---|---|---|---|---|---|
| Animal | Eukaryotic | Absent | Multicellular | Heterotrophic (ingest food) | Mammals, birds, insects |
| Plant | Eukaryotic | Present (cellulose) | Multicellular | Autotrophic (photosynthesis) | Trees, grasses, ferns |
| Fungus | Eukaryotic | Present (made of chitin, not cellulose) | Mostly multicellular (yeasts are unicellular) | Heterotrophic — mostly saprotrophic (digest dead matter externally) | Mushrooms, yeasts, moulds |
| Prokaryote | Prokaryotic — no nucleus | Present (not cellulose) | Unicellular | Varies (some autotroph, some heterotroph) | Bacteria |
| Protoctist | Eukaryotic | Present in some, absent in others | Mostly unicellular; some simple multicellular | Varies (some autotroph, some heterotroph) | Amoeba, Paramecium, algae, Plasmodium |
Despite being fixed in position and looking somewhat plant-like, fungi differ from plants in three crucial ways:
- No chloroplasts / no photosynthesis — fungi cannot make their own food
- Cell walls of chitin, not cellulose
- Heterotrophic nutrition — they digest dead organic matter externally and absorb the products (saprotrophic), or feed as parasites
Plant Kingdom Subgroups — Extended
Within the plant kingdom, the syllabus limits you to ferns and flowering plants (and within flowering plants, monocotyledons and dicotyledons).
| Feature | Ferns | Flowering plants |
|---|---|---|
| Reproductive structures | Spores (no flowers, no seeds) | Flowers and seeds (often inside fruits) |
| Vascular tissue | Present (xylem and phloem) | Present (xylem and phloem) |
| Typical leaf form | Fronds, often divided | Highly variable |
| Examples | Bracken, tree ferns | Roses, oaks, grasses, daffodils |
Within Flowering Plants — Monocotyledons vs Dicotyledons
| Feature | Monocotyledons (monocots) | Dicotyledons (dicots) |
|---|---|---|
| Cotyledons (seed leaves) per seed | 1 | 2 |
| Leaf venation | Parallel veins | Branching network of veins |
| Flower parts | In multiples of 3 (e.g. 3 or 6 petals) | In multiples of 4 or 5 (e.g. 4 or 5 petals) |
| Examples | Grasses, lilies, palms, orchids | Roses, beans, oak trees, sunflowers |
Viruses — Extended
Viruses are not placed in any of the five kingdoms because they are not made of cells. The syllabus limits the features of viruses to:
- A protein coat (called a capsid) surrounding…
- …some genetic material (DNA or RNA)
Viruses cannot reproduce on their own — they must invade a host cell and use its machinery to make copies of themselves. Examples include the influenza virus, HIV, and the various coronaviruses.
Which feature is found in fungi but not in plants?
- A. Cell wall present
- B. Eukaryotic cells
- C. Cell wall made of chitin
- D. Multicellular body
Explain why viruses are not classified as living organisms in the five-kingdom system. [4 marks]
- Viruses are not made of cells — the five-kingdom system classifies cellular organisms
- They consist only of a protein coat surrounding genetic material (DNA or RNA)
- They cannot reproduce on their own — they require a host cell
- They do not carry out respiration independently
- They show no nutrition, growth, sensitivity or excretion outside a host cell
- They lack the cellular structures (cytoplasm, ribosomes, etc.) characteristic of all five kingdoms
❌ "Spider is an insect" → No. Spiders are arachnids — 8 legs, 2 body regions, no antennae. Insects have 6 legs, 3 body regions, 1 pair of antennae.
❌ "Whales are fish" → No. Whales are mammals — they have hair (sometimes only as embryos), give birth to live young, produce milk, and breathe air with lungs. Living in water doesn't make an animal a fish.
❌ "Bats are birds" → No. Bats are mammals — they have fur, give birth to live young, and produce milk. Wings alone do not make a bird.
❌ Ext: "Fungi are plants because they don't move" → No. Fungi differ from plants in three ways: no chloroplasts (no photosynthesis), cell walls of chitin not cellulose, heterotrophic nutrition.
❌ Ext: "Viruses are prokaryotes" → No. Prokaryotes are cells (with cell walls, ribosomes, cytoplasm). Viruses are not cells at all.
❌ Ext: Confusing monocots and dicots → Memory tip: "mono" = "one" — one cotyledon, one number system (multiples of 3). Parallel veins in monocots; branching veins in dicots.
Comprehensive Practice Questions
Mixed questions covering all of Topic 1, in the style of Cambridge IGCSE 0610 Papers 1–4. Try each before revealing the answer.
A robot is programmed to walk and to "learn" from its environment by adjusting its programming. Which characteristic of life does this robot still lack?
- A. Movement
- B. Sensitivity
- C. Reproduction
- D. Growth (it can be enlarged through upgrades)
Which combination of features identifies an organism as a bird rather than a mammal?
- A. Maintains a constant body temperature; lays eggs with hard shells
- B. Feathers; lays eggs with hard shells; beak instead of teeth
- C. Maintains a constant body temperature; has hair
- D. Lays eggs; has wings
Two organisms have very similar DNA base sequences but look completely different. The most likely explanation is that they:
- A. Belong to different kingdoms
- B. Share a recent common ancestor but have evolved different physical features
- C. Have just had random base mutations that match by coincidence
- D. Are not related at all — DNA is unreliable
A student is given two unfamiliar organisms, P and Q. P has 6 legs, 1 pair of antennae and a body divided into three regions. Q has 8 legs, no antennae and a body divided into two regions.
(a) State the arthropod group to which each organism belongs. [2 marks]
(b) Construct a simple dichotomous key that could distinguish between P and Q. [2 marks]
(c) State two features that all arthropods share. [2 marks]
- P is an insect (6 legs + 3 body regions + 1 pair of antennae) [1 mark]
- Q is an arachnid (8 legs + 2 body regions + no antennae) [1 mark]
- 1a. Has antennae → P
- 1b. No antennae → Q
- (or equivalent using leg count, body regions — any objective feature that separates them) [1 mark for valid mutually-exclusive feature; 1 mark for correctly leading to each organism]
- Jointed legs
- External skeleton (exoskeleton)
- Segmented body
- Bilateral symmetry
Yeasts are unicellular organisms with cell walls and no chloroplasts. They obtain energy by breaking down sugars.
(a) Identify the kingdom to which yeasts belong, and justify your answer. [3 marks]
(b) Yeast cells and bacterial cells are both unicellular and have cell walls. State two ways their cell structures differ. [2 marks]
(c) Explain why a virus, which can also infect yeast cells, is not placed in the same kingdom as yeasts. [2 marks]
- Has cell wall — but made of chitin (not cellulose), so not a plant [1 mark]
- Has no chloroplasts → cannot photosynthesise → not a plant [1 mark]
- Heterotrophic nutrition (breaks down sugars for energy) — characteristic of fungi (although yeast is unicellular, fungi as a kingdom include unicellular forms like yeasts and multicellular forms like moulds and mushrooms) [1 mark]
- Yeast cells are eukaryotic (have a nucleus); bacteria are prokaryotic (no nucleus, DNA in cytoplasm)
- Yeast cell walls contain chitin; bacterial cell walls contain peptidoglycan (or "different chemical composition")
- Yeast has membrane-bound organelles (mitochondria, ER); bacteria do not
- Yeast has linear DNA in chromosomes; bacteria have a single circular DNA molecule and may have plasmids
- Viruses are not made of cells — they only have a protein coat and genetic material [1 mark]
- The five-kingdom classification system applies only to cellular organisms; viruses cannot reproduce or carry out metabolism without a host cell [1 mark]
High-Frequency Mistakes — Topic 1 Overall
- 📖Vague definitions for the seven characteristicsThe syllabus expects precise wording. "Excretion is getting rid of waste" is too vague — full credit requires "removal of the waste products of metabolism and substances in excess of requirements". Memorise the syllabus phrasing word-for-word.
- 🚫Calling faeces "excretion"Faeces is undigested food that has never entered the body's metabolism — it is egestion, not excretion. The classic excretion examples are CO₂ from the lungs and urea in urine.
- 🌬"Respiration = breathing"Respiration is the chemical reaction in cells that releases energy. Breathing (ventilation) is the physical movement of air in and out of the lungs. Plants respire too — but they don't breathe.
- 📝Wrong binomial formatGenus capitalised, species lowercase, both italicised (or underlined). Homo sapiens is correct; "Homo Sapiens", "homo sapiens", and "Homo sapiens" (no italics) all lose marks.
- 🐝Mistaking a spider for an insectInsects: 6 legs · 3 body regions · 1 pair of antennae. Arachnids: 8 legs · 2 body regions · NO antennae. Always check leg count and body region count first.
- 🦀Mistaking a crustacean for an insectThe most reliable distinguishing feature is the antennae count: crustaceans have 2 pairs of antennae; insects have only 1 pair. Crustaceans also have ≥5 pairs of legs.
- 🐋"Whales are fish; bats are birds"Both are mammals — they have hair, give birth to live young, and produce milk. Habitat (water) and ability to fly are not classification features.
- 🍄Ext: "Fungi are plants"Three differences make fungi their own kingdom: (1) no chloroplasts → no photosynthesis; (2) cell walls of chitin (not cellulose); (3) heterotrophic nutrition (saprotrophic).
- 🦠Ext: "Viruses are bacteria"Bacteria are prokaryotic cells. Viruses are not cells at all — only protein coat plus genetic material. They are excluded from the five-kingdom system.
- 🌱Ext: Mixing up monocots and dicots"Mono" = one cotyledon, parallel leaf veins, flower parts in 3s. "Di" = two cotyledons, branching veins, flower parts in 4s or 5s. Examples: grasses are monocots; oaks are dicots.
- 🧬Ext: Misreading DNA similarity dataHigher % DNA similarity = more recent common ancestor (less time for mutations to accumulate). Lower % DNA similarity = more distant common ancestor.
Topic 1 carries fewer marks than the larger topics (~3–5% of Papers 1–4 directly), but its terms recur throughout the syllabus — every later unit uses the species concept, classification, and kingdom-level features. Highest-yield items for the exam: precise definitions of the seven characteristics, binomial naming conventions, dichotomous key construction, and arthropod group identification.
Extended candidates additionally need to know: 5-kingdom features and exemplars, why fungi are their own kingdom, the structure and exclusion of viruses, monocot/dicot distinction, and the use of DNA base sequences as evidence of evolutionary relationships.