Characteristics and Classification of Living Organisms
This is usually the first topic taught and one of the most heavily examined, because it supplies the vocabulary every later topic depends on. It is also unusually easy to score well on, provided the definitions are exact.
1. The characteristics of living organisms
There are seven characteristics, and the standard mnemonic is MRS GREN:
| Letter | Characteristic | Definition |
|---|---|---|
| M | Movement | An action causing a change of position or place |
| R | Respiration | The chemical reactions in cells that break down nutrient molecules to release energy |
| S | Sensitivity | The ability to detect and respond to changes in the internal or external environment |
| G | Growth | A permanent increase in size and dry mass |
| R | Reproduction | The processes that make more of the same kind of organism |
| E | Excretion | The removal of waste products of metabolism and substances in excess of requirements |
| N | Nutrition | Taking in of materials for energy, growth and development |
The details that earn the marks:
- Respiration is not breathing. It is the chemical reaction that releases energy from glucose, happening inside cells. Breathing is ventilation — a different thing entirely.
- Growth is an increase in size and dry mass. Dry mass matters because taking in water increases size without being growth.
- Excretion is the removal of waste products of metabolism. Removing undigested food (egestion) is not excretion, because that material was never part of the body’s chemical reactions.
- Sensitivity covers detecting and responding.
2. Why we classify
Classification organises the enormous number of living organisms into groups, so they can be identified and studied and so that relationships between them can be understood.
Organisms are placed into groups based on similarities and differences. Traditionally these were shared physical features; increasingly, classification uses DNA base sequences, and species that are more closely related have more similar base sequences.
Species
A species is a group of organisms that can reproduce to produce fertile offspring.
Both halves matter, and the classic example shows why:
A horse and a donkey can interbreed, and they produce a mule. But a mule is infertile — it cannot reproduce. So horses and donkeys satisfy the first half of the definition but not the second, and they remain two different species.
Leaving out “fertile” makes the definition wrong, not merely incomplete. It is one of the most common one-mark losses in the whole subject.
3. The classification hierarchy
Groups run from the largest and most general to the smallest and most specific:
Kingdom → Phylum → Class → Order → Family → Genus → Species
A mnemonic most students find sticks: King Philip Came Over For Good Spinach.
Human classification is worth memorising, as it is asked directly:
| Rank | Human |
|---|---|
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Mammalia |
| Order | Primates |
| Family | Hominidae |
| Genus | Homo |
| Species | sapiens |
A common mix-up: Mammalia is a class, not a phylum. The phylum is Chordata — animals that have a notochord at some stage of development, which in vertebrates becomes the vertebral column (backbone).
4. The binomial system
The binomial system is an internationally agreed system in which the scientific name of an organism is made up of two parts: the genus and the species.
The rules for writing it — each is examinable:
| Rule | Example |
|---|---|
| Genus first, with a capital letter | Homo |
| Species second, all lower case | sapiens |
| Written in italics, or underlined if handwritten | Homo sapiens / Homo sapiens |
So homo Sapiens, Homo Sapiens and an un-underlined handwritten Homo sapiens are all wrong.
The advantage of the system is that it is international — the same organism has the same scientific name everywhere, avoiding the confusion of common names that differ between languages and regions.
5. Dichotomous keys
A dichotomous key identifies an organism through a series of steps, each offering two contrasting descriptions.
Di- means two: at each stage there are exactly two options, and each choice sends you either to another numbered step or to a name.
How to use one without losing marks:
- Start at step 1 — never in the middle
- Read both statements before choosing
- Follow exactly where that choice directs you
- Continue until you reach a name, not a number
- Base each decision only on what you can actually see in the picture or specimen
A key describes visible features. If a question specifies visible features, characteristics you cannot see — such as whether an organism produces scent, or its diet — will not score even if true.
6. The five kingdoms
| Kingdom | Key features | Examples |
|---|---|---|
| Animal | Multicellular; no cell walls; no chloroplasts; feed on organic substances made by other organisms | Mammals, insects, fish |
| Plant | Multicellular; cell walls made of cellulose; chloroplasts present; feed by photosynthesis | Flowering plants, ferns |
| Fungus | Usually multicellular; cell walls not made of cellulose; no chloroplasts; feed by saprotrophic or parasitic nutrition; body often made of hyphae forming a mycelium | Mushrooms, Penicillium, yeast (single-celled) |
| Prokaryote | Single-celled; no nucleus; circular DNA loose in the cytoplasm; plasmids often present; cell wall not cellulose; no mitochondria | Bacteria |
| Protoctist | Mostly single-celled; do have a nucleus; some have chloroplasts and photosynthesise, some do not | Amoeba, Chlorella, Plasmodium |
Prokaryotes and protoctists are not the same thing. Bacteria are prokaryotes — no nucleus. Protoctists have a nucleus and are eukaryotes. Calling bacteria protoctists is a straightforward error.
The quickest way to separate the first three: all three are multicellular, so look at the cell wall and chloroplasts. Cell wall + chloroplasts = plant. Cell wall, no chloroplasts = fungus. No cell wall = animal.
7. Vertebrate groups
Vertebrates are animals with a backbone, in the phylum Chordata.
| Group | Skin | Reproduction | Temperature | Gas exchange |
|---|---|---|---|---|
| Fish | Wet scales | External fertilisation; soft eggs in water | Variable | Gills |
| Amphibian | Moist smooth skin | External fertilisation; jelly-coated eggs in water | Variable | Lungs and skin |
| Reptile | Dry scales | Internal fertilisation; leathery-shelled eggs on land | Variable | Lungs |
| Bird | Feathers; scales on legs | Internal fertilisation; hard-shelled eggs | Constant | Lungs |
| Mammal | Hair / fur | Internal fertilisation; live young; mammary glands produce milk | Constant | Lungs |
8. Arthropods
Arthropods have an exoskeleton, jointed legs and a segmented body. The four groups are distinguished mainly by the number of legs and body parts.
| Group | Legs | Body parts | Other features | Example |
|---|---|---|---|---|
| Insect | 6 (3 pairs) | 3 — head, thorax, abdomen | Usually wings; one pair of antennae | Butterfly, ant |
| Arachnid | 8 (4 pairs) | 2 | No antennae; no wings | Spider, scorpion |
| Crustacean | More than 8, often 10 | 2 main regions | Two pairs of antennae; often chalky exoskeleton; mostly aquatic | Crab, woodlouse |
| Myriapod | Many — one or two pairs per segment | Many similar segments | One pair of antennae | Centipede, millipede |
A memory aid for the groups: CAM I — Crustaceans, Arachnids, Myriapods, Insects.
Insects have six legs, not eight. Spiders have eight. This gets swapped surprisingly often under exam pressure — the easiest check is to picture the animal.
9. Plant groups
| Group | Features | Examples |
|---|---|---|
| Ferns | Have leaves called fronds; reproduce by spores; no flowers | Bracken |
| Flowering plants | Reproduce by seeds produced in flowers; seeds are in a fruit | Roses, grasses, oak |
Flowering plants divide further:
| Monocotyledons | Dicotyledons | |
|---|---|---|
| Cotyledons in seed | One | Two |
| Leaf veins | Parallel | Branching / net-like |
| Flower parts | In multiples of 3 | In multiples of 4 or 5 |
| Example | Grass, palm, maize | Sunflower, bean |
10. Viruses
Viruses do not fit into the five kingdoms. They are not considered living organisms because they do not carry out the seven characteristics of living organisms on their own.
Their structure is very simple: genetic material (DNA or RNA) surrounded by a protein coat. They have no cytoplasm, no nucleus and no organelles.
A virus can only reproduce inside a host cell, using the host cell’s machinery — which is why all viruses are pathogens.
11. Mistakes that cost marks
Defining a species without “fertile”.
Saying respiration is breathing. Respiration releases energy in cells.
Defining growth as an increase in size only. It is size and dry mass.
Confusing excretion with egestion. Excretion removes waste from metabolism.
Writing the binomial name wrongly — capital on the species, no italics or underline, or both names capitalised.
Calling Mammalia a phylum. It is a class.
Saying bacteria are protoctists. Bacteria are prokaryotes and have no nucleus.
Giving insects eight legs.
Using invisible features in a “visible features” answer.
Starting a dichotomous key partway through instead of at step 1.
Frequently asked questions
What does MRS GREN stand for? Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
Why aren’t viruses classified as living? Because they do not carry out the seven characteristics of living organisms independently — they can only reproduce inside a host cell.
How do I write a scientific name correctly? Genus with a capital letter, species in lower case, the whole name in italics or underlined: Homo sapiens.
What is the difference between a genus and a species? The genus is the larger group; the species is the specific one within it. In a binomial name, the genus comes first.
Why can’t a mule reproduce? It comes from two different species, so it is infertile — which is exactly why horses and donkeys are classified as separate species.
What is the main difference between a prokaryote and a protoctist? Prokaryotes have no nucleus; protoctists have one.
How do I tell an insect from an arachnid? Insects: 6 legs, 3 body parts, usually wings. Arachnids: 8 legs, 2 body parts, no wings or antennae.
Quick revision checklist
- I can name all seven characteristics and define each precisely
- I know respiration is not breathing, and growth includes dry mass
- I can distinguish excretion from egestion
- I can define a species, including “fertile offspring”
- I can explain the horse/donkey/mule example
- I can list the classification hierarchy in order
- I can give the full classification of humans
- I can write a binomial name with correct capitals and italics/underlining
- I can say why the binomial system is useful
- I can use a dichotomous key correctly from step 1
- I can give the features of all five kingdoms
- I know prokaryotes have no nucleus and protoctists do
- I can give features of all five vertebrate groups
- I can distinguish insects, arachnids, crustaceans and myriapods
- I can distinguish ferns from flowering plants, and monocots from dicots
- I can explain why viruses are not classified as living organisms
These notes cover topic 1 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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