Key Idea: Life is hugely diverse, so biologists need shared rules to group, name and identify organisms. It starts with the species — the basic unit — which gets a two-part Latin name. Species are then sorted into kingdoms by their cell features and how they feed, recognised by a short list of identifying features, and pinned down in the field using a dichotomous key. Even a single cell counts: a unicellular organism still carries out all functions of life. This topic is a heavy hitter on Paper 1A (classify an organism, read a binomial, follow a key) and appears on Paper 2 (define species, give recognition features) — 23 past-paper questions across its six micros.
🧬 What is a species? (1.6.1)
A species is a group of organisms that can interbreed to produce fertile offspring. Both parts matter — simply mating is not enough, the young must themselves be able to breed. The classic exception is the mule (horse × donkey): the parents are different species, so the mule is sterile. The concept is also hard to apply to fossils and to organisms that reproduce asexually, so karyotyping (comparing chromosome number and shape) is used as extra evidence of whether two populations are the same species.
Same species needs both: they interbreed AND the offspring are fertile. Fertile young = same species; sterile young (a hybrid like the mule) = different species.
🏷️ Naming species: binomial nomenclature (1.6.2)
Common names are messy (a 'robin' means different birds in different countries), so every species gets a two-part Latin name — the binomial system introduced by Carl Linnaeus. The first word is the genus (capital letter), the second is the species (lower case), and the whole name is italicised or underlined. Two species that share the same genus (the same first word) are more closely related than species placed in different genera. Linnaeus also used a morphological species concept — grouping organisms by shared physical features.
| Part of the name | What it tells you | How to write it |
|---|---|---|
| First word | the genus (the wider group it belongs to) | Capital letter, italic/underlined — e.g. Felis |
| Second word | the species (the specific organism) | lower case, italic/underlined — e.g. catus |
| Same first word | the two species share a genus → closely related | compare the genus to judge relatedness |
🍄 Classifying eukaryotes: kingdoms & nutrition (1.6.3)
Eukaryotes (cells with a nucleus) are sorted into four kingdoms using cell features — such as whether there is a cell wall and what it is made of, and whether there are chloroplasts. Plant walls are cellulose; fungal walls are chitin; animals have no wall; protists are the diverse 'everything else' group.
| Kingdom | Key cell features | Mode of nutrition |
|---|---|---|
| Plants | cellulose wall + chloroplasts | autotroph (photosynthesis) |
| Animals | no cell wall, no chloroplasts | heterotroph (holozoic — eats and digests inside) |
| Fungi | chitin wall, no chloroplasts | heterotroph (saprotrophic — digests dead matter outside) |
| Protists | varied — the 'everything else' eukaryotes | autotroph, heterotroph or mixotroph |
Both are heterotrophic — the difference is where digestion happens. Holozoic (animals): food taken inside and digested internally. Saprotrophic (fungi): enzymes released outside onto dead matter, then the products are absorbed. A mixotroph can feed either way depending on conditions.
🐦 Recognising the major groups (1.6.4)
Each major group has a short list of identifying (recognition) features — characteristics its members share that other groups lack. Learn two or three give-away features per group and match the organism against them, rather than memorising everything.
| Group | Give-away identifying features |
|---|---|
| Mammals | fur/hair; feed young on milk (mammary glands) |
| Birds | feathers; beak with no teeth; lay hard-shelled eggs |
| Fish | scales; gills; fins |
| Flowering plants | true roots, stems and leaves; vascular tissue; flowers and seeds |
| Mosses (bryophytes) | no true roots or vascular tissue; reproduce by spores; stay small and damp |
| Cnidarians / Molluscs | stinging cells + single gut opening / soft body often with a shell |
🔀 Dichotomous keys for identification (1.6.5)
A dichotomous key identifies an unknown organism through a chain of either/or choices, each based on an observable feature you can see or measure. At every step you follow only the choice that matches your organism, then go to exactly where it sends you — repeating until you reach a named organism rather than another step. 'Dichotomous' means splitting into two — every step offers exactly two opposite options, so there is one clear route to the answer.
Always start at Step 1, follow only the matching branch, and trust the key — never skip a step. Use observable features (legs, shell, scales), not invisible ones.
🦠 Functions of life in unicellular organisms (1.6.6)
A unicellular organism is made of just one cell, yet that single cell must carry out all the functions of life by itself. A handy memory hook is MRS H GREN: Movement, Reproduction, Sensitivity, Homeostasis, Growth, Respiration, Excretion, Nutrition. In the exam you match an observed clue in a single cell (e.g. in a Paramecium) to its function.
| Function of life | What you might observe in one cell |
|---|---|
| Nutrition | sweeping food into a food vacuole / making food by photosynthesis |
| Respiration | releasing energy from food inside the cell (mitochondria) |
| Homeostasis | a contractile vacuole pumping out excess water to keep balance |
| Excretion | removing the waste products of metabolism (e.g. CO₂) |
| Movement / Sensitivity | beating cilia or flagella; moving toward food or light |
A contractile vacuole shows homeostasis (controlling internal water), not excretion. Excretion is removing the waste of metabolism — keep the two separate.
✍️ Worked examples
IB-style question — classify by cell features and feeding
An unknown organism has cells with a wall made of chitin and no chloroplasts, and it feeds by releasing enzymes onto a fallen log and absorbing the digested products. State its kingdom and its mode of nutrition. [2]
Model answer:
Use the cell features. A chitin wall with no chloroplasts rules out plants (cellulose + chloroplasts) and animals (no wall) — this is the fungi kingdom.
Use the feeding method. Releasing enzymes onto dead matter and absorbing the products = digestion outside the body = saprotrophic nutrition.
Kingdom: fungi. Mode of nutrition: saprotrophic (a heterotrophic mode using external digestion).
IB-style question — read a binomial
A field guide lists four beetles: Carabus auratus, Carabus violaceus, Harpalus affinis and Pterostichus niger. State which two are most closely related and explain why. [2]
Model answer:
Read the first word (genus) of each name — the genera are Carabus, Carabus, Harpalus and Pterostichus.
Find the pair that shares a genus. Only Carabus auratus and Carabus violaceus share the first word (Carabus).
Explain. Sharing the same genus means they are more closely related than species placed in different genera.
Carabus auratus and Carabus violaceus — they share the same genus (Carabus), so they are more closely related than the others.
IB-style question — functions of life in one cell
A student watches a single Paramecium and sees it (i) sweep food particles into a food vacuole and (ii) steadily empty a contractile vacuole. State the function of life shown by each observation. [2]
Model answer:
Clue (i): sweeping food into a food vacuole is taking in food → nutrition.
Clue (ii): emptying a contractile vacuole pumps out excess water to keep internal balance → homeostasis (not excretion).
(i) Nutrition; (ii) Homeostasis (the contractile vacuole controls internal water balance).
✅ Quick self-check
Tap each card to check yourself.
Define a species. A group of organisms that can interbreed to produce fertile offspring. (A mule is sterile, so horse and donkey are different species.)
Which word in a binomial is the genus, and how is it written? The first word is the genus, written with a capital letter and italicised/underlined. Sharing a genus means two species are closely related.
How do you tell a fungus from a plant by its cell? Both have a wall, but a fungal wall is chitin with no chloroplasts (saprotrophic), while a plant wall is cellulose with chloroplasts (autotroph).
Holozoic vs saprotrophic nutrition? Both are heterotrophic. Holozoic (animals) digests food inside the body; saprotrophic (fungi) releases enzymes onto dead matter and digests it outside, then absorbs it.
How do you use a dichotomous key? Start at Step 1, pick the one matching observable feature, follow it to the step it sends you to, and repeat until you reach a named organism.
Which function does a contractile vacuole show? Homeostasis — it pumps out excess water to keep internal water balance stable. It is not excretion (the removal of metabolic waste).
Exam Tips
- Define species with BOTH parts: interbreed AND produce fertile offspring — drop 'fertile' and you lose the mark.
- Binomial: genus first (capital, italic), species second (lower case). Same genus = most closely related.
- Classify by cell features first: cellulose+chloroplasts = plant, chitin+no chloroplasts = fungus, no wall = animal.
- Holozoic = digests inside (animals); saprotrophic = digests outside then absorbs (fungi). State WHERE digestion happens.
- For recognition features, give identifying ones (fur+milk, feathers+beak) — not features many groups share.
- In a dichotomous key, follow ONE branch per step from Step 1 and never skip; use only observable features.
- A contractile vacuole is homeostasis, NOT excretion. Match each observed clue to one function of life.