Home › Learning Hub › IGCSE Biology › 1 Characteristics and classification
Topic 1 · 1.1–1.3

Characteristics and classification of living organisms

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the seven characteristics of living organisms.
  • Define a species, describe the binomial system, and construct and use dichotomous keys.
  • State the features used to classify animals and plants, the vertebrate groups and the arthropod groups.
  • State the features of the five kingdoms, of ferns and flowering plants, and of viruses EXTENDED.
  • Explain how classification reflects evolutionary relationships, and how DNA base sequences are used EXTENDED.

📚The biology

The characteristics of living organisms

Movement: an action by an organism or part of an organism causing a change of position or place
Respiration: chemical reactions in cells that break down nutrient molecules and release energy for metabolism
Sensitivity: the ability to detect and respond to changes in the internal or external environment
Growth: a permanent increase in size and dry mass
Reproduction: the processes that make more of the same kind of organism
Excretion: removal of the waste products of metabolism and substances in excess of requirements
Nutrition: taking in materials for energy, growth and development

A car moves, “breathes” air and gets rid of waste gases, but it does not grow or reproduce, so it is not alive. Learn the definitions word for word: examiners look for “permanent” and “dry mass” in growth, and “waste products of metabolism” in excretion.

Species, naming and keys

Organisms are classified into groups by the features they share. A species is a group of organisms that can reproduce to produce fertile offspring. A horse and a donkey can breed, but the mule they produce is infertile, so they are different species.

The binomial system is an internationally agreed system in which each species has a two-part scientific name: the genus then the species. The Bali starling is Leucopsar rothschildi: genus Leucopsar, species rothschildi. The genus has a capital letter; the whole name is written in italics (or underlined by hand).

A dichotomous key identifies organisms by a series of questions, each with exactly two answers, based on features you can see.

Five leaves labelled A to E. A: a simple oval leaf with a smooth edge. B: a simple leaf with a toothed edge. C: a simple leaf with a wavy, lobed edge. D: a compound leaf with five leaflets on one stalk. E: long thin needle leaves.
Five leaves to identify. The worked example below builds a key for them.

Features of animals and plants

Animals are multicellular, have no cell walls and no chlorophyll, and feed on other organisms. Plants are multicellular, have cell walls made of cellulose and chloroplasts, and make their food by photosynthesis. The main vertebrate groups:

Mammals: fur or hair, external ears (pinnae), mammary glands to feed milk to young, most give birth to live young
Birds: feathers, beak, wings, lay eggs with hard shells
Reptiles: dry scaly skin, lay eggs with rubbery shells on land
Amphibians: moist skin without scales, lay jelly-covered eggs in water, larvae (tadpoles) live in water
Fish: scales, fins and gills, live in water, lay eggs in water

Arthropods are invertebrates with jointed legs, a segmented body and an exoskeleton. There are four groups:

Four arthropods. An insect with three body parts, six legs, one pair of antennae and two pairs of wings. An arachnid (spider) with two body parts, eight legs and no antennae. A crustacean (crab) with a hard shell, claws, more than four pairs of legs and two pairs of antennae. A myriapod (centipede) with many segments, legs on every segment and one pair of antennae.
Count legs, body parts and antennae. Insects: 6 legs; arachnids: 8 legs and no antennae; crustaceans: more than 8 legs and 2 pairs of antennae; myriapods: many segments.

The five kingdoms EXTENDED

Five panels. Animals: a cell with a nucleus and no wall; many cells, no chlorophyll, feed on other organisms. Plants: a cell with a thick wall, chloroplasts and a vacuole; cellulose cell walls, photosynthesis. Fungi: branching thread-like hyphae with nuclei; cell walls not made of cellulose, feed by digesting dead matter. Prokaryotes: a bacterium with circular DNA and no nucleus. Protoctists: a single irregular cell with a nucleus.
The deciding features: a nucleus or not (prokaryotes have none), cell walls and what they are made of, chlorophyll, and single-celled or many-celled. EXTENDED

EXTENDED Fungi have a nucleus and cell walls (not of cellulose) but no chlorophyll; they feed by secreting enzymes on to dead or living material and absorbing the products. Yeast is a single-celled fungus. Prokaryotes (bacteria) are single cells with a cell wall, circular DNA and plasmids but no nucleus. Protoctists are mostly single-celled organisms with a nucleus; some (like Amoeba) are animal-like, others (like algae) contain chloroplasts.

EXTENDED Within the plant kingdom:

Ferns: leaves called fronds; no flowers; reproduce by spores
Flowering plants: produce flowers and seeds
Monocotyledons: one seed leaf (cotyledon), parallel leaf veins, flower parts in threes (grasses, maize)
Dicotyledons: two cotyledons, a branching network of veins, flower parts usually in fours or fives (beans, hibiscus)

EXTENDED Viruses are not placed in any kingdom: they are not cells. A virus is a protein coat surrounding genetic material.

A virus drawn as a hexagonal protein coat with short spikes, containing a coiled strand of genetic material.
The two features you need: a protein coat and genetic material. EXTENDED

Classification and evolution EXTENDED

Classification systems aim to reflect evolutionary relationships. The sequence of bases in DNA is now used to classify organisms: groups that share a more recent common ancestor have DNA base sequences that are more similar than groups that share only a distant ancestor.

The same 20-base stretch of DNA from four species, P, Q, R and S, with bases that differ from species P highlighted. Q differs by 1 base, R by 3 and S by 7. Below, a tree joins P and Q first, then R, then S.
Q differs from P at 1 position, R at 3 and S at 7, so Q is P’s closest relative and S its most distant. (Illustrative sequences.) EXTENDED

✏️Worked example

(a) Construct a dichotomous key to identify leaves A–E in the figure above. [4] (b) EXTENDED Use the DNA figure to state which species is most closely related to species P, and explain your answer. [2]

(a) One correct key (each step splits the group into two using a visible feature):

  1. Leaf divided into separate leaflets … D. Leaf not divided … go to 2.
  2. Leaf long, thin and needle-like … E. Leaf broad and flat … go to 3.
  3. Edge of leaf lobed (deep, rounded bulges) … C. Edge not lobed … go to 4.
  4. Edge of leaf toothed … B. Edge smooth … A.

(b) Species Q: its sequence differs from P’s at only 1 base (R at 3, S at 7), so P and Q share the most recent common ancestor.

Check it. Five organisms need four steps, and each organism should appear exactly once at the end of a branch. Run each leaf through your key to be sure it lands in the right place.
Features that are not visible or not fixed. “Is it large?” or “Is it poisonous?” cannot be judged reliably from the specimen. Use features anyone can see: shape, edge, number of parts.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which is the definition of growth? A: an increase in the number of cells. B: a permanent increase in size and dry mass. C: an increase in mass due to water uptake. D: the development of adult features.
B. Water uptake (C) is not permanent, and dry mass rules it out.
2. (Theory.) A horse and a donkey can mate and produce a mule, but mules cannot reproduce. Explain whether horses and donkeys belong to the same species. [2]
No. Members of a species reproduce to produce fertile offspring; the mule is infertile.
3. (Theory.) The Komodo dragon is Varanus komodoensis. State the genus, and give one advantage of using the binomial system rather than common names. [2]
Varanus. It is internationally agreed, so scientists everywhere use the same name for the same species (common names vary between languages and places).
4. (Theory.) An animal has moist skin without scales and lays jelly-covered eggs in water; its young breathe using gills. Name its vertebrate group. [1]
Amphibians.
5. (Theory.) An arthropod has two body parts, eight jointed legs and no antennae. Name its group, and state one feature shared by all arthropods. [2]
Arachnids. Any: jointed legs; segmented body; exoskeleton.
6. (Theory.) EXTENDED Name the kingdom of (a) a single-celled organism with circular DNA and no nucleus, (b) a mushroom. [2]
(a) Prokaryotes. (b) Fungi.
7. (Theory.) EXTENDED A plant has leaves with parallel veins and flowers with parts in threes. Is it a monocotyledon or a dicotyledon? Give one other feature you would expect. [2]
Monocotyledon. Its seeds have one cotyledon.
8. (Theory.) EXTENDED State the two structural features of a virus and explain why viruses are not placed in any of the five kingdoms. [3]
A protein coat and genetic material (DNA or RNA). They are not made of cells, and cannot carry out the characteristics of life (e.g. reproduction) outside a host cell.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Natural History Museum — identification keys for leaves, insects and other groups
  • OneZoom — an explorable tree of life showing how groups are related