LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- describe the seven characteristics shown by living organisms
- explain how shared features allow organisms to be classified
- define a species and use the binomial naming system correctly
- construct and follow a dichotomous key using visible features
- explain how classification can show evolutionary relationships
- relate similarity in DNA base sequences to recent common ancestry
- recognise the main features of kingdoms, animal groups and plant groups
- state the structural features of a virus.
THE BIG IDEA
Life is varied, but it is not disorganised
Imagine finding an unfamiliar object on a forest floor. It grows slowly, releases spores and has thread-like structures beneath it. Is it a plant, an animal or something else? Biologists answer questions like this by looking for evidence. They check the processes the object carries out, compare its structures with known organisms and, when needed, examine its DNA.
Classification is therefore more than giving organisms labels. It is a system for arranging biological information so that organisms can be identified, compared and studied. Modern classifications also aim to show how groups are related through evolution.
SECTION 01
What makes something living?
No single action is enough to prove that something is alive. A flame can spread and use oxygen, yet it is not an organism. Living organisms are recognised by a connected set of life processes.
Movement
An action by a whole organism, or part of it, that changes position or place.
Respiration
Chemical reactions inside 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 produce more organisms of the same kind.
Excretion
Removal from an organism of metabolic waste products and substances present in excess of requirements.
Nutrition
Taking in materials needed for energy, growth and development.
SECTION 02
Why classify organisms?
There are millions of known species. Classification makes this diversity manageable by placing organisms into groups according to features they share.
Observe
Record features that can be compared, such as body covering, number of legs or the presence of flowers.
Group
Place organisms with important shared features together and separate those with clear differences.
Test relationships
Use additional evidence, including DNA base sequences, to check how closely groups are related.
Classification helps biologists communicate precisely. It also allows a newly discovered organism to be compared with known groups, helps researchers predict features it might have, and provides clues about evolutionary history.
Classification
The organisation of living organisms into groups using features that they share.
SECTION 03
Species and scientific names
Species
A group of organisms that can reproduce with one another and produce fertile offspring.
Members of one species are not identical. They can differ in colour, height or many other characteristics. What defines the species is their ability to interbreed successfully and produce offspring that can also reproduce.
The binomial system
Common names vary between languages and regions, so scientists use an internationally agreed two-part name for each species. The first word gives the genus; the second identifies the species within that genus.
WRITING RULES
- Write the genus first and begin it with a capital letter.
- Write the species word second and use a lower-case letter.
- Use italics when typing: Mangifera indica.
- Underline each word separately when writing by hand.
SECTION 04
Dichotomous keys
A dichotomous key identifies an organism through a sequence of choices. Each step offers two contrasting statements, so only one route should match the specimen.
Dichotomous key
An identification tool made from paired, contrasting statements about observable features.
How to use a key
- 1
Begin at the first pair of statements.
- 2
Study the specimen and choose the statement that matches it.
- 3
Follow the instruction to another numbered pair or to an identification.
- 4
Continue until the organism has been named.
Identifying four garden animals
Suppose the possible organisms are a bee, spider, snail and earthworm. A useful key could be:
| Step | Observation | Go to / identify |
|---|---|---|
| 1a | Jointed legs are present | Go to 2 |
| 1b | Jointed legs are absent | Go to 3 |
| 2a | Three pairs of legs | Bee |
| 2b | Four pairs of legs | Spider |
| 3a | A shell is present | Snail |
| 3b | No shell is present | Earthworm |
Following the key: an animal with no jointed legs and a shell follows 1b, then 3a, so it is identified as a snail.
SECTION 05
Classification and evolutionary relationships
Early classification relied mainly on visible structures. These observations remain useful, but similar appearances do not always mean close relationship. Unrelated organisms can evolve similar features when they face similar environmental challenges.
Modern systems aim to reflect evolutionary relationships. Scientists compare DNA base sequences because DNA is inherited. Two groups with very similar base sequences are likely to share a more recent common ancestor than two groups with many sequence differences.
A T G C C A T G AA T G C C A T G GG C T A A G C T CIn this simplified comparison, A and B differ at only one position, while C differs at many positions. This is evidence that A and B are more closely related. Real analyses compare far longer sequences and use several kinds of evidence.
SECTION 06
The five kingdoms
At this level, organisms are placed into five broad kingdoms. Their cell structure and method of nutrition are especially useful for separating the groups.
| Kingdom | Main identifying features | Example |
|---|---|---|
| Animals | Multicellular; cells have no cellulose walls or chloroplasts; obtain nutrients by eating other organisms; most can move from place to place. | Butterfly |
| Plants | Multicellular; cells have cellulose walls; photosynthetic cells contain chloroplasts; carbohydrates may be stored as starch. | Maize plant |
| Fungi | Usually made of a network of hyphae called a mycelium; chitin cell walls; no chlorophyll; extracellular digestion followed by absorption. Yeast is single-celled. | Bread mould |
| Prokaryotes | Single-celled; no nucleus; circular DNA lies free in the cytoplasm; may contain plasmids; have cell walls. | Bacterium |
| Protoctists | Mostly single-celled organisms with a nucleus; some are animal-like and some contain chloroplasts and are plant-like. | Amoeba |
SECTION 07
Groups within the animal kingdom
Vertebrates
Vertebrates have a backbone. The five groups can be identified using features such as body covering, gas-exchange organs, reproduction and temperature regulation.
| Group | Useful identifying features |
|---|---|
| Fish | Scales, fins and gills; eggs usually laid in water; body temperature varies with the surroundings. |
| Amphibians | Moist skin without scales; larvae use gills and adults usually use lungs; jelly-like eggs laid in water. |
| Reptiles | Dry, scaly skin; lungs; leathery-shelled eggs usually laid on land; body temperature varies with the surroundings. |
| Birds | Feathers and a beak; forelimbs form wings; hard-shelled eggs; maintain a nearly constant internal body temperature. |
| Mammals | Hair or fur; females have mammary glands that produce milk; lungs and a diaphragm; maintain a nearly constant internal body temperature. |
Arthropods
All arthropods have a segmented body, jointed legs and a hard external skeleton. Count body regions, legs and antennae to distinguish the main groups.
Example: beetle
Example: scorpion
Example: crab
Example: centipede
An unknown land animal
It has a hard external skeleton, two main body regions, four pairs of jointed legs and no antennae.
Conclusion: the external skeleton and jointed legs place it among the arthropods. Four pairs of legs and no antennae identify it as an arachnid.
SECTION 08
Groups within the plant kingdom
Reproduce using spores
Ferns have roots, stems and leaves called fronds. They contain vascular tissue but do not produce flowers or seeds. Spores are commonly formed in structures on the underside of fronds.
Reproduce using seeds
Flowering plants have roots, stems and leaves with vascular tissue. Flowers are reproductive structures, and seeds develop after fertilisation.
Monocotyledons and dicotyledons
The name refers to the number of seed leaves, or cotyledons, in the seed. Several visible features should be considered together when classifying an unknown flowering plant.
| Feature | Monocotyledon | Dicotyledon |
|---|---|---|
| Cotyledons | One | Two |
| Leaf veins | Usually parallel | Usually form a branching network |
| Flower parts | Usually in multiples of three | Usually in multiples of four or five |
| Root system | Usually many similar fibrous roots | Often one main taproot with branches |
| Vascular bundles in stem | Scattered | Arranged in a ring |
SECTION 09
Viruses
Viruses are not made of cells. At this syllabus level, remember two structural features:
- genetic material
- a surrounding protein coat.
They can reproduce only inside living host cells, using the host cell’s machinery. Outside a host cell they do not carry out the full set of life processes.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Living organisms show movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.
- Classification groups organisms using shared features and aims to reflect evolutionary relationships.
- A species can interbreed to produce fertile offspring.
- A binomial name contains the genus followed by the species.
- Dichotomous keys use pairs of contrasting, observable statements.
- Greater similarity in DNA base sequences suggests a more recent common ancestor.
- Cell structure and nutrition distinguish the five kingdoms.
- Observable features distinguish vertebrates, arthropods, ferns and flowering plants.
- A virus contains genetic material inside a protein coat and is not made of cells.