2.1 Concept and use of a classification system
Classification is sorting organisms into groups, based on the features that they share. It makes it easier to study the huge number of different living things, and it shows how closely related they are.
A horse and a donkey can breed, but the offspring (a mule) is sterile, so they are different species.
The binomial system
Humans are Homo sapiens: the genus is Homo and the species is sapiens. Using one agreed name avoids confusion, because common names differ between languages and places.
Dichotomous keys
A dichotomous key is a series of paired statements, each with two choices, used to identify an organism from features that you can see. Each choice leads either to another pair of statements or to the name of the organism.
Example of how a key is used:
- (a) Has wings: go to 2. (b) No wings: go to 3.
- (a) One pair of wings: fly. (b) Two pairs of wings: bee.
- (a) Six legs: ant. (b) Eight legs: spider.
To construct a key: choose features that are easy to see and that divide the group into two, and write the statements so that each pair is clear. Use features that do not change (such as the number of legs), and not features such as size or colour that can vary.
2.2 Features of organisms
The five kingdoms
| Kingdom | Main features |
|---|---|
| animal | multicellular; no cell walls or chloroplasts; feed on other organisms (heterotrophic); move around |
| plant | multicellular; cellulose cell walls; chloroplasts; make their own food by photosynthesis |
| fungus | mostly multicellular; cell walls (not cellulose); no chloroplasts; feed by absorbing nutrients from dead or living organisms; body made of threads called hyphae |
| prokaryote | single-celled; no nucleus (DNA lies free); includes the bacteria |
| protoctist | mostly single-celled with a nucleus; some have chloroplasts (algae) and some do not (such as Amoeba) |
Animals: vertebrates
Vertebrates have a backbone.
| Group | Features |
|---|---|
| mammals | hair or fur; give birth to live young (most); feed young on milk from mammary glands; constant body temperature |
| birds | feathers; wings; lay eggs with hard shells; beak and no teeth; constant body temperature |
| reptiles | dry scaly skin; lay eggs with leathery shells (on land); body temperature varies with surroundings |
| amphibians | moist, smooth skin; lay jelly-covered eggs in water; larval stage (tadpole) lives in water, adult on land |
| fish | scales; gills for gas exchange; fins; lay eggs in water; body temperature varies with surroundings |
Animals: arthropods
Arthropods have jointed legs, a hard exoskeleton and a segmented body.
| Group | Features |
|---|---|
| insects | 3 body parts (head, thorax, abdomen); 6 legs; usually 2 pairs of wings; one pair of antennae |
| arachnids | 2 body parts; 8 legs; no antennae and no wings |
| crustaceans | more than 8 legs (usually 10 or more); two pairs of antennae; usually live in water |
| myriapods | many segments, each with one or two pairs of legs; one pair of antennae (centipedes and millipedes) |
Plants
| Group | Features |
|---|---|
| ferns | have leaves called fronds; no flowers; reproduce using spores |
| flowering plants | have flowers; reproduce using seeds inside fruits |
Flowering plants are divided into two groups.
| Dicotyledons | Monocotyledons | |
|---|---|---|
| seed leaves (cotyledons) | two | one |
| leaf veins | branching network | parallel |
| flower parts | in fours or fives | in threes |
| examples | beans, roses, oak | grasses, maize, lilies |
It has jointed legs, so it is an arthropod, and eight legs and two body parts make it an arachnid (for example a spider).
Viruses
Viruses are not placed in the five kingdoms, because they are not made of cells and do not carry out the characteristics of living things on their own.
- A virus has a protein coat around its genetic material (DNA or RNA).
- Viruses can only replicate inside living cells of a host. They take over the cell's machinery to make more viruses.
- Antibiotics do not work against viruses.
Practise this chapter
Every past-paper question on these topics, with the mark scheme.