Classification of organismsIB MYP Biology: Revision notes
Section 1
The five kingdoms
Scientists sort organisms into five kingdoms using their features:
- Animals: multicellular, no cell walls, no chloroplasts, feed on other organisms and most can move.
- Plants: multicellular, cellulose cell walls, chloroplasts, make food by photosynthesis.
- Fungi: cell walls, no chloroplasts, thread-like hyphae, feed on dead or living material.
- Protists: mostly single-celled with a nucleus, e.g. amoeba and algae.
- Prokaryotes: single-celled with no nucleus, e.g. bacteria.
Section 2
Vertebrate groups
Vertebrates are animals with a backbone. There are five groups:
- Fish: gills, scales and fins; lay eggs in water.
- Amphibians: moist skin; lay eggs in water; adults often live on land.
- Reptiles: dry scaly skin; lay soft-shelled eggs on land.
- Birds: feathers and a beak; lay hard-shelled eggs; warm-blooded.
- Mammals: hair or fur; mammary glands feed young on milk; most give birth to live young; warm-blooded.
Flying does not make an animal a bird. Bats fly, but they have fur and feed their young on milk, so they are mammals.
Section 3
Invertebrates and arthropods
Invertebrates have no backbone. The largest group is the arthropods. All arthropods have a hard exoskeleton, jointed legs and a segmented body. Main groups:
- Insects: 6 legs, 3 body parts (head, thorax, abdomen), antennae, usually wings.
- Arachnids: 8 legs, 2 body parts, no antennae or wings, e.g. spiders.
- Crustaceans: many legs, a hard shell, e.g. crabs.
- Myriapods: many legs and many body segments, e.g. centipedes.
Section 4
Binomial naming
Every species has a two-part scientific name, the binomial name. The first part is the genus (capital letter) and the second is the species (lower case). It is written in italics or underlined, e.g. Homo sapiens.
Scientists in every country use the same name, which avoids confusion because common names differ between languages and places.
Genus has a capital letter and species does not: Homo sapiens, never homo Sapiens.
Section 5
Dichotomous keys
A dichotomous key is a series of paired questions with two possible answers each. Each answer leads to another question or to the name of an organism.
To use a key, start at question 1 and follow the answers.
To build a key:
- Choose a clear feature that splits the organisms into two groups.
- Split each group again with another feature.
- Continue until every organism has its own name at the end.
Use features that can be seen, such as wings or legs, not size or colour that varies.
Must Know
- Five kingdoms: animals, plants, fungi, protists, prokaryotes
- Vertebrate groups: fish, amphibians, reptiles, birds, mammals
- Arthropods: exoskeleton, jointed legs, segmented body
- Binomial name: genus then species
- Dichotomous keys use paired questions
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Classification of organisms
- A biologist in Santiago sorts the organisms she has collected from a pond and its banks into the five kingdoms. Her collection includes a mushroom growing on a rotting log, a bacterium from the pond water, a water lily, a pond snail and an amoeba.Outline two differences between plants and animals.2 marks
- A zoo in Singapore is preparing information boards for four of its animals: a crocodile, a frog, a penguin and a fruit bat. The zoo wants to show visitors which vertebrate group each animal belongs to.Explain why the fruit bat is classified as a mammal and not as a bird, although it can fly.2 marks
- An entomologist in Kuala Lumpur sorts invertebrates collected from a rainforest. Among them are a beetle, which has six legs and a body in three parts, a spider, which has eight legs and a body in two parts, a crab and a centipede. All four belong to the same large group of invertebrates.State three features that all arthropods have.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).