Topic 2- Classification of living organisms

This information is based on the IGCSE Biology Study Guide released by CIE

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  • Created on: 11-10-15 15:36

What is Classification?

Classification makes the identification of living organisms easier, especially as there are already more than a million different species already identified!
It involves sorting organisms into groups according to the features they have in common.
The biggest group is a kingdom. There are five kingdoms, each with their own special and obvious features.

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They are very small and single-celled with cell walls but no nucleus.

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They are single-celled with a nucleus, some also have chloroplasts.

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Many of these are made of hyphae, with nuclei and cell walls, containing chitin, but no chloroplasts.

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They are multicellular organisms with the ability to make thier own food through photosynthesis, due to the presence of chlorophyll. Their cells have walls containing cellulose.

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These are also multicellular organisms, but animals have to obtian their food. Their cells do not have cell walls.

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Each kingdom is divided into smaller groups, which include genus and species. Organisms can exist in only one group at each level of classification. For example, an organism can only belong to one kingdom or genus.

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Classification of Vertebrates

Vertebrates are animals with backbones. Vertebrates are divided into five groups called classes, these classes are:

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Body covering- Scales
Movement- Fins (also used for balance)
Reproduction- Usually produces jelly-covered eggs in water
Sense Organs- Eyes but no ears, lateral line along body for detecting vibrations in water
Other details- Cold-blooded, gills for breathing
Examples- Herring, perch, shark

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Body covering- moist skin
Movement- Four limbs, back feet often webbed to make swimming more efficiant
Reproduction- Produces jelly-covered eggs in water
Sense organs- Eyes and ears
Other details- Cold-blooded, lungs and skin for breathing
Examples- Frog, toad, salamander

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Body covering- Dry, with scales
Movement- Four legs (apart from snakes)
Reproduction- Eggs with rubbery, waterproof shell- laid on sand
Sense organs- Eyes and ears
Other Details- Cold-blooded, lungs for breathing
Examples- Crocodile, python

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Body covering- Feathers, scales on legs
Movement- Wings, two legs
Reproduction- Eggs with hard shell
Sense organs- Eyes and ears
Other details- Warm-blooded, lungs for breathing, beak
Examples- Flamingo, penguin

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Body covering- Fur (apart from whales)
Movement- Four limbs (apart from whales)
Reproduction- Live young
Sense organs- Eyes, ears with pinna (external flap)
Other details- Warm-blooded, lungs for breathing, females have mammary glands to produce milk to feed young, four types of teeth
Examples- Elephant, mouse

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Classification of Flowering Plants

All flowering plants are multicellular organisms. Their cells have cellulose cell walls and sap vacuoles. Some of the cells contain chloroplasts.
They have roots, stems and leaves. Reproduction can be by producing seeds, although asexual reproduction is also possible.
There are two groups:
The term 'cotyledon' means leaf

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Classification of Arthropods

They are invertebrates- they have no backbone.
They have an exoskeleton, whihc is waterproof, making arthropods an extremely successful group as they can exist in very dry places- they are not confined to water or moist places like most invertebrates.
Their bodies are segmented.
They have joined limbs (the exoskeletion would prevent movement).
There are more arthropods than any other group of animals, so they are divided into classes:

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E.g. Dragonfly, locust
Key features:
Three pairs of legs
Usually have two pairs of wings
One pair of antennae
Body divided into head, thorax and abdomen
A pair of compound eyes

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E.g. Spider, tick
Key features:
Four pairs of legs
Body divided into cephalothorax and abdomen
Several pairs of simple eyes
Chelicerae for biting and poisoning prey

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E.g. Crab, woodlouse
Key features:
Five or more pairs of legs
Two pairs of antennae
Body divided into cephalothorax and abdomen
Exoskeleton often calcified to form a carapace (hard)
Compound eyes

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E.g. Centipide, millipide
Key features:
Ten or more pairs of legs (usually one pair per segment)
One pair of antennae
Body not obviously divided into thorax and abdomen
Simple eyes

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Classification of other groups of invertebrates

Although there are many groupls of invertebrates, you only need to know the details of the external features of:

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E.g. Earthworm
Key features:
Elongated, cylindrical body covered with a mucus layer
Segmented body
Mouth and **** present
Bristles (chaetae) usually present
May have a clitellum (reproductive structure)

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E.g. Ascaris
Key features:
Elongated, cylindrical body
Body not segmented
Body pointed at both ends

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E.g. Snail
Key features:
Most have a shell, hardened by calcium carbonate
Have a muscular food, used for movement or burrowing
Many have eyes on tentacles

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Classification of Viruses

E.g. HIV
Key features:
They are very small (100 times smaller than bacteria)
They do not have a typical cell structure
They contain a strand of DNA or RNA
They are surrounded by a protein coat called a capsid
The only life process they show is reproduction (in host cells)

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Classification in Bacteria

E.g. Escherichia coli
Key features:
They are small (1000 times smaller than a plant cell)
They have a cell wall, made of a material other than cellulose or chitin
They have a cell membrane and cytoplasm, containing glycogen granules
They have no nucleus, with DNA in the form of a single, coiled chromosome
Some have a slime capsule
Some have one or more flagellae

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Classification of Fungi

E.g. Mucor
Key features:
Multicellular fungi are composed of hyphae (thread-like structures) which form a network called a mycelium
Their cell walls are made of cellulose or chitin
Cells merge together, with nuclei spread along the cytoplasm
They do nto produce chlorophyll
They secrete enzymes to digest food outside the cells, absorbing the products
Some produce spores in a sporangium or a cap

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