Linnaean Classification System

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Traditional Classification System

Classification in biology involves the identifying, naming and sorting of organisms in a process called taxonomy. Organisms are classified into groups called taxa.

Taxa

Closely related organisms are grouped together into groups called taxa (singular = taxon).
The phylogeny of organisms (their evolutionary relationships) is important for determining which taxa an organism is a part of.

Hierarchy

Each taxon is arranged into a hierarchy.
The hierarchy divides into smaller, more specific taxa at each level.
There is no overlap between taxa at the same level.
- Organisms can only belong to one taxon at each level.

Stages in the Linnaean hierarchy

The traditional classification system, devised by Carl Linnaeus, categorises organisms based on their structure and characteristics. Linnaeus created 7 different groups:
- Kingdom.
- Phylum.
- Class.
- Order.
- Family.
- Genus.
- Species.
The five kingdoms are : Animalia, Plantae, Protoctista, Fungi and Prokaryotae.

E.g. Humans

Humans can be classified in the hierarchy system as:
- Kingdom = Animalia.
- Phylum = Chordata.
- Class = Mammalia.
- Order = Primates.
- Family = Hominidae.
- Genus = Homo.
- Species = sapiens.

Binomial names

The naming system used in classification is called the binomial system.
The binomial system consists of the genus and the species of an organism.
The names are always written in italics if they are typed or they are underlined if hand-written.
- E.g. The binomial name for humans is Homo sapiens.

Animals and Plants

Animals and plants are eukaryotes; their cells contain mitochondria (for respiration) and a nucleus (containing DNA).

Animals

Animal cells have no cell wall and no chloroplasts (so do not carry out photosynthesis).
Animals are multicellular (made of many cells) and they don't make their own food, but they store carbohydrates as glycogen.
They often have nervous coordination (using nerves) and are able to move around.
Mammals (like whales) and insects (like mosquitoes) are examples.

Plants

Plant cells are surrounded by cell walls (made of a substance called cellulose) and contain chloroplasts (carry out photosynthesis).
Plants are multicellular (many cells) and they make their own carbohydrate food, which they store as sucrose or starch.
Plants can be divided into flowering and non-flowering plants.
- Flowering plants include cereals (e.g. maize) and herbaceous legumes (e.g. peas).
- Non-flowering plants include pines and ferns.

Fungi, Protoctists and Bacteria

Fungi and protoctists are eukaryotic organisms, but bacteria are prokaryotes.

Fungi

Fungi cells have cell walls (that contain a substance called chitin) and no chloroplasts.
Fungi can be multicellular or unicellular and they are saprotrophic (they feed by secreting digestive enzymes and absorbing broken down decaying matter), and often store carbohydrates as glycogen.
Their bodies are made up of mycelium, which contains many thread-like structures called hyphae.
Mushrooms and single-celled yeast are examples.

Bacteria

Bacteria are unicellular and they have cell walls and circular loops of DNA (which is not inside a nucleus).
Some bacteria photosynthesise, and some feed off decaying matter.
Bacteria can be rod-shaped (like Lactobacillus which turns milk into yoghurt)or spherical (like Pneumococcus which causes pneumonia).

Protoctists

Protoctists are unicellular, and their cells have nuclei (membrane-bound structures that contain the cell's genetic materials).
- Some, like Amoeba, are similar to animal cells.
- Some, like Chlorella, are like plant cells and have chloroplasts.