18.2.2

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.

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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.
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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.
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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.
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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.
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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).

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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.
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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.

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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.
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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).
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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.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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