AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Classification of living organisms: Archaea (primitive bacteria usually living in extreme environments)

    Archaea are single-celled prokaryotes placed in their own domain, separate from true bacteria.

    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Archaea are single-celled prokaryotes placed in their own domain, separate from true bacteria. Their cells have no nucleus and no membrane-bound organelles, and their ribosomes and cell-wall chemistry differ from those of Bacteria. Many archaea live in extreme environments: hot springs, salt lakes, deep-sea vents and acidic bogs. Others live in ordinary soils, oceans and even the human gut, so 'usually' matters. A useful method is to compare a hot-spring archaeon with a bacterium from the same habitat: both are prokaryotic, but the archaeon's membrane lipids and cell-wall material are distinct, and its enzymes stay active at high temperature. This is why classification uses molecular evidence, such as base sequences in ribosomal RNA, alongside visible features.

    What earns the marks

    • Archaea are prokaryotic: cells lack a nucleus and membrane-bound organelles.
    • Archaea form a domain separate from Bacteria and Eukaryota.
    • Their cell membranes, cell walls and ribosomes differ in chemistry from those of true bacteria.
    • Many archaea are adapted to extreme environments such as hot springs, salt lakes or deep-sea vents.
    • Some archaea live in moderate environments, so 'extreme' describes the typical case, not every species.
    • Classification uses molecular evidence, for example ribosomal RNA base sequences, as well as observable features.

    Common mistakes and how to put them right

    • Calling archaea 'ancient bacteria' and treating them as a type of bacterium.

      Put it right by stating they are a separate domain, not a subgroup of Bacteria.

    • Assuming every archaeon lives in an extreme environment.

      Put it right by noting that archaea also occur in soils, oceans and guts.

    • Saying archaea have a nucleus because they are 'more advanced'.

      Put it right by confirming they are prokaryotic and have no nucleus.

    Exam tips

    • Use the word 'domain' when naming the rank of Archaea, and contrast it with Bacteria in the same sentence.
    • Give one named extreme habitat and one ordinary habitat to show the 'usually' in the statement is understood.
    • If asked to justify a classification, refer to molecular evidence such as RNA base sequences rather than appearance alone.

    Try the lesson

    The start of the Classification of living organisms: Archaea (primitive bacteria usually living in extreme environments) lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which group of organisms is described as primitive bacteria that usually live in extreme environments?

    Listen

    Archaea are single-celled prokaryotes placed in their own domain, separate from true bacteria. Their cells have no nucleus and no membrane-bound organelles, and their ribosomes and cell-wall chemistry differ from those of Bacteria. Many archaea live in extreme environments: hot springs, salt lakes, deep-sea vents and acidic bogs. Others live in ordinary soils, oceans and even the human gut, so 'usually' matters. A useful method is to compare a hot-spring archaeon with a bacterium from the same habitat: both are prokaryotic, but the archaeon's membrane lipids and cell-wall material are distinct, and its enzymes stay active at high temperature. This is why classification uses molecular evidence, such as base sequences in ribosomal RNA, alongside visible features. A strong answer earns marks for these points. Archaea are prokaryotic: cells lack a nucleus and membrane-bound organelles. Archaea form a domain separate from Bacteria and Eukaryota. Their cell membranes, cell walls and ribosomes differ in chemistry from those of true bacteria. Many archaea are adapted to extreme environments such as hot springs, salt lakes or deep-sea vents. A common slip: Calling archaea 'ancient bacteria' and treating them as a type of bacterium. Instead, stating they are a separate domain, not a subgroup of Bacteria. Exam tip: Use the word 'domain' when naming the rank of Archaea, and contrast it with Bacteria in the same sentence.

    See it

    A flowchart with Archaea at the top branching into six points. First, Archaea are prokaryotic, with no nucleus and no membrane-bound organelles. Second, they form their own domain, separate from Bacteria and Eukaryota. Third, their cell membranes, cell walls and ribosomes differ in chemistry from those of true bacteria. Fourth, many are adapted to extreme environments such as hot springs, salt lakes, deep-sea vents and acidic bogs. Fifth, some live in moderate environments including soils, oceans and the human gut. Sixth, classification uses molecular evidence such as ribosomal RNA base sequences as well as observable features.
    It shows the key features of Archaea: their prokaryotic cells, their separate domain, their differing cell chemistry and ribosomes, their extreme and moderate habitats, and the molecular evidence used to classify them.

    Key words

    Say what each word means before you turn the card.

    Archaea

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    Prokaryotic cell structure

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    Extreme-environment adaptations

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    Use of molecular evidence in classification

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    See every definition
    Archaea
    Single-celled prokaryotes placed in their own domain, separate from true bacteria, usually living in extreme environments.
    Prokaryotic cell structure
    Cells that have no nucleus and no membrane-bound organelles.
    Extreme-environment adaptations
    Features such as membrane lipids and cell-wall material that are distinct, and enzymes that stay active at high temperature.
    Use of molecular evidence in classification
    Classification uses base sequences in ribosomal RNA, alongside visible features, to justify grouping.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart with Archaea at the top branching into six points. First, Archaea are prokaryotic, with no nucleus and no membrane-bound organelles. Second, they form their own domain, separate from Bacteria and Eukaryota. Third, their cell membranes, cell walls and ribosomes differ in chemistry from those of true bacteria. Fourth, many are adapted to extreme environments such as hot springs, salt lakes, deep-sea vents and acidic bogs. Fifth, some live in moderate environments including soils, oceans and the human gut. Sixth, classification uses molecular evidence such as ribosomal RNA base sequences as well as observable features.

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    • 3Box 3

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