Oxygen tolerance at Higher Level: The same statement as SL. HL questions name a microorganism and ask you to classify it by how it tolerates oxygen.
Practise this as you read
- Identify the group from a description.
- Distinguish between the three groups.
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The big idea: Organisms differ in how they tolerate oxygen gas: some need it, some are harmed by it, and some manage with or without it.
The points to remember
- Obligate aerobe: must have oxygen, for aerobic respiration (most animals and plants).
- Obligate anaerobe: oxygen kills it or stops its growth; it lives only where there is none.
- Facultative anaerobe: uses oxygen when it is there, but can still make ATP without it (E. coli, yeast).
- Where a species can live depends on its tolerance of oxygen: part of its niche.
Remember it as: Obligate = no choice; facultative = can switch.
Real example: methanogens. Methane-producing archaea are obligate anaerobes: they live in waterlogged rice paddies, marsh mud and the stomachs of cattle, where there is no oxygen, and release methane.
Trap: Your muscles can respire anaerobically for a short time, but humans are still obligate aerobes: the body cannot survive without oxygen.
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How it is asked: Expect a named organism to classify, a condition an obligate aerobe needs, or the three groups to tell apart.
IB-style questionDistinguish[3 marks]
Distinguish between obligate aerobes, obligate anaerobes and facultative anaerobes.
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