Soil microorganisms and soil health — A/L Biology

How do I practise Soil microorganisms and soil health?

1 question with a worked solution

1. Which bacterial genus forms symbiotic root nodules in leguminous crops and reduces atmospheric dinitrogen into ammonia using an oxygen-sensitive nitrogenase enzyme complex protected by leghemoglobin?
   1. A) Nitrosomonas
   2. B) Rhizobium
   3. C) Pseudomonas denitrificans
   4. D) Azotobacter
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   Correct answer: B) Rhizobium

   Rhizobium enters legume root hairs and induces the formation of root nodules containing bacteroids. Within the nodules, the nitrogenase enzyme complex fixes N₂ into NH₃, while plant-synthesized leghemoglobin buffers oxygen concentration to prevent irreversible inactivation of nitrogenase.

   Show worked solution

   Step 1: Differentiate nitrogen cycle bacteria: - Nitrosomonas: oxidizes NH₄⁺ to NO₂⁻ (nitrification). - Azotobacter: free-living aerobic nitrogen-fixing bacterium. - Pseudomonas denitrificans: reduces NO₃⁻ to N₂ (denitrification). - Rhizobium: symbiotic nitrogen fixer forming legume root nodules. Answer: Rhizobium.
   Modelled on: the A/L Biology examination pattern

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Where does Soil microorganisms and soil health fit in the syllabus?

Year 2 · Term 3