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 Form 3 Biology questions and answers on ecology

Describe the nitrogen cycle.

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Answer Text:
• During thunderstorms/lightning nitrogen gas combines with oxygen to form nitrogen oxides
• Nitrogen oxides dissolve in water to form nitric acid
• Acid is deposited in the soil by rain
• Nitric acid combines with chemical substances to
form nitrates or nitric acid dissociates to form nitrates which are absorbed by plants
• Symbiotic bacteria (Rhizobium) which are found in root nodules of leguminous plants fix free nitrogen to nitrates
• Free living bacteria (clostridium and Azotobacter) fix nitrogen to nitrates
• Nostoc algae (Anabaema chlorella) fix nitrogen to
nitrates
• Plants use nitrates to form plant proteins
• Animals feed on plants and convert plant proteins into animal proteins
• Plants and animals die and are decomposed by
putrefying bacteria, fungi(saprophytes)
• Decomposing plants, animals and nitrogenous
wastes release ammonia which is converted to nitrites by Nitrosomonas and nitrococcus bacteria
• Nitrites are converted to nitrates by nitrobacter
bacteria
• Nitrates in the soil can be converted to free nitrogen (denitrification) by some fungi, pseudomonas and theobaccilus bacteria generally called denitrifying bacteria


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