Nitrogen Cycle Worksheet featuring a diagram and multiple-choice questions about bacterial processes in the nitrogen cycle.

Let’s go step by step through each question.

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Question 1: Which group of bacteria convert ammonia into nitrates in the nitrogen cycle?

We know that in the nitrogen cycle:
- Ammonia (NH₃) is first turned into nitrite (NO₂⁻), then into nitrate (NO₃⁻).
- The bacteria that do this two-step process are called nitrifying bacteria.
- Example: *Nitrosomonas* turns ammonia → nitrite.
- Then *Nitrobacter* turns nitrite → nitrate.
- So, both steps together are done by nitrifying bacteria.

Other options:
- Anaerobic bacteria — work without oxygen; not specifically for turning ammonia to nitrate.
- Denitrification bacteria — turn nitrates back into nitrogen gas (opposite direction!).
- Nitrogen-fixation bacteria — take N₂ from air and turn it into ammonia or related compounds (like in root nodules).

✔ Correct answer: C nitrifying bacteria

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Question 2: The change of nitrite ion (NO₂⁻) to nitrate ion (NO₃⁻) takes place by the action of ___

This is the second step of nitrification. As mentioned above:
- First step: ammonia → nitrite (by some nitrifying bacteria)
- Second step: nitrite → nitrate (by other nitrifying bacteria, like *Nitrobacter*)

So again, this is done by nitrifying bacteria.

Check options:
- A: Bacteria in root nodules — those fix nitrogen (N₂ → NH₃), not involved here.
- B: Denitrifying bacteria — they remove nitrates, not make them.
- C: Nitrifying bacteria — YES!
- D: Nitrogen-fixation bacteria in soil — same as A, wrong process.

✔ Correct answer: C nitrifying bacteria

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Question 3: The diagram shows part of the nitrogen cycle. Which stage depends on nitrifying bacteria?

Look at the diagram flow:

We have:
- “ammonia” → “nitrate” via arrow labeled C
- “nitrate” → “plant protein” via arrow A (plants absorb nitrate to make proteins)
- “dead matter” → “ammonia” via B (decomposition)
- “nitrate” → “nitrogen” via D (denitrification)

Nitrifying bacteria convert ammonia → nitrate. That’s exactly what arrow C shows.

So stage C depends on nitrifying bacteria.

✔ Correct answer: C

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Final Answer:
1. C
2. C
3. C