This exam tips covers Exam Tips: The Nitrogen Cycle within The Nitrogen Cycle for GCSE Biology. The nitrogen cycle: nitrogen-fixing, nitrifying, denitrifying bacteria, ammonification, and the role of legumes It is section 13 of 14 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
💡 Exam Tips: The Nitrogen Cycle
🎯 Common Question Types
- "Describe the role of bacteria in the nitrogen cycle" — 3-4 marks
- "Explain why farmers rotate legume crops with cereal crops" — 3-4 marks
- "Explain what happens to nitrogen in waterlogged soil" — 2-3 marks
- "Compare the nitrogen cycle and carbon cycle" — 4-6 marks (often a 6-marker)
- "Explain how nitrogen in dead organisms is returned to the soil" — 2 marks
📝 Key Command Words
- State/Name: Just the name of the bacteria type or process — no explanation needed
- Describe: Give the sequence of what happens — include what is converted to what
- Explain: Give the mechanism AND the reason — "because", "so", "therefore"
- Compare: Find similarities AND differences between the two cycles — use a table structure in your answer
- Suggest: Apply your knowledge to an unfamiliar context — e.g. why is a new species of bacteria found in a peat bog likely to be anaerobic?
⚠️ Common Mistakes to Avoid
- Saying "bacteria convert nitrogen" — always specify WHICH type and WHAT they convert (N₂ → ammonia, or ammonia → nitrates, etc.)
- Confusing nitrifying bacteria (make nitrates, aerobic) with denitrifying bacteria (destroy nitrates, anaerobic)
- Forgetting that plants absorb nitrates via ACTIVE TRANSPORT (requires energy)
- Saying "all bacteria increase soil fertility" — denitrifying bacteria DECREASE it
- Describing the nitrogen cycle as if it only involves decomposition — nitrogen fixation is equally important
- Not linking crop rotation back to nitrogen fixation in root nodules of legumes
Quick Check: A student writes: "Plants can get nitrogen from both the soil and the air." Identify the error in this statement and correct it.
The error is that plants cannot absorb nitrogen gas directly from the air. Atmospheric nitrogen exists as N₂, which has a very strong triple bond that plants lack the enzymes to break. Plants can only absorb nitrogen from the soil in the form of nitrates (NO₃⁻), which they take up through root hair cells via active transport. The correct statement should be: "Plants absorb nitrogen from the soil as nitrates. The nitrogen in those nitrates originally came from the atmosphere, but it was converted into a usable form by nitrogen-fixing bacteria (such as Rhizobium) and nitrifying bacteria."
Practice questions for The Nitrogen Cycle
What percentage of the atmosphere is made up of nitrogen gas (N₂)?
Explain the difference between nitrifying bacteria and denitrifying bacteria, including the conditions in which each type thrives.