This topic summary covers Knowledge Organiser within The Nitrogen Cycle for GCSE Biology. The nitrogen cycle: nitrogen-fixing, nitrifying, denitrifying bacteria, ammonification, and the role of legumes It is section 14 of 14 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 14 of 14
Practice
15 questions
Recall
15 flashcards
Knowledge Organiser
Key Terms
- Nitrogen fixation: Conversion of atmospheric N₂ into ammonia (NH₃) by nitrogen-fixing bacteria — makes nitrogen available to living organisms
- Nitrification: Conversion of ammonia → nitrites → nitrates by nitrifying bacteria (aerobic process)
- Ammonification: Decomposers break down proteins and urea in dead organisms and waste → release ammonia into the soil
- Denitrification: Denitrifying bacteria convert nitrates → N₂ gas (anaerobic, in waterlogged soils) — returns nitrogen to the atmosphere and reduces soil fertility
- Root nodules: Swellings on the roots of legume plants containing Rhizobium bacteria; the relationship is mutualistic — bacteria fix nitrogen (benefit: carbon compounds from plant); plant gains usable nitrogen compounds
- Legumes: Plants (peas, beans, clover, soya) with nitrogen-fixing Rhizobium bacteria in root nodules; used in crop rotation to restore soil nitrate levels
Must-Know Facts
- The atmosphere is 78% N₂ but plants cannot absorb nitrogen gas — they can only absorb nitrate ions (NO₃⁻) dissolved in soil water
- Plants absorb nitrates by active transport through root hair cells — this requires energy (ATP) from respiration
- Plants use nitrogen to make: proteins, DNA, and chlorophyll
- Nitrifying bacteria are aerobic — they need oxygen; found in well-aerated soils
- Denitrifying bacteria are anaerobic — they thrive in waterlogged, oxygen-poor soils and reduce soil fertility
- Crop rotation: growing legumes in a field one year naturally replenishes nitrates, reducing the need for artificial fertiliser
The Four Bacteria Types
- Nitrogen-fixing bacteria (e.g. Rhizobium): N₂ → NH₃ — in root nodules of legumes OR free-living in soil; increases available nitrogen
- Nitrifying bacteria: NH₃ → NO₂⁻ → NO₃⁻ — aerobic; convert ammonia to nitrates that plants can absorb; increases soil fertility
- Decomposers (bacteria and fungi): proteins/urea → NH₃ (ammonification) — return nitrogen from dead organisms to the soil as ammonia
- Denitrifying bacteria: NO₃⁻ → N₂ — anaerobic; return nitrogen to the atmosphere; reduce soil nitrate levels and therefore reduce soil fertility
Common Mistakes
- Writing "plants absorb nitrogen from the air": Plants cannot use atmospheric nitrogen gas (N₂) directly. They absorb nitrate ions (NO₃⁻) from the soil through their root hair cells by active transport. Only nitrogen-fixing bacteria can convert N₂ into a usable form.
- Confusing nitrifying and denitrifying bacteria: Nitrifying bacteria convert ammonia into nitrates — they increase soil fertility and are aerobic. Denitrifying bacteria convert nitrates back into N₂ gas — they decrease soil fertility and are anaerobic (found in waterlogged soils). A flooded field loses nitrates because denitrifying bacteria become more active.
- Forgetting that nitrate uptake requires active transport: Nitrates are at a lower concentration in the soil than inside root cells, so they cannot enter by diffusion. Active transport (using ATP from respiration) is required. This is a specific 1-mark point that students frequently omit.
- Saying all soil bacteria benefit crop farmers: Nitrogen-fixing and nitrifying bacteria both increase soil nitrate levels and help plants grow — farmers want more of these. Denitrifying bacteria convert nitrates back to N₂ and reduce soil fertility — farmers want fewer of these, which is why waterlogged soils reduce crop yields. Always distinguish between the two groups.
- Grade 7+ separator — explaining the mutualistic relationship fully: The relationship between Rhizobium and legumes must be described as mutualistic with benefits named for both partners: bacteria receive carbohydrates (sugars) from the plant for energy; the plant receives ammonia/nitrogen compounds from the bacteria. Stating only "both benefit" without naming the specific benefit for each organism will not score full marks.