This definitions covers Key Definitions within The Nitrogen Cycle for GCSE Biology. The nitrogen cycle: nitrogen-fixing, nitrifying, denitrifying bacteria, ammonification, and the role of legumes It is section 7 of 14 in this topic. Make sure you can use the exact wording confidently, because definition marks are often lost through vague language.
Topic position
Section 7 of 14
Practice
15 questions
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15 flashcards
📖 Key Definitions
Nitrogen-fixing bacteria: Bacteria (e.g. Rhizobium) that convert atmospheric nitrogen gas (N₂) into ammonia (NH₃). They live either freely in soil or in root nodules of legume plants in a mutualistic relationship.
Nitrifying bacteria: Bacteria in the soil that convert ammonia into nitrites, and nitrites into nitrates. They are aerobic and require oxygen to function. Their activity increases the concentration of plant-available nitrates in the soil.
Denitrifying bacteria: Bacteria that convert nitrates in the soil back into nitrogen gas (N₂), which is released into the atmosphere. They are anaerobic and thrive in waterlogged, oxygen-poor soils — reducing soil fertility by removing nitrates.
Ammonification: The process by which decomposers (bacteria and fungi) break down nitrogen-containing organic molecules (proteins and nucleic acids from dead organisms and waste) into ammonia. This returns nitrogen to the soil in a form bacteria can further convert to nitrates.
Nitrates: Inorganic ions (NO₃⁻) found dissolved in soil water. They are the primary form in which plants absorb nitrogen, taking them up via active transport through root hair cells. Nitrates are used to synthesise amino acids and proteins.
Root nodules: Small swellings on the roots of legume plants (peas, beans, clover) that contain nitrogen-fixing bacteria (Rhizobium). This is a mutualistic relationship — the bacteria fix nitrogen for the plant; the plant provides sugars for the bacteria.
Legumes: Plants (including peas, beans, lentils, clover) that have a mutualistic relationship with nitrogen-fixing bacteria in their root nodules. Legume crops enrich the soil with nitrogen, which is why farmers rotate them with other crops.
Nitrogen fixation: The conversion of atmospheric nitrogen gas (N₂) into ammonia or ammonium ions. This is carried out by nitrogen-fixing bacteria and, to a small extent, by lightning. It is the entry point of nitrogen from the atmosphere into the soil.