This deep dive covers The Big Picture: Nitrogen's Journey within The Nitrogen Cycle for GCSE Biology. The nitrogen cycle: nitrogen-fixing, nitrifying, denitrifying bacteria, ammonification, and the role of legumes It is section 2 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
🔬 The Big Picture: Nitrogen's Journey
The nitrogen cycle describes how nitrogen atoms move between the atmosphere, soil, living organisms, and back to the atmosphere. Unlike the carbon cycle, which is driven largely by photosynthesis and respiration, the nitrogen cycle depends almost entirely on bacteria performing chemical transformations that no other organisms can do.
There are four main groups of bacteria, each playing a distinct role:
- Nitrogen-fixing bacteria — convert atmospheric N₂ into ammonia (NH₃) or ammonium (NH₄⁺)
- Nitrifying bacteria — convert ammonia into nitrites, then nitrites into nitrates
- Decomposers (bacteria and fungi) — break down dead organisms and waste products, releasing ammonia (ammonification)
- Denitrifying bacteria — convert nitrates back into N₂ gas, returning nitrogen to the atmosphere
Think of these bacteria as the maintenance crew of the nutrient system — without them, nitrogen would stay locked in forms that plants and animals simply cannot access.
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.