Ecology and the Environment · 8 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 12% of your exam marks.
Food chains, energy transfer, and ecological definitions are regularly tested, often as short-answer questions.
Nitrogen is needed by every living thing to make proteins (and DNA, which contains nitrogen-rich bases). The atmosphere is 78% nitrogen gas (N₂), but this form is chemically unreactive and most organisms cannot use it directly. The nitrogen cycle shows how nitrogen is converted into usable forms and back again.
| Process | What it does | Carried out by |
|---|---|---|
| Nitrogen fixation | Converts N₂ from the air into ammonium (NH₄⁺) and then nitrate ions (NO₃⁻) that plants can use | Nitrogen-fixing bacteria in soil and in legume root nodules; also lightning; also industrial fertiliser production |
| Nitrification | Converts ammonia (from decomposition) into nitrites, then into nitrates | Nitrifying bacteria in the soil |
| Decomposition (ammonification) | Breaks down nitrogen-containing waste (urea, dead bodies) into ammonia in the soil | Decomposers (bacteria and fungi) |
| Denitrification | Converts nitrates in the soil back to N₂ gas, releasing it to the atmosphere | Denitrifying bacteria, in anaerobic conditions (waterlogged soil) |
| Group | Job | Example |
|---|---|---|
| Nitrogen-fixing bacteria | Fix N₂ gas from the air into ammonium/nitrate compounds | Rhizobium (in legume root nodules), Azotobacter (free-living) |
| Nitrifying bacteria | Convert ammonia into nitrites then nitrates | Nitrosomonas (ammonia → nitrite), Nitrobacter (nitrite → nitrate) |
| Denitrifying bacteria | Convert nitrates back to N₂ gas | Pseudomonas |
Crops remove nitrogen from the soil with every harvest. If farmers do nothing, the soil eventually runs out of nitrogen and crop yields fall. Solutions include:
