Draw a labelled diagram of the nitrogen cycle, and explain how nitrogen moves from the air into living things and back, naming the bacteria that do the work.

1. 1 · Define. The nitrogen cycle is the cyclic movement of nitrogen between the air, the soil and living things.
2. 2 · Fixation. Lightning and microbes (Rhizobium in legume root nodules, blue-green algae) change N₂ into nitrites and nitrates.
3. 3 · Assimilation. Plants absorb nitrates to make amino acids and proteins; animals get them as food.
4. 4 · Ammonification. Excreta and dead bodies are broken down into ammonia by ammonifying bacteria.
5. 5 · Nitrification. Nitrosomonas changes ammonia into nitrites; Nitrobacter changes nitrites into nitrates.
6. 6 · Denitrification. Pseudomonas changes nitrates back into nitrogen gas, which returns to the air.
7. 7 · Draw it. [Cycle: N₂ in air → fixation → nitrates in soil → plants → animals → dead matter and excreta → ammonia → nitrates; nitrates → N₂ by denitrification]
8. 8 · Close in one line. So nitrogen is used again and again and never runs out.

Scoring tip: Write the name of the bacterium on the arrow of each step; a well-labelled cycle earns most of the marks.