How It Works: Why Bacteria Are Essential
Part of The Nitrogen Cycle · GCSE GCSE Biology revision
This how it works covers How It Works: Why Bacteria Are Essential within The Nitrogen Cycle for GCSE Biology. The nitrogen cycle: nitrogen-fixing, nitrifying, denitrifying bacteria, ammonification, and the role of legumes It is section 5 of 14 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 5 of 14
Practice
15 questions
Recall
15 flashcards
⚙️ How It Works: Why Bacteria Are Essential
Plants and animals cannot break the triple bond in N₂ — it requires too much energy. Only certain bacteria (and industrial processes like the Haber process for making fertiliser) have the enzyme nitrogenase, which can do this job. This makes nitrogen-fixing bacteria the critical gateway through which atmospheric nitrogen enters the food chain.
The mutualistic relationship between Rhizobium and legumes is a beautiful example of co-evolution. The plant actively recruits the bacteria by secreting chemical signals from its roots. The bacteria colonise the root hair cells, forming characteristic pink nodules (the pink colour comes from a molecule called leghaemoglobin, related to haemoglobin in our blood). Inside the nodule, the bacteria receive a steady supply of glucose from the plant's photosynthesis; in return they produce ammonium ions which the plant can use directly for amino acid synthesis.
This is why farmers practice crop rotation — alternating legumes (peas, beans, clover) with cereals or other crops. After a legume crop, the soil is enriched with nitrogen left in root nodules and decomposed plant matter. The following crop benefits without the farmer needing to apply as much artificial fertiliser.
The two competing forces in the nitrogen cycle — nitrogen fixation (adding reactive nitrogen to soil) and denitrification (removing it) — determine the total amount of plant-available nitrogen. When soils are well-aerated, nitrifying bacteria dominate and nitrate levels are high. When soils become waterlogged, denitrifying bacteria take over in the anaerobic conditions and nitrate levels fall, explaining why flooding devastates crop yields.