Lesson Notes

## Introduction to Nitrogen Assimilation Nitrogen constitutes 78% of the Earth's atmosphere. However, atmospheric nitrogen is chemically unreactive and cannot be directly utilized by most living organisms. ### Key Biological Pathways 1. **Nitrogen Fixation**: Conversion of atmospheric N2 into ammonia (NH3) or ammonium (NH4+). This is catalyzed by symbiotic bacteria (e.g., *Rhizobium* in root nodules of legumes) and free-living nitrogen fixers (e.g., *Azotobacter*). 2. **Nitrification**: A two-step aerobic process where soil bacteria oxidize ammonia into nitrite (NO2-) and then nitrate (NO3-). • *Nitrosomonas* oxidize ammonium to nitrite. • *Nitrobacter* oxidize nitrite to nitrate. 3. **Denitrification**: The anaerobic reduction of soil nitrate back into gaseous nitrogen (N2), completed by denitrifying bacteria such as *Pseudomonas denitrificans*. ### Classroom Practical Demonstrations Uproot a cowpea plant to analyze and microscope the root nodules harboring *Rhizobium* colonies under a high-power lens.