1. Figure 4.4 The Nitrogen Cycle

2. While N 2 is the ultimate source and sink of biospheric nitrogen, several oxidized and reduced forms occur in the environment. Nitrogen Fixation Figure 15.17

3. Nitrogen-fixing bacteria may be free- living in soil or water, or they may form symbiotic associations with plants. - Rhizobium and legumes The Nitrogen Cycle Figure 4.5

4. FeMoCo

6. The nitrogenase complex includes two kinds of subunits: The Mechanism of Nitrogen Fixation - A protein with an iron-sulfur core (Fe protein) - A protein containing a complex of molybdenum, iron, and sulfur protein (FeMo protein) Electrons acquired by Fe protein (with energy from ATP) are transferred to FeMo protein to reduce nitrogen. Figure 15.19A

7. Figure 15.20

9. Figure 15.21B

10. Nitrogen gas (N 2 ) is fixed into ammonium ion (NH 4 + ) only by some species of bacteria and archaea. Aquatic cyanobacteria develop special cells called heterocysts to fix N 2. - Photosynthesis is turned off to maintain anaerobic conditions. Nitrogen Fixation Figure 15.18

14. Figure 21.33