pp. 720-727 only

Nitrogen most often the limiting nutrient

essential for assembling proteins, nucleic acids

80% N in atmosphere, but inaccessible to life in this form

short-term, nitrogen is recycles via microbial decomposition of dead organisms (Fig. 37.8)

only certain prokaryotes in soil capable of "fixing" into organic form (Fig. 37.8)

N2 + 8 e- + 8 H+ + 16 ATP = 2 NH3 + H2 + 16 ADP + 16 Pi

catalyzed by nitrogenase

very energy-expensive (8 ATP/ NH3 fixed)

1. free-living nitrogen-fixing bacteria (Fig. 37.8)

occurs in rich organic soils (provide fuel for cellular respiration)

2. symbiotic nitrogen fixation--also important (Fig 37.8, 9, 10)

legumes and specific Rhizobium species--mutualistic (coevolution)

evident in leghemoglobin--plant and bacterium each make subunit of the macromolecule

binds and releases O2 for intense respiratory demands of N2 fixation, also keeps nitrogenase from being inhibited by O2

nodules make a.a.'s, transport to shoots, leaves via xylem

excrete excess NH4+ into soil, increasing fertility (basis for intercropping; crop rotation: "green fertilizer", plowed under)