1. Assimilation of mineral nutrients (Chapter 12) Incorporation of inorganic forms of essential elements into organic compounds necessary for growth and development. Nitrogen assimilation is quantitatively the most important and is energetically expensive. Two main ways that plants get reduced N 1.Assimilation of NO 3 - or NH 4 + absorbed by roots 2.Symbiotic N 2 fixation

2. The Nitrogen Cycle Fig. 12.1

3. Microorganisms have a central role in almost all aspects of nitrogen availability and thus for life support on earth: * some bacteria can convert N2 into ammonia by the process termed nitrogen fixation; these bacteria are either free-living or form symbiotic associations with plants or other organisms (e.g. termites, protozoa) * other bacteria bring about transformations of ammonia to nitrate, and of nitrate to N2 or other nitrogen gases * many bacteria and fungi degrade organic matter, releasing fixed nitrogen for reuse by other organisms.

4. The Nitrogen Cycle

5. Nitrate and ammonium assimilation involves their conversion into amino acids Nitrate assimilation requires two reduction steps NO 3 - + NAD(P)H + H + + 2e - --> NO 2 - + NAD(P) + + H 2 O enzyme is nitrate reductase NO 2 - + 6 Fd red + 8H + + 6e - --> NH 4 + + 6Fd ox + 2H 2 O enzyme is nitrite reductase

7. Biological Nitrogen Fixation converts N 2 gas into chemical forms usable by microbes and plants.

11. Examples of N 2 fixing plants of the Southwest Mesquite, Acacia, Palo Verde Symbionts are Rhizobium & Bradyrhizobium Lupines - Bradyrhizobium

12. Root nodules

14. Nitrogenase is the key enzyme in biological N fixation. A bacterial enzyme, not made by plants

15. Biological N fixation is energetically expensive, 16 ATP/N 2. Note that Molybdenum is a cofactor