Assimilation of mineral nutrients Chapter 12 Incorporation of

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

The Nitrogen Cycle Fig. 12. 1

Microorganisms have a central role in almost all aspects of nitrogen availability and thus for life support on earth: * some bacteria can convert N 2 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 N 2 or other nitrogen gases * many bacteria and fungi degrade organic matter, releasing fixed nitrogen for reuse by other organisms.

The Nitrogen Cycle

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

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

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

Root nodules

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

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