Plant Physiology Before deciduous trees drop their leaves in the fall, one of th
ID: 210748 • Letter: P
Question
Plant Physiology
Before deciduous trees drop their leaves in the fall, one of the first things that occurs is that chlorophyll is broken down and the nitrogen is transported to the roots for storage. If chlorophyll was not broken down, but was instead retained in the leaves as they dropped from the plant, what pathway would the nitrogen in the chlorophyll have to follow in order to be taken back up by the plant and reincorporated into new chlorophyll in new leaves the next year. Describe the types of microorganisms involved, and where the plant would have to directly expend ATP or divert resources from ATP production to reassimilate the nitrogen into amino acids.
Explanation / Answer
If the plant's leaves fall off with the chlorophyll intact , the nitrogen is in organic form. Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (NH4+), a process called ammonification.
The ammonium is then converted to nitrate, primarily by soil-living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2-). Other bacterial species such as Nitrobacter,oxidize the nitrites (NO2-) into nitrates (NO3-). The nitrates are highley soluble in water, the nitrates enter the ground water and from there they can either be absorbed by the plants or they can be further brokendown into nitrogen gas.
Nitrate is taken up by nitrate transporters in the root, that use a proton gradient to power the transport. Nitrogen is transported from the root to the shoot via the xylem in the form of nitrate, dissolved ammonia and amino acids. Usually most of the nitrate reduction is carried out in the shoots while the roots reduce only a small fraction of the absorbed nitrate to ammonia. Ammonia is incorporated into amino acids via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway.
Nitrate reduction is carried out in two steps: Nitrate is first reduced to nitrite (NO2-) in the cytosol by nitrate reductase using NADH or NADPH. Nitrite is then reduced to ammonia in the chloroplasts by a ferredoxin dependent nitrite reductase.
In the chloroplasts, glutamine synthetase incorporates ammonia as the amide group of glutamine using glutamate as a substrate. Glutamate synthase (Fd-GOGAT and NADH-GOGAT) transfer the amide group onto an 2-oxoglutarate molecule producing two glutamates. Further transaminations are carried out to make other amino acids (most commonly asparagine) from glutamine.
Nitrogen fixation:
When atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase in the bacteria. The nitrogen fixing bacteria can be free living or symbiotic.
The reaction for nitrogen fixation is:
N2 + 8 H+ + 8 e 2 NH3 + H2.
The process is coupled to the hydrolysis of 16 ATP and H2 is the byproduct of this reaction.
The enzyme nitrogenase is a Mo-Fe containing protein and binds with molecule of nitrogen (N2) at its binding site. This molecule of nitrogen is then acted upon by hydrogen (from the reduced coenzymes) and reduced in a stepwise manner.
It first produces dimide (N2H2) then hydrazime (N2H4) and finally ammonia(2NH3).
NH3 is toxic to the cells and therefore the NH3 combines with organic acids in the cell and form amino acids.