Nitrogen Metabolism - PP Exam 2 Flashcards
Explain the assimilation process of nitrate (NO3-).
Root cells uptake nitrate ions -> nitrate (NO3-) reduced to nitrite (NO2-) by the enzyme ‘nitrate reductase’ -> nitrite (NO2-) reduced to ammonium (NH4+) by the enzyme ‘nitrite reductase’
nitrate -> nitrite -> ammonium -> glutamine -> glutamate
Explain the assimilation process of ammonium (NH4+).
Root cells uptake ammonium ions, or obtained from nitrite reductase (plasitdial) -> Glutamine is synthesized by the enzyme Glutamine synthetase (GOGAT) -> glutamate is formed by the enzyme glutamate dehydrogenase (GDH)
ammonium -> glutamine -> glutamate
Do nitrate and ammonium assimilation take place in root cells, or are they transported and assimilated in other parts of the plant?
Assimilation does take place in the root cells; the nitrogen can be distributed throughout the plant afterward.
Explain the steps that lead to formation of nodules in the symbiotic association between legumes and Rhizobia. [Paragraph answer]
Nodules are formed when Rhizobia is present. FLAVANOIDS are produced, which attract the rhizobia, who then produce the NODULATION FACTORS. These factors are sensed by the ROOT HAIRS of the legume plant. The root hairs respond by CURLING around the Rhizobia, which allows for a successful infection. INFECTION THREADS form, and the Rhizobia enters then and then enters the root tissues. Once the Rhizobia reaches the ROOT CORTEX, it starts to proliferate, and the nodules begin to form.
Where does nitrate reductase convert nitrate to nitrite?
the cytoplasm
Where does nitrite reductase convert nitrite to ammonium?
the plastids
Is nitrate toxic to plants?
no
Is nitrate toxic to animals?
yes, in humans it binds to hemoglobin and inhibits the binding of oxygen to hemoglobin, disease called ‘methemoglobinemia’
Is ammonium toxic? What can it damage/destroy?
yes, it destroys the pH gradient created by the H+ pumps across the plasma membrane and tonoplast
Where is ammonium assimilated?
wherever it’s produced: either assimilated in the soil, or where it’s converted from nitrite
What is the amino acid for nitrogen storage and transport in plants?
asparagine
What pathway synthesizes aromatic amino acids?
shikimate pathway
What is the precursor of important secondary metabolites (like flavonoids)?
phenylalanine
How do herbicides function?
they inhibit amino acid biosynthesis
What regulates amino acid levels?
regulation of their synthesis and degradation
How is amino acid biosynthesis inhibited?
inhibited by feedback mechanisms
What concentration of oxygen does nitrogen fixation occur in?
low oxygen concentration
Explain the steps that lead to formation of nodules in the symbiotic association between legumes and Rhizobia. [Bullet points answer]
- plant secretes flavanoids into soil, attracting Rhizobia
- nodulation genes are activated in rhizobia
- nodulation genes encode proteins that synthesize nodulation factors
- receptors on root hairs of plant bind to nodulation factors
- Rhizobia attach to cell wall of root hairs
- root hairs curl, forming infection threads
- cells in root cortex and in pericycle proliferate, nodules start forming
- bacteria release inside the proliferating cells, and they are covered peribacteroid membrane derived from the plasma membrane
**nodulin genes expressed in proliferating cells, these encode leghemoglobin (protein that binds to oxygen), also transporters that transport ammonia into plant cell and the sugars into the bacteria, and the enzymes that make special nitrogen compounds (ureids)
**nodules have right environment has low oxygen conc., so bacterial nitrogenase works in those molecules, converts molecular nitrogen into ammonia
symbiotic
What is short-term storage for nitrogen in plants?
nitrate in vacuoles
What is medium-term storage for nitrogen in plants?
asparagine and non-protein amino acids
What is long-term storage for nitrogen in plants?
storage proteins
What are the three advantages of compartmentalization?
1) metabolic reactions requiring different conditions can happen at the same time in different compartments
2) same reactions can happen in different compartments at different rates
3) a metabolite can be used in different reactions without direct competition between reactions
What is the difference between coarse and fine control relating to the control of biochemical reactions?
coarse control determines the number of enzyme proteins in the cell, while fine control determines the extent to which each enzyme protein is active
How much of the atmosphere does molecular nitrogen make up?
78%
What is nitrification?
When nitrifying bacteria convert NH4+ (ammonium) into nitrite (NO2-) and nitrate (NO3-)
What is denitrification?
_____________
Denitrifying bacteria use NO3- (nitrate) as electron acceptor during respiration, instead of oxygen
these bacteria convert nitrate into molecular nitrogen and nitric oxide
Where does the electron transport chain reside?
it is on the thylakoid membrane in the chloroplast, cuz the site of photosynthesis is the chloroplast
Where does the Calvin cycle take place?
in the stroma in the chloroplast
Where is cellulose synthesized?
plasma membrane
Where is pectin from the lumen synthesized?
endoplasmic reticulum
Where is sucrose synthesized?
cytosol
How is nitrate uptaken through the roots? (not reducted)
- nitrate taken into the symplast by epidermal & cortical cells in the root, mostly through symporters
- nitrate actively transported into vacuole, where its stored
How are nitrate ions transported into the cell? What is the H+ concentration inside and outside the cell?
proton pumps pump hydrogen ions out of the cell and symporters actively trasnport nitrate ions into the cell, using the potential energy of the pH gradient
low H+ conc. inside cell cuz the H+ is being transported out, high H+ conc. outside cell cuz thats where the hydrogen ions being trasnported to
What factors can increase nitrate reductase activity? How do they increase activity?
light, nitrate, sucrose, triose-P, and hexose-P
increasing gene transcription / keeping nitrate reductase dephosphorylated
What factors can decrease nitrate reductase activity? How do they decrease activity?
darkness & glutamine
either decreasing gene transcription / increasing nitrate reductase kinase activity (phosphorylates nitrate reductase)
How is nitrate reductase activity regulated by coarse control?
light & sucrose activate transcription of the nitrate reductase gene, to make more of the protein
How is nitrate reductase activity regulated by fine control?
-when nitrate reductase is phosphorylated (a phosphate group added), it can bind a certain protein (14-3-3 but we dont need to know the name) and become inactivated
-so in order to keep the nitrate reductase active, it is kept in the non-phosphorylated state
-the sugars, triose, and hexose phosphates do this by inhibiting the ‘nitrate reductase kinase’ that adds the phosphate group to nitrate reductase, inactivating it (also, protein phosphatase removing the new phosphate group from nitrate reductase)
What do the two GOGAT enzymes oxidize, and where is each located?
one of them oxidizes ferredoxin and is located in plastids in leaves, the other one oxidizes NADH and it is located in plastids in roots and cotyledons
Where is starch synthesized?
in the chloroplast check if correct
What are the sources of ammonium in plant cells?
-photorespiration
-absorption through roots
-synthesis of amino acids
-synthesis of lignin
-deamination of amino acids during seed germination
Why can’t ammonium be immediately translocated through the plant? How is it toxic to the plant?
it must be assimilated wherever it is produced because it is toxic to the plant
it is toxic to the plant because it destroys the pH gradient created by the H+ cells on the plasma membrane
What part of the cell is nitrate stored in at high concentrations? What parts of the plant do the nitrate-storing cells reside in?
stored at high conc. in the vacuole
stored in the roots, stems, and leaf midribs
Why does keeping nitrate reductase dephosphorylated increase enzyme activity?
It increases enzyme activity because with the addition of a phosphate group, nitrate reductase can bind the 14-3-3 protein which inactivates it. If it is kept in the dephosphorylated state, it will not be able to bind this protein and it will therefore stay activated. It is kept in the dephosphorylated state by triose-P and hexose-P, which inhibit the nitrate reductase kinase. This kinase is responsible for adding a phosphate group to nitrate reductase, so if it is inhibited, nitrate reductase cannot get phosphorylated and cannot be bound by the protein and inhibited.
What process controls nitrite reductase activity? How?
photosynthesis
light reactions from photosynthesis lead to electron transport on the thylakoid membrane, which reduces ferredoxin (gains electron) and this reduces nitrite reductase (gains the electron), which activates the enzyme
What are nodulin genes produced by? What do they encode for?
produced by infected plant cells
encode for:
-leghemoglobin: protein that binds oxygen
-transporters: allow molecule exchange across peribacteroid membrane
-enzymes: that make special N compounds, like ureids
