Nitrogen Metabolism - PP Exam 2

Question 1

Explain the assimilation process of nitrate (NO3-).

Answer 1

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

Question 2

Explain the assimilation process of ammonium (NH4+).

Answer 2

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

Question 3

Do nitrate and ammonium assimilation take place in root cells, or are they transported and assimilated in other parts of the plant?

Answer 3

Assimilation does take place in the root cells; the nitrogen can be distributed throughout the plant afterward.

Question 4

Explain the steps that lead to formation of nodules in the symbiotic association between legumes and Rhizobia. [Paragraph answer]

Answer 4

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.

Question 5

Where does nitrate reductase convert nitrate to nitrite?

Answer 5

the cytoplasm

Question 6

Where does nitrite reductase convert nitrite to ammonium?

Answer 6

the plastids

Question 7

Is nitrate toxic to plants?

Answer 7

no

Question 8

Is nitrate toxic to animals?

Answer 8

yes, in humans it binds to hemoglobin and inhibits the binding of oxygen to hemoglobin, disease called ‘methemoglobinemia’

Question 9

Is ammonium toxic? What can it damage/destroy?

Answer 9

yes, it destroys the pH gradient created by the H+ pumps across the plasma membrane and tonoplast

Question 10

Where is ammonium assimilated?

Answer 10

wherever it’s produced: either assimilated in the soil, or where it’s converted from nitrite

Question 11

What is the amino acid for nitrogen storage and transport in plants?

Answer 11

asparagine

Question 12

What pathway synthesizes aromatic amino acids?

Answer 12

shikimate pathway

Question 13

What is the precursor of important secondary metabolites (like flavonoids)?

Answer 13

phenylalanine

Question 14

How do herbicides function?

Answer 14

they inhibit amino acid biosynthesis

Question 15

What regulates amino acid levels?

Answer 15

regulation of their synthesis and degradation

Question 16

How is amino acid biosynthesis inhibited?

Answer 16

inhibited by feedback mechanisms

Question 17

What concentration of oxygen does nitrogen fixation occur in?

Answer 17

low oxygen concentration

Question 18

Explain the steps that lead to formation of nodules in the symbiotic association between legumes and Rhizobia. [Bullet points answer]

Answer 18

• 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

Question 19

What is short-term storage for nitrogen in plants?

Answer 19

nitrate in vacuoles

Question 20

What is medium-term storage for nitrogen in plants?

Answer 20

asparagine and non-protein amino acids

Question 21

What is long-term storage for nitrogen in plants?

Answer 21

storage proteins

Question 22

What are the three advantages of compartmentalization?

Answer 22

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

Question 23

What is the difference between coarse and fine control relating to the control of biochemical reactions?

Answer 23

coarse control determines the number of enzyme proteins in the cell, while fine control determines the extent to which each enzyme protein is active

Question 24

How much of the atmosphere does molecular nitrogen make up?

Answer 24

78%

Question 25

What is nitrification?

Answer 25

When nitrifying bacteria convert NH4+ (ammonium) into nitrite (NO2-) and nitrate (NO3-)

Question 26

What is denitrification?

Answer 26

_____________

Denitrifying bacteria use NO3- (nitrate) as electron acceptor during respiration, instead of oxygen

these bacteria convert nitrate into molecular nitrogen and nitric oxide

Question 27

Where does the electron transport chain reside?

Answer 27

it is on the thylakoid membrane in the chloroplast, cuz the site of photosynthesis is the chloroplast

Question 28

Where does the Calvin cycle take place?

Answer 28

in the stroma in the chloroplast

Question 28

Where is cellulose synthesized?

Answer 28

plasma membrane

Question 28

Where is pectin from the lumen synthesized?

Answer 28

endoplasmic reticulum

Question 28

Where is sucrose synthesized?

Answer 28

cytosol

Question 29

How is nitrate uptaken through the roots? (not reducted)

Answer 29

• nitrate taken into the symplast by epidermal & cortical cells in the root, mostly through symporters
• nitrate actively transported into vacuole, where its stored

Question 30

How are nitrate ions transported into the cell? What is the H+ concentration inside and outside the cell?

Answer 30

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

Question 31

What factors can increase nitrate reductase activity? How do they increase activity?

Answer 31

light, nitrate, sucrose, triose-P, and hexose-P

increasing gene transcription / keeping nitrate reductase dephosphorylated

Question 32

What factors can decrease nitrate reductase activity? How do they decrease activity?

Answer 32

darkness & glutamine

either decreasing gene transcription / increasing nitrate reductase kinase activity (phosphorylates nitrate reductase)

Question 33

How is nitrate reductase activity regulated by coarse control?

Answer 33

light & sucrose activate transcription of the nitrate reductase gene, to make more of the protein

Question 34

How is nitrate reductase activity regulated by fine control?

Answer 34

-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)

Question 35

What do the two GOGAT enzymes oxidize, and where is each located?

Answer 35

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

Question 36

Where is starch synthesized?

Answer 36

in the chloroplast check if correct

Question 37

What are the sources of ammonium in plant cells?

Answer 37

-photorespiration
-absorption through roots
-synthesis of amino acids
-synthesis of lignin
-deamination of amino acids during seed germination

Question 38

Why can’t ammonium be immediately translocated through the plant? How is it toxic to the plant?

Answer 38

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

Question 39

What part of the cell is nitrate stored in at high concentrations? What parts of the plant do the nitrate-storing cells reside in?

Answer 39

stored at high conc. in the vacuole

stored in the roots, stems, and leaf midribs

Question 40

Why does keeping nitrate reductase dephosphorylated increase enzyme activity?

Answer 40

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.

Question 41

What process controls nitrite reductase activity? How?

Answer 41

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

Question 42

What are nodulin genes produced by? What do they encode for?

Answer 42

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