Chapter 5: Nutrient Assimilation

Question 1

nutrient assimilation

Answer 1

The incorporation of mineral nutrients into carbon compounds such as pigments, enzyme cofactors, lipids, nucleic acids, or amino acids.

Question 2

what other elements are more abundant than nitrogen?

Answer 2

oxygen, carbon, and hydrogen

Question 3

how much nitrogen makes up the atmosphere?

Answer 3

78%

• not directly available
• Has to break the stable triple bond to form ammonia (NH3) and nitrate (NO3 -) - which is called nitrogen fixation (high energy process)

Question 4

Haber-Bosch process

Answer 4

N2 combines with hydrogen to form ammonia under elevated temperature (200 C) and high pressure (200 atm) with the presence of a metal catalyst like iron

Question 5

What are the natural processes that can fix nitrogen and how much does it produce? Why is biological nitrogen fixation important?

Answer 5

Lightning = 8%
Photochemical reactions = 2%
Biological nitrogen fixation = 90%
Industrially produced nitrogen is economically and environmentally costly

Question 6

is ammonium toxic? why or why not? How does it effect plants?

Answer 6

Ammonium is toxic to both plants and animals if it accumulates to high concentrations in the tissues.
Disrupts transmembrane proton gradients
Unlike the case with ammonium, plants can store high levels of nitrate, and they can translocate it from tissue to tissue without deleterious effect
- they do this by storing excess ammonium in their vacuoles via a pH gradient

Question 7

How is nitrogen converted to organic compounds?

Answer 7

nitrate is converted into organic compounds
Convert nitrate to nitrite in the cytosol by a reduction reaction. Nitrate reductase catalyzes this reaction using NADH or NADPH as an electron donor

Question 8

how is nitrate reductase regulated?

Answer 8

Nitrate, light, and carbohydrates influence nitrate reductase at the transcription and translation levels
Subject to post translational modification
Regulation of nitrate reductase activity through phosphorylation and dephosphorylation provides more rapid control than can be achieved through synthesis or degradation of the enzyme (minutes versus hours).

Question 9

how does nitrate reductase convert nitrate to ammonium?

Answer 9

Because nitrite is reactive and toxic, it is immediately transported for reduction
In these organelles, the enzyme nitrate reductase reduces nitrate to ammonium, a reaction that involves the transfer of six electrons,
Chloroplasts and root plastids contain different forms of nitrite reducates that consists of an iron-sulfur cluster and a specialized heme to bind nitrite and reduce it to ammonium
Small percentage is released as nitrous oxide, a greenhouse gas

Question 10

is nitrate assimilated by the roots or shoots?

Answer 10

Nitrate is primarily reduced by roots
But when more and more nitrate is assimilated, it travels to the shoot to be assimilated
Generally, species native to temperate regions rely more heavily on nitrate assimilation by the roots than do species of tropical or subtropical origins.

Question 11

how do plants avoid ammonium toxicity?

Answer 11

Plant cells avoid ammonium toxicity by rapidly converting the ammonium generated from nitrate assimilation or photorespiration into amino acids.

Question 12

what two enzymes are needed to convert ammonium to amino acids?

Answer 12

Glutamine synthetase (GS) combines ammonium with glutamate to form glutamine
Requires hydrolysis of one ATP and divalent cation like magnesium, manganese, or cobalt as a cofactor

Question 13

What are the two classes of GS?

Answer 13

in the cytosol
The cytosolic forms are expressed in germinating seeds or in the vascular bundles of roots and shoots and produce glutamine for intercellular nitrogen transport
and the other in the root plastids or shoot chloroplasts
Generates amid nitrogen for local consumption
Reassimilated photorespiratory ammonium
Light and carbohydrates can affect this reaction - but not in the cytosol

Question 14

What stimulates the activity of GS?

Answer 14

glutamate synthase  (GOGAT)
This enzyme transfers the amide group of glutamine to 2-oxoglutarate, yielding two molecules of glutamate
Two types: one accepts electrons from NADH and the other accepts from ferredoxin
NADH: in plastids of nonphotosynthetic tissues like roots and vascular tissue; assimilation of ammonium from the rhizosphere 
Ferredoxin: in chloroplasts and serves in photorespiratory nitrogen metabolism

Question 15

what is an alternative pathway ammonium can be assimilated?

Answer 15

Glutamate dehydrogenase (GDH) catalyzes a reversible reaction that synthesizes or deamination glutamate 
NADH dependent form of GDH is in the mitochondria and NADPH dependent is found in chloroplasts 
primary function is in deaminating glutamate during the reallocation of nitrogen

Question 16

how is nitrogen added into amino acids?

Answer 16

Once assimilated into glutamine and glutamate, nitrogen is incorporated into other amino acids via transamination reactions.
Aminotransferases catalyzes
Found in cytoplasm (significant role in amino acid biosynthesis in plants), chloroplasts, mitochondria, glyoxysomes, and peroxisomes

Question 17

what is the most common symbiosis?

Answer 17

The most common type of symbiosis occurs between members of the plant family Fabaceae (Leguminosae) and soil bacteria of rhizobia

Question 18

actinorhizal symbiosis

Answer 18

Another symbiosis is between woody plant species like alder trees and soil bacteria of the genus frankia.

Question 19

why does nitrogen fixation require anaerobic conditions?

Answer 19

Because nitrogen fixation takes up so much energy, nitrogenase enzymes catalyze these reactions by exchanging high energy electrons
Oxygen can damage electron exchange and inhibit nitrogenase, so it is done under anaerobic conditions
Operate anaerobically or creates an internal anaerobic environment (microaerobic)

Question 20

explain nodules and give examples of different types

Answer 20

Specialized organs of a plant host that contain symbiotic nitrogen-fixing bacteria.
Gunnera: in stem glands and are separate from symbiote
Legumes and actinorhizal: induce the plant to form nodules
Grasses: no nodules, achor to root surface at the elongation zone

Question 21

is symbiosis between plants and rhizobia obligatory?

Answer 21

no - it occurs under nitrogen limiting conditions
This signaling, the subsequent infection process, and the development of nitrogen-fixing nodules involve specific genes in both the host and the symbionts.

Question 22

nodulin genes

Answer 22

plant genes specific to nodules

Question 23

nodulation (nod) genes

Answer 23

rhizobia genes that participate in nodule formation

Question 24

how is the rhizoidal symbiotic relationship formed?

Answer 24

Migration of bacteria toward the root - chemostatic response
Chemical attractant secreted by the root activate the rhizobial NodD gene, which induces transcription of other nods
Nod factors produced by bacteria act as signals for symbiosis
Nodule formation involves phytohormones
Rhizobia infect root hairs by releasing nod factors to curl around the rhizobia
Cell wall disintegrates, allowing access to the plasma membrane

Question 25

What two processes occur simultaneously during root nodule formation?

Answer 25

infection and nodule organogenesis

Question 26

infection thread

Answer 26

An internal tubular extension of the plasma membrane of root hairs through which rhizobia enter root cortical cells.

Question 27

does biological nitrogen produce ammonia?

Answer 27

yes through the nitrogenase enzyme complex

Question 28

what are the two components of the nitrogenase complex?

Answer 28

Fe protein and MoFe protein
Ferredoxin serves as an electron donor to the Fe protein, which will make ATP and reduce MoFe protein. MoFe protein reduces other substance to react with N2 and H

Question 29

describe the Fe protein in the nitrogenase complex

Answer 29

Smaller
Has two identical subunits
Contains iron-sulfur cluster that participates in a redox reaction that converts N2 to NH3
Inactivated by O2

Question 30

describe the MoFe protein in the nitrogenase complex

Answer 30

Four subunits
Each subunit has two MoFeS clusters
Inactivated by O2

Question 31

what is the most common symbiosis?

Answer 31

occurs in members of legume family (fabaceae) forming nodules with soil bacteria
- process requires 16 ATP rather than 12 but it provides access to nitrogen

Question 32

what happens to ammonium after being converted from nitrite?

Answer 32

• assimilated into glutamine by glutamine synthetase and glutamate is produced by glutamate synthetase
• glutamate is transported to cytosol and amino acid biosynthesis proceeds (in roots or shoots)

Question 33

what happens to nitrate after being assimilated into the cytosol?

Answer 33

• transported to the plastids in the root cells or to the chloroplasts in the leaf cells where it is converted to ammonium by nitrite reductase

Question 34

nitrate assimilation

Answer 34

• expends 12 ATP to convert NO3 to glitamine
• plant roots absorb nitrate in their root cells through nitrate-proton cotransporters
• nitrate is then transported to root cells or to the xylem into leaf mesophyll cells and then reduced to nitrite in the cytosol by nitrate reductase