Nitrogen Fixation

Question 1

Give the first step of the nitrogen cycle

Answer 1

Fixation (reduction) of atmospheric nitrogen by nitrogen-fixing bacteria to yield ammonia (NH3 or NH4+).

Question 2

The process of ammonia converted to nitrate is called what?

Answer 2

Nitrification

Question 3

The process whereby Nitrate is reduced to N2 under anaerobic conditions is called?

Answer 3

Denitrification

Question 4

Bacteria that perform denitrification use what rather than O2 as the ultimate electron acceptor?

Answer 4

NO3- is used

Question 5

What is the name of the bacteria that converts Ammonia and Nitrate into atmospheric N2?

Answer 5

Annamox bacteria

Question 6

90% of the NH4- generated by vascular plants, algae and microorganisms comes from what process?

Answer 6

Nitrate Assimilation, a two-step process.

Question 7

Give the first step of nitrogen assimilation

Answer 7

First NO3- is reduced to NO2- by nitrate reductase.

Question 8

Give step two of Nitrate Assimilation

Answer 8

NO2- is reduced to NH4+ in a six-electron transfer catalyzed by nitrate reductase.

– in plants, electrons pass from ferredoxin through a 4Fe- 4S center to a novel hemelike molecule (siroheme) before reducing NO2−

– nonphotosynthetic microbes have a distinct enzyme for which NADPH is the electron donor

Question 9

Where in plants is Nitrate Reductase located?

Answer 9

The chloroplasts.

Question 10

Nitrite reductase receives its electrons from?

Answer 10

Ferredoxin (which is reduced in the light-dependent reactions of photosynthesis)

Question 11

What is the name of bacteria that can fix nitrogen?

Answer 11

Diazotrophs

Question 12

What is the most important product of nitrogen fixation?

Answer 12

Ammonia

Question 13

Is the reduction of nitrogen to ammonia endergonic or exergonic?

Answer 13

Exergonic.

Question 14

Why does nitrogen fixation have an extremely high activation energy?

Answer 14

Because the Nitrogen triple bond is very stable

Question 15

Nitrogen fixation is carried out by a complex of proteins called what?

Answer 15

The nitrogenase complex.

Question 16

Give the enzymes of the Nitrogenase complex

Answer 16

Dinitrogenase (a and b subunits) and Dinitrogenase reductase

Question 17

Why is the presence of molybdenum in the Dinitrogenase heterotetramer important?

Answer 17

Because it catalyzes nitrogen fixation

Question 18

Give the Nitrogenase Complex Rxn

Answer 18

N2 + 10H+ + 8e- + 16ATP —> 2NH4+ 16 ADP + 16 Pi

Question 19

Is Dinitrogenase reductase a dimer or tetramer?

Answer 19

A dimer of two identical subunits.

Question 20

How many electrons are required for nitrogen fixation?

Answer 20

Eight

Question 21

Where are the eight electrons for nitrogen fixation derived from?

Answer 21

4 CoA and 4 Pyruvate

Question 22

Define ‘nitrification’.

Answer 22

It is the process of converting ammonia to nitric oxide, nitrite, and finally nitrate.

Question 23

Define ‘denitrification’.

Answer 23

It is the reduction of nitrate and nitrite to N under anaerobic conditions.

Question 24

Where does Nitrate reductase receive its electrons from?

Answer 24

From NADH through cysteine, FAD, a cytochrome (cyt b557), and a novel cofactor containing molybdenum.