Lecture 7: Assimilation of nitrogen by plants

Question 1

why is nitrogen needed in plants

Answer 1

needed for amino acids, proteins and other cellular constituents.

Question 2

nitrogen deficiency results in

Answer 2

stunted growth & chlorotic leaves

Question 3

what is generally the most limiting nutrient to plant growth

Answer 3

Nitrogen

Question 4

N2 is an extremely ____ molecule

Answer 4

unreactive

Question 5

The industrial Haber-Bosch process: equation + use of global energy

Answer 5

N2 + 3H2 –> 2NH3 requires high temperatures (300-600 degrees C ), high pressures (20-80MPa) and has a low yield
- uses 1-2% of global energy

Question 6

_ tonnes of oil are needed to produce 1 tonne of fertiliser

Answer 6

2

Question 7

____ is the point of entry of nitrogen into organic combination

Answer 7

Ammonia

Question 8

Ammonia is generated in two ways:

Answer 8

1) reduction of N2 by micro-organisms, often in symbiotic association with other plants
2) reduction of nitrate (the predominant form of soil in N) to ammonia, catalysed by nitrate and nitrite reductase in roots or shoots

Question 9

nitrate reduction accounts for about __ Mt per annum, compared with N2 fixation at ___ Mt

Answer 9

nitrate reduction = 2 X 10^4 Mt

N2 fixation = 200 Mt

Question 10

pollutants such as NO2 are also

Answer 10

assimilated via the pathway of nitrate reduction

Question 11

how many species micro-organisms (bacteria & cyanobacteria) accomplish nitrogen bacteria with the aid of nitrogenase

Answer 11

over 250

EQUATION:
N2 + 16ATP +8e +10H+ –> 2NH4+ + H2 + 16ADP + 16Pi

Question 12

There are three problems facing organisms which reduce nitrogen:

Answer 12

• reaction very sensitive to oxygen (which inactivates the nitrogenase)
• side reaction - the reduction of 2H+ to H2

These two features are reminiscent of Rubisco

• requires a very large input of energy in the form of ATP and reductant

Question 13

Gloeocapsa sp. in the cyanobacterium when does photosynthesis + N2 fixation occur

Answer 13

photosynthesis, and accompanying O2 evolution, occurs during the day, and N2 fixation at night

Question 14

explanation of nitrogen-fixing symbioses

Answer 14

Although nitrogen fixation is confined to micro‐organisms, there is a wide range of symbiotic associations of these organisms with, e.g., lichens, diatoms, bryophytes, pteridophytes (Azolla), cycads, Alnus, and legumes (peas, beans, clover, many tropical trees).

Question 15

in legumes for every 100 carbon atoms fixed in the leaves, ___ go to the nodules

Answer 15

1/3.

5 C atoms for growth and maintenance of the nodule.
photosynthate
12 C atoms for respiratory production of ATP and NADH (to fix nitrogen).
15 C atoms to provide carbon substrates for the assimilation of ammonia (these largely return to the shoot).

Question 16

Darkened plants fix N2 at very ____

Answer 16

low rates

Question 17

CO2 enrichments affect on N2 fixation

Answer 17

CO2 enrichment, which inhibits photorespiration, enhances N2 fixation.

Question 18

Oxygen supply in legume nodules: O2 is maintained in nodules at a ___

Answer 18

low and buffered concentration by an O2 carrier - leghaemoglobin

Question 19

leghaemoglobin facilities the diffusion of

Answer 19

O2 within the nodule, but the free concentration of O2 is about 10 nM in soybean nodules, compared with 280 microM in air-saturated water

Question 20

oxygen supply in legume nodules allows

Answer 20

rapid rates of respiration and hence production of ATP and reductant, but reduces the danger of inactivation of nitrogenase by O2

Question 21

What is the first organic product of ammonia assimilation

Answer 21

Glutamine

Question 22

Nitrogen assimilation:

Answer 22

Nitrogen assimilation is the formation of organic nitrogen compounds like amino acids from inorganic nitrogen compounds present in the environment

Question 23

how is ammonia assimilated into amino acids?

Answer 23

(cycle)

Glutamate + ATP + NH3 –> Glutamine (under glutamine synthesise) ADP released

glutamine (under glutamate synthase) + 2-oxoglutarate produce glutamate [ferredoxin(red) goes to ferredpxin(ox) in this step]

RECYCLE

Glutamate + glutamine can lead on to form a.a.

Question 24

amides + ureides are characterised by

Answer 24

a high N:C ratio

Question 25

Ureides are

Answer 25

tropical legumes

Question 26

other ways of acquiring nutrients =

Answer 26

carnivore in N-poor environments

venus fly trap etc