Lecture 38: Nitrogen Cycle Fixation

Question 1

What are the most abundant biological elements?

Answer 1

H, C, N, O, P

Question 2

What are the available form in the enviroment of H, O, and P

Answer 2

H20
O2
Pi

Question 3

What is the availabe form in the environment of C?

Answer 3

CO2

Question 4

CO2`

Answer 4

unreactive (very stable)

• Primarily fixed by photosynthesis
• C-O bond energy is 351 kJ/mol

Question 5

What is the available form in the environment of N

Answer 5

N2

Question 6

N2

Answer 6

unreactive (very stable)

-only a few strains of bacteria can convert to a metabolically useful form

Question 7

What is the body energy for a N/N triple bond?

Answer 7

945 kJ/mol

Question 8

Nitrogen

Answer 8

required in proteins and nucleic acids

Question 9

The formation of NH3 from N2 and 3H2

Answer 9

• exergonic reaction
• problem is that N2 is a very table molecule so the rate of formation of NH3 is negligible except under extreme conditions

Question 10

NH3 in aqueous solution

Answer 10

• must be made under anaerobic conditions
• those organisms that fix N2 either grow under anaerobic conditions or burn off O2 by uncoupling ATP synthesis from oxidation of carbohydrates

Question 11

NH3

Answer 11

• plants utilize for growth

- toxic to animals and is incorporated for dietary proteins

Question 12

Anammox

Answer 12

• unusual organism

- reduce nitrite and nitrate to N2H4 (Hydrazine) and then oxidize hydrazine to N2

Question 13

Nitrogen Cycle

Answer 13

NH3(NH4+) is oxidized to nitrites and nitrates by other special of bacteria and archaea
-nitrates are reduced to N2 by denitrifying organisms (most are green plants, bacteria, archaea, and fungi)

Question 14

Organisms that can fix atmospheric nitrogen

Answer 14

1) Free living bacteria
2) Cyanobacteria
3) Symbionts

Question 15

Free living bacteria

Answer 15

• Aerobic (Azotobacter)

- Anaerobic ( Desulfovibrio)

Question 16

Cyanobacteria

Answer 16

Nostoc (common in lichen)

-N2 fixation occurs in specialized anaerobic cells (heterocysts)

Question 17

Symbionts

Answer 17

live in symbiosis with certain plants

• with legumes (Rhizobium)
• with other plants (Frankia)

Question 18

Problem: Nitrogenase inactivated by O2

Answer 18

utilize anaerobic organisms or maintain anaerobic conditions for nitrogen fixation

Question 19

Symbiosis

Answer 19

Relationship is beneficial to both microbe and plant

1) Bacteria Fixes N2
2) Plant provides anaerobic environment by making Leghemoglobin which sequesters O2
3) Plant also supplies ATP

Question 20

Symbiotic relationship: Benefits to bacteria

Answer 20

• requires anaerobic environments since nitrogenase is inactivated by O2
• N2 fixation requires a lot of energy-provided by the plant
• Provides organic acid precursors of amino acids

Question 21

Symbiotic relationships: benefits to the plant

Answer 21

-provides nitrogen to plant in the form of amino acids

Question 22

N2 fixation (reduction) reaction

Answer 22

• reaction with one N2 molecule occurs at one of the MoFe7S9N centers in dinitrogenase
• dinitrogenase has to reach a highly reduced state before the reaction can begin, which occurs from a transfer of e-s coming from the oxidation of pyruvate
• an intermediary step involves dinitrogenase reductase donating e-; this only happens if the enzyme is in a conformation induced by ATP
• there is ATP hydrolysis upon passage of e-s

Question 23

What three problems does the symbiotic growth of microorganisms with plants in nodules solve?

Answer 23

• the plant helps supply energy needed for the N2 fixation reaction and helps provide leg hemoglobin that sequesters O2
• ensures an anaerobic enviroment
• in return NH3 is provided to the plant

Question 24

NH3 assimilation for mammals: what amino acid is important and how is it accomplished?

Answer 24

Glutamine

• high concentrations of L-Glutamate are obtained from dietary proteins which are catabolized to the level of amino acids
• amino acids are used in amino transfer reactions that transfer the NH3 to alpha-ketogluterate to make L-glutamate
• The L-glutamine will be made from L-Glutamate by glutamine synthetase