Photosynthesis, The Nitrogen Cycle and the Nitrogenases

Question 1

What is photosynthesis and why does it happen?

Answer 1

Photosynthesis takes place inside chloroplasts and involves metalloproteins embedded inside a membrane system (thylakoids)
A proton gradient is set up across the membrane using an electron transport chain driving ATP synthesis

Question 2

What is the second main step of photosynthesis?

Answer 2

• At PS I the electrons gain energy and leads to production of NADPH from NADP⁺

Question 2

What is the first main step of photosynthesis?

Answer 2

• The absorption of light by PSII makes the reaction centre, P680 give up electrons + triggers splitting of water
• The excited electrons pass through an ETC which pumps electrons across membrane
• This proton gradient drives ATP synthesis

Question 3

As part of the PSII unit, there is a Mn₄Ca centre which…

Answer 3

… undertakes the reaction:
2H₂O → O₂ + 4H⁺ + 4e⁻
Electrons are used as part of the electron transport chain and promoted in energy

Question 4

Describe the structure of the Mn₄Ca centre relative to hard or soft ligands/acids

Answer 4

• Oxygen is a electron donor ligand which is bound to Mn
• Mn is in the Mn(III) and Mn(IV) oxidation states and is a hard acid
• O donor is a hard ligand

Question 5

The nitrogen cycle involves a series of processes, each of which is catalysed by one (or more) metalloenzyme
The process from NO₃⁻ to N₂ (denitrification) requires

Answer 5

The loss of energy (in the same way we put electrons onto oxygen)
Bacteria put their electrons from their electron transport pathway from nitrate to for dinitrogen

Question 6

The process from NO₃⁻ → NH₄⁺ is called nitrogen assimilation
What is the reverse process called?

Answer 6

Nitrification
Allows nitrogen to be returned to the soil

Question 7

Nitrogen assimilation is catalysed by…

Answer 7

Nitrate reductases
The removal of one O atom from NO₃⁻ is achieved at a Mo centre bound by two special ligands “molybdopterin”

Question 8

Nitrogen fixation is…

Answer 8

Converting N₂ → NH₄⁺
Can then be converted into ammonia
(catalysed by nitrogenases)

Question 9

We can do nitrification in real life as part of the Haber process
What is the difference between the Haber-Bosch and Biogenic process

Answer 9

• Industrial requires high temp and pressure just for a 17% conversion
• Biogenic process uses a FeMo catalyst
• (Nitrogen fixation accounts for ca. 50% of Earth’s supply of nitrogen)

Question 10

What does the following graph tell us about nitrification?

Answer 10

Associated with a high activation energy
Hence the nitrogenases for this process are special as they can cleave the strongest homopolar chemical bond at normal temperatures and pressures

Question 11

Nitrogenases can also reduce

Answer 11

• Acetylene and ethene
• and also produce dihydrogen gas
• (this process involves hydroloysis of 2x ATP for every electron = 16ATP total and hence very energy intensive)

Question 12

What are the key elements involved in the nitrogenase centre?

Answer 12

• Irons connected to 3 sulfurs
• All the irons connected to a central carbon
• FeMoco (homocitrate)
• However still may questions about the structure and mechanism