Phototrophy

Question 1

Photolithoautotrophy
Quick summary of what light and dark reactions make.

Answer 1

• “light reactions” generate Δp (thus ATP) and NADPH.
• “dark reactions” assimilate CO2 into biomass consuming ATP and NADPH.
• Eukarya assimilate CO2 using the Calvin-Benson-Bassham (CBB) cycle. Use water (H2O) as their electron donor and thus produce molecular oxygen (O2). Obligate aerobes.

Question 2

What assimilates CO2 using the Arnon-Buchanon cycle and uses hydrogen sulfide (H2S) as their electron donor?

Answer 2

green sulfur bacteria

Question 3

Purple sulfur bacteria (the Chromatiales)

Answer 3

They assimilate their CO2 using the CBB cycle and use H2S as their electron donor and thus produce S8/SO. Obligate anaerobes.

Question 4

Calvin-Benson-Bassham cycle
What is RuBisCO and what does it catalyse. Give main products

Answer 4

• CO2 assimilation into biomolecules occurs at ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO, EC 4.1.1.39 – has at least 9 isoenzymes across all 3 Domains of Life) which catalyses:
  D-ribulose 1,5-bisphosphate + CO2 + H2O → 2 × 3-phospho-D-glycerate + 2H+
• Next 2 steps basically convert 3-phospho-D-glycerate into D-glyceraldehyde 3-phosphate
• 1/6th of D-glyceraldehyde 3-phosphate (GAP) produced (rest is used up in the cycle) is extruded from the plastid into cytoplasm and is used to make hexose sugars which are fed into glycolytic pathways and Krebs’ cycle to make building blocks for amino acidbiosynthesis.

Question 5

What happens when RuBisCO reacts with oxygen?
How do cyanobacteria prevent this?
How do streptophyta of viridiplantae prevent this?
How do Bryophyta prevent this?

Answer 5

D-ribulose 1,5-bisphosphate + O2 → 3-phospho-D-glycerate +
2-phospho-D-glycolate + 2H+ (only one molecule)
* This means autotrophs can waste half of RuBP given 2-phospho-D-glycolate can’t continue in the cycle
(2C lost) and no CO2 was assimilated (net 3C lost)!
* Several carbon concentrating mechanisms (CCMs) have evolved e.g.
* Many “Cyanobacteria” (and chemolithoautotrophic Bacteria) contain carboxysomes (protein ‘houses’ for RuBisCO and carbonic anhydrase) to keep pCO2 high around RuBisCO and to exclude oxygen.

• The Streptophyta of the Viridiplantae use the Hatch-Slack pathway.
• Some Bryophyta (viz. Anthocerotophyta) and Chlorophyta of the Viridiplantae use pyrenoids to concentrate CO2
  similarly to carboxysomes in general principles but not in structure or function.

Question 6

Light reactions in chloroplasts overview

Answer 6

• ATP is made by ATP synthase
• Δp is maintained inside of the thylakoid lumen and not in a space between two membranes per respiration.
• two types of light reaction happen:
• non-cylic photophosphorylation, involving photosystems II and I.
• cyclic photophosphorylation, involving photosystem I.

Question 7

Explain photosystem I

Answer 7

• photosystem I (PSI) is a large protein complex containing antenna complexes that harvest photons using Chl a and carotenes; a P700
  reaction centre containing P700 (modified Chl a, λmax = 700 nm – sometimes P700 is called Chl a′) and phylloquinone (vitamin K1).

Question 8

Explain photosystem II

Answer 8

• photosystem II (PSII) has antenna complexes that harvest photons using Chl a, Chl b, carotenes with addition of pheophytin as an accessory pigment.
  Reaction centre is P680 (modified Chl a-dimer). A manganese centre catalyses the oxidation of water (electron donor).

Question 9

Quinone pool

Answer 9

• quinone pool functions just like in respiration but comprises
  plastoquinone [(PQ) carries two e-] which is reduced to plastoquinol (PQH2).
• in place of cytochrome c from respiration is the functionally similar plastocyanin (carries 1 e- each), a blue copper protein and water soluble electron carrier.

Question 10

What is in the photosynthetic electron transport chain?

Answer 10

photosystem II
b6f complex plastocyanin
Quinone pool
photosystem I
ferredoxin
ferredoxin-NADP+ reductase

Question 11

Explain the process of non-cyclic phosphorylation. Electrons down the chain

Answer 11

1) Light hits photosystem II ( water:plastoquinone reductase of wavelength 680nm) as that happens some electrons leave water on the manganese Centre producing oxygen gas and some protons in the thylakoid lumen and those electrons are deposited into the quinone pool. Energy from light pushes electrons off.
2) quinone pool donates its electrons to the b6f complex which translocates 4 protons across
3) electrons carry on to plastocyanin and they then travel to photosystem I
4) if light of 700 nm hits reaction center these electron can go to ferodoxin
5) if its noncyclic these electrons go to ferredoxin-NADP+ reductase and turn 2NADP+ into 2NADPH
*This is then to fuel the calvin cycle

Question 12

Explain the process of cyclic phosphorylation. Electrons down the chain

Answer 12

• Light hits PSI aka plastocyanin: ferredoxin reductase catalyses
  the reduction of ferredoxin (another soluble electron carrier):
• ferredoxin donates electrons to the quinone pool:
• In the “b6f complex” plastoquinol: plastocyanin reductase donates the electrons to plastocyanin then back to the reaction center.

Question 13

where are phylloquinone and plastoquinone located in chloroplasts?

Answer 13

phylloquinone is found in photosystem I, which in turn is located in the thylakoid membrane, whereas plastoquinone is directly dissolved in the membrane.