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MBB266 > Johnson (Photosynthesis) > Flashcards

Johnson (Photosynthesis) Flashcards

1
Q

How does photosynthesis power biosphere?

A
  • source of all food, O and most energy (≈88%)
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2
Q

How did photosynthesis change the world?

A

increase in atmospheric O2 conc allowed multicellular organisms to appear

  • spike in [O2] is coniferous period
  • many trees etc. died and prod fossil fuels present today
  • at this time plants also dev lignin in cell walls
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3
Q

Why is ps the basis of food chain?

A
  • virtually all life depends on it to provide energy in form of red C molecules
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4
Q

What are the types of photosynthetic organism, and eg.s?

A
  • euk oxygenic ps (chloroplasts) = eg. plants, mosses
  • prok oxygenic ps = eg. cyanobacteria (-)
  • prok anoxygenic ps = eg. purple bacteria (-)
  • archaeal ps = halobacteria
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5
Q

Where does photosynthesis take place in euks?

A
  • chloroplast thylakoid membrane
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6
Q

How is thylakoid membrane specialised for photosynthesis?

A
  • highly folded

- providing huge area for light absorption and e- transport

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7
Q

How does oxygenic photosynthesis occur in plants?

A
  • e- transport in thylakoid membrane
  • enzymatic machinery responsible for CO2 fixation located in stroma
  • main aldehyde product is glyceraldehyde-3-phosphate
  • light and ‘dark’ reactions
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8
Q

Are dark reactions really in the dark?

A
  • no, only occur in light

- ie, don’t cont if remove light after ATP and NADPH formed

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9
Q

What is the photosynthetic e- transfer (PET) chain?

A
  • 2 light driven reactions in chlorophyll-protein complexes PSII and PSI
  • result in e- transfer via chain of acceptors from water to NADP+, w/ O formed as by product
  • e- transfer coupled to pmf formation for ATP synthesis
  • none of complexes pump protons, all translocate
  • NADPH/NADP+ has v -ve redox pot and H2O/O2 v +ve
  • H+ released into lumen
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10
Q

What does it mean to say complexes translocate protons instead of pumping them?

A
  • get net redistribution by performing reactions on both sides of membrane
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11
Q

What is the function of photosystems?

A
  • carry out light dep e- transfer
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12
Q

What is the structure of photosystems?

A

2 parts:

  • reaction centre = where photochemical redox reactions take place
  • light harvesting antenna system = responsible for light absorption and transfer of captured light energy to reaction centre
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13
Q

What is the key light absorbing pigment molecule in both structures of photosystems, and how is it bound?

A
  • chlorophyll

- non covalently bound to these proteins

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14
Q

What is the basic structure of photosystems?

A
  • antenna complex formed of 100s of chlorophylls

- transfer absorbed light energy to special pair chlorophylls of reaction centre that are redox active

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15
Q

What do antenna chlorophylls transfer?

A
  • energy, NOT e-s
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16
Q

What 2 parts is chlorophyll formed of?

A
  • tetrapyrrole ring = similar to haem, but coords Mg2+

- hydrophobic phytyl tail region

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17
Q

What is the conjugated π e- system in tetrapyrrole ring of chlorophyll responsible for?

A
  • light absorption

- when chlorophyll absorbs light, e- in this region promoted to higher energy level

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18
Q

What occurs in the reaction centre chlorophyll molecules, and how does this vary between PSI and PSII?

A
  • 1° donor oxidised upon excitation
    • -> P680 for PSII
    • -> P700 for PSI
  • e- transferred to acceptor, which is red
    • -> lipid soluble plastoquinone for PSII
    • -> soluble protein ferredoxin for PSI
  • 1°donor re-red by 2°donor
    • -> H2O for PSII
    • -> plastocyanin for PSI
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19
Q

What is the redox ‘Z-scheme’ for photosynthesis?

A
  • light energy used by reaction centres to drive +ΔG reactions that transfer e-s from donor w/ +ve redox pot (water) to acceptor w/ more -ve redox pot (NADP+)
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20
Q

What occurs in PSII?

A
  • uses light energy to transfer e-s from special pair P680 to lipid soluble PQ
  • P680+ drives splitting of water into e-, H+ and O2 (photolysis) by Mn cluster attached to PSII
  • protons released into lumen, while 2e-s used to red P680+ to P680
  • once red, PQ binds 2H+ from stroma side of membrane
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21
Q

What is the overall reaction in PSII?

A
  • H2O + PQ + 2H+stroma –> 1/O2 + PQH2 + 2H+lumen
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22
Q

What occurs in cytochrome b6f?

A
  • similar to complex III in mito

- carries out Q cycle

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23
Q

What is the Q cycle?

A
  • complex series of reactions that ox PQ and transfer e- to plastocyanin, a small soluble e- transfer protein located on lumen side of thylakoid membrane
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24
Q

What is the overall Q cycle reaction?

A
  • PQH2 + 2PCox + 2H+stroma –> 2PCred + 4H+lumen
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25
Q

What occurs in PSI?

A
  • uses light energy to transfer e-s from special pair P700 to ferredoxin
  • P700+ drives ox of plastocyanin (PC) on lumen side of membrane regen P700
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26
Q

What is ferredoxin?

A
  • small soluble protein on stroma side of thylakoid membrane
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27
Q

What is the overall reaction is PSI?

A
  • PCred + Fdox —-light—> PCox + Fdred
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28
Q

What is plastocyanin?

A
  • small soluble e- transfer protein
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29
Q

What is the role of plastocyanin?

A
  • Cu ion bound at active site coord by several His residues and acts as e- carrier
  • Cu ox from Cu+ –> Cu2+ by PSI and red back to cytochrome b6f
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30
Q

What is the role of ferredoxin?

A
  • binds 2Fe-2S cluster at active site

- bound by several Cys residues that act as e- carrier

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31
Q

What occurs in ferredoxin-NADP+ reductase?

A
  • contains FAD cofactor which sequentially ox 2 molecules of Fd
  • then uses e-s to red NADP+ –> NADPH, w/ H+ taken up from stroma
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32
Q

What is the overall reaction in ferredoxin-NADP+ reductase?

A
  • 2Fdred + NADP + H+stroma –> 2Fdox + NADPH
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33
Q

Are the structures of chloro and mito ATPases similar?

A
  • v similar
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34
Q

What is the difference between chloro and mito ATPases?

A
  • stoichiometry of H+ of ATP much higher in chloro

- 14 c subunits, instead of 8

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35
Q

What is the overall reaction in CF1CF0 ATPase?

A
  • 14H+lumen + 3Pi + 3ADP –> 14H+stroma + 3ATP + 3H2O
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36
Q

What is the meaning of Jablonski diagrams?

A
  • molecules only absorb photons w/ energy equal to energy gap between e- orbitals
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37
Q

What are the diff excitation states in Jablonski diagrams?

A
  • S0 = ground state
  • S1 = 1st excited state
  • S2 = 2nd excited state
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38
Q

How do red and blue photons vary in terms of Jablonski diagrams?

A
  • red photons match S0 -> S1 energy gap
  • blue photons match S0 -> S2 energy gap
  • red are higher energy photons w/ lower wavelength
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39
Q

What timescale does light absorption occur on?

A
  • femtosecond (10^-15)
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40
Q

What do electronic, vibrational and rotational energy levels show?

A
  • some energy levels more favourable
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41
Q

Does photosynthesis need green light, and why?

A
  • yes, underneath leaf

- as chloroplasts filter out all blue and red light, so green req

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42
Q

What is the fate of S2 excited state, and timescale?

A
  • e- v rapidly loses part of absorbed energy as heat internal conversion
  • falls to S1 state
  • picosecond timescale (10^-12)
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43
Q

What are fates of S1 excited state, and timescales?

A
  • slower, nanosecond (10^-9), process as closer to nucleus so lower energy and excited state more stable
  • internal conversion to S0 slow enough that fluorescence can compete as alt channel of de-excitation
  • if another chlorophyll in close prox then FRET
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44
Q

How does energy transfer occur between chlorophylls, and when can this happen?

A
  • FRET
  • excited energy transferred from excited donor chlorophyll to acceptor chlorophyll in ground state
  • can occur when 2 chlorophylls in close prox and have overlapping excited state energy levels
  • keep flipping around and energy cascades between them
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45
Q

How is FRET distance dep?

A
  • efficiency varies w/ 6th power of distance
  • ie, if distance between donor and acceptor doubles, FRET transfer time increases 64x
  • ∴ only efficient over short distances
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46
Q

Why are antenna needed?

A
  • increase reaction centre rate by 2 orders of magnitude

- acts to capture and concentrate light energy

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47
Q

What features should antenna have?

A
  • high as poss pigment conc
  • wide spectral cross section (as many colours as poss)
  • wide spatial cross section
  • modularity build up in low light, red in high light (don’t need as many antenna complexes in high light as more excitation)
  • provide directionality to energy transfer
  • min losses of excitation energy to heat and fluorescence, and prevent e- transfer
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48
Q

Why is there a pigment variety in plants?

A
  • variation in length of conjugated π e- system affects wavelength of light absorbed by each pigment
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49
Q

What is the pigment absorption spectra?

A
  • combo of multiple pigment types in antenna broadens spectral cross section of light energy that is absorbed and transferred to RC chlorophylls
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50
Q

What pigments are bound to antenna proteins, and how?

A
  • antenna proteins non covalently bind pigments at v high conc to ensure efficient light absorption
  • LHCII 1 of most abundant membrane proteins
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51
Q

What is the antenna structure of PSII?

A
  • multiple antenna proteins provide large spatial cross section for light absorption
  • ≈ 157 chlorophylls/RC
  • PSII forms dimeric supercomplex = 1x LHCII trimer per RC, 2x PSII, 2x LHCII monomers per RC and O evolving complex (catalyst)
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52
Q

What is the antenna complex of PSI?

A
  • generally don’t have dimers
  • multiple antenna proteins provide large spatial cross section for light absorption
  • ≈155 chlorophylls/RC
  • PSI RC and 4 x LHCI monomers per RC
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53
Q

What makes the structure of antenna modular?

A
  • can build up or down
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54
Q

What is the modular structure of antenna?

A
  • leaves –> thylakoid membranes –> solubilisation of membranes in detergent –> separation of sucrose grad ultracentrifugation
  • [LHCII] increases in low light and decreases in high light
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55
Q

What makes each pigment binding site of LHCII unique?

A
  • env of each pigment affects π e- system

- so pigments excited state properties inc energy, spectra and excited state lifetime

56
Q

What does excited state lifetime mean?

A
  • how long excited state lasts before returning to ground state by photon emission
57
Q

What is the effect of binding site heterogeneity in LHCII?

A
  • range of binding site energies further broadens spectral cross section and creates directionality in energy flow
58
Q

How does the range of site energies provide directionality?

A
  • chlorophylls closer to RC have excited state at lower energies than those further out in antenna
  • so excitation energy cascades downhill towards RC by FRET
  • where its trapped as e- transfer reactions
59
Q

How is FRET demands balanced w/ ET for stopping LHCs becoming RCs?

A
  • antenna proteins like LHCII evolved to keep far enough apart to avoid raped e- transfer, but close enough to allow FRET
  • RCs behave as 1 pigment –> req overlap of mol wavelengths
60
Q

How do you calc energy light contains?

A
  • ΔG = (Nhc) / λ
  • N = avogadro’s constant
  • h = constant
  • c = speed of light in vacuum
  • λ = wavelength of light
61
Q

PSII is the only biological enzyme known capable of doing what?

A
  • ox H2O to O2
62
Q

How does PSII prevent e-s going astray?

A
  • e- transfer efficiency decays exponentially w/ distance
  • RCs evolved to allow FRET from antenna to RC, but no e- transfer from RC to antenna
  • gap in middle of protein separates RC from bulk reaction pigments
  • isolating RC done by all PS
  • don’t want so close that e- could go back to antenna complex and cause damage in oxidative reactions
  • but close enough to transfer excitation energy = few nms
63
Q

Why are e-s lost from tetrapyrrole ring in RC chlorophyll?

A
  • upon excitation e- lost rather than from Mg2+, which is redox active
64
Q

What is the result of e-s being lost from tetrapyrrole ring?

A
  • resulting hole/e- delocalised over ring
  • this is key diff from haem cofactors found in cytochromes where Fe2+ redox active instead
  • in chlorophyll Mg2+ tunes absorption spectrum of molecule and provides coord site for interaction w/ protein
65
Q

What provides the free energy input for PSII reaction?

A
  • 4x 680nm photons
66
Q

What are the 2 branches in PSII RC, and in which does e- transfer occur?

A
  • a and b

- e- transfer only down a, b is ‘ghost’ branch

67
Q

Why does PSII want to separate e-s spatially and energetically?

A
  • prevent recombination
68
Q

What happens if e- recombines in PSII redox scheme?

A
  • energy lost as heat
69
Q

What happens in the redox scheme of PSII?

A
  • to achieve photolysis redox pot of over +820mV req
  • P680+/P680 is high enough to act as oxidants of H2O/O2
  • P680 left w/ really +ve charge so can rip e- out of anything
  • rips e- out of Tyr as closest residue
  • e- ripped out of water bound to Mn cluster by Tyr
70
Q

What is the result of photochemical reaction of PSII RC?

A
  • 2e-s end up on QB
  • QB binds 2H+ from stroma and leaves binding site as QH2
  • hole does work and is transferred in direction of water and Mn cluster
  • e-s transferred to plastoquinone to make plastoquinol (2e- carrier so red twice)
  • 2 charge separations driven by 2 photons
  • need 4 turnovers to make 1 O molecule
71
Q

What is pheophytin?

A
  • chlorophyll molecule w/ 2H+ instead of Mg2+ at centre
72
Q

What are the starting and final reactions in cascade of photochemical reactions of PSII RC?

A
  • P680 excited (P680*) and cascades along series

- e- transferred until Y2+ receives from Mn cluster and cycle repeats

73
Q

What is plastoquinone, and what is its role?

A
  • lipid soluble e- carrier
  • 2e-s used to red 2 C=O C atoms from +2 to +1, forming 2 OH groups
  • 2 H+ req taken up from stroma, once PQH2 formed at QB site its exchanged for PQ from membrane pool
74
Q

How is charge separation stabilised (PSI and PSII)?

A
  • chain of closely packed e- acceptors energetically ‘downhill’ from P680* ensures e- and hole rapidly separated
  • reverse reactions uphill so slow
  • 60% loss of energy necessary price of stabilising charge separation
75
Q

What is the ‘hole’?

A
  • +ve charge
76
Q

How is water oxidation catalysed by Mn cluster?

A
  • PSII binds Mn cluster

- P680+ provides thermodynamic driving force for water ox

77
Q

What is the structure of Mn cluster?

A
  • 4 Mn ions and Ca2+ bridged together by O atoms

- 2 water molecules bound

78
Q

Why can Mn cluster give lots of e-s away?

A
  • can exist in multiple ox states from +2 to +5
79
Q

What occurs during the water oxidation (S state cycle)?

A
  • 4 light driven turnovers of P680 drive evo of 1 O2
  • Mn ions progressively oxidised to provide e-s, released from cluster and used to red P680+
  • e-s restored by water in final step that sees O=O bond formation, returning catalyst to original state
  • protons released deposited into lumen and contribute to pmf
80
Q

How does cytochrome b6f differ from cytochrome bc1 in mito?

A
  • v similar

- haem c replaced by haem f

81
Q

How is no. of H+ translocated by PSI doubled?

A
  • recycling 1 of 2 e-s from each plastoquinone via low pot chain
82
Q

What does PSI function as?

A
  • light dep plastocyanin (lumen) - ferredoxin (stroma) oxidoreductase
83
Q

What is the antenna structure of PSI?

A
  • similar to PSII
  • multiple antenna proteins provide large spatial cross section for light absorption
  • ≈155 chlorophylls/RC
  • PSI RC and 4x LHCI monomers per RC
84
Q

What is ox and red in PSI reaction?

A
  • ox plastocyanin
  • red ferredoxin
  • 1 e- transfer
85
Q

What are the pigments of PSI, and their roles?

A
  • binds several key cofactors which take part in e- transfer reactions
  • a and b branches active
  • unlike PSII quinones tightly bound and don’t swim away after reaction
  • no pheophytin
  • e-s passed between FeS clusters, then to Fd
  • like PSII separate e- and hole spatially and energetically
86
Q

What is role of Fd?

A
  • powerful reductant

- can red NADP+ to NADPH and NO3- to NH4+

87
Q

What are the starting and final reactions in cascade of photochemical reactions of PSII RC?

A
  • P700 excited (P700*)

- P700+ re-red by e- from plastocyanin and cycle begins again

88
Q

How do no. e- gates vary between PSI and PSII?

A
  • PSI has 1

- PSII has 2

89
Q

Why is b branch switched off in PSII?

A
  • disfavoured by small energy gap between P680 and ChIB and large gap between QB and PheoB
  • simultaneously prevents charge separation in b branch and back reaction of P680+QB- via PheoB
90
Q

What is the result of deactivating b branch?

A
  • creates 2 e- gate
  • makes sure both e-s end up on QB and not 1 on QA
  • ensuring always quinone to accept e-s (avoiding energy losses)
91
Q

Why does photosynthesis work at all?

A
  • in PSI and PSII most energetically favourable reaction appears to be direct recombination of 1° and 2° radical pairs
92
Q

What is the Marcus theory?

A
  • DIAGRAM*
  • explains rates of ET reactions where participants don’t undergo large structural changes
  • prediction is ‘inverted reaction’ where large driving forces between redox couples drastically slows ET rates
  • direct recombination in this region, so rate v slow
  • when e- transferred molecules around e- donor/acceptor have to move to accom change or charge = ‘reorganisation energy’
  • ET rate optimal when driving force ΔG = reorganisation energy
93
Q

What does the Calvin cycle use ATP and NADH for?

A
  • to convert CO2 into carbs

- regen ADP, Pi and NADP+

94
Q

What are the 3 main parts of Calvin cycle?

A
  • carboxylation
  • reduction
  • regeneration
95
Q

What does Calvin cycle use/prod for every complete turn?

A
  • 3 CO2 red
  • using 9 ATP and 6 NADPH
  • net output 1 GAP
96
Q

What are the consequences of the Calvin cycle being so similar to Krebs cycle?

A
  • KC would run in reverse if had enough CO2

- some bacteria use this instead of Calvin cycle

97
Q

Why is rubisco not that great?

A
  • slow
  • relatively low affinity for CO2
  • so v high concs needed to match pot supply of ATP and NADPH
98
Q

How many subunits does rubisco have, and what are there functions?

A
  • 8 large catalytic subunits

- 8 small regulatory subunits

99
Q

What happens during carboxylation in CC?

A
  • ribulose-1,5-bisphosphate –> enediolate intermediate +CO2 ——rubisco—->unstable intermediate +H2O –> 2 3-phosphoglycerate
  • H+ prod in 1st step
100
Q

Is carboxylation in CC energetically favourable?

A
  • yes
101
Q

What happens during reduction in CC?

A
  • 3-PGA + ATP –> 1,3-bisphosphoglycerate
    cat by phosphoglycerate kinase
  • 1,3-bisphosphoglycerate + NADPH –> GAP + NADP+ + Pi
102
Q

How does reduction in CC relate to glycolysis?

A
  • reverse of steps 6 and 7 of glycolysis
103
Q

What is major fate of GAP?

A
  • sucrose synthesis in cyto
104
Q

What is the result of the fact that both reactions of reduction stage run quite close to equilibrium?

A
  • small changes in ATP/ADP able to push reaction in certain direction
  • -> sensitive to mass action effects
105
Q

What happens during regeneration in CC?

A
  • ribulose 5-phosphate + ATP –> ribulose 1,5-bisphosphate + ADP
    –> cat by phosphoribulose kinase
  • for every 3x 5C RuBP and 3x CO3, 6v GAP formed
  • 5/6 req to regen RuBP
  • involves complex series of reactions that form 3x RuBP:
    3C + 3C –> 6C
    6C and 3C –> 4C + 5C
    4C + 3C –> 7C
    7C + 3C –> 5C + 5C
106
Q

Why is GAP important in plants?

A
  • starting point for multiple metabolic pathways that lead to AA, lipid and nt synthesis
107
Q

How do plants sustain metabolism at night, in roots etc.?

A
  • mito as well as chloro which can ox sugar molecules to prod ATP
108
Q

What is the role of phloem and xylem?

A
  • phloem transports water

- xylem transports sucrose

109
Q

What makes sucrose more stable?

A
  • bond making it disaccharide of glucose and fructose
110
Q

What is the fate of 1 GAP not used in regeneration in CC?

A
  • exported into chloro in exchange for Pi from cyto
  • using translocator protein in chloroplast IM
  • converted into 2 types of 6C sugars = glucose-1-phosphate and fructose-6-phosphate
  • alt GAP can be converted to glucose-1-phosphate in stroma, then polymerised into starch for storage
111
Q

How are light and dark reactions linked?

A
  • many enzymes of CC also involved in glycolysis or PPP –> so must be carefully reg to avoid futile cycling
  • reg achieved by light reactions, mod env of dark reactions
  • pmf formation increases pH and [Mg2+] in stroma
112
Q

Why is changing concs when running out of light not an option?

A
  • plants can’t store light

- reactions can start to run in reverse

113
Q

What is the role of thioredoxin?

A
  • regulatory protein
  • senses change in redox state of stroma, caused by red Fd and NADP+
  • reg activity of several CC enzymes, ensuring activity of light and dark reactions closely coord
114
Q

How is rubisco reg by light and dark reactions?

A
  • active site contains Lys, reacts w/ CO2 to form carbamate anion, then able to bind Mg2+
  • both Mg2+ and alkaline conditions req for carbamate formation provided by light reactions
  • Mg2+ activates RuBP so readily reacts w/ CO2
115
Q

What is essential for catalytic function of rubisco?

A
  • Mg2+
116
Q

What happens if chloros incubated in low pH medium, then rapidly transferred to high pH medium, and what is this proof of?

A
  • ATP rapidly formed when ADP+Pi added, even in absence of light
  • Mitchells’ chemiosmotic theory
117
Q

What is virtually all pmf stored as in chloros?

A
  • ΔpH
118
Q

How can Δp and ΔμH+ be interconverted using Faraday’s constant?

A
  • ΔμH+ = -FΔp

- Δp = -ΔμH+ / F

119
Q

How is Δψ dissipated by counterion movements?

A
  • counterions move via VG cation and anion channels in thylakoid membranes
  • allows ΔpH to build up
120
Q

Why doesn’t ΔpH really affect chloros?

A
  • lumen doesn’t contain many enzymes
121
Q

Can no. c-subunits vary in CF1CF0 ATPase?

A
  • varies from structure resolved by atomic force microscopy
122
Q

How does c-ring size vary in CF1CF0 ATPase?

A
  • larger
123
Q

How does Δp relate to ΔGp?

A
  • larger the Δp, the fewer mol H+ spent per mol ATP, so fewer c-subunits req per ATPase
  • smaller pmf req larger force multiplier
124
Q

What ratio of ATP:NADPH is req by CC?

A
  • 1.5ATP:NADPH
125
Q

How many ATPs formed per NADPH in photosynthesis, and why is this an imbalance?

A
  • 1.28
  • CC req 1.5 ATP per NADPH
  • imbalance must be corrected for efficient functioning of ps
  • energy balance attained by 2nd type of e- transport taking place in chloroplasts, cyclic e- transport, that prod only ATP
126
Q

What is the role of cyclic e- transport (CET) chain in plants?

A
  • gen ATP, but no NADPH
  • so rebalances ATP/NADPH ratio
  • protein PGRL1 acts as ferredoxin-plastoquinone oxidoreductase
  • alt pathway of CET may involve chloro NADPH-plastoquinone oxidoreductase (NDH-1), similar to mito complex I
127
Q

How does membrane folding divide CET form LET?

A
  • thylakoid membranes divided roughly 80:20 between grana and stroma lamellae thylakoids
  • reflects division of labour between CET and LET req to achieve 1.5 ATP/NDPH ratio
128
Q

What is lateral heterogeneity?

A
  • PSII-LHCII residues almost entirely in grana
  • PSI-LHCI in 2 pops
    • -> 1 in margins of grana that form LET domain w/ cytochrome b6f
    • -> 2nd in stromal lamellae w/ cytochrome b6f, forming CET domain
129
Q

What is photorespiration?

A
  • rubisco can cat wasteful competition reaction between RuBP and O2
  • phosphoglycerate molecule prod is metabolic ‘dead-end’, must be converted to CO2
  • at 25° rate of carboxylation 4x that of oxygenation (decreases at lower temps)
  • photoresp raises req ATP:NADPH ratio
130
Q

How do C4 plants avoid photoresp?

A
  • evolved w/ energy dep CO2 concentrating method = C4 pathway
  • assoc w/ special leaf anatomy = Kranz anatomy
131
Q

What are most plants we eat?

A
  • C3
132
Q

Where is photoresp a major cause of inefficient photosynthesis?

A
  • warmer climates
133
Q

How do C3 and C4 plants vary?

A
  • C4 have bundle sheath of cells surrounding central vein w/in leaf, which in turn are surrounded by mesophyll cells
134
Q

What occurs during the C4 pathway?

A
  • mesophyll cells fix CO2 into malate
  • malate exported to bundle sheath cells
  • decarboxylated to pyruvate
  • regen NADPH and CO2
  • mesophyll cells don’t have enzymatic machinery of CC
  • high CO2 conc resulting in bundle sheath allows rubisco to min photoresp
135
Q

How does ATP req for C4 pathway vary from C3, and where does extra ATP come from?

A
  • 15 ATP req for each GAP synthesised (9 in C3)
  • extra ATP prod by CET
  • C4 costs more but C3 not efficient in tropics

MBB266 (7 decks)

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