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MBB266 > Photosynthesis > Flashcards

Photosynthesis Flashcards

1
Q

Humans currently use 15TW of energy per year. How much energy does the sun deliver to the planet per year?

A

100,000 TW.

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2
Q

How much energy do chlorophyll use annually for photosynthesis?

A

100 TW.

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3
Q

How many tonnes of CO2 does photosynthesis take in?

A

200 billion tonnes.

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4
Q

What are the three biggest problems humankind face according to the UN?

A
  1. Not enough food.
  2. Not enough energy.
  3. Less CO2.
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5
Q

What are the three types of photosynthetic organisms?

A
  1. Eukaryotic oxygenic photosynthesis.
  2. Prokaryotic oxygenic photosynthesis.
  3. Prokaryotic an oxygenic photosynthesis.
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6
Q

Eukaryotic oxygenic photosynthesis examples…

A

Plants, moses red/ green/ brown algae.

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7
Q

Prokaryotic oxygenic photosynthesis example…

A

Cyanobacteria.

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8
Q

Prokaryotic oxygenic photosynthesis examples….

A

Purple sulphur/ non sulphur bacteria, Green sulphur bacteria, Green gliding bacteria.

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9
Q

What do prokaryotic anoxygenic organisms use in photosynthesis ?

A

Cyclic sulphur.

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10
Q

Where does photosynthetic electron transport occur?

A

Thylakoid membrane.

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11
Q

Where is the enzyme machinery responsible for CO2 fixation found?

A

Stroma.

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12
Q

What stage of photosynthesis are the light reactions involved in?

A

1.

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13
Q

What lipid soluble electron carrier is involved in photosynthesis?

A

Plastoquinone.

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14
Q

What is the Q cycle equivalent in photosynthesis?

A

Cytochrome B6-f.

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15
Q

Where does E’ come from in photosynthesis?

A

Covering water to NADPH.

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16
Q

What makes up the photostem?

A

Antenna complex and the reaction centre.

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17
Q

What part of the photostem is involved in light harvesting?

A

Antenna complexes.

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18
Q

Chlorophyll contains a _______ ring which coordinates a ____ ion. The structure contains a ______ tail.

A

Tetrapyrolle, Mg2+, phytyl.

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19
Q

What in chlorophyll is reusable for light absorption?

A

Conjungetd pie electron system if the tetrapyrolle ring which gets promoted to a higher level when light is absorbed.

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20
Q

When will molecules absorb photons?

A

When the energy is equal to the gaps between the electron orbitals/ electron state.

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21
Q

What is the excited electron state also known as?

A

S1/ Lumo.

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22
Q

What is the ground electron state also known as?

A

S0/ Humo.

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23
Q

Absorption of what wavelength photon promotes an electron from the ground to the excited state?

A

656nm.

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24
Q

What does each electron state of chlorophyll contain?

A

Multiple vibration sub levels.

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25
Q

What colour photon matches the energy gap between S0 and S1?

A

Red.

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26
Q

What colour photon matches the energy gap between S1 and S2?

A

Blue.

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27
Q

Each vibrational sub level of the excited states have slightly different energies. What does this mean?

A

They are all slightly different colours and different photons excite them.

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28
Q

What do photons with a longer wave lengths have?

A

Lower energy.

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29
Q

Why is absorption stronger at some wavelengths than others?

A

Each wavelength has a slightly different probability.

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30
Q

What happens instantaneously to an electron when it is promoted to the S2 excited state?

A

It very rapidly looses some of the absorbed energy thought vibration relaxation. The electron will fall to the lowest sub level of the electronic state.

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31
Q

What is internal conversion?

A

When an electron falls to the next lowest excited state, e.g. S2- S1.

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32
Q

How fast is vibrational relaxation and internal conversion?

A

10-12s.

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33
Q

Between what states is internal conversion slower and why?

A

Between S1 and S0 internal conversion is slower (10-9) because it is closer to the nucleus making it more stable.

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34
Q

Why can fluorescence (emission of a photon) compete with internal conversion between S1 and S0?

A

Because it is slower.

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35
Q

Where does fluorescence always occur from?

A

The lowest vibrational sub level of the S1 state.

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36
Q

What do the antenna complexes transfer light energy as and how?

A

Excitation energy by FRET.

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37
Q

Where does excitation always occur?

A

At the lowest part of the ground state.

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38
Q

Internal conversion looses 40% of the energy as heat from the blue photon. Why?

A

Allows the shape to be maintained.

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39
Q

How much energy absorbed from a blue photon is lost via internal conversion as heat?

A

40%.

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40
Q

Why do photons emitted as fluorescence have a lower energy?

A

They are emitted from the first excited state. This prevents too much energy being lost.

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41
Q

Why does FRET occur faster than fluorescence?

A

To ensure that it occurs first.

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42
Q

What sort of mechanism does FRET use?

A

Dipole-dipole.

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43
Q

What does FRET stand for?

A

Forster Resonance Energy Trasfer.

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44
Q

When can FRET occur?

A

When two chlorophylls are close to each other, with excited states that overlap.

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45
Q

What is transferred in FRET

A

ENERGY not electrons.

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46
Q

When is FRET very efficient?

A

When chlorophylls are 5nm apart or less.

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47
Q

Every time the distance doubles how many times longer does FRET take?

A

64 (Sixth power of distance).

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48
Q

Over what distance will FRET not occur due to the fact that it might not be quick enough to outcompete fluorescence ?

A

7nm.

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49
Q

Why are the antennas needed in the photostems?

A

To capture and concentrate energy.

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50
Q

By how many orders of magnitude do the antennas increase RC excitation by?

A

2.

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51
Q

It makes more sense for a plant to make lots of chlorophyll instead of RC molecules. Why?

A

Maximises light absorption in the shade and less energetically expensive to make.

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52
Q

What are 7 features of the antenna?

A
  1. High pigment concentration.
  2. Wide spectral cross section.
  3. Modularity.
  4. Provides directionality for electron transfer.
  5. Minimises loss of energy through fluoresce.
  6. Prevents electron transfer.
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53
Q

Why is it important that the antenna complex is modular?

A

Allows the antenna to be built up when the light concentration is low.

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54
Q

What are the three main types of pigments are found in plants?

A
  1. Chlorophyll A.
  2. Chlorophyll B.
  3. Carotenoids.
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55
Q

What are the 5 types of carotenoids?

A
  1. B-carotene.
  2. Zeaxanthin.
  3. Lutein.
  4. Vicolaxanthin.
  5. Neoaxanthin.
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56
Q

What carotenoids is the only one to be found in the RC and antenna?

A

B-carotene.

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57
Q

What varies between the pigments?

A

The length of the conjuncted pie electron systems. This effects the wavelength of the light absorbed.

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58
Q

What broadens the spectral cross section of light energy absorbed and transferred to the reaction centre?

A

The combination of multiple pigment types in the antenna.

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59
Q

What is the most abundant pigment bound to the atenaa proteins?

A

LHCII.

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60
Q

What does the LHCII contain?

A

4 carotenoids, 6 Chlorophyll B, 8 chlorophyll A.

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61
Q

What type of complex does PS2 form?

A

A dimeric super complex made of 157 chlorophylls/RC.

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62
Q

What concentration do antenna proteins bind chlorophyll molecules at to maximise light absorption?

A

0.25M.

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63
Q

How many chlorophylls/RC does PS1 contain?

A

197.

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64
Q

Describe the modular structure of the antenna was determined.

A

Leaves were grinded with osmotic shock/ centrifugation. The thylakoid membranes were then solubilised with a detergent and the complexes were separated with a sucrose gradient.

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65
Q

In a low light what is the concentration of the LHCII complex?

A

High.

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66
Q

What does the primary structure of the protein in the antenna determine in regards to the pigment?

A

The binding, orientation and the environment. This can affect the pigments spectral and excitation state properties.

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67
Q

How does chlorophyll bind to the antenna?

A
  1. Polar ester carbonyl and kept groups can H bond to the proteins side chains.
  2. Mg2+ can form coordinate bonds with unpaid electrons i N, S and O in side chains such as histidine.
  3. The hydrophobic phytyl tail provides a large surface area for Van De Walls interactions with the protein.
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68
Q

The pigment binding sites in LHCII are all similar. True or false?

A

False they are all unique,

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69
Q

What does the environment that each pigment is in effect?

A

The pi electron system and with it the pigments excited state properties and with it its energy spectra and the lifetime of the excited state.

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70
Q

What does binding site heterogeneity allow in LHCII?

A

A range of binding site energies broadening the spacial cross section. This creates directionality in the energy flow.

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71
Q

Why is back transfer in FRET disfavoured?

A

It requires a positive delta G. It also needs heat energy of the environment and the probability decreases exponentially with the energy gap.

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72
Q

Why have antenna proteins evolved to keep chlorophyll molecules at a certain distance apart?

A

Need to be close enough for FRET but far enough apart to prevent electron transfer (electron transfer requires overlap of wave functions. Dipole dipole interactions does not require this)

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73
Q

In what state does transfer to the RC occur?

A

S1.

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74
Q

Chlorophyll is present in LHCII at 0.25M, at same concentration in pure lipid the excited state lifetime of the S1 state is 250 ps- fast enough to compete with FRET, thus wasting energy as heat. How does LHCII combat this?

A

Each chlorophyll molecule is orientated precisely maintaining the lifespan at 4ns.

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75
Q

What is electron transport coupled to photosynthesis?

A

Proton translocation from the stroma to the thylakoid space (lumen.)

76
Q

What is light energy ultimately used for in photosynthesis?

A

To allow uphill redox reactions/ drive unfavourable reactions.

77
Q

What redox potentials does photostem 2 stretch over?

A

1200 to -600mv.

78
Q

What redox potentials does photostem 1 stretch over?

A

600- 1200mv.

79
Q

What is the only biological enzyme known that is capable of oxidising H20 to 02?

A

PS11. It couples this with the reduction of PQ.

80
Q

What does PSII use as the primary electron donor?

A

Pair of P680 chlorophylls in the RC.

81
Q

What does this equation represent?

2H20 + 2PQ +4H+ (stroma) –> O2+ 2PQH2 + 4H+ (lumen).

A

The overall reaction of PSII.

82
Q

What organism was the first to carry out oxygenic photosynthesis?

A

Cyanobacteria.

83
Q

Why was there a peak where atmospheric oxygen was 35% instead of todays 21%?

A

Multicellular life, such as giant insects dominant as decomposers could not digest ligin in plant cell walls, this meant that decomposers did not use the O2 in respiration.

84
Q

What does electron transfer efficiency decay exponentially in regards too?

A

Distance.

85
Q

Upon excitation of the RC chlorophyll is the electron lost from the tetrapyrole ring or from the magnesium ion?

A

The tetrapyrolle ring. The resulting electrons are delocalised over the tetrapyrolle ring. This results in delocalisation over the ring.

86
Q

What is the role of Mg2+ in chlorophyll?

A

Tunes the absorption spectrum of the molecule and provides a coordination site for interactions with the protein.

87
Q

What colour does chlorophyll change to when no Mg2+ is bound?

A

Brown.

88
Q

What is chlorophyll called when it has no Mg2+?

A

Pheophytin.

89
Q

What is the delta G value of PSII?

A

285kj

90
Q

How many photons are needed in PSII and how much energy do they contain?

A

4.

91
Q

What percentage of energy is lost from PSII?

A

60%.

92
Q

To achieve water splitting potential what redox value is needed?

A

More than 820+.

93
Q

What is the redox potential of the P680+/P680 couple?

A

+1200mv.

94
Q

What does Yz+/Yz have a redox potential of?

A

+950mv.

95
Q

What can Yz+/Yz act as oxidants of?

A

P680+/P680.

96
Q

What is the redox potential of P680*/P680+?

A

-630mv.

97
Q

What is the shape of both photosystems?

A

Horseshoe?

98
Q

What tyrosine residues is involved in PSII?

A

161.

99
Q

Why do the pigments allow election transfer?

A

They all touch.

100
Q

Where does Yz+ receive the electrons from?

A

The Mn cluster.

101
Q

Does QH or QH2 have a lower affinity to the binding site of PSII?

A

QH2.

102
Q

How does the oxidation state of the carbon change in plastoquinone?

A

+2 to +1.

103
Q

The chain of electron donors is energetically downhill from P680. What does this ensure?

A

That the electron and the electron hole are spatially separated, this lows down reverse reactions and stabilises charge separation.

104
Q

What does PSII bind which acts like a catalyst for water oxidation?

A

Manganese cluster.

105
Q

What provides the thermodynamic driving force for water oxidation?

A

P680+.

106
Q

What are the oxidation states of magnese?

A

+2 to +5.

107
Q

What happens in the S1 stage regarding the magnese cluster?

A

One hydrogen is lost from the water molecule bound to Mn2+. This changes to Mn3+ through the loss of an electron.

108
Q

What happens in the S2 stage regarding the magnese cluster?

A

The Mn3+ in the the cube bound to three oxygens looses an electron and becomes Mn4+.

109
Q

What happens in the S3 stage regarding the magnese cluster?

A

The final H+ is lost from the water molecule now bound to Mn3+. This forms an oxygen double bond and the Mn3+ looses an electron making it Mn4+.

110
Q

What happens in the S4 stage regarding the magnese cluster?

A

The Mn4+ formed in the S3 phase looses an electron to form Mn5+. This is then attacked by the other H20 oxygen returning the Mn5+ to Mn3+.

111
Q

How does the magnese cluster reset itself after the S4 phase?

A

Two water molecules attack. O2 and H+ released.

112
Q

What are the electrons released from the magenese cluster used to do?

A

Reduce P680 via Yz+.

113
Q

What are the protons released from the magnenese cluster used for?

A

To contribute to deltaP in the lumen.

114
Q

What is the role of Ca2+ in the magnese cluster?

A

Polarises one of the water molecules so it acts as a nucleophile. Polarisation of a molecule in an active site is normally carried out by Zn2+.

115
Q

Why would mimicking the magnese cluster be a huge scientific breakthrough?

A

The system is composed of inexpensive Earth materials, works at an ambient temperature and has an efficient turnover of 50 O2 per second meaning it could be an ideal technique for splitting water.

116
Q

What is cytochrome b6f, found in photostem 1 very similar to?

A

Bc1 in complex 3 of the mitochondria.

117
Q

What haem group is found in cytochrome b6f instead of haem C, which is found in complex 3?

A

Haem f.

118
Q

What three haem groups are found in cytochrome b6f?

A

Haem bh, haem bl and haem f.

119
Q

Where does the Q cycle occur in photosynthesis?

A

In cytochrome b6f found in photosystem 1.

120
Q

What is the purpose of the Q cycle?

A

It doubles the number of protons translocated for every plastoquinone oxidised.

121
Q

What is the overall reaction of cytochrome b6f?

A

PQH2 + 2PCox +2H+( stroma) –> PQ + 2PCred + 4H+(lumen).

122
Q

What is photosystemI?

A

Ferrodoxin oxidoreductase.

123
Q

What photosystem is a light dependant plastocyanin?

A

PSI.

124
Q

What does photosystem I catalyse?

A

PCred + Fdox –> Pcox + Fdred.

125
Q

How many photons does photosystem I require for catalysis?

A

1.

126
Q

What nm of photon provides the free energy at PSI?

A

700nm.

127
Q

What is the free energy of PSI ?

A

-96kjmol, this means that there is a 57% energy loss.

128
Q

What is the redox potential of the redox couple P700+/ P700?

A

+450mv. This is insufficient in oxidising water.

129
Q

What is the redox potential of the P700*/P700+ couple?

A

-1320mv.

130
Q

What can the redox couple P700/ P700+ reduce?

A

2Fe-2S cluster of ferrodoxin (-530mv).

131
Q

Ferrodoxin is a power reductant. What can it reduce?

A

NADP+ to NADPH and NO3- to NH4+.

132
Q

In what photosystem are both quinone molecules tightly bound?

A

Both phylloquinone molecules are tightly bound in 1. in 2 only one plastoquinone is tightly bound.

133
Q

Where can charge separation occur in photosystem 1?

A

A or B branch.

134
Q

Where can charge separation occur in photosystem 2?

A

A.

135
Q

What stabilises charge separation?

A

By the electron acceptor being downhill, slowing down reversible reactions. If they were only spatially separated the reverse reactions could still occur.

136
Q

What are plastocyanin and ferrodoxin, found in PSI?

A

Small (10kda) soluble electron transfer proteins.

137
Q

How does plastocyanin act as an electron carrier?

A

It contains a copper ion bound at the active site to several his residues. The copper ion can be oxidised from Cu+ to Cu2+ by PSI and reduced back by cytochrome b6f.

138
Q

How does ferrodoxin act as an electron carrier?

A

Binds to a 2Fe-2S cluster at its active site, bound by several cys residues that act as an electron carrier.

139
Q

What photosystem has two active branches?

A

1.

140
Q

What photosystem has two extra chlorophyll a molecules instead of pheophytin?

A

1.

141
Q

What photosystem contains a Mn4CaO5 cluster?

A

2.

142
Q

What photosystem contains 3 x 4Fe4S clusters?

A

1.

143
Q

What photosystem contains plastoquinones?

A

2.

144
Q

What photsytstem contains phylloquinones?

A

1.

145
Q

What two reason mean that photosystem 1 can not oxidise water?

A

There is no Mn cluster or sufficient thermodynamic driving force.

146
Q

Why is the B branch in photosystem 2 switched of?

A

There is a small energy gap between P680 and Chub an a a large gap between Qb and Pheob.

147
Q

What does blocking the B branch in photosystem 2 ensure?

A

That both electrons end up at Qb. This minimises energy loss.

148
Q

Why do you want to avoid charge recombination in the photosystems?

A

So energy isn’t lost as heat.

149
Q

What is the name of the theory that explains the rate of electron transfer?

A

Marcus theory.

150
Q

When is the electron transfer rate at a maximum?

A

When the reorganisation energy equals the driving force.

151
Q

What defines the ATP ratio?

A

The number of C subunits?

152
Q

How many C subunits are in yeast mitochondria?

A

10.

153
Q

How many C subunits are in beef heart mitrochondia?

A

7.

154
Q

How many C subunits are in spinach chloroplasts?

A

14.

155
Q

The larger _____ or a smaller _____ the fewer moles of H+ are spent per mole of ATP therefore fewer subunits needed?

A

DeltaP or smaller DeltaGp.

156
Q

What is the overall equation for photosynthesis?

A

H2O+ NADP+ + 5H+ (stroma) –> 1/2O2 + NADPH +6H+ (lumen). 4 light photons needed.

157
Q

How many ATPS are formed per ATP in NADPH in photosynthesis?

A

1.28 ATPs. However the Calvin Cycle requires 1.5 ATP per NADPH.

158
Q

The Calvin Cycle requires more ATP per NADH than is actually produced. How is this redox imbalance corrected?

A

Cyclic electron transport occurring in the chloroplasts.

159
Q

Why can cyclic electron transport help maintain the redox balance in photosynthesis?

A

CET generates ATP but not NADH.

160
Q

What two possible pathways have been suggested for CET?

A
  1. The PGRL1 protein acts as a ferrodoxin-plastoquinone oxidoreductase.
  2. Through a NADPH- plastoquinone oxidoreductase with the electrons coming from complex 1.
161
Q

What does CO2 fixation involve?

A

Reduction of carbohydrates in the stroma.

162
Q

What type of organism does CO2 fixation happen in?

A

MOST plants.

163
Q

What is the other name for the Calvin Cycle?

A

Reductive Pentose Phosphate Pathway.

164
Q

What does the Reductive Pentose Phosphate Pathway produce?

A

Glyceraldehyde 3 phosphate.

165
Q

What is the fate of the Glyceraldehyde 3 phosphate that is produced in the Calvin Cycle?

A

Converted into sucrose (glucose and fructose) or stored as starch (polysaccharide storage polymer.)

166
Q

What are the three main steps in the Calvin Cycle?

A
  1. Carboxylation.
  2. Reduction.
  3. Regeneration.
167
Q

What happens in each complete turn of the Calvin Cycle?

A

3 CO2 molecules are reduced, 6 NADPH molecules and ( ATP molecules are used and one molecule 1G3P molecule is produced (net).

168
Q

Describe the experiment that lead to the discovery of the Calvin Cycle.

A

A glass lollipop, filled with algal suspension, is supplied with 14CO2 leading to its illumination. The contents of the flask are then drained into hot alcohol to stop any reactions. The radiolaballed chemical components are then separated by chromatography. At 10 seconds most of the radioactivity is found in the 3-phosphoglycerate and after two minutes phosphorylated carbon sources, glucose and fructose, were synthesised as well as a number of amino acids.

169
Q

Is CO2 directly polymerised in the carboxylation step of the Calvin Cycle?

A

No.

170
Q

Describe the steps of carboxylation in the Calvin Cycle.

A
  1. CO2 is directly added to the 5 carbon sugar ribulose 5 phosphate.
  2. Rubisco carries out energetically favourable carboxylation..
  3. Two molecule of 2-phosphoglycerate are produced.
171
Q

What is special about Rubsico?

A

It is the most abundant protein on Earth. It makes up almost 30% of the total leaf protein.

172
Q

What is the rate of Rubisco?

A

3 (s-1), it is a very slow enzyme.

173
Q

Why does there need to be a high concentration of CO for Rubisco to work?

A

It has a low substrate affinity for CO2.

174
Q

Rubisco has a slight affinity for O2 which can lead to photorespiration. True or false?

A

False, it has almost the same affinity for O2 as it does for CO2 (both these affinities are low.)

175
Q

What is produced in photorespiration?

A

PhosphoGLYCORATE.

176
Q

Why can phosphoglycorate, produced in photorespiration, be described as a metabolic dead end?

A

As it needs ATP to change into CO2.

177
Q

What effect does temperature have on photorespiration?

A

At 25C the rate of decarboxylation is four times the rate of oxygenation. This difference decreases as temperature increases a 25% of the yield can be lost.

178
Q

What are the steps of the reduction stage in the Calvin Cycle?

A

3 Phosphoglycerate +ATP –> 1,3 bisphosphoglycerate +ADP by the enzyme phospoglycerate kinase.

1,3 bisphosphoglycerate is reduced to glyceraldehyde 3 phosphate (X6) by glyceraldehyde 3 phosphate hydrogenase.

179
Q

What can glyceraldehyde 3 phosphate (produced in the reduction step of the Calvin Cycle) be converted to?

A

Glucose 1 phosphate and Fructose 6 phosphate (these can combine to make sucrose.)

180
Q

Gylceraldehyde 3 phosphate can be exported from the chloroplast for exchange for what through the use of a translocator protein?

A

Pi from the cytoplasm.

181
Q

How is starch formed in stroma?

A

Glyceraldehye 3 phosphate and Glucose 1 phosphate.

182
Q

Sugar produced in photosynthesis are the starting points for multiple metabolic pathways. What can be produced by these?

A

Amino acids, lipids and nucleotides.

183
Q

What does the regeneration step of photosynthesis involve?

A

5 3 carbon sugars forming 3 molecules of the 5C sugar ribulose 5-phosphate.

184
Q

Regeneration in photosynthesis leads to the production of ribulose 5-phosphate. What happens to this?

A

IT can then be phosphorylated by phosphoribulose kinase using ATP to make ribulose 1-5 bisphosphate.

185
Q

What are the 4 regeneration steps in the Calvin Cycle?

A

3C +3C = 6C
6C + 3C = 4C +5C
4C + 3C = 7C
7C + 3C = 5C + 5C.

MBB266 (7 decks)

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