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Molecules to Cells - Biochemistry > Lecture 18. Photosynthesis > Flashcards

Lecture 18. Photosynthesis Flashcards

1
Q

Where does carbon fixation occur ?

A

Stroma or chloroplast matrix

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2
Q

What does the fixation of C02 require ?

A

2 NADPH and three ATP molecules

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3
Q

What is plastocyanin ?

A

A mobile carrier that can bind to the surface of the photosystem 1 and reduce P700 +

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4
Q

What is the acceptor of electrons in cyanobacteria and plants ?

A

The blue copper containing protein called plastocyanin

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5
Q

What can the transfer of electrons from PQH2 to plastocyanin also generate ?

A

The transfer is coupled to the movement of H+ across the membrane from the stromal side into the lumen via a photosynthetic Q cycle. Generating a proton electrochemical gradient across the membrane which can be used to drive a chloroplast F1F0-ATPase and phosphorylate ADP to generate ATP

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6
Q

How are electrons transferred from PQH2 to plastocyanin ?

A

Through an electron transport chain consisting of iron sulphur centers and cytochrome f and cytochrome b6

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7
Q

Where are electrons from the reduced plastoquinone (PQH2) transferred to ?

A

The cytochrome bf complex, which is analogous to the chytochrome bc complex III of mitochondria

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8
Q

What can Qb reduce ?

A

PQ picking up a protein from the stroma side

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9
Q

Where is the redox-active tyrosine YZ found?

A

On the D1 protein and the associated OEC is located on the lumen side

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10
Q

What are plastoquinones structurally similar to ?

A

Ubiquinone or coenzyme Q10

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11
Q

Where do inhibitors of PS2 generally bind ?

A

At the Qb site

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12
Q

Is Qa plastoquinone or Qb plastoquinone free to move in and out of the D1 protein at the Qb site ?

A

Qb plastoquinone, Qa remains relatively fixed

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13
Q

Where is the special pair found ?

A

Between D1 and D2 proteins

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14
Q

What are the most important subunits of PS2 ?

A

A pseudo-symmetric heterodimer of two homologous proteins D1 and D2

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15
Q

What are the electron transfer events restricted to ?

A

The major proteins of the central core

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16
Q

What is photosystem II composed of

A

More than 25 polypeptides and is surrounded by a variety of chlorophyll a and chlorophyll b binding proteins

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17
Q

In PS2, what happens after the first of the light quanta is absorbed ?

A

An electron is translocated from the special pair (P680*) through an accessory chlorophyll and a pheophytin molecule to the tightly bound quinone QA. The mobile quinone QB is reduced to QB ^- in the thylakoid membrane

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18
Q

What does OEC stand for ?

A

Oxygen evolving complex

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19
Q

What is phenophytin ?

A

The primary acceptor in a chlorophyll molecule lacking a central Mg+2 ion

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20
Q

What led to the emergence of green plants ?

A

Acquisition of cyanobacteria by an early eukaryote

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21
Q

What does PS2 contain

A

A type 2 reaction center

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22
Q

What does PS1 contain

A

A type 1 reaction centre

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23
Q

How much does PS2 absorb

A

Max 680nm

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24
Q

How much does PS1 absorb ?

A

max 700nm

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25
What evidence is their for two photosystems ?
1. Emerson enhancement effect demonstrates a mixture of light is more effective than either alone 2. Robin Hill used artificial acceptors and donors to establish the presence of two photosystems
26
What is the result of second boost of electrons by photons ?
Generate NADPH
27
What is the result of first boost of electrons by photons ?
To make ATP via electron transport
28
Why does it take 8 photons to produce 1 O2 ?
Each of the 4 electrons must be boosted uphill twice by light during photosynthesis by green plants
29
How many photons are required to produce one 02 molecules ?
8
30
What are the three states that chlorophyll molecules can exist in ?
1. Ground state 2. Excited state 3. Oxidised state
31
What is an excitonic dimer ?
They are excited as if they were a single molecule
32
What do the special pair of chlorophylls form ?
An excitonic dimer
33
What are the two chlorophyll molecules participating in the photochemical act called ?
The special pair
34
How many chlorophylls participate in the photochemical act ?
2
35
What position are chlorophyll pigments in for fluorescence resonance energy transfer ?
Chlorophyll are oriented very precisely in the membrane to allow for FRET to happen by noncovalent binding to integral membrane proteins
36
What happens when molecules are far away in fluorescence resonance energy transfer ?
Efficiency decreases with separation distance with an inverse 6th power law
37
When can fluorescence resonance energy transfer occur ?
When molecules are very close together
38
Does fluorescence resonance energy transfer have energy loss ?
no, there is no energy loss
39
What is fluorescence resonance energy transfer ?
How a photon is absorbed and transferred to neighboring molecules and eventually to reaction center
40
What acts as light harvesting antenna ?
300 chlorophyll molecules and 40 or so beta carotenes and other accessory pigments
41
What is the concept of the photosynthetic unit ?
Most chlorophyll is not involved in the photochemistry reaction, they absorb light and transfer to reaction center
42
What is the role of the accessory pigment carotenoid in plants ?
To protect thylakoid membranes from photoxidation
43
What is the light gathering function associated with ?
The tetrapyrrole ring and its network of conjugated double bonds
44
How is chlorophyll b related to chlorophyll a ?
A displaced maxima
45
What is chlorophyll a sharp maxima ?
660nm and 420nm
46
Where is chlorophyll a found ?
In reaction centers
47
What are the two types of chlorophyll ?
1. Chlorophyll a | 2. Chlorophyll b
48
Who discovered chlorophyll ?
A. Engelmann and Julius Sachs
49
What is chlorophyll ?
Light gathering pigments that absorb blue and red light and emit green light
50
What are the 5 steps of oxygenic photosynthesis mechanism ?
1. Light gathering reaction 2. Oxidation of water and transfer of electrons eventually to NADP+ to generate NADPH 3. Generation of proton electrochemical gradient 4. Phosphorylation of ADP to ATP 5. Fixation of C02 to generate carbohydrates
51
What is used to fix CO2 to generate carbohydrates ?
NADPH and ATP
52
What is ATP from light reactions used for ?
Dark reactions
53
What must light reactions allow for ?
Generation of ATP from light energy
54
How often are electrons transferred uphill in most plants ?
Usually twice
55
What can a strong reducing agent be used for ?
To generate NADPH
56
What does a strong reducing agent have ?
A very negative redox potential
57
How is a strong reducing agent generated ?
Electrons are transferred uphill to a more negative redox potential using light energy
58
What is required to remove 4 electrons from water ?
A very strong oxidising agent
59
What do light reactions have to use ?
The energy of light
60
What are other names for dark reactions ?
1. Calvin- Benson or C3 cycle
61
Where do dark reactions occur ?
Stroma matrix
62
What happens during the dark reactions ?
The reducing power of NADPH and the free energy of hydrolysis of ATP are used to covert C02 into carbohydrate
63
What are dark reactions ?
Are not directly dependent on light
64
What are the two ways electrons are transferred ?
1. Via an electron transport chain or proton pump to generate a chemical electrochemical gradient which drives phosphorylation of ADP to ATP via a CF1CFR0-ATPase (photophosphorylation) 2. Eventually to generate reducing equivalence in the form of 2 molecules of NADPH
65
What does oxygenic photosynthesis involve ?
Using light energy to drive the oxidation of water to generate O2 and then uphill to transfer 4 electrons
66
What are grana ?
Thylakoid disks stacked on one another
67
What is the lumen topologically equivalent to ?
Mitochondrial inner membrane
68
What are thylakoid membranes the site of ?
Light reactions
69
What are thylakoids ?
Flattened structures that make up a continuous internal phase sealed from the chloroplast matrix or stroma
70
Do chloroplasts contain their own genome ?
Yes
71
What is the origin of chloroplasts ?
Endosymbiotic
72
What length are chloroplasts ?
5-10 um
73
What is required to remove 4 electrons from water ?
A very strong oxidising agent
74
What is used to fix CO2 to generate carbohydrates ?
NADPH and ATP

Molecules to Cells - Biochemistry (17 decks)

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