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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
Q

What evidence is their for two photosystems ?

A
  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
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26
Q

What is the result of second boost of electrons by photons ?

A

Generate NADPH

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27
Q

What is the result of first boost of electrons by photons ?

A

To make ATP via electron transport

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28
Q

Why does it take 8 photons to produce 1 O2 ?

A

Each of the 4 electrons must be boosted uphill twice by light during photosynthesis by green plants

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29
Q

How many photons are required to produce one 02 molecules ?

A

8

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30
Q

What are the three states that chlorophyll molecules can exist in ?

A
  1. Ground state
  2. Excited state
  3. Oxidised state
31
Q

What is an excitonic dimer ?

A

They are excited as if they were a single molecule

32
Q

What do the special pair of chlorophylls form ?

A

An excitonic dimer

33
Q

What are the two chlorophyll molecules participating in the photochemical act called ?

A

The special pair

34
Q

How many chlorophylls participate in the photochemical act ?

A

2

35
Q

What position are chlorophyll pigments in for fluorescence resonance energy transfer ?

A

Chlorophyll are oriented very precisely in the membrane to allow for FRET to happen by noncovalent binding to integral membrane proteins

36
Q

What happens when molecules are far away in fluorescence resonance energy transfer ?

A

Efficiency decreases with separation distance with an inverse 6th power law

37
Q

When can fluorescence resonance energy transfer occur ?

A

When molecules are very close together

38
Q

Does fluorescence resonance energy transfer have energy loss ?

A

no, there is no energy loss

39
Q

What is fluorescence resonance energy transfer ?

A

How a photon is absorbed and transferred to neighboring molecules and eventually to reaction center

40
Q

What acts as light harvesting antenna ?

A

300 chlorophyll molecules and 40 or so beta carotenes and other accessory pigments

41
Q

What is the concept of the photosynthetic unit ?

A

Most chlorophyll is not involved in the photochemistry reaction, they absorb light and transfer to reaction center

42
Q

What is the role of the accessory pigment carotenoid in plants ?

A

To protect thylakoid membranes from photoxidation

43
Q

What is the light gathering function associated with ?

A

The tetrapyrrole ring and its network of conjugated double bonds

44
Q

How is chlorophyll b related to chlorophyll a ?

A

A displaced maxima

45
Q

What is chlorophyll a sharp maxima ?

A

660nm and 420nm

46
Q

Where is chlorophyll a found ?

A

In reaction centers

47
Q

What are the two types of chlorophyll ?

A
  1. Chlorophyll a

2. Chlorophyll b

48
Q

Who discovered chlorophyll ?

A

A. Engelmann and Julius Sachs

49
Q

What is chlorophyll ?

A

Light gathering pigments that absorb blue and red light and emit green light

50
Q

What are the 5 steps of oxygenic photosynthesis mechanism ?

A
  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
Q

What is used to fix CO2 to generate carbohydrates ?

A

NADPH and ATP

52
Q

What is ATP from light reactions used for ?

A

Dark reactions

53
Q

What must light reactions allow for ?

A

Generation of ATP from light energy

54
Q

How often are electrons transferred uphill in most plants ?

A

Usually twice

55
Q

What can a strong reducing agent be used for ?

A

To generate NADPH

56
Q

What does a strong reducing agent have ?

A

A very negative redox potential

57
Q

How is a strong reducing agent generated ?

A

Electrons are transferred uphill to a more negative redox potential using light energy

58
Q

What is required to remove 4 electrons from water ?

A

A very strong oxidising agent

59
Q

What do light reactions have to use ?

A

The energy of light

60
Q

What are other names for dark reactions ?

A
  1. Calvin- Benson or C3 cycle
61
Q

Where do dark reactions occur ?

A

Stroma matrix

62
Q

What happens during the dark reactions ?

A

The reducing power of NADPH and the free energy of hydrolysis of ATP are used to covert C02 into carbohydrate

63
Q

What are dark reactions ?

A

Are not directly dependent on light

64
Q

What are the two ways electrons are transferred ?

A
  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
Q

What does oxygenic photosynthesis involve ?

A

Using light energy to drive the oxidation of water to generate O2 and then uphill to transfer 4 electrons

66
Q

What are grana ?

A

Thylakoid disks stacked on one another

67
Q

What is the lumen topologically equivalent to ?

A

Mitochondrial inner membrane

68
Q

What are thylakoid membranes the site of ?

A

Light reactions

69
Q

What are thylakoids ?

A

Flattened structures that make up a continuous internal phase sealed from the chloroplast matrix or stroma

70
Q

Do chloroplasts contain their own genome ?

A

Yes

71
Q

What is the origin of chloroplasts ?

A

Endosymbiotic

72
Q

What length are chloroplasts ?

A

5-10 um

73
Q

What is required to remove 4 electrons from water ?

A

A very strong oxidising agent

74
Q

What is used to fix CO2 to generate carbohydrates ?

A

NADPH and ATP

Molecules to Cells - Biochemistry (17 decks)

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