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CHEM 441A+B > Chapter 12: Photosynthesis > Flashcards

Chapter 12: Photosynthesis Flashcards

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1
Q

What is the balanced equation of photosynthesis?

A

6 CO2 +6 H2O -> C6H12O6 + 6 O2

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2
Q

Where does photosynthesis occur?

A

Electron transport system takes place on the thylakoid membrane of the chloroplast

Calvin cycle (carbon fixation) takes place in the stroma of chloroplast

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3
Q

Compare and contrast a mitochondria and chloroplast.

A

Mitochondria:
- Animal
- Two membranes
- Protons in inter membrane space (low pH) and electrons in matrix

Chloroplast:
- Plant
- Three membranes
- Protons in lumen electrons in stroma

Both:
- Have DNA which is distinct to the rest of the cell
- Produce energy for the cell

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4
Q

What is Photosystem II composed of? Photosystem I?

A

PS II and PS I both have light harvesting complexes that surround the reaction center. On the outermost part are LHC II which facilitate resonance to LHC I and then lead to the reaction center. At the reaction center there is a special pair which can be either P680 (PS II) or P700(PS I)

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5
Q

How do the photosystems capture and convert photon energy?

A
  • A photon is absorbed by a pigment
  • Energy is directed toward Chl reaction center
  • Energy reaches Chl causing an increase in redox potential which
    results in electron transfer
  • Electron jumps to primary electron acceptor
  • Electron then travels through a chain of e- acceptors to another PS reaction center
  • Electron is excited and results in the reduction of NADP+ to NADPH
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6
Q

How is the proton gradient established and what does it do?

A

Photons excite electrons from water which travel through cytochrome to pump protons into the lumen and reduce NADP+.

The protons then exit the lumen through ATP synthase which generates ATP.

ATP and NADPH will be used to power the Calvin cycle

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7
Q

How do the gradients differ in a mitochondria vs a chloroplast?

A

Mitochondria:
- Protons in inter-membrane space
- Electrons in matrix

Chloroplast:
- Protons are in the lumen
- Electrons in Stroma

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8
Q

What two processes are a part of photosynthesis? What is their purpose?

A

Electron transport system and photophosphorylation
- Generates ATP and NADPH (O2 and glyceraldehyde-3-P

Calvin cycle
- Converts CO2 to sugar using ATP and NADPH

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9
Q

Which membrane(s) is/are permeable to water in the chloroplast?

A

Inner membrane and thylakoid membrane are impermeable

Outer membrane is permeable to water

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10
Q

Where is the electron transport system located?

A

On the surface of the thylakoid membrane

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11
Q

What is the final electron acceptor?

A

NADP+ -> NADPH (in stroma)

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12
Q

Outline the steps involved in the electron transport system.

A
  1. 4 photons enter Chlorophyll of PSII (680nm)
  2. Photons excite 4 electrons from 2 H2O -> O2 + 4H+
  3. Electrons are picked up by PQ (plastoquinone) which carries electrons to cytochrome b6F
  4. 8 protons cross from the stroma to lumen with the help of 4 electrons
  5. 4 electrons are picked up by PC (plasocyanin) which takes electrons to PSI
  6. With the help of 4 photons, 4e- pass to FdR (reductase) to reduce 2NADP+ + 2H+ -> 2NADPH
  7. 12 protons from step 2 and 4 are used by ATP synthase to generate 3 ATP
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13
Q

What is the ideal wavelength for plants?

A

Plants absorb Blue (380-480) and red (650-750) light well which is why plants are usually green

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14
Q

What structure does chlorophyll resemble?

A

Hemoglobin but instead of Fe, Mg is used

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15
Q

What are three other chlomophores besides chlorophyll A and B?

A

Beta Carotene
Phycocyanobilin
Phycoerythribilin

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16
Q

What three things can result if a photon is absorbed?

A

Flourescence: No useful work for photosynthesis

Photooxidation: Excited electron jumps from Chl to Pheo turning it to Pheo-

Resonance energy transfer: Energy from Chl- turns to Chl as Chl2 becomes Chl2-

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17
Q

Describe the arrangement of chromophores in photosynthetic membranes and what purpose they serve.

A
  • Arranged to maximize light absorption
  • Light harvesting complexes have a lot of chlorophyll molecules
  • Arrangement allows resonance transfers from LHCII to LHCI to reaction center
  • Photooxidation occurs in reaction center
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18
Q

What is (Chl)2 and what does it do?

A
  • It is a special pair of chlorophyll that sit in closely in the reaction center and absorb the excited electron
  • Energy from excited electron makes Chl2 a strong e- donor which sends electron to the primary acceptor
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19
Q

What are the key points of the thermodynamics of electron transfer in photosynthesis?

A
  • Occurs in solid state
  • Highly efficient 90%
  • Fast and thermodynamically favorable
  • Irreversible
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20
Q

What is the electron donor of the electron transport system?

A

H2O

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21
Q

What is the balanced equation of the Z-scheme?

A

2 H2O + 2 NADP+ + 8 photons -> O2 + 2 NADPH + 2H2+

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22
Q

What happens if NADPH concentration is high in the Z-scheme?

A

Electrons will be diverted from and back to PQb where the electrons will repeat the cycle. The electrons passing through cytochrome b6F will cause 8 protons to pass into the lumen and generate ATP

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23
Q

What is the key requirement for photooxidation?

A

A reductant to replace the electron lost by photooxidation (ex.H2O or AH)

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24
Q

How can PSII be inhibited?

A

DCMU - a potent herbicide

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25
Q

How does an electron flow in PSII?

A

2H2O -> OEC -> Chlorophyll P680 -> Pheo ->PQa -> PQb -> Cytochrome b6F via Q cycle

26
Q

How are Cytochrome b6F and Complex III in Oxidative phosphorylation similar?

A

both use the energy from electrons to pump protons creating a gradient.

27
Q

How does an electron flow in PSI?

A
  1. PC transports 4 electrons to PSI
  2. 4 photons absorbed causing electrons to jump through a series of transporters
  3. Electrons reach Ferrodoxin
    4a. Ferrodoxin is sent back to the Q cycle in PSII where 8 protons will be pumped for 2 ATP
    4b. Ferrodoxin passes electrons to FdR (ferredoxin-NADP+ reductase) where 2NADP+ are reduced to 2NADPH
28
Q

What is Paraquat and what does it do?

A
  • Paraquat is a Herbicide that target PSI
  • Accepts electrons from PSI and donates them to O2
  • Blocks NADPH production
  • Produces superoxide anion
29
Q

What are the two components of ATP synthase in a chloroplast?

A

CFo - Membrane bound Fo unit
CF1 - Catalytic complex

30
Q

What occurs in cyclic phosphorylation and why does it occur?

A
  • Ferredoxin takes electrons from PSI and returns them to the Q cycle after PSII to be recycled
  • Occurs when NADP+ is limiting
  • Does not produce O2 or NADPH, just ATP
  • Controls ATP/NADPH ratio
31
Q

How are the complexes of photosynthesis distributed on the membrane?

A
  • PSII associated with Grana
  • PSI and ATP synthase associated with Lamellae
  • Cytochrome b6F and PC are evenly distributed
32
Q

How is the rate of photooxidation controlled?

A

Redistribution of LHCII between the grana and lamellae

33
Q

What does the Calvin Cycle do?

A

Uses ATP and NADPH to convert CO2 into a triose phosphate

34
Q

What are the three stages of the Calvin Cycle

A
  1. Fixation
  2. Reduction
  3. Regeneration
35
Q

Where does the Calvin Cycle take place?

A

Stroma of the chloroplasts

36
Q

How many times does the ETS and photophosphorylation have to occur for the Calvin Cycle to occur once?

A

3 times (9 ATP and 6 NADPH)

37
Q

What are the inputs, outputs, and transformations that occur in Fixation?

A

Inputs:
3 CO2

Outputs:
N/A

Transformations:
3(Rubulose-1,5-bisphosphate) + 3 CO2 –Rubisco–> 6(3 Phosphoglycerate)

38
Q

What are the inputs, outputs, and transformations that occur in Reduction?

A

Inputs:
6 ATP
6 NADPH

Outputs:
6 ADP
6 NADP+

Transformations:
6(3-Phosphoglycerate) + 6 ATP –Phosphoglycerate kinase–> 6(1,3-bisphosphoglycerate) + 6 NADPH –G3P dehydrogenase–> 6 G3P

39
Q

What are the inputs, outputs, and transformations that occur in Regeneration?

A

Inputs:
3 ATP

Outputs:
3 ADP
1 G3P

Transformations:
6 G3P —-> 5 G3P + 3 ATP —-> Rubulose-1,5-bisphosphate

40
Q

What is Rubisco and what does it do?

A
  • An enzyme that turns a molecule of Ru1,5BP to 2 3PG
  • Bad enzyme
  • 3 molecules of CO2 fixed per second
  • Plants have high [rubisco]
  • Most abundant enzyme in biosphere
  • Oxygenase activity = useless
41
Q

Describe the structure of Rubisco.

A
  • 8 large subunits
  • 8 small subunits to hold the large units together
  • Mg2+ is critical cofactor
  • CAP is Transition state analog
42
Q

What is the role of Mg2+ in Rubisco

A
  • Stabilizes Ru1,5BP binding and faciliates Nu attack on CO2
  • Rubisco activity dependent on carbamoylation of Lys201 which helps coordinate Mg2+
43
Q

What enzymes are responsible for the reduction of 3-PG to G3P in the reduction phase?

A

Phosphoglycerate Kinase and G3P dehydrogenase

44
Q

What is Triose Phosphate isomerase?

A

An enzyme that transforms G3P to dihydroxyacetone phosphate

45
Q

What molecule must be regenerated in stage three for the Calvin cycle to continue?

A

Rubulose 1,5-BP

46
Q

What are the net products of the Calvin Cycle?

A

1 G3P

47
Q

What is G3P used for?

A

Make sugars such as sucrose

48
Q

How many ATP and NADPH are used to make one G3P?

A

9 ATP and 6 NADPH or three rounds of ETS + Photophosphorylation

49
Q

Why can animals NOT convert CO2 to glucose?

A
  • Lack Rubisco
  • Lack SBPase
  • Lack Rubulose 5-phosphate kinase
50
Q

How is photosynthesis regulated?

A

During Day:
- ATP and NADPH generated
- G3P produced via Calvin Cycle

During Night:
- ATP regen via Glycolysis and Ox-Phos
- NADPH regenerated by Pentose phosphate pathway

51
Q

What happens to Rubisco at night and why?

A
  • Inhibited by nocturnal inhibitor CAP
  • CAP synthesized in the dark
  • Inhibits carbamoylated Rubisco
  • Calvin cycle unable to occur
52
Q

What happens to Rubisco and Fructose 1, 6-Bisphosphate during the day?

A
  • Elevated pH and Mg2+ levels stimulate Rubisco and Fructose 1,6-Bisphosphate
  • Elevated pH and Mg2+ concentration result in activation of photosynthetic ETS in thylakoid lumen
53
Q

What happens in the Thioredoxin-Mediated reduction?

A
  • When Ferredoxin is in a reduced state due to PSI but there is an absence of NADP+, Ferredoxin will donate electrons to Ferredoxin-thioredoxin reductase
  • Involves 2 Cystine -SH
  • Electrons go from Ferredoxin –> Ferredoxin thioredoxin reductase —> Thioredoxin —> activates Calvin cycle enzymes
54
Q

What is photorespiration?

A
  • Reaction driven by light that results from Rubisco activity
  • Consumes O2 and makes CO2
  • Does not conserve energy
55
Q

Why is the oxygenase activity of Rubisco so wasteful?

A
  • O2 competes with CO2 to bind and as heat rises O2 gets better affinity
  • Oxygenase results in a 2 phosphoglycerate which is expensive to recover carbons from
  • High atmospheric [O2] relative to [CO2] means a significant amount of O2 fixation by Rubisco
  • CO2 has a x30 better (Km) binding affinity
56
Q

What two factors increase O2:CO2 ratio in the stroma?

A
  • Decrease in CO2 as a result of carbon fixation in intense light
  • Increase temperature favors O2 binding
57
Q

Compare C3 and C4 plants

A

C3
- First step in Calvin cycle fixation results in 3-C product
- Wheat, Barley, Potatoes
- Tend to grow faster in cold temperatures

C4
- Avoids Photorespiration
- Fixes 4-C compound - Oxaloacetate
- Grow well in heat
- Sugar cane, corn, sorhum

58
Q

How does a C4 plant work?

A
  • Occurs in 2 different cells
  • CO2 is captured in oxaloacetate (4C) in mesophyll cell
  • OAA passes carbon to bundle sheath cell where CO2 is released
  • Higher energy cost
  • As temp increases efficiency outweighs cost
59
Q

What does the Glyoxylate cycle do?

A
  • Mechanism for fats stored in seed to be converted to Glucose
  • Uses Acetyl Co-A to turn OAA to citrate to isocitrate which is split into succinate and glyoxylate
  • Succinate is take to mito and Glyoxylate + Acetyl CoA results in malate which is oxidized to OAA
60
Q

What two enzymes do animals not have which don’t allow the glyoxylate cycle to occur?

A

Isocitrate lyase and Malate synthase

CHEM 441A+B (22 decks)

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