CH350 > CH 12 > Flashcards

CH 12 Flashcards

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1
Q

What do the light reactions accomplish?

A

Convert light energy → chemical energy
Produce ATP, NADPH, and O₂ from water
Energize electrons using PSII and PSI

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2
Q

What does the Calvin‑Benson cycle accomplish?

A

Uses ATP + NADPH to reduce CO₂ → G3P (glyceraldehyde‑3‑phosphate)
G3P is used to build glucose, starch, sucrose, cellulose

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3
Q

What is the real product of photosynthesis?

A

G3P (triose phosphate)
—not glucose directly.

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4
Q

Why is photosynthesis essential for life on Earth?

A

Provides carbohydrate fuel for nonphotosynthetic organisms
Produces atmospheric O₂
Feeds carbon into biosynthetic pathways (gluconeogenesis → glucose etc.)

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5
Q

Where do the light reactions occur?

A

Thylakoid membrane of chloroplasts.

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6
Q

Where does the Calvin cycle occur?

A

Stroma of chloroplasts.

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7
Q

What are the main compartments of a chloroplast and what occurs in each?

A

Thylakoid membrane: light reactions (PSII, PSI, Cyt b₆f, ATP synthase)
Thylakoid lumen: proton accumulation
Stroma: Calvin cycle, carbohydrate synthesis
Grana: stacks of thylakoids for efficient light capture

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8
Q

Why does the ATP synthase face outward into the stroma?

A

Because the proton gradient is built inside the thylakoid lumen, so protons flow outward → stroma → ATP synthesis.

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9
Q

Describe the endosymbiotic theory for chloroplasts.

A

Chloroplasts originated from engulfed cyanobacteria
Evidence: own DNA, double membrane, prokaryote‑like ribosomes, divide by fission

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10
Q

What is the evolutionary connection between cyanobacteria and plant chloroplasts?

A

Both perform oxygenic photosynthesis (H₂O → O₂)
Share similar ETC components (PSII, PSI, Cyt b₆f, PQ cycle)

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11
Q

What are the three possible fates of excited chlorophyll?

A

Resonance energy transfer (antenna → reaction center)
Photooxidation (electron transfer → productive)
Fluorescence (wasted energy)

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12
Q

How does resonance energy transfer work?

A

Energy (not the electron) moves from one chlorophyll to another until reaching the reaction center.

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13
Q

What pigments do plants use, and what wavelengths do they absorb?

A

Chlorophyll a: blue & red
Chlorophyll b: blue & red‑orange
Carotenoids: blue (appear orange)
Phycobilins: middle of spectrum (marine organisms)

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14
Q

Why are plants green?

A

Chlorophyll reflects green wavelengths and absorbs blue & red.

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15
Q

What is LHCII?

A

Light‑Harvesting Complex II: a “solar panel” that collects photon energy → transfers it to PSII & PSI reaction centers via resonance energy transfer.

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16
Q

What are the two major reaction center chlorophylls and what do they do?

A

P680 (PSII): strongest oxidant in biology; oxidizes water
P700 (PSI): re‑excites electrons to reduce NADP⁺ → NADPH

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17
Q

What is the Z‑scheme?

A

The energy diagram showing electrons boosted twice:
1st boost at PSII
Electron drops through Cyt b₆f, pumping protons
2nd boost at PSI
Electrons reduce NADP⁺ → NADPH

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18
Q

What happens at the OEC (O₂‑Evolving Complex)?

A

Located in PSII
Contains Mn
Splits water → 4 e⁻ + 4 H⁺ + O₂

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19
Q

Describe electron flow through PSII.

A

Light excites P680 → Pheo → PQA → PQB → PQBH₂ → Cyt b₆f

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20
Q

Describe electron flow through PSI.

A

Light excites P700 → electron carriers (Fe–S clusters FX, FA, FB) → ferredoxin → FNR → NADPH

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21
Q

What is the role of plastocyanin (PC)?

A

PC donates an electron to P700⁺ to restore it after photooxidation.

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22
Q

What are PQ and Cyt b₆f’s roles in proton pumping?

A

PQ carries electrons + protons; Cyt b₆f pumps protons into the lumen; PQ cycle adds 12 H⁺ to lumen per cycle.

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23
Q

What does paraquat do?

A

Blocks electron transfer at PSI → prevents NADPH production → generates ROS → plant death.

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24
Q

What generates the proton gradient in chloroplasts?

A

Water splitting at PSII (adds 4 H⁺ to lumen)
PQ Cycle (translocates 8 H⁺ per cycle)
Cytochrome b₆f proton pumping
Total: 12 H⁺ added to lumen per PQ cycle

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25
What is photophosphorylation?
Light‑driven conversion of ADP + Pi → ATP using ATP synthase powered by the thylakoid proton gradient.
26
Where is ATP synthesized in chloroplasts?
In the stroma, as protons flow from lumen → stroma through ATP synthase.
27
Compare cyclic vs non‑cyclic photophosphorylation.
Cyclic: PSI only Electrons cycle back to Cyt b₆f Produces ATP only No NADPH, no O₂ Used when NADPH levels are high Non‑cyclic: PSII → PSI (Z‑scheme) Produces ATP, NADPH, O₂ Source of electrons: water
28
Why does cyclic electron flow exist?
Calvin cycle requires more ATP than NADPH, so cyclic flow provides extra ATP without making NADPH.
29
Why does plant ATP synthase face the stroma?
Because proton gradient accumulates inside lumen, and ATP must be synthesized where Calvin cycle occurs (stroma).
30
What are the 3 stages of the Calvin cycle?
Fixation: RuBP + CO₂ → 2 × 3‑PGA (via RuBisCO) Reduction: 3‑PGA → G3P (using ATP + NADPH) Regeneration: 5 G3P rearranged to regenerate 3 RuBP (requires ATP)
31
What is the input and output for 3 turns of the Calvin cycle?
Inputs: 3 CO₂ 6 NADPH 9 ATP Outputs: 1 G3P Regenerated RuBP
32
Why is RuBP regeneration essential?
Without RuBP, the cycle cannot accept CO₂ → Calvin cycle stops completely.
33
What is the "carbon shuffle"?
A complex set of rearrangements that convert 5 G3P (3C) → 3 RuBP (5C).
34
What is the real energetic cost for producing ONE G3P?
9 ATP 6 NADPH
35
What reaction does RuBisCO catalyze?
Carboxylation of RuBP + CO₂ → 2 × 3‑PGA.
36
Why is RuBisCO considered inefficient?
Very slow (3–10 reactions/sec) Frequently accepts O₂ instead of CO₂ (~25% of the time) O₂ reaction produces toxic intermediates → requires costly salvage
37
Why is RuBisCO the most abundant enzyme on Earth?
Plants must compensate for its low catalytic rate by producing enormous amounts of it.
38
What is required to activate RuBisCO?
Mg²⁺ at the active site A special activator CO₂ attached to a lysine residue → carbamate formation Occurs in the daylight
39
Why is RuBisCO inactive in the dark?
The activator CO₂ detaches at night → enzyme turns OFF.
40
What is photorespiration?
When RuBisCO reacts with O₂ instead of CO₂, creating a product that must be salvaged in energy‑consuming reactions.
41
What are the two main triose phosphates made during the Calvin cycle?
G3P (glyceraldehyde‑3‑phosphate) DHAP (dihydroxyacetone phosphate)
42
What can triose phosphates be used for inside the chloroplast?
To make starch for energy storage.
43
What can triose phosphates be used for in the cytosol?
To synthesize sucrose or enter glycolysis.
44
What is the purpose of the glyoxylate cycle?
Allow seeds to convert stored lipids → carbohydrates (sucrose) before they can photosynthesize.
45
Where does the glyoxylate cycle occur?
In specialized organelles called glyoxysomes.
46
What are the two key enzymes unique to the glyoxylate cycle?
Isocitrate lyase Malate synthase
47
What major intermediate links lipid breakdown to carbohydrate synthesis?
Succinate, which travels to mitochondria → converted to malate → used to make glucose.
48
Why is the glyoxylate cycle crucial for seed germination?
Seeds cannot yet perform photosynthesis, so this cycle provides the sugars needed for early growth.
49
What is the overall net reaction of the light reactions (photosynthetic electron transport)?
2 H₂O + 8 photons + 2 NADP⁺ + ~3 ADP + ~3 Pi → O₂ + 2 NADPH + ~3 ATP
50
What is the overall net reaction of the Calvin‑Benson cycle for 1 G3P?
3 CO₂ + 6 NADPH + 9 ATP + 6 H₂O → G3P + 6 NADP⁺ + 9 ADP + 9 Pi
51
Why are ATP and NADPH used in different amounts in the Calvin cycle?
Because carbon fixation and sugar synthesis require more phosphorylation steps than reduction steps → ATP demand > NADPH demand (reason cyclic photophosphorylation exists).
52
What is the source of electrons for the whole photosynthetic ETC?
Water, split by the O₂‑Evolving Complex (OEC) in PSII → electrons, protons, and O₂.
53
Where do electrons end up after passing through the photosynthetic ETC?
They reduce NADP⁺ → NADPH (via ferredoxin‑NADP⁺ reductase).
54
Which processes contribute protons to the thylakoid lumen?
Water splitting (PSII) PQ shuttle carries protons Cytochrome b₆f proton pumping
55
What is the total proton gain per electron‑pair through PSII + PQ cycle?
12 H⁺ added to lumen 4 from water 8 from PQ/cyt b₆f cycle
56
Why does PSI need its own photon to excite electrons a second time?
Because electrons lose energy traveling through the cyt b₆f complex → must be re‑energized at PSI to reach the reduction potential needed to form NADPH.
57
What is the difference between PQ (plastoquinone) and ubiquinone?
Both are lipid‑soluble electron carriers, but: PQ is used in chloroplasts Ubiquinone (Q) is used in mitochondria
58
Why do plants need both PSII and PSI?
PSII provides strong oxidation potential to split water; PSI provides strong reduction potential to make NADPH. Together, they form the Z‑scheme.
59
What is the role of phylloquinone (QK) and Fe‑S centers in PSI?
They are sequential electron carriers inside PSI that help transfer electrons to ferredoxin.
60
Why is paraquat deadly to plants?
It steals electrons from PSI before NADP⁺ can be reduced → forms ROS → damages membranes and kills cells.
61
Why does RuBisCO sometimes fix O₂ instead of CO₂?
Because RuBisCO has a dual specificity active site and O₂ competes with CO₂. Approximately 25% of reactions are with O₂.
62
Why is photorespiration costly?
The O₂‑fixation product must be salvaged via multi‑step, energy‑consuming pathways → reduces net carbon gain.
63
Why is the activator CO₂ required for RuBisCO?
It binds to an essential lysine, forming a carbamate → coordinates Mg²⁺ → activates the enzyme.
64
Why are seeds able to grow before they can photosynthesize?
Because the glyoxylate cycle converts stored lipids → sucrose for energy + transport.
65
What organelles cooperate during seedling lipid‑to‑sugar conversion?
Glyoxysomes (glyoxylate cycle) Mitochondria (succinate → malate) Cytosol (gluconeogenesis → sucrose export)
66
What is the single triose phosphate that exits the Calvin cycle per 3 CO₂ fixed?
G3P (glyceraldehyde‑3‑phosphate)
67
Why is ATP synthase orientation NOT reversed in chloroplasts vs mitochondria?
The gradient direction differs, but ATP synthase is ALWAYS oriented so ATP is made into the compartment with biosynthetic enzymes (matrix/stroma/cytosol).
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