Photosynthesis - chloroplast structure and light reactions Flashcards
(20 cards)
What is the substructure of a chloroplast?
Outer and inner envelope membranes: Control exchange with the cytoplasm.
Thylakoid membrane: Site of the light-dependent reactions.
Grana: Stacks of thylakoids (appressed regions).
Lamellae: Unstacked thylakoid membranes connecting grana.
Stroma: Fluid-filled space where the Calvin cycle (light-independent reactions) occurs.
Thylakoid lumen: Internal space of the thylakoid where protons accumulate.
How does chloroplast structure enable electron transport and ATP/NADPH production? (what is embedded in membrane, proton gradient, reducing power?)
Thylakoid membranes house the electron transport chain and photosystems.
Photosystems I and II, cytochrome b6f, and ATP synthase are embedded in the thylakoid membrane.
Protons (H⁺) are pumped into the thylakoid lumen, creating a proton gradient.
This gradient powers ATP synthase, which produces ATP.
Electrons are transferred through protein complexes to ultimately reduce NADP⁺ to NADPH.
What is the role of pigments and what wavelengths are important? (4 key pigments, absorptions?)
Pigments absorb light energy for photosynthesis:
Chlorophyll a (420–660 nm) – core pigment in all higher plants and algae.
Chlorophyll b (435–643 nm) – accessory pigment.
Carotenoids (e.g., β-carotene) – broaden light absorption and protect from photo-damage.
Phycobilins – in cyanobacteria and red algae.
Each pigment absorbs different wavelengths, maximizing energy capture across the light spectrum.
What is ‘resonance energy transfer’ and how does it feature in photosynthesis?
Resonance energy transfer is the non-radiative transfer of energy between adjacent pigment molecules.
It enables light-harvesting complexes to pass energy to a reaction center chlorophyll (P680 or P700).
No electrons move here—only energy is transferred until the reaction center is excited.
What happens to the electron(s) during electron transport? (what are the 4 structures passed to, how does it then make NADPH)
Excited electrons from P680 (Photosystem II) are passed to:
Pheophytin
Plastoquinone A (QA)
Plastoquinone B (QB)
Plastoquinol (PQH₂) – carries electrons and protons to the cytochrome b6f complex
From there, electrons go to plastocyanin, then Photosystem I, then ferredoxin, then to NADP⁺ reductase, which makes NADPH.
How is the electron in P680 replaced? (Splitting of water by manganese cluster)(OEC)
The oxygen-evolving complex (OEC) in Photosystem II splits water:
2H2O→4H++4e−+O22H2 O→4H++4e−+O2
The manganese cluster provides the oxidation power to extract electrons from water.
These electrons replace those lost by P680 during light excitation.
What is the role of cytochrome b6f complex and how does it increase the proton gradient?
Accepts electrons from plastoquinol (PQH₂).
Transfers electrons to plastocyanin.
Pumps protons (H⁺) from the stroma into the thylakoid lumen, increasing the proton gradient.
This proton motive force is used by ATP synthase to make ATP.
What is the fate of the electron(s) from electron transport and the proton gradient?
Electrons reduce NADP⁺ to NADPH (used in the Calvin cycle).
The proton gradient powers ATP synthase (CF₀-CF₁), generating ATP from ADP + Pi.
Both ATP and NADPH are used to fix CO₂ into glucose during the light-independent reactions.
Summary Reaction of Photosynthesis (ATP, NADPH)
6CO2+6H2O+light→C6H12O6+6O26CO2 +6H2 O+light→C6 H12 O6 +6O2
ATP: Made via H⁺ gradient across thylakoid membrane.
NADPH: Made via electron transport, storing high-energy electrons.
Both are used to synthesize glucose.
What is LHCII (Light Harvesting Complex II)?
A tetrameric pigment-protein complex embedded in the thylakoid membrane.
Contains high concentrations of chlorophyll and carotenoids to absorb light.
Transfers absorbed energy to Photosystem II (PSII).
Can move laterally in the membrane and associate with PSI or PSII depending on the phosphorylation state, helping to balance excitation between the two photosystems (called state transitions).
What happens in the 3 stages of the carbon cycle?
The Calvin cycle (or PCR cycle) consists of three stages, using ATP and NADPH from the light reactions:
- Carboxylation:
RuBISCO fixes CO₂ to ribulose-1,5-bisphosphate (RuBP).
Produces 3-phosphoglyceric acid (PGA).
- Reduction:
PGA is phosphorylated by ATP → 1,3-bisphosphoglycerate (BPG).
BPG is reduced by NADPH → G3P (glyceraldehyde-3-phosphate) and DHAP (dihydroxyacetone phosphate).
These 3-carbon molecules are collectively called triose phosphate.
3. Regeneration:
5 out of 6 triose phosphates are used to regenerate RuBP via a complex enzyme network, requiring ATP.
What is RuBISCO and what does it do?
RuBISCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase):
Catalyzes the first step of carbon fixation: CO₂ + RuBP → 2 PGA.
Also catalyzes oxygenation, starting photorespiration.
It has:
8 large subunits (catalytic) from the chloroplast genome.
8 small subunits (structural) from the nuclear genome, imported via TOC/TIC complexes.
RuBISCO is regulated by light via thioredoxin and RuBISCO activase.
Why are these not truly “dark” reactions?
Despite the name, light is indirectly required because:
Many enzymes (like RuBISCO) are activated by light-regulated systems (e.g., via thioredoxin).
The Calvin cycle depends on ATP and NADPH from the light reactions. So, they are more accurately called “light-independent” reactions, not “dark.”
What happens to triose phosphate?
Triose phosphate has two major fates depending on plant needs:
Sucrose synthesis (in the cytosol):
When carbohydrate demand is high, triose phosphate is exported.
Pi is imported in exchange via the triose phosphate–phosphate translocator (antiporter).
Starch synthesis (in the chloroplast):
When carbohydrate needs are met, triose phosphate is retained.
Starch is synthesized in the stroma.
Only 1 out of 6 triose phosphates is used for sugar synthesis per cycle; the rest go back to regenerate RuBP.
What is the triose phosphate antiporter?
A transporter protein in the inner chloroplast membrane.
Exports triose phosphate (G3P/DHAP) to the cytosol in exchange for inorganic phosphate (Pi).
This maintains phosphate balance and supports sucrose synthesis.
If Pi is low, export slows and triose phosphate is used to make starch inside the chloroplast.
How is RuBP regenerated?
Regeneration uses 5 out of 6 triose phosphates.
A multi-step process involving rearrangements (e.g., aldolase, transketolase).
Requires ATP to phosphorylate intermediates and complete regeneration of RuBP (a 5-carbon sugar).
What is photorespiration?
RuBISCO sometimes adds O₂ to RuBP instead of CO₂.
This produces phosphoglycolate, a toxic by-product.
Plants must convert this into useful molecules via the photorespiratory pathway:
Involves chloroplasts, peroxisomes, and mitochondria.
Energy-intensive and releases CO₂, reducing photosynthetic efficiency.
Photorespiration is favored at high temperatures and O₂ levels.
What are C4 photosynthesis?
CO₂ initially fixed by PEP carboxylase into malate (4C) in mesophyll cells.
Malate is transported to bundle sheath cells.
RuBISCO operates in the bundle sheath, where O₂ is low, reducing photorespiration.
Common in hot, sunny environments (e.g., maize, sugarcane).
What are CAM photosynthesis?
Stomata open at night to take in CO₂, stored as malate.
During the day, stomata close to save water, and CO₂ is released from malate for fixation.
Found in succulents and desert plants (e.g., cacti).
