Chloroplast & Light Reactions (Thylakoid Membrane)
This interactive diagram covers 16 structures of the chloroplast and the light-dependent reactions of photosynthesis (AP Biology Unit 3). The upper section shows the overall chloroplast architecture; the lower zoom reveals the four protein complexes of the thylakoid electron transport chain in the order electrons flow: PSII → Cyt b6f → PSI → ATP synthase.
Outer Membrane
The smooth, freely permeable outer boundary of the chloroplast envelope. Porins allow small molecules (up to ~10 kDa) to pass without active transport. Does not control selective metabolite exchange — that role belongs to the inner membrane.
Inner Membrane
The selectively permeable inner layer of the chloroplast envelope. Transporter proteins — especially the triose phosphate/phosphate translocator — regulate the export of Calvin cycle products (G3P) to the cytoplasm and the import of inorganic phosphate. Not the site of the light reactions.
Stroma
The aqueous fluid inside the chloroplast (enclosed by the inner membrane). Site of the Calvin cycle (carbon fixation, reduction, RuBP regeneration). Receives ATP and NADPH from the thylakoid membrane and releases G3P. Contains chloroplast DNA, ribosomes, and Calvin cycle enzymes including RuBisCO.
Granum (Thylakoid Stack)
A stack of 5–20 flattened thylakoid discs interconnected by stroma lamellae. Stacking increases membrane surface area for light-harvesting complexes. A single chloroplast contains 10–100 grana.
Thylakoid Membrane
The inner membrane system where all four light-reaction protein complexes are embedded. Creates a sealed lumen compartment that accumulates H⁺, driving ATP synthesis. Analogous to the inner mitochondrial membrane.
Thylakoid Lumen
The aqueous interior space enclosed by the thylakoid membrane. Receives H⁺ from water splitting (at PSII) and from plastoquinol oxidation (at Cyt b6f), becoming acidic (pH ~5) during illumination. This H⁺ reservoir powers CF₁ ATP synthase.
Photosystem II (PSII)
P680 — the first complex in linear electron flow. Absorbs photons, uses the energy to oxidize water (\(\ce{2H2O -> O2 + 4H+ + 4e-}\)), and passes electrons to plastoquinone. The oxygen-evolving complex on the lumenal face coordinates a Mn₄CaO₅ cluster to accomplish water splitting.
H₂O → O₂ (Water Splitting)
Photolysis of water at the oxygen-evolving complex of PSII. Two water molecules yield one O₂ molecule, four H⁺ ions (released into the lumen), and four electrons (passed to P680). All atmospheric O₂ on Earth originates from this reaction.
Plastoquinone (PQ)
A small, hydrophobic mobile carrier embedded in the thylakoid membrane lipid bilayer. Accepts 2e⁻ and 2H⁺ from PSII to become plastoquinol (PQH₂), diffuses laterally to the cytochrome b6f complex, and releases its H⁺ into the lumen — directly contributing to the proton gradient that drives ATP synthesis.
Cytochrome b6f Complex
The central "hub" of the light reactions — analogous to Complex III of the mitochondrial ETC. Oxidizes plastoquinol, pumps additional H⁺ into the lumen via the Q-cycle, and reduces plastocyanin. The rate-limiting step of linear electron flow. Also the branching point for cyclic electron flow.
Plastocyanin (PC)
A small, copper-containing soluble protein in the thylakoid lumen. Shuttles one electron at a time from the cytochrome b6f complex to the P700⁺ reaction center of PSI. Its copper alternates between Cu⁺ (reduced) and Cu²⁺ (oxidized) with each electron transfer.
Photosystem I (PSI)
P700 — the second light-driven reaction center. Re-energizes electrons received from plastocyanin and passes them via ferredoxin to FNR, which reduces \(\ce{NADP+ + H+ + 2e- -> NADPH}\). PSI is also the site of cyclic electron flow, which produces additional ATP without NADPH or O₂.
Ferredoxin (Fd)
A small, soluble iron-sulfur protein on the stromal face of PSI. Accepts high-energy electrons from P700 one at a time and delivers them to FNR (ferredoxin-NADP⁺ reductase) for NADPH synthesis. Also donates electrons to the cytochrome b6f complex during cyclic electron flow.
NADP⁺ / NADPH
The terminal electron acceptor of the light reactions. FNR pairs two single-electron transfers from ferredoxin to reduce one NADP⁺ to NADPH. NADPH is consumed in the Calvin cycle to reduce 3-PGA to G3P via the enzyme GAPDH. It is analogous to NADH in cellular respiration but carries electrons at a higher energy state.
ATP Synthase (CF₁)
Chloroplast ATP synthase (CF₀CF₁) uses H⁺ flow from lumen to stroma to phosphorylate ADP + Pᵢ → ATP in the stroma. The CF₀ transmembrane domain forms the H⁺ channel; the CF₁ catalytic head protrudes into the stroma. Rotation of the central γ-subunit drives conformational changes in the three catalytic β-subunits, each cycling through open, loose, and tight conformations (binding change mechanism).
Proton Gradient (H⁺ Flow)
The proton-motive force across the thylakoid membrane is the driving force for photophosphorylation. H⁺ accumulates in the lumen from three sources: water splitting at PSII, the Q-cycle at cytochrome b6f, and H⁺ consumption in the stroma by NADPH synthesis and Calvin cycle reactions. The resulting ΔpH (~3 units) provides the energy for ATP synthase to produce ATP from ADP + Pᵢ.