Table of Contents
Overview: From Light Capture to ATP and NADPH
In the light-dependent reactions of photosynthesis, light energy is converted into chemical energy stored in $ATP$ and $NADPH$. This sequence follows a defined path through the thylakoid membrane, mainly involving photosystem II (PSII), the cytochrome $b_6f$ complex, photosystem I (PSI), and ATP synthase.
Here we focus on the chronological order of events—what happens first, next, and last—without re-explaining structures or concepts that are treated in other chapters (e.g., chloroplast structure, photosystem architecture, or the Calvin cycle).
1. Excitation and Water Splitting in Photosystem II
- Photon absorption by antenna pigments (PSII)
- Light hits pigment molecules in the light-harvesting complex of PSII.
- The energy is transferred between pigments until it reaches the reaction center (P680).
- Excitation of P680 and primary charge separation
- The special chlorophyll pair P680 absorbs energy and is excited to $P680^\*$.
- $P680^\*$ donates a high-energy electron to a primary electron acceptor (often denoted $Q_A$).
- P680 becomes oxidized to $P680^+$ (a very strong oxidant).
- Water splitting (photolysis) and oxygen evolution
- The oxygen-evolving complex (OEC), also called the water-splitting complex, is attached to PSII.
- It extracts electrons from water to reduce $P680^+$ back to P680, step by step:
$\,H_2O \rightarrow 4\,H^+ + 4\,e^- + O_2$$ - Electrons go to $P680^+$, protons ($H^+$) are released into the thylakoid lumen, and molecular oxygen ($O_2$) diffuses away.
Result at this stage:
- One electron has been moved from P680 to the primary acceptor.
- P680 has been re-reduced using electrons from water.
- Protons have been added to the lumen, and $O_2$ has been produced.
2. Electron Transfer from PSII to the Cytochrome $b_6f$ Complex
- Plastoquinone binding and reduction
- The primary acceptor passes the electron to a mobile plastoquinone ($PQ$) at the PSII $Q_B$ site.
- $PQ$ gets reduced stepwise to plastoquinol ($PQH_2$):
$$PQ + 2\,e^- + 2\,H^+_{\text{stroma}} \rightarrow PQH_2$$ - The protons taken up come from the stroma.
- Diffusion of $PQH_2$ in the membrane
- $PQH_2$ is lipid-soluble and moves within the thylakoid membrane to the cytochrome $b_6f$ complex.
So far:
- Electrons have moved from PSII to $PQH_2$.
- Protons have been removed from the stroma and are now carried by $PQH_2$.
3. Proton Pumping via Cytochrome $b_6f$
- Oxidation of $PQH_2$ and proton release into the lumen
- At cytochrome $b_6f$, $PQH_2$ is oxidized back to $PQ$:
$$PQH_2 \rightarrow PQ + 2\,e^- + 2\,H^+_{\text{lumen}}$$ - The two protons are released into the thylakoid lumen, increasing the proton concentration there.
- Electron transfer within cytochrome $b_6f$
- The electrons move through redox centers in cytochrome $b_6f$ (including cytochromes and iron–sulfur proteins).
- Part of this transfer is via a “Q cycle”-like mechanism that enhances proton translocation (details are not needed here in full).
- Transfer to plastocyanin (PC)
- Finally, electrons are transferred from cytochrome $b_6f$ to plastocyanin, a small water‑soluble copper protein in the lumen.
- Reduced plastocyanin (PC–red) diffuses along the lumen side of the membrane toward PSI.
At this point:
- Additional protons have been added to the lumen (from water splitting and from $PQH_2$).
- Electrons are now carried by plastocyanin toward PSI.
4. Excitation and Electron Transport in Photosystem I
- Delivery of electrons to PSI
- Reduced plastocyanin donates its electron to oxidized PSI reaction center (P700).
- P700 receives the electron and is in its ground (non-excited) state, ready for a new light event.
- Photon absorption by PSI
- Light is absorbed by antenna pigments in PSI, and energy is funneled to the reaction center chlorophyll pair P700.
- P700 becomes excited to $P700^\*$.
- Primary charge separation at PSI
- $P700^\*$ transfers its high‑energy electron to a primary acceptor (e.g., a chlorophyll or phylloquinone, often denoted $A_0$ or $A_1$).
- P700 becomes oxidized to $P700^+$.
- Refilling P700 from plastocyanin
- The oxidized P700$^+$ is reduced back to P700 by accepting an electron from reduced plastocyanin.
- Thus, electrons arriving from PSII (via $PQH_2$ and cytochrome $b_6f$) ultimately “refill” P700, allowing repeated excitation cycles.
Outcome so far:
- The PSI reaction center has boosted electron energy a second time.
- An energized electron now moves “downhill” through the PSI acceptor chain.
5. Reduction of NADP⁺ to NADPH
- Electron flow through PSI acceptors
- From the primary acceptor, electrons are passed through a series of iron–sulfur centers (e.g., $F_X, F_A, F_B$) in PSI.
- They eventually reach the soluble protein ferredoxin (Fd) on the stromal side.
- Action of ferredoxin–NADP⁺ reductase (FNR)
- Reduced ferredoxin carries electrons to the enzyme ferredoxin–NADP⁺ reductase, which is associated with the stromal side of the thylakoid membrane.
- FNR catalyzes the transfer of two electrons and one proton to $NADP^+$:
$$NADP^+ + 2\,e^- + H^+_{\text{stroma}} \rightarrow NADPH$$
Result:
- Each pair of high‑energy electrons coming through PSI reduces one molecule of $NADP^+$ to $NADPH$.
- The proton for $NADPH$ is taken from the stroma, slightly lowering stromal proton concentration.
6. Building the Proton Gradient
While electrons flow from water to $NADP^+$, a proton gradient is built across the thylakoid membrane. The main contributors are:
- Water splitting at PSII
- Adds protons directly into the lumen.
- Plastoquinone/PQH₂ cycle at PSII and cytochrome $b_6f$
- Protons are taken up from the stroma by $PQ$ and released into the lumen when $PQH_2$ is oxidized.
- Small effect from NADPH formation
- A proton is consumed in the stroma when $NADP^+$ is reduced, which also contributes (modestly) to the gradient.
Altogether, this establishes:
- High [H⁺] in the lumen
- Low [H⁺] in the stroma
This proton gradient represents stored potential energy (a proton‑motive force) across the thylakoid membrane.
7. ATP Synthesis by ATP Synthase
- Protons flow through ATP synthase
- Protons move down their concentration and electrical gradient from the lumen to the stroma through CF₀CF₁‑ATP synthase.
- Chemiosmotic coupling to ATP formation
- The flow of protons drives rotation and conformational changes in ATP synthase.
- This mechanical/structural change catalyzes:
$$ADP + P_i \rightarrow ATP$$
Thus, the proton gradient generated by electron flow is converted into the chemical bond energy of $ATP$.
8. Overall Linear Electron Flow (“Z‑Scheme”)
Putting the sequence together:
- Light excites PSII (P680) → electron transferred to primary acceptor.
- Water is split → electrons replace those lost from PSII; $O_2$ and $H^+$ in lumen are produced.
- Electrons move: PSII → $PQ \rightarrow PQH_2$ → cytochrome $b_6f$ → plastocyanin.
- Protons are pumped/accumulated in the lumen via water splitting and $PQH_2$ oxidation.
- Light excites PSI (P700) → high‑energy electrons are passed to ferredoxin.
- Ferredoxin reduces $NADP^+$ to $NADPH$ (via FNR).
- The proton gradient drives ATP synthase, forming $ATP$.
The overall linear (non‑cyclic) flow of electrons can be summarized as:
$$
2\,H_2O \;+\; 2\,NADP^+ \;+\; n\,ADP \;+\; n\,P_i \;+\; \text{light}
\;\longrightarrow\;
O_2 \;+\; 2\,NADPH \;+\; n\,ATP \;+\; 2\,H^+
$$
Here $n$ indicates the number of $ATP$ molecules formed per pair of electrons (this depends on system details and is not fixed to a single number).
9. Note on Cyclic Electron Flow Around PSI
In addition to the linear electron flow described above, cyclic electron flow can occur:
- Electrons from ferredoxin are routed back to the cytochrome $b_6f$ complex instead of to $NADP^+$.
- This route:
- Pumps additional protons into the lumen.
- Increases $ATP$ production.
- Produces no $NADPH$ and no $O_2$.
Sequence (simplified) for cyclic flow:
- PSI → ferredoxin → cytochrome $b_6f$ → plastocyanin → back to PSI.
This cyclic path modifies the ratio of ATP to NADPH produced by the light‑dependent reactions and is especially important when more ATP than NADPH is needed for downstream processes.