Chapter 2: Photosynthesis — Light Reactions

Part I — Energy & Transport

2.1 Chlorophyll & Light Absorption

Chlorophylls are magnesium-containing tetrapyrrole pigments that absorb light in the blue (430–453 nm) and red (642–662 nm) regions, reflecting green light. Chlorophyll a (P680, P700) is the primary photochemical pigment; chlorophyll b acts as an accessory antenna. Carotenoids (carotenes, xanthophylls) absorb 400–530 nm and transfer energy to chlorophyll by resonance energy transfer (Förster mechanism).

Photosystem Organization:

  • Each PSII core contains ~250–300 chlorophyll molecules
  • LHCII trimer: most abundant membrane protein in biosphere
  • Energy funnels from antenna → reaction center in ~100 ps
  • Quantum efficiency of energy transfer: ~95–99%

Excitation Energy Transfer:

Förster resonance energy transfer rate:

\[k_{FET} = \frac{1}{\tau_D}\left(\frac{R_0}{r}\right)^6\]

R₀ = Förster radius (~4–8 nm for chlorophylls), τ_D = donor excited state lifetime, r = donor–acceptor distance.

2.2 Photosystem II: Water Splitting

PSII (P680) is the only enzyme that oxidizes water, releasing O₂ into the atmosphere. The oxygen-evolving complex (OEC) contains a Mn₄Ca₁O₅ cluster that cycles through five oxidation states (S₀–S₄, the Kok cycle):

\[2H_2O \rightarrow O_2 + 4H^+ + 4e^-\quad (\Delta G^{\circ} = +237\text{ kJ mol}^{-1})\]

Electron Transport from PSII:

  1. P680 absorbs photon → P680* (excited state)
  2. Charge separation: P680* → Pheo⁻ in ~3 ps
  3. Electron to QA (plastoquinone), then QB
  4. QB²⁻ + 2H⁺ → PQH₂ (plastoquinol)
  5. P680⁺ oxidizes TyrZ → OEC → water

Kok S-state Cycle:

Four sequential photochemical steps accumulate charge:

\(S_0 \xrightarrow{h\nu} S_1 \xrightarrow{h\nu} S_2 \xrightarrow{h\nu} S_3 \xrightarrow{h\nu} S_4 \rightarrow S_0 + O_2\)

Each Sₙ→Sₙ₊₁ transition removes one electron from Mn cluster. O₂ released at S₃→S₀.

2.3 Thylakoid Electron Transport Chain

Electrons flow from water through PSII to PSI in a thermodynamically downhill process (except at the two photochemical uphill steps), coupled to proton pumping across the thylakoid membrane.

ComponentEm (mV)Function
H₂O/O₂ (OEC)+820Electron donor — water oxidation
P680⁺/P680+1100Strongest biological oxidant known
Plastoquinone (PQ/PQH₂)0 to +100Mobile electron + proton carrier in membrane
Cytochrome b6f complex+100 to +370Q-cycle; pumps 2H⁺/e⁻; links PSII to PSI
Plastocyanin (PC)+370Soluble Cu-protein; shuttles e⁻ to PSI
P700⁺/P700*−1200PSI reaction center; strongest reductant in biology
Ferredoxin (Fd)−420Iron–sulfur protein; reduces NADP⁺
FNR (NADP⁺)−320Flavoenzyme; Fd:NADP⁺ oxidoreductase

2.4 Chemiosmotic ATP Synthesis

Proton pumping by PSII (via PQ/PQH₂ exchange) and cyt b6f (Q-cycle) creates a proton electrochemical gradient across the thylakoid membrane, expressed as the proton motive force:

\[\Delta\tilde{\mu}_{H^+} = -2.3RT\,\Delta pH + F\,\Delta\Psi\]

In chloroplasts, ΔpH dominates (~3–3.5 units, lumen pH ~5 vs stroma pH ~8), while ΔΨ is relatively small due to counterion movement (Cl⁻ efflux, Mg²⁺ influx). The chloroplast ATP synthase (CF₀CF₁) uses H⁺ flow down the gradient to synthesize ATP:

\[n_{H^+/ATP} \approx 4.7 \text{ (in vivo)}\quad\Rightarrow\quad ATP_{synthesized} = \frac{\Delta\tilde{\mu}_{H^+} \cdot n_{H^+}}{F}\]

CF₀CF₁ Structure:

  • CF₀: integral membrane, H⁺ channel (a-subunit + c-ring)
  • CF₁: peripheral catalytic domain (α₃β₃γδε)
  • c-ring of 14 subunits → ~4.7 H⁺/ATP
  • Rotary catalysis: binding change mechanism

ATP:NADPH Ratio:

Linear electron flow produces:

  • 1 NADPH per 2 electrons (from 2 photons in PSI)
  • ~1.28–1.43 ATP per NADPH (theoretical)
  • Calvin cycle requires ATP:NADPH = 1.5
  • Cyclic electron flow around PSI & PTOX make up deficit

Simulation: Light Reactions & Z-Scheme

Chlorophyll absorption spectra, light response curve with electron transport rate, and the Z-scheme redox potential diagram showing electron flow from water to NADP⁺.

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← Ch 1: Water TransportCh 3: Calvin Cycle →