explain that during photophosphorylation: energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected), the released energy is used to transfer protons across the thylakoid membrane, prot

Photosynthesis as an Energy‑Transfer Process (Cambridge 9700 A‑Level)

Learning Objective

Explain how, during photophosphorylation, energetic electrons released from chlorophyll transfer their energy to pump protons across the thylakoid membrane, how the resulting proton‑motive force drives ATP synthesis, and how this links to NADPH formation, the Calvin‑Benson cycle and the overall stoichiometry of photosynthesis.

1. Organisation of the Chloroplast (Syllabus 13.1)

• Double‑membrane envelope – encloses the organelle and separates the internal stroma from the external cytosol.
• Stroma – aqueous matrix that contains the enzymes of the Calvin‑Benson cycle, chloroplast DNA, ribosomes and soluble electron carriers (e.g. ferredoxin).
• Thylakoid system – internal membrane system where light‑dependent reactions occur.

  • Granal thylakoids (stacks = grana) – provide a large surface area for PSII, the oxygen‑evolving complex and the bulk of the electron‑transport chain. Function: high density of PSII maximises water‑splitting and initial electron donation.
  • Stromal (lamellar) thylakoids – unstacked membranes that house PSI and the ATP synthase complex. Function: spatial separation allows the two photosystems to operate optimally and prevents interference between water oxidation and NADP⁺ reduction.
  • The thylakoid membrane is the site of all light‑dependent reactions; the lumen is the interior aqueous space.

• Pigment localisation – chlorophyll a, chlorophyll b and carotenoids are embedded in the protein complexes of PSI and PSII within the thylakoid membrane, positioning them to capture light energy efficiently.

2. Light‑Dependent Reactions (Syllabus 13.1)

2.1. Photon Absorption and Excitation

• Blue (≈ 430 nm) and red (≈ 660 nm) photons are absorbed by the pigment‑protein complexes (action spectrum of chlorophyll a).
• Absorption promotes an electron in the reaction‑centre chlorophyll to an excited state:

  • PSII: P680 → P680*
  • PSI: P700 → P700*

• The excited electron possesses a higher redox potential; as it moves to lower‑energy carriers it releases energy that is used to pump protons.

2.2. Water‑Splitting (Oxygen‑Evolving Complex, OEC) – PSII

• The oxidised P680⁺ extracts electrons from H₂O via the Mn‑Ca cluster of the OEC (four‑metal centre).
• Overall reaction:
  

  2 H₂O → 4 H⁺ + 4 e⁻ + O₂

• Four protons are released into the thylakoid lumen, contributing directly to the proton gradient.
• Electrons enter the photosynthetic electron‑transport chain (ETC).

2.3. Linear (Non‑Cyclic) Electron Transport – Photophosphorylation

1. PSII → Plastoquinone (PQ) – The excited electron from P680* is transferred to the primary electron acceptor and then to PQ, reducing it to plastoquinol (PQH₂). The reduction releases enough energy to pump H⁺ from the stroma into the lumen.
2. PQH₂ → Cytochrome b₆f complex – PQH₂ is oxidised; the electrons flow to the b₆f complex. The b₆f complex uses the energy of this electron transfer to pump additional protons into the lumen (Q‑cycle).
3. b₆f → Plastocyanin (PC) – Electrons are handed to the soluble copper protein PC, which diffuses to PSI.
4. PSI → Ferredoxin (Fd) – Light absorbed by PSI excites P700*. The high‑energy electron is passed to Fd.
5. Fd → NADP⁺ (Ferredoxin‑NADP⁺ reductase, FNR) – Fd reduces NADP⁺ + H⁺ to NADPH. No further proton pumping occurs at this step.

During each electron transfer step the drop in redox potential releases energy that is harnessed by the membrane‑embedded complexes to move protons from the stroma into the lumen, building the proton‑motive force.

2.4. Cyclic Electron Transport – ATP‑Only Pathway

• When the Calvin cycle requires more ATP than NADPH, reduced Fd can return its electron to the PQ pool instead of reducing NADP⁺.
• The electron then follows the same route through the b₆f complex and PC back to PSI, pumping additional protons each time it passes b₆f.
• Result: extra H⁺ gradient → extra ATP; no NADPH is produced.

2.5. Proton‑Motive Force (pmf) and ATP Synthesis

• Proton pumping creates:

  • a concentration gradient (ΔpH, higher [H⁺] in the lumen)
  • an electrical potential (Δψ, lumen becomes positively charged relative to the stroma)

• The combined electrochemical gradient is the proton‑motive force.
• ATP synthase (CF₁CF₀) provides a channel for H⁺ to flow back to the stroma. The flow drives rotation of the γ‑subunit, catalysing ADP + Pᵢ → ATP.
• Approximately 3 H⁺ are required to synthesise one ATP molecule in chloroplasts.

3. Quantitative Aspects (Syllabus 13.1 – stoichiometry)

4. Connection to the Light‑Independent (Calvin‑Benson) Cycle (Syllabus 13.2)

• ATP and NADPH diffuse from the thylakoid membrane into the stroma.
• They supply the chemical energy (ATP) and reducing power (NADPH) required for CO₂ fixation:

  • 3 ATP + 2 NADPH are needed to convert 3 CO₂ into one molecule of glyceraldehyde‑3‑phosphate (G3P).
  • Six G3P molecules yield one glucose; overall requirement is 18 ATP and 12 NADPH per glucose.

• The Calvin cycle also regenerates ribulose‑1,5‑bisphosphate (RuBP), completing the cycle and allowing continuous CO₂ assimilation.

5. Pigment Absorption & Action Spectra (Syllabus 13.1)

• Chlorophyll a – peaks at ≈ 430 nm (blue) and ≈ 660 nm (red).
• Chlorophyll b – peaks at ≈ 453 nm and ≈ 642 nm; extends the range of usable light.
• Carotenoids – absorb 400–500 nm; protect the photosystems from excess light and pass energy to chlorophyll a.
• Students should be able to interpret a typical absorption spectrum and explain why green light is largely reflected (chlorophylls absorb poorly in the 500–600 nm region).

6. Experimental Investigations (Cambridge 13.2)

7. Summary Table – From Photon to Sugar

8. Key Points to Remember (Exam Checklist)

• The thylakoid membrane separates light‑dependent (energy capture) and light‑independent (CO₂ fixation) stages.
• Non‑cyclic photophosphorylation produces both ATP and NADPH; cyclic photophosphorylation produces ATP only.
• Water is the electron donor; O₂ is a by‑product of PSII.
• Energetic electrons do not store energy in the carriers; the energy is released as they move to lower‑energy carriers and is used to pump protons.
• The proton gradient (ΔpH + Δψ) is the immediate energy store; ATP synthase converts this into chemical energy.
• Understanding pigment absorption, quantitative yields, and the purpose of experimental investigations is essential for Cambridge exam questions.