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

Published by Patrick Mutisya · 8 days ago

Photosynthesis – Energy Transfer Process

Photosynthesis as an Energy Transfer Process

Learning Objective

Explain how photophosphorylation couples the flow of energetic electrons through the electron transport chain (ETC) to the synthesis of ATP by ATP synthase.

Key Concepts

  • Energetic electrons released from water are excited by light energy.

  • As electrons move through the ETC, they lose energy.

  • The energy released is used to pump protons (H⁺) from the stroma into the thylakoid lumen, creating a proton electrochemical gradient.

  • Protons flow back to the stroma through ATP synthase by facilitated diffusion.

  • The flow of protons drives the conversion of ADP + Pᵢ into ATP.

Step‑by‑Step Description of Photophosphorylation

  1. Photon absorption and water splitting: Light excites electrons in photosystem II; water is oxidised, releasing O₂, electrons and H⁺ that enter the ETC.

  2. Electron transport: Electrons pass through a series of carriers in the thylakoid membrane, releasing energy at each step.

  3. Proton pumping: The released energy is used to move H⁺ from the stroma into the thylakoid lumen, building up a proton gradient.

  4. ATP synthesis: H⁺ return to the stroma through ATP synthase; the energy of this flow drives the phosphorylation of ADP.

Energy Transformations

The overall reaction for photophosphorylation can be written as:

\$\text{ADP} + \text{P}{i} + n\text{H}^{+}{\text{lumen}} \;\xrightarrow{\text{ATP synthase}}\; \text{ATP} + n\text{H}^{+}_{\text{stroma}}\$

where n represents the number of protons that pass through ATP synthase for each ATP formed (typically 3–4).

Summary Table

StageLocationMain EventEnergy Outcome
Photon absorption & water splittingPhotosystem II (thylakoid membrane)Light excites electrons; H₂O → O₂ + 2e⁻ + 2H⁺Conversion of light energy to electronic excitation
Electron transportThylakoid membrane (ETC)Electrons move through carriers, losing energyEnergy released for proton pumping
Proton pumpingThylakoid lumenH⁺ moved from stroma → lumenCreation of electrochemical gradient (ΔpH, Δψ)
ATP synthesisATP synthase (thylakoid membrane)H⁺ flow back to stroma via ATP synthaseADP + Pᵢ → ATP (chemical energy)

Suggested diagram: Schematic of the thylakoid membrane showing the electron transport chain, proton gradient, and ATP synthase.

Key Points to Remember

  • The ETC does not store energy; it transfers it from high‑energy electrons to the proton gradient.

  • ATP synthase works like a rotary motor; the passage of protons drives conformational changes that bind ADP and Pᵢ and release ATP.

  • Photophosphorylation is a type of chemiosmosis – coupling an electrochemical gradient to ATP synthesis.