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 · 14 days ago

Cambridge A-Level Biology 9700 – Photosynthesis as an Energy Transfer Process

Photosynthesis as an Energy Transfer Process

Learning Objective

Explain how, during photophosphorylation, energetic electrons release energy as they pass through the electron transport chain (ETC), how this energy is used to move protons across the thylakoid membrane, and how the return flow of protons through ATP synthase drives the synthesis of ATP.

Key Concepts

  • Light energy excites electrons in chlorophyll molecules of photosystem II.

  • Excited (high‑energy) electrons travel through a series of carriers in the thylakoid membrane (the ETC).

  • The drop in electron energy is harnessed to pump protons (\$\mathrm{H^+}\$) from the stroma into the thylakoid lumen.

  • A proton gradient (electrochemical potential) is established across the thylakoid membrane.

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

  • The energy released by this proton flow is used to phosphorylate ADP to ATP.

Step‑by‑Step Description of Photophosphorylation

  1. Excitation of electrons: Photons are absorbed by chlorophyll a in photosystem II, raising electrons to a higher energy level.

  2. Electron transport: The high‑energy electrons move through the ETC. As they pass from one carrier to the next, they lose energy.

  3. Proton pumping: The energy released from the electrons is used to transport \$\mathrm{H^+}\$ ions from the stroma into the thylakoid lumen, creating a proton gradient.

  4. Formation of the proton motive force: The accumulation of \$\mathrm{H^+}\$ inside the lumen generates both a concentration gradient and an electrical potential across the membrane.

  5. ATP synthesis: \$\mathrm{H^+}\$ ions flow back into the stroma through the channel of ATP synthase. The movement of protons drives the conversion of ADP + \$P_i\$ into ATP.

Overall Reaction for the Light‑Dependent Stage

\$\$

2\,\text{H}2\text{O} + 2\,\text{NADP}^+ + 3\,\text{ADP} + 3\,Pi + \text{light energy}

\;\longrightarrow\;

O_2 + 2\,\text{NADPH} + 3\,\text{ATP}

\$\$

Summary Table

ProcessEnergy SourceResulting ChangeBiological Significance
Electron excitation in PSIIPhotons (light)Electrons raised to higher energyInitiates electron flow
Electron transport through ETCEnergy of excited electronsRelease of energy; electrons move to lower‑energy carriersProvides energy for proton pumping
Proton pumping into thylakoid lumenEnergy released from electronsBuilds \$\mathrm{H^+}\$ gradient (proton motive force)Stores energy as electrochemical gradient
Proton flow through ATP synthaseProton motive forceADP + \$P_i\$ → ATPProvides chemical energy for the Calvin‑Benson cycle

Suggested diagram: Schematic of the thylakoid membrane showing photosystem II, the electron transport chain, proton pumping into the lumen, and ATP synthase allowing proton return to the stroma.

Key Points to Remember

  • The ETC does not store energy; it transfers it from excited electrons to protons.

  • Only the movement of protons across the membrane stores usable energy (the proton gradient).

  • ATP synthase acts like a molecular turbine: the flow of protons drives the synthesis of ATP.

  • Details of the individual electron carriers and the structural subunits of ATP synthase are not required for this objective.