Electron transport chain presented as four molecule structures aligned in one row, embedded inside the thylakoid membrane. The molecules, in order from left to right, are Photosystem II, plastoquinone, cytochrome b6f, plastocyanin, photosystem I, and ATP synthase. On both photosystem I and II, two yellow beams are falling from the top of the image indicating light energy from photons. The space above the row of molecules is called ‘chloroplast stroma’, and the lower part is called ‘thylakoid lumen’. Two water molecules are split into an oxygen molecule, four protons, and 2 electrons in the thylakoid lumen. These two electrons are energized by the light energy from the photons in the photosystem II reaction center chlorophylls and move to plastoquinone, cytochrome b6f, plastocyanin and finally photosystem I where they are energized again through light energy. Attached to the chloroplast stroma end of photosystem I are ferredoxin and ferredoxin-NADP reductase. These two enzymes accept the two electrons and reduce NADP+ to NADPH. Finally, ATP synthase uses the protons accumulated in the thylakoid lumen to pump the enzyme and phosphorylate ADP into ATP.

Figure 1: Electron transport chain.

From photosystem II the electrons are first transferred to a plastoquinone which shuttles the electrons on the electron transport chain. The energy of the electrons is used to actively pump protons into the thylakoid lumen. The low energy electrons are then transferred to photosystem I, which uses light energy to re-energize the electrons to reduce NADP+ to NADPH.

Each electron carrier in the electron transport chain is both an electron acceptor and an electron donor. Plastoquinone, for example, is reduced by the reaction center, because it has a higher reduction potential than the previous electron donor in the reaction center. The next complex in line, the cytochrome proton pump, has once again a higher reduction potential. This drives the electrons to move along the electron transport chain and the energy of the electrons is used to pump protons into the thylakoid lumen.

This is the chemical equation for the light reaction:

2 H2O + 2 NADP+ + nADP + nPi → O2 + 2 NADPH + 2 H+ + nATP