Why plastoquinone receives hydrogen ion from stroma and release it in the lumen during light reaction?

When an electron leaves PSII, it is transferred first to a small organic molecule (plastoquinone, Pq), then to a cytochrome complex (Cyt), and finally to a copper-containing protein called plastocyanin (Pc). As the electron moves through this electron transport chain, it goes from a higher to a lower energy level, releasing energy. Some of the energy is used to pump protons (H+\text H^+H+start text, H, end text, start superscript, plus, end superscript) from the stroma (outside of the thylakoid) into the thylakoid interior.

This transfer of H+\text H^+H+start text, H, end text, start superscript, plus, end superscript, along with the release of H+\text H^+H+start text, H, end text, start superscript, plus, end superscript from the splitting of water, forms a proton gradient that will be used to make ATP (as we'll see shortly).

The light-dependent reactions involve two photosytems (II and I) and an electron transport chain that are all embedded in the thylakoid membrane. Light that is harvested from PSII causes an excited electron of the chlorophyll a special pair to be passed down an electron transport chain (Pq, Cyt, and Pc) to PSI. The electron lost from the chlorophyll a special pair is replenished by splitting water.

The passing of the electron in the first part of the electron transport chain causes protons to be pumped from the stroma to the thylakoid lumen. A concentration gradient formed (with a higher concentration of protons in the thylakoid lumen than in the stroma). Protons diffuse out of the thylakoid lumen through the enzyme, ATP synthase, producing ATP in the process.

Once the electron reaches PSI, it joins its chlorophyll a special pair and re-excited by the absorption of light. It proceeds down a second part of the electron transport chain (Fd and NADP+^++start superscript, plus, end superscript reductase) and reduces NADP+^++start superscript, plus, end superscript to form NADPH. The electron lost from the chlorophyll a special pair is replenished by electrons flowing from PSII.

 

Once an electron has gone down the first leg of the electron transport chain, it arrives at PSI, where it joins the chlorophyll a special pair called P700. Because electrons have lost energy prior to their arrival at PSI, they must be re-energized through absorption of another photon.

Excited P700 is a very good electron donor, and it sends its electron down a short electron transport chain. In this series of reactions, the electron is first passed to a protein called ferredoxin (Fd), then transferred to an enzyme called NADP+^++start superscript, plus, end superscriptreductase. NADP+^++start superscript, plus, end superscript reductase transfers electrons to the electron carrier NADP+^++start superscript, plus, end superscript to make NADPH. NADPH will travel to theCalvin cycle, where its electrons are used to build sugars from carbon dioxide.

The other ingredient needed by the Calvin cycle is ATP, and this too is provided by the light reactions. As we saw above, H+\text H^+H+start text, H, end text, start superscript, plus, end superscript ions build inside the thylakoid interior and make a concentration gradient. Protons "want" to diffuse back down the gradient and into the stroma, and their only route of passage is through the enzyme ATP synthase. ATP synthase harnesses the flow of protons to make ATP from ADP and phosphate (Pi\text P_iPi​start text, P, end text, start subscript, i, end subscript). This process of making ATP using energy stored in a chemical gradient is called chemiosmosis.