Plastoquinone Qb (magenta) transports two electrons to the second complex in the photosynthetic ETC--Cytochrome b6-f.

When it reaches cytochrome b6-f (yellow), Qb releases the two protons it picked up from the stroma into the lumen space and transfers the two electrons it picked up from photosystem II to the cytochrome complex.

Coupled with the transfer of each electron, is the pumping of another hydrogen ion across the thylakoid membrane into the lumen by the cytochrome complex.

After cytochrome b6-f pumps the additional protons across the membrane, each electron is transferred to plastocyanin, the next mobile carrier (dark blue).