Plastoquinone is found within photosystem II in two specific binding sites, known as Q and Q. The plastoquinone at Q, the primary binding site, is very tightly bound, compared to the plastoquinone at Q, the secondary binding site, which is much more easily removed. Q is only transferred a single electron, so it has to transfer an electron to Q twice before Q is able to pick up two protons from the stroma and be replaced by another plastoquinone molecule. The protonated Q then joins a pool of free plastoquinone molecules in the membrane of the thylakoid. The free plastoquinone molecules eventually transfer electrons to the water-soluble plastocyanin so as to continue the light-dependent reactions. There are additional plastoquinone binding sites within photosystem II (Q and possibly Q), but their function and/or existence have not been fully elucidated.