After excitement, $e^-$ is passed from PS-II ($P_{680}$) to primary electron acceptor (Pheophytin). From primary $e^-$ acceptor, $e^-$ is passed to plastoquinone. Plastoquinone (PQ) in turn transfer its $e^-$ to Cyt $b_6f$ complex. Therefore plastoquinone facilitates the transfer of electrons from PS-II to Cyt $b_6f$ complex.