We show with the model for the parton distribution in nuclei, that nuclear Fermi motion fully accounts for the collective motion of partons in nuclear medium. The sea parton distributions are described by additional virtual pions in interacting nucleon in such a way as to reproduce the nuclear lepton pair production data and saturate the energy-momentum sum rule. The influence of Fermi motion changes the nucleon rest energy and consequently the transverse momentum square of partons inside bound nucleons.

PACS numbers: 24.85.+p, 25.30.Mr