  Using differential cross-section data of inclusive $W$ and $Z/\gamma^\ast$
boson production and of their production in association with jets, and
differential cross-section data of top-quark pair production in the lepton +
jets and dilepton channels in proton$-$proton collisions at the CERN Large
Hadron Collider at center-of-mass energies of $\sqrt{s}=7$ and 8 TeV, together
with deep inelastic scattering data from electron$-$proton collisions at the
HERA collider, several determinations of parton distribution functions of the
proton have been performed by the ATLAS experiment. The results are presented
here. The emphasis is given to the determination of the strange sea quark
distribution and its evolution over time.
