  The distributions of transverse momentum and longitudinal momentum fraction
of charged particles in jets are measured in Pb+Pb and pp collisions with the
ATLAS detector at the LHC. The distributions are measured as a function of jet
transverse momentum and rapidity. The analysis utilises an integrated
luminosity of 0.14 nb$^{-1}$ of Pb+Pb data and 4.0 pb$^{-1}$ of pp data
collected in 2011 and 2013, respectively, at the same centre-of-mass energy of
2.76 TeV per colliding nucleon pair. The distributions measured in pp
collisions are used as a reference for those measured in Pb+Pb collisions in
order to evaluate the impact on the internal structure of jets from the jet
energy loss of fast partons propagating through the hot, dense medium created
in heavy-ion collisions. Modest but significant centrality-dependent
modifications of fragmentation functions in Pb+Pb collisions with respect to
those in pp collisions are seen. No significant dependence of modifications on
jet $p_{\mathrm{T}}$ and rapidity selections is observed except for the
fragments with the highest transverse momenta for which some reduction of
yields is observed for more forward jets.
