  We develop a "hybrid" model of the proton inelastic structure functions
applicable in a wide region of invariant mass of produced states $W$ and
invariant momentum transfer $Q$ including deep inelastic scattering (DIS),
nucleon resonance production as well as the region close to inelastic
threshold. DIS is described in terms of the parton distributions together with
higher-twist corrections from an available global QCD fit. The resonant part is
addressed in terms of the Breit-Wiegner contributions from five states
including the $\Delta(1232)$ resonance, the $N(1440)$ Roper resonance, and
three effective resonances describing the second and third resonance regions.
The couplings of the nucleon resonances to photon are described in terms of
helicity amplitudes. The nonresonant background is addressed in terms of DIS
structure functions smoothly extrapolated to low-$W$ and low-$Q$ values with
the proper behavior at the real photon limit $Q^2=0$ as well as near the
inelastic threshold. We independently treat the transverse $F_T$ and the
longitudinal $F_L$ structure function and fix the model parameters from a
global analysis of the world hydrogen electroproduction and photoproduction
cross-section data. We demonstrate a very good performance of the model by
comparing our predictions with data on differential cross sections and the
structure functions $F_2$ and $R=F_L/F_T$.
