This work presents a simple molecular model of membrane transport processes generated by hydrostatic (DeltaP) and osmotic (Delta Pi) pressure. We postulate that the porous membrane permeability is determined by the size and distribution of its pores. From this postulate and a mechanistic interpretation of the flows we obtain equations for the volume flux J(v), the solute volume flux J(vs), and the solvent volume flux J(vw): J(v) = L-P DeltaP - L(P)sigma Delta Pi J(vs) = (1 - sigma) (c) over bar(v) over bar L-s(P)(Delta Pi + DeltaP) J(vw) = J(v) - (1 - sigma)(c) over bar(v) over bar L-s(P)(Delta Pi + DeltaP) We also observe that the porous membrane properties are described by two phenomenological parameters (L-P and sigma), as opposed to three parameters (L-P, sigma, and omega) in the standard Kedem-Katchalsky formalism. The extra parameter was eliminated using the correlation relation for the transport parameters found previously and modified in this work. It has a form: omega = (1 - sigma)(c) over barL(P)