Work done (E_z) to raise a unit test mass from the mid-plane to any vertical height as a function of z (shown upto z = 1 kpc here) at R = 8.5 kpc (solar neighbourhood) for the stars-alone case against its self-gravity (solid curve), for the gas-alone case against its self-gravity (dashed curve), and for stars or gas in the stars-plus-gas coupled system against their coupled gravity (dash-dotted curve). The work done required to raise a unit mass at any vertical height in the last case is highest. This shows that stars or gas in the coupled system are more strongly bound to the mid-plane than in the corresponding single-component cases.