It shouldn't be able to diffuse through the membrane. For example, in the nephron, urea can be reabsorbed from the collecting duct in the presence of ADH but not via diffusion through the membrane. A transporter must be present; otherwise, urea reabsorption wouldn't be regulated since it would passively diffuse through the membrane all the time depending on the concentration gradient (which is high in the collecting duct and low in the interstitial fluid).