Neutral molecules are transpoed by

Simple diffusion occurs from higher to lower concentration. This does not require any energy. It is a very slow process.Diffusion of gases such as O2, CO2, NO and CO occurs at a rate that is solely dependent upon concentration gradients.Lipophilic molecules will also diffuse across membranes at a rate that is directly propoional to the solubility of the compound in the membrane.Ref: D M Vasudevan 7th edition Page no: 17