<prompt> A factor favouring the transport across a cell membranePolar substanceHigh concentration gradientThick membraneBig particle size  <response>: Simple diffusion refers to the diffusion of the molecule across the membrane following a concentration gradient or chemical gradient, but without the help of any carrier protein. The rate of diffusion depends on the following factors : -A The rate of diffusion is directly proportionatei The concentration chemical gradientii The cross-sectional area of the membrane through which diffusion takes placeiii Lipid solubility of the substanceB The rate of diffusion is inversely proportionate i The thickness of the diffusion membraneii Size of the particleiii Charge or polarity of the substance So, simple diffusion is favoured by small size, lipid solubility and absence of polarity non-polar substance and charge neutral molecule through a thin, large membrane where the concentration gradient is more. 