The smaller they are the faster. Ions and large polar molecules will NOT pass. Concentration change in one direction. Overall movement down the gradient even though molecules are moving randomly. Continues until a dynamic equilibrium is reached.
Many molecules move across membranes via diffusion.
Cell does not expend energy, Energy is from potential energy of conc. Water diffuses down its concentration gradient. Diffusion with the help of a transport protein. Set up by an electrogenic pump. Transport of Large Molecules and particles.