Smaller molecules diffuse faster than larger molecules. The smaller molecules are able to move more quickly at a given temperature than larger molecules, allowing them diffuse across the membrane with greater speed.
The size of a molecule is not the only factor that can influence the rate of diffusion. Molecules diffuse faster as temperature increases. Molecules also diffuse across a membrane with greater speed as the difference of the concentrations on both sides of the membrane increases. Molecules diffuse quickly across larger surface areas and move more slowly at greater distances. Some membranes can be more permeable than others, increasing the rate of diffusion.
When molecules are released into an environment, they tend to spread out to less concentrated areas. This process is called diffusion. Diffusion is used to move molecules both in and out of a cell. It is a type of passive transport that does not require the cell to expend energy. Channels exist in the cell membrane that allow some molecules to diffuse, while other molecules are allowed passage in and out of the cell through special carrier proteins.
Osmosis is the diffusion of water. Water moves freely across the cell membrane, equalizing concentrations on both sides of the membrane when other particles are unable to diffuse.