Diffusion refers to a process wherein substances and molecules are inclined to spread or move to another space. This particular process basically occurs without any force acting on the substances to move them from one space to another. And since there is no force required to cause the diffusion of molecules, this process is considered a basic example of passive transport. Literally, molecules are passively transported from one area or space to another. Under normal conditions, passive transport processes involve the movement of substances through a membrane from an area with a very high concentration into another space with a low concentration.
In the case of electrolytes in the cells, for example, the various molecules present in one space will have the tendency to be transported into another space where there is less concentration of the same molecules. The rate of this type of passive transport also depends on how permeable the cell membrane is. Some cells have membranes that readily allow certain molecules to pass through. With this kind of permeability, cell membranes are called highly permeable.There are also cell membranes that are classified as semi-permeable, and this simply means that some molecules are allowed to pass through while some are not. In both ways, diffusion will still occur, and the only difference is the rate of the transport process.
Osmosis is a good example of diffusion or passive transport. This process is specifically applied to water molecules, though, but is the same with simple diffusion in a sense that the movement of water molecules does not require energy or force. Water molecules also have the tendency to go from a space with a higher concentration to another space with a lower concentration of the same molecules. Water molecules also have an easier time transferring in-between cell spaces because of easier permeability compared to other types of molecules and substances.