There are four different types of passive transport: diffusion, facilitated diffusion, filtration and osmosis. Diffusion is the movement of substances from an area of high concentration to an area with lower concentration. The difference of concentration between these two areas is called concentration gradient.
Diffusion is described as moving substances down the concentration gradient. This differs with active transport, which moves substances from a low concentration area to a higher one, a process referred to as moving substances against the concentration gradient.
Osmosis, on the other hand, is a type of diffusion whereby water molecules move through a cell membrane from a hypotonic solution to a hypertonic solution. A hypotonic solution is a solution with a low concentration of solutes, whereas a hypertonic solution has a high concentration of solutes.
Another passive process is filtration, which is the movement of solute molecules and water across the cell membrane by normal cardiovascular pressure. Certain functions of the kidneys and liver are based upon filtration. With facilitated diffusion, movement of molecules across the cell membrane occurs through carrier proteins, which are embedded in the cell membrane. The solutes in facilitated diffusion move down the concentration gradient and do not use energy to move.