Passive transport is a means of moving biochemicals, and other atomic or molecular substances, across membranes. Unlike active transport, this process does not involve chemical energy. Passive transport is dependent on the permeability of the cell membrane, which, in turn, is dependent on the organization and characteristics of the membrane lipids and proteins. The four main kind of passive transport are diffusion, facilitated diffusion, filtration and osmosis.
Main article: Diffusion
Diffusion is the net movement of material from an area of high concentration of that material to an area with lower concentration. The difference of concentration between the two areas is often termed as the concentration gradient, and diffusion will continue until this gradient has been eliminated. Since diffusion moves material from area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient" (compared with active transport, which often moves material from area of low concentration to area of higher concentration, and therefore referred to as moving the material "against the concentration gradient").
If and when the concentration gradient have been eliminated, no net exchange of material occurs. Although material may move forth from one area to the other, it will be balanced by movement of the same amount of material to the opposite direction.
Diffusion is biologically important because it enables the abolishment of concentration gradients in the body. For example, metabolic activity will consume oxygen, which will reduce its concentration in the bloodstream; diffusion of oxygen in the alveoli of the lungs allows it to be replenished.
Main article: Facilitated diffusion
Facilitated diffusion is movement of molecules across the cell membrane via special carrier proteins that are embedded within the cellular membrane. A lot of large molecules, such as glucose, are insoluble in lipids and too large to fit through the membrane pores. Therefore, it will bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane. Bear in mind, however, that facilitated diffusion is a passive process, and the solutes still move down the concentration gradient.
Main article: Filtration
Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system. Depending on the size of the membrane pores, only solutes of a certain size may pass through it. For example, the membrane pores of the Bowman's capsule in the kidneys are very small, and only albumin, the smallest of the proteins, have any chance of being filtered through. On the other hand, the membrane pores of liver cells are extremely large, to allow a veriety of solutes to pass through and be metabolized.
Main article: Osmosis
Osmosis is basically diffusion of water molecules. Most cell membranes are permeable to water, and since the diffusion of water plays such an important role in the biological functioning of any living being, a special term has been coined for it -- osmosis.
Water molecules "stick" together via weak hydrogen bonds; therefore, unlike most solutes, water molecules move around in large clumps, a phenomenon known as bulk flow.