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Passive transportPassive transport is a means of movingbiochemicals, and other atomicor molecularsubstances, across membranes. Unlike active transport, this process does not involve chemicalenergy. 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, filtrationand osmosis.
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 alveoliof the lungs allows it to be replenished. Facilitated diffusion
Facilitated diffusion is movement of moleculesacross the cell membranevia special transport proteinsthat are embedded within the cellular membrane. Many large molecules, such as glucose, are insoluble in lipidsand 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 receptorsite 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. Filtration
Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressuregenerated 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 capsulein 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 livercells are extremely large, to allow a veriety of solutes to pass through and be metabolized. Osmosis
Osmosis is the diffusionof a solventacross a membrane to a region of higher soluteconcentration. (In biological processes then, it usually is diffusion of water molecules) Most cell membranesare 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. Watermolecules "stick" together via weak hydrogen bond.sl:pasivni transport Categories: Transport phenomena| Biochemistry| Physiology
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