Passive transport 417014 221842438 2008-06-26T10:02:54Z 68.104.117.229 /* Simple Diffusion */ '''Passive transport''' means [[Motion (physics)|moving]] [[biochemistry|biochemical]]s and other atomic or molecular substances across [[atome|membranes]]. Unlike [[active transport]], this process does not involve [[chemistry|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 kinds of passive transport are [[diffusion]], [[facilitated diffusion]], [[filtration]] and [[osmosis]]. ==Diffusion== [[Image:Scheme_simple_diffusion_in_cell_membrane-en.svg|left|thumb|240px|Passive diffusion on a cell membrane.]] {{main|Diffusion}} ==Simple Diffusion== Simple Diffusion is the net movement of material from an area of high concentration 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 materials from an 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"). ==Facilitated Diffusion== [[Image:Scheme_facilitated_diffusion_in_cell_membrane-en.svg|left|thumb|240px|Facilitated diffusion on a cell membrane.]] {{main|Facilitated diffusion}} Facilitated diffusion, also called carrier-mediated diffusion, is the movement of [[molecule]]s across the [[cell membrane]] via special [[transport protein]]s that are embedded within the cellular membrane. Many large molecules, such as [[glucose]], are insoluble in [[lipid]]s 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 (biochemistry)|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]]. ==Filtration== [[Image:Filtration diagram.svg|left|thumb|238px]] {{main|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 [[albumins]], 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 variety of solutes to pass through and be metabolized. ==Osmosis== [[Image:Osmotic pressure on blood cells diagram.svg|left|thumb|249px|Effect of osmosis on blood cells under different solutions.]] {{main|Osmosis}} Osmosis is the [[diffusion]] of water [[molecules]] across a partially permeable membrane. The net movement of water molecules through a partially permeable membrane from a solution of high water potential to an area of low water potential. A cell with a less negative water potential will draw in water but this depends on other factors as well such as solute potential (pressure in the cell e.g. solute molecules) and pressure potential (external pressure e.g. cell wall). ==See also== *[[Active transport]] *[[Transport phenomena]] {{Membrane transport}} [[Category:Transport phenomena]] [[Category:Cellular processes]] [[Category:Membrane biology]] [[Category:Physiology]] [[ko:수동수송]] [[it:Trasporto passivo]] [[ja:受動輸送]] [[sl:Pasivni transport]] [[sr:Пасиван транспорт]]