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Passive transport

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Title: Passive transport  
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Subject: Renal physiology, Facilitated diffusion, Membrane transport, Active transport, Membrane potential
Collection: Cellular Processes, Membrane Biology, Physiology, Transport Phenomena
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Passive transport

Passive diffusion across a cell membrane.

Passive transport is a movement of lipids and proteins. The four main kinds of passive transport are diffusion, facilitated diffusion, filtration and osmosis.

Contents

  • Diffusion (simple) 1
  • Facilitated diffusion 2
  • Filtration 3
  • Osmosis 4

Diffusion (simple)

Passive diffusion on a cell membrane.

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"). Simple diffusion and osmosis are similar. Simple diffusion is the passive movement of solute from a high concentration to a lower concentration until the concentration of the solute is uniform throughout and reaches equilibrium. Osmosis is much like simple diffusion but it specifically describes the movement of water (not the solute) across a membrane until there is an equal concentration of water and solute on both sides of the membrane. Simple diffusion and osmosis are both forms of passive transport and require none of the cell's ATP energy.

Facilitated diffusion

Depiction of facilitated diffusion.

Facilitated diffusion, also called carrier-mediated diffusion, is the movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane. Many 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. Facilitated diffusion is a passive process: The solutes move down the concentration gradient and don't use extra cellular energy to move.

Filtration

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|right|thumb|249px|Effect

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