<P> 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 an area of lower concentration, 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"). However, in many cases (e.g. passive drug transport) the driving force of passive transport cannot be simplified to the concentration gradient . If there are different solutions at the two sides of the membrane with different equilibrium solubility of the drug, the difference in degree of saturation is the driving force of passive membrane transport . It is also true for supersaturated solutions which are more and more important owing to the spreading of the application of amorphous solid dispersions for drug bioavailability enhancement . </P> <P> Simple diffusion and osmosis are in some ways 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 selectively permeable 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 . </P> <P> Facilitated diffusion, also called carrier - mediated osmosis, is the movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane . Large, insoluble molecules, such as glucose, vesicles and proteins require a carrier molecule to move through the plasma membrane . 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 their concentration gradient and do not require the expenditure of cellular energy for this process . Carrier proteins and channel proteins allow for the diffusion of molecules across the cell membrane . Carrier proteins undergo conformational alterations to allow molecules to pass, while channel proteins form unblocked pores . </P> <P> Facilitated diffusion may be achieved as a consequence of charge gradients in addition to concentration gradients . Plant cells create an unequal distribution of charge across their plasma membrane by actively taking up or excluding ions . Active transport of protons by H ATPases alters membrane potential allowing for facilitated passive transport of particular ions such as Potassium down their charge gradient through high affinity transporters and channels . </P>

How do carrier proteins facilitate passive transport of molecules across a membrane
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