transport of a substance across a cell membrane by diffusion; expenditure of energy is not required. There is diffusion, simple diffusion, osmosis, facilitated diffusion, channel-mediated passive transport, and carrier-mediated passive transport.

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The passive movement of molecules or particles along a concentration gradient, or from region of higher to regions of lower concentration.

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Molecular diffusion, often called simply diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero.

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a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane.

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Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is a process of passive transport, facilitated by transport proteins

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Particles diffuse through membrane channels 

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Particles diffuse through carrier mechanisms in a membrane

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