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Definition

noun

1. Thepassivemovement ofmolecules or particles along aconcentrationgradient, or fromregions of higher toregions of lowerconcentration.

2. Thespontaneousnetmovement of particles down theirconcentrationgradient (i.e. difference in theconcentrations of substances ormolecules between twoareas).

3. (Cell biology): a type ofpassivetransport, therefore, it is anetmovement ofmolecules in and out of thecell across thecell membrane along aconcentrationgradient.

Supplement

Unlikeactive transport, diffusion does not involvechemicalenergy. Whenmolecules move (diffuse) via specialtransportproteins found within thecell membrane, it is calledfacilitated diffusion, otherwise it is only simple diffusion. An example of diffusion in biological system is diffusion ofoxygen andcarbon dioxide across the alveolar-capillarymembrane in mammalianlungs.

Word origin: From Latindiffusionem (accusative ofdiffusio), from verbdiffundere.
Related forms: diffusional (adjective).
Compare:active transport,osmosis.
See also:passive transport.