| passive diffusion | See: facilitated transport. (05 Mar 2000) |
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| classical diffusion | <radiobiology> In plasma physics, diffusion due solely to scattering (collisions) of charged particles (with unlike charges) via electrical (Coulomb) interactions. (See also diffusion.) (09 Oct 1997) |
| population diffusion coefficient | <cell biology> Coefficient that describes the tendency of a population of motile cells to diffuse through the environment. Its use presupposes that the cells move in a random walk. (18 Nov 1997) |
| neoclassical diffusion | <radiobiology> In a magnetised plasma, _classical_ diffusion refers to transport of particles due to Coulomb collisions, taking the spiral orbits in the magnetic field into account. In a toroidal magnetic field, the actual rate of diffusive transport is much higher due to slow changes in the positions of the centres of the spirals, known as banana orbits. This faster transport is called _neo-classical_. With very few exceptions the transport in toroidal devices is observed to be 10-100 times larger still, presumably due to small-scale turbulence. The observed transport is called _anomalous_ (although it actually is the normal state). (09 Oct 1997) |
| diffusion | The process of becoming diffused or widely spread, the spontaneous movement of molecules or other particles in solution, owing to their random thermal motion, to reach a uniform concentration throughout the solvent, a process requiring no addition of energy to the system. (18 Nov 1997) |
| diffusion anoxia | Diffusion hypoxia severe enough to result in the absence of oxygen in alveolar gas. (05 Mar 2000) |
| diffusion chambers, culture | Devices used in a technique by which cells or tissues are grown in vitro or, by implantation, in vivo within chambers permeable to diffusion of solutes across the chamber walls. The chambers are used for studies of drug effects, osmotic responses, cytogenic and immunologic phenomena, metabolism, etc., and include tissue cages. (12 Dec 1998) |
| diffusion coefficient | For the translational diffusion of solutes, diffusion is described by Fick's First Law, that states that the amount of a substance crossing a given area is proportional to the spatial gradient of concentration and the diffusion constant (D), that is related to molecular size and shape. A useful derived relationship is that the mean square distance moved by molecules in time t is 6Dt. (18 Nov 1997) |
| diffusion constant | For the translational diffusion of solutes, diffusion is described by Fick's First Law, that states that the amount of a substance crossing a given area is proportional to the spatial gradient of concentration and the diffusion constant (D), that is related to molecular size and shape. A useful derived relationship is that the mean square distance moved by molecules in time t is 6Dt. (18 Nov 1997) |
| diffusion hypoxia | Abrupt transient decrease in alveolar oxygen tension when room air is inhaled at the conclusion of a nitrous oxide anaesthesia, because nitrous oxide diffusing out of the blood dilutes the alveolar oxygen. (05 Mar 2000) |
| diffusion limitation | The boundary layer hypothesis, that the proliferation of cells in culture is limited by the rate at which some essential component (almost certainly a growth factor) diffuses from the bulk medium into the layer immediately adjacent to the plasma membrane. By spreading out, a cell obtains a supra threshold level of the factor and can divide, if unable to spread (because of crowding or poor adhesion) then the cell will remain in the G0 stage of the cell cycle. (18 Nov 1997) |
| diffusion method | A method for the study of bacterial enzymes in which agar is mixed with the material (e.g., starch or milk) which is to serve as an indicator of the enzyme action and is inoculated and plated; if the bacteria produce enzymes digesting the admixed material, there will be a zone of clearing in the medium about each colony. Synonym: diffusion method. (05 Mar 2000) |
| diffusion of innovation | The broad dissemination of new ideas, procedures, techniques, materials, and devices and the degree to which these are accepted and used. (12 Dec 1998) |
| diffusion potential | Potential arising from different rates of diffusion of ions at the interface of two dissimilar fluids, a junction potential. (18 Nov 1997) |
| diffusion respiration | Maintenance of oxygenation during apnea by intratracheal insufflation of oxygen at high flow rates. Synonym: apneic oxygenation. (05 Mar 2000) |