DIFFUSION PRINCIPLES IN BIOLOGICAL SYSTEMS
Sele£ted Dellnltlons •
Diffusion: It is defined as a mass transfer of individual molecules of a substance caused by random molecular motion. It is associated with a driving force such as the concentration gradient.
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Flux: It is defined as number of atoms diffusing across the unit area per unit time
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Fick's first law or diffusion: It states that rate of diffusion across a surface is proportional to the concentration gradient and to the area of the surface
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Fick's second law or diffusion: It states that the rate of change of diffusant concentration with time at a definite location in unit time.
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Steady state: A system is said to be in steady state when concentration profile does not change with time.
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Dlf'fusant: It is a substance that can diffuse through something.
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Sink condition: It is the state in which concentration in receptor compartment is maintained at lower level compared to its concentration in donor compartment.
3. 1 INTRODUCTION Diffusion is defined as a mass transfer of individual molecules of a substance caused by random molecular motion and associated with a driving force such as the concentration gradient. Diffusion is a time-dependent process. The movement is based on the kinetic energy (velocity), the charge and the mass of molecules. A number of drugs are absorbed by passive diffusion process. The passage of drug molecule across a cell membrane from high concentration region to lower concentration region is called passive diffusion. But Some drugs or nutrients have low partition coefficient i.e. poor absorption through gastrointestinal tract. In this case, a special absorption mechanism is developed. This special mechanism is known as active transport.