Diffusion
Molecules are transported around the body via a combination of diffusion and bulk flow.
Bulk flow refers to molecules that are moved within a medium such as air, gastric fluid, blood or lymph. Diffusion refers to the movement of molecules along their concentration gradient.
Passive diffusion is the process of molecules moving from an area of high to low concentration along their concentration gradient.
Diffusion across a membrane is affected by the membrane permeability. Membrane permeability may be defined as the passive diffusion rate of permeated molecules across the membrane. It is determined by the membrane composition, membrane thickness and diffusion coefficient.
Bulk flow refers to molecules that are moved within a medium such as air, gastric fluid, blood or lymph. Diffusion refers to the movement of molecules along their concentration gradient.
Passive diffusion is the process of molecules moving from an area of high to low concentration along their concentration gradient.
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Diffusion across a plasma membrane. click on image to enlarge
Factors affecting diffusion
J = -DA (ˆ†C ÷ ˆ†x)
J = diffusion flux vector (amount of substance transferred per unit time) D = diffusion coefficient (negative reflects movement down the concentration gradient), A = surface area, ˆ†C = difference in concentration, ˆ†x = diffusion distance
The diffusion coefficient is affected by:
J = -DA (ˆ†C ÷ ˆ†x)
J = diffusion flux vector (amount of substance transferred per unit time) D = diffusion coefficient (negative reflects movement down the concentration gradient), A = surface area, ˆ†C = difference in concentration, ˆ†x = diffusion distance
The diffusion coefficient is affected by:
- Molecule size
- Temperature
- Solvent viscosity
- Molecular weight (Inversely proportional to the cubed root of molecular) weight
Diffusion across a membrane is affected by the membrane permeability. Membrane permeability may be defined as the passive diffusion rate of permeated molecules across the membrane. It is determined by the membrane composition, membrane thickness and diffusion coefficient.