See also:
A constant which is a measure of how well a dissolved substance can spread, or diffuse, through a concentration gradient (a fluid which has a higher concentration on one end and a lower concentration on the other end of the container). This depends on the particle size of the substance, the temperature, and the polarity of the substance. The constant has the symbol of k(subscript D), and is used to calculate diffusion rates of the substance.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode. | 
