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Diffusion occurs from a region of {{high}} concentration to a region of {{low}} concentration. The primary factor driving this {{passive}} process comes from the kinetic energy of {{Brownian motion}}, the random process of movement imparted by increasing {{temperature}}.
Diffusion is good for rapid movement over short distances because the time taken to diffuse increases with the square of the {{distance}}.
With respect to the medium through which the particle is diffusing, two factors needing consideration are the total {{surface area}} available for the particle to diffuse across, and the thickness of the medium, or more correctly, the {{viscosity}} of the medium.
One factor related to the particle under consideration, independent of whether the area through which the particle is diffusing is a cell membrane or a fluid or a gas medium is the {{size}} of the particle.
Another particle-related factor which is relevant only if considering the diffusion of the particle through a cell membrane is the solubility of the particle in {{lipids}}.
For any particular molecule trying to traverse a specific membrane at a particular temperature, a number of these factors are fixed as the {{permeability}} constant.
To calculate the net flux of a solute across a membrane at any particular temperature, we can use an equation known as {{Fick's}} law.
This law tells us that if we are considering a specific cell membrane at a given temperature, then we can compare the the relative permeability of the membrane to different molecules by using the molecule-specific properties of {{mass and lipid}} solubility.