Although diffusion (from now on any reference to diffusion will refer to net diffusion) is a very common in biological systems, it is also a very slow process (on a macroscopic scale). There several physical factors that affect the rate at which particles diffuse. Organisms have been able to utilize some of these factors to regulate the transport of items. Some of these factors are:

1.

Size of the particle (molecular weight): Remember that the source of energy for the movement of particles in diffusion is the heat in the environment. At a given temperature, a smaller particle (molecule) moves faster than a larger one. This is analogous to a person pushing an object. A person can push a wheelbarrow much faster than he/she can push a minivan. This is due to the size of the object being pushed. With the same amount of force, a smaller object can be pushed faster than a larger object. Thus the rate of diffusion is inversely proportional to the size of the particle. That is, a smaller particle will diffuse faster than a larger one.2.

Temperature: As the temperature increases, the amount of energy available for diffusion is increased. This means that a given particle will move faster (there is more energy >pushing= it) at a higher temperature. Thus the rate of diffusion will be faster as the temperature increases.3.

Concentration Difference: When a substance is diffusing between two compartments, the greater the concentration difference between the two compartments, the faster the substance will diffuse (faster rate of diffusion).4.

Diffusion Distance: For a particle at a given temperature (and thus moving at a constant speed), it takes longer for the particle to diffuse a farther distance. Thus the farther a particle must diffuse, the longer it will take and the slower the rate of diffusion.5.

Surface Area: When a substance is diffusing between two compartments (through a membrane, for example), the greater the surface area of the membrane, the greater the probability that a particle will pass through it. Therefore, the larger the surface area, the greater the rate of diffusion.6.

Permeability: Obviously, if a substance is not permeable through the membrane, it will not be able to diffuse through it. The more permeable the a substance is, the faster it can diffuse through it.

These >rules= of diffusion are generally logical and you are required to remember them. As you learn more about biology, you should be able to recognize adaptations that act to increase the rate of diffusion.

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