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Diffusion is the movement of molecules or ions from a region of high concentration to a region of low concentration aided by a concentration gradient.

Diffusion continues to occur as long as there is a difference in concentration between two regions (concentration gradient). Diffusion is a process that occurs inside living organisms as well as the external environment. Does not require energy. The rate of Diffusion affected by the following reasons:-

1. Concentration Gradient: An increase in the concentration of molecules at one region results in a steeper concentration gradient which in turn increases the rate of diffusion.

2. Temperature: High temperature increases kinetic energy of molecules. They move faster hence resulting in an increase in rate of diffusion, and vice versa.

3. Size of Molecules or Ions: The smaller the size of molecules or ions, the faster their movement hence higher rate of diffusion.

4. Density: the denser the molecules or ions diffusing, the slower the rate of diffusion, and vice versa.

5. Medium: The medium through which diffusion occurs also affects diffusion of molecules or ions. For example, diffusion of molecules through gas and liquid media is faster than through a solid medium.

6. Distance; This refers to the thickness or thinness of surface across which diffusion occurs. Rate of diffusion is faster when the distance is small i.e, thin surface.

7. Surface Area to Volume Ratio: The larger the surface area to volume ratio, the faster the rate of diffusion. For example, in small organisms such as Amoeba the surface area to volume ratio, is greater hence faster diffusion than in larger organisms

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