The exchange of substances between cells and the environment occur in ways that require metabolic energy (active transport) and in ways that do not (passive transport-Energy from natural, inbuilt motion of particles). Diffusion is an example of passive transport. Diffusion is the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower.
Figure 1Figure 2
Diffusion is only possible because:
All particles are constantly in motion due to the kinetic energy that they possess, This motion is random, with no set pattern to the way the particles move around, Particles are constantly bouncing off one another as well as off other objects e.g. the sides of the vessel in which they are contained.
At first diffusion will occur more a rapidly because there is more space for the particles to move to and even out, a higher concentration gradient, looking at figure one the particles will move right to left because of the concentration difference. (All in random motion) After a short time the particles can now move left to right but there still is a higher chance that they will move right to left as there still is a higher concentration on the right, look at figure 2. Also there is a smaller concentration gradient so diffusions is slower. Sometime later the particles have evenly distributed out, there is no diffusion occurring, this is called equilibrium. At a later stage the particles are still evenly distributed but the individual particles are continuously moving to each side (Number of particles on each side is still equal) this is called a dynamic equilibrium.
Rate of Diffusion
There are several factors affecting the rate at which diffusion occurs: Concentration gradient: The greater the difference in the concentration of molecules or ions the faster diffusions will occur, if there is little difference diffusion will be slower. Area over which...