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Diffusion
Diffusion is the movement of a substance from a region of higher concentration
to a region of lower concentration.
Diffusion is important to whole organisms because it is the process by
which useful molecules enter the body cells and waste products are removed.
For example digested food molecules (amino acids, glucose) move down a
"concentration gradient" from the intestine to the blood and then to any
cells which have a shortage. Waste products such as carbon dioxide or
urea do exactly the same thing in the reverse direction.
Another example is that oxygen moves from high concentration (in the air)
to a lower concentration (in the blood), Take a look at this happening
in the following animation.
In the same way examples of substances which enter and leave cells by
diffusion are; dissolved food (e.g. amino acids, glucose, fatty acids),
oxygen. carbon dioxide and water.
