Diffusion and Osmosis

Diffusion is when molecules move from high concentration to low concentration.

Let's think of this in terms of students in a classroom. If we were to fit 100 students in our classroom, and there was no one in the common area, student would naturally leave the classroom to spread out and stand in the common area so each student would have more space.

Food coloring is another good example of diffusion. When food coloring is first placed into a beaker of water, the coloring is highly concentrated in one area. Then, you will observe it spreading out and eventually becoming evenly spaced throughout the beaker. 

Diffusion doesn't cost the cell any energy- it happens naturally!

Osmosis is a special kind of diffusion, with water molecules only.This can be a little tricky. Just keep in mind that the water molecules always move from high concentration to low concentration- so they go from where there are many water molecules to where there are less (just like food coloring does).

Take a look at the diagram of osmosis above. The red molecules with white around them represent water (H2O- note that there are 2 hydrogens--white and 1oxygen--red in each). In the first diagram, there are many more water molecules on the left than on the right. Then, some of these molecules move over to the right, so that the water molecules are more evenly spaced out. 

If you look at the above diagram, you will note that the purple dots represent a solute (which is any molecule that dissolves in water- such as salt for example). If you look at the left side of this diagram, you can see that the left part of the tube has more water molecules and less solute molecules, while the right part of the tube has less water molecules and more solute molecules. So, water moves from the left to the right, as you can see in the diagram to the right of the arrow.

A few important terms when learning about osmosis are HYPERTONIC and HYPOTONIC. 

-HYPERTONIC means more solute, less water.

-HYPOTONIC means less solute, more water.

In the diagram below, you can see that the outside of the cell is HYPERTONIC to the inside of the cell..because there are many more solutes in the outside and less water.

You could also say that the inside of the cell is HYPOTONIC to the outside of the cell, because there are many less solutes and more water molecules.

So, in osmosis of this cell, water molecules will move from where there are more water molecules to less, meaning that they will move from inside the cell to outside the cell.

The opposite is true for the diagram below. The inside of the cell is HYPERTONIC to the outside of the cell (since there are more solutes and fewer water), or the outside of the cell is HYPOTONIC to the inside of the cell.

Therefore, in this example, water will move from outside the cell (where there are more water molecules) to inside the cell (where there are less water molecules)

Take a look at the following animation on osmosis

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

and a similar animation on diffusion 

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html