Osmosis & Diffusion
 

Osmosis and diffusion are similar in that they require no energy to take place.  Well...That is not exactly true.  The more correct statement would be that osmosis and diffusion require no EXTRA energy input from the cell for that process to take place. 

The Kinetic Theory of Matter states that all molecules are moving, constantly bumping into one another.  This causes very small movements to take place at the molecular level.  This bumping and pushing occurs more in areas that are highly concentrated with a substance because there is more of that substance to bump into.  This is like putting people in a room in such great numbers that the people can only elbow each other.  If the door is open, pretty soon the people will eventually go to where there are fewer people, thus going from an area of high concentration of people to low concentration of people.  They have traveled down a concentration gradient of people.  A concentration gradient is that area of high concentration and low concentration and everything in between. 

Diffusion is the movement of materials from an area of high concentration to an area of low concentration gradient.

Osmosis is the movement of water from an area of high concentration to low concentration across a semi-permeable membrane.  Osmosis is dependent on osmotic pressure.  This osmotic pressure creates the pressure for plant cells so they can stand upright.  As seen with a plant cell under a microscope, if salt were added to water the cell would shrink.  This occurs because there is more water inside the cell than outside the cell.  Water goes from high concentration to low concentration.  The solution is hypotonic and the cell looses water to the environment.  Likewise if pure water was added to a plant cell the cell would swell because there is more water on the outside of the cell than inside the cell.  The solution is a hypertonic solution.  If a cell were placed in an environment that is just like the inside of the cell, the solution is then said to be isotonic and there is not net movement of water.