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What is the difference between diffusion and osmosis?

Diffusion and osmosis are both examples of passive transport. Passive transport does not require any energy to happen. It is a natural process that moves materials from an area of higher concentration to an area of lower concentration. The differences in concentration of materials is called a concentration gradient, and substances naturally move down (higher to lower) the gradient.  For example, when you put a drop of food coloring in a glass of water, the food coloring will begin moving from where there is a high concentration of food coloring to where there is a lower concentration of food coloring.  This will occur until equilibrium is reached and the food coloring is evenly spread out.  Osmosis and diffusion both work passively based on a concentration gradient. 


A difference between osmosis and diffusion is that diffusion can occur in any mixture (with or without a semipermeable membrane), but osmosis always occurs across a semipermeable membrane.


Another key difference is that osmosis is only the movement of water across the membrane. 


When I teach this concept to my students, I stress that osmosis is the diffusion of water.  I also like to say that all osmosis is diffusion, but not all diffusion is osmosis.  

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