What is the difference between diffusion and osmosis?

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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...

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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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Diffusion and osmosis are both examples of transport phenomena.  In other words, particles are transported from one area to another without a net infusion of energy or mechanical intervention.  The difference between the two is what is transported and through what medium.  Diffusion is the movement of solute particles in a given solution from an area of high concentration to an area of low concentration.  The solute particles ultimately spread themselves evenly throughout the solvent to form a uniform concentration.  Osmosis is the movement of solvent particles through a semipermeable membrane to even out any concentration gradient on both sides of the membrane.  So the key differences between the two are that diffusion involves movement of solute particles while osmosis involves movement of solvent particles and osmosis involves a semipermeable membrane while diffusion does not.

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