Small molecules move from a region of high concentration to one of lower concentration in diffusion. Diffusion is the random movement of molecules or particles and occurs when gases mix, as in air, or when molecules mix in liquids, such as water. In osmosis, water molecules move across a semipermeable membrane from a low concentration of solute, or dissolved particles, to one of high concentration of solute. Water movement stops when solute concentrations are equal on both sides.
Diffusion Follows a Downhill Concentration Gradient
Gases and substances dissolved in a liquid diffuse from an area of high concentration to one of low concentration. For example, if you spray perfume into the air, the volatile perfume molecules will spread out in the air from the concentrated point of origin. Diffusion also takes place with or without a permeable membrane in a liquid, such as water. Diffusion of small molecules across plant or animal cell membranes follows a concentration gradient. For example, if oxygen is higher on the outside of a cell, it will diffuse into the cell until the oxygen concentrations are equal on the outside and inside of the cell.
Osmosis Follows an Uphill Concentration Gradient
During osmosis, water flows from a low solute concentration across a semipermeable membrane to a high solute concentration. For example, if you add water to a blood sample, consisting of plasma and red blood cells, water will enter the red blood cells and cause them to swell, because the blood plasma has become less concentrated than the inside of the red blood cells. However, if you add sugar or salt to the blood sample, water will leave the red blood cells and cause them to shrink and pucker.
Both Processes Require No Energy
Diffusion and osmosis are passive processes, meaning they don't need energy to occur. Both are spontaneous processes. Diffusion depends on the random movement of particles or molecules. It increases with a rise in temperature, because heat increases the random movement of molecules. In osmosis, water freely moves across a membrane from an area of low solute concentration, or hypotonic solution, to one of high solute concentration, or hypertonic solution. When the solute concentration is equal on both sides of the membrane, the solution is said to be "isotonic." Osmosis does not achieve isotonicity in plant cells, because they are surrounded by a rigid covering, causing pressure to build up within cells.
Moving Molecules Differ
Diffusion across a membrane depends on the size and electric charge of molecules. Smaller molecules diffuse faster than larger molecules. Charged molecules do not diffuse across animal or plant cell membranes; they need to enter or leave cells by other mechanisms, because cell membranes are made up of hydrophobic lipids and repel charged molecules similar to how oil repels vinegar. Osmosis is the flow of water molecules and depends on the particle concentration -- not the type of molecule on either side of the membrane.
Osmosis Requires a Semipermeable Membrane
Diffusion occurs with or without a membrane between two areas of different concentrations of molecules. However, osmosis only occurs across a semipermeable membrane, a membrane that prevents many particles or molecules to freely travel between the two sides, while allowing water to pass through. In nature, osmosis is essential for many biological processes that depend on the movement of water, such as cell shape or pressure.