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Diffusion and Osmosis Diffusion • Diffusion- the movement of molecules from an area of higher concentration to an area of lower concentration. – A difference in the concentrations of molecules across a space is called a concentration gradient. – Diffusion is driven by the kinetic energy (movement) of molecules. Equilibrium • Equilibrium occurs when the concentration of molecules is the same throughout a space. – Molecular movement still occurs, but in all directions the balance is maintained. Membranes • Some molecules can diffuse across a membrane (cell membrane). If a molecule can move through a membrane, that membrane is permeable to that molecule. • Not all molecules can diffuse across all membranes. Molecular movement depends on the size and type of the molecule as well as the membrane structure. Osmosis… • Osmosis- process by which water molecules diffuse across a cell membrane from an area of high concentration of water to areas of low concentration. – The direction of water movement depends on the concentrations of water and solute dissolved in the solution. • Solution- mixture in which one or more substances are uniformly dissolved in another substance. • Solute- substance dissolved in a solution. Water flows “downhill” – If the concentration of solute molecules outside the cell is lower than the concentration in the cell, the solution is hypotonic to the cell. Water moves into the cell until equilibrium is reached. • Hypo means “lower” Water flows “downhill” continued… – If the concentration of solute molecules outside the cell is higher than the concentration inside the cell, the solution is hypertonic to the cell. Water moves out of the cell until equilibrium is reached. • Hyper mean “higher” Water flows “downhill” continued… • When concentration of solute molecules outside the cell is equal to the concentration inside the cell, the solution is isotonic to the cell. Water moves into and out of the cell at an equal rate. Pressure – If water is moving into the cell, turgor pressure increases. If the cell takes too much water in it can burst. This is termed cytolysis. – If water is moving out of the cell, turgor pressure drops and the cell wilts. This is termed plasmolysis.