1 2 nd Law of Thermodynamics All things tend toward entropy (randomness). Molecules move (diffuse) from an area of high concentration to areas of low concentration. Eventually, molecules become randomly distributed unless acted on by something else.

2 Osmosis Osmosis: a special case of diffusion –Water flows from where it is more concentrated (a dilute solution) to where it is less concentrated (a solution with many solute molecules) Osmosis requires a “semi-permeable” membrane –One which water, but not dissolved substances, can pass through. Cells typically have lots of dissolved substances; the net flow of water is into the cell (unless resisted).

3 Osmosis Yellow spots cannot move through membrane in middle. Water moves into compartment where spots are most concentrated, trying to dilute them, make concentration on both sides of the membrane the same. In this example, gravity limits how much water can flow. In a bacterium, the peptidoglycan provides the limit.

4 Osmosis definitions Movement of water across a semi permeable membrane. If the environment is: Isotonic: No NET flow. Hypertonic: Water flows OUT of cell. Hypotonic: Water flows IN. Water can flow both ways; we are considering NET flow. Terms are comparative terms, like the word “more”.

Inside a cell Every dissolved molecule or ion creates osmotic pressure –Cells contain lots of proteins, salts, all sorts of small organic molecules. –All contribute toward “osmotic pressure” –The concentration of all the dissolved molecules can be in expressed as “osmolarity”. 5