Texts: Part II. Selective Permeability of Membranes - Diffusion and Osmosis 3. Selectively permeable membranes let some solutes diffuse but not others. Given a problem where two different solutions are separated by a selectively permeable membrane, you are expected to predict what will happen next. Solving these problems will always involve the same set of steps: 1. Determine which solutes will diffuse and which solutes will NOT diffuse. 2. For each solute that will diffuse, calculate the equilibrium concentration on each side. 3. Add up the total solutes on each side to determine if equilibrium solutions are isotonic or if not, label each side as hypertonic or hypotonic. a. Problem: In an experiment, an artificial membrane that is permeable to ions but not larger molecules like sugars separates two solutions: 20 mM NaCl and water. Determine if there will be osmosis, and if so, draw an arrow showing the net flow of water. Process: You know NaCl will dissolve in water to give Na+ and Cl- ions; there is 20 mM NaCl, so there will be 20 mM Na+ and 20 mM Cl-. These are the initial conditions (A, below). The problem tells you that ions will diffuse through the membrane, so you need to do diffusion. Calculate the equilibrium concentrations of each solute: in this problem, 20 + 0 / 2 = 10 mM on each side for both Na+ and Cl-. These are the equilibrium concentrations (B, below). Will there be osmosis? Osmosis will occur if there is a difference in the total amount of solute. In this case, there is none, so there is no osmosis (under Figure B). You do not need to draw an arrow. 10 mM Na+ plus 10 mM Cl- = 20 mM total on each side - they are the same. Go ahead and label each side as isotonic. A. Initial Conditions: 20 mM Na+ 0 mM Cl- B. Equilibrium Concentrations: 10 mM Na+ 10 mM Cl-

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