Labs 6 & 7

1. Labs 6 & 7 Diffusion and Osmosis

2. Diffusion • dialysis tubing filled with 0.15mg/ml KMnO4 • tubing placed into a beaker of water • one beaker kept at into room temp water, the other placed into ice cold water bath

3. Diffusion • KMnO4 particles should diffuse out of the dialysis bag and into the surround beaker water • take a sample of water from each beaker at defined times and measure its absorbance at Abs 545nm • detecting the presence of KMnO4 particles • because you did a standard curve – you could plot this as concentration vs. time • diffusion should be higher at room temperature vs. cold

4. KMnO4 Standard Curve and Unknown slope = 0.284 – 0.044 0.0156-0.002 slope = 17.554 molar absorptivity = 17.554

5. Diffusion • you have already calculated the molar absorptivity for KMnO4 (see spec lab) • but you did a standard curve and you could re-calculate it • for each absorbance value – divide by the molar absorptivity to get the concentration • the units are the same as the standard – which was mg/ml • plotting concentration vs. time looks just like absorbance vs. time

6. Diffusion • place a crystal of KMnO4 in a petri dish of water and measure its spread through the water over time • diffusion results in the increasing diameter of the KMnO4 “cloud” in the water • place a drop of NaOH in an agar dish and measure its diffusion through the agar • agar is a colloid and will slow the diffusion of NaOH vs. water • same concept at the dialysis tubing study • high concentration to low concentration

7. Osmometer • based on saturated sucrose solutions • submerged in pure water – i.e. hypotonic solution (high [water], low [solute]) • low osmotic pressure • water will move from high [water] to low [water] • volume of the sucrose solution increases • sucrose solution moves up the tube • SO: water movement into the osmometer pushes the sucrose solution up • you measured distance over time as a way of measuring osmotic pressure • many osmometers are attached to pressure transducers that measure the pressure physically “pushing” the sucrose solution up

8. Osmometer • measured the distance the red sucrose solution travelled up the pipette and plotted it versus time • the slope of the graph is essentially the osmotic pressure of the water (or sucrose solution) • i.e. the driving pressure that causes water to move

9. Potatoes and Turgidity • immersed pieces of potato in increasing molarities of sucrose • 0.0Msucrose = pure water • up to 1.0M sucrose • each sucrose solution has a defined number of sucrose particles and free water molecules • the higher the concentration – the higher the osmotic pressure of the solution • the osmotic pressure of the potato is defined also • it has to be compared to the OP of the surrounding sucrose solution in order to figure out which way water will flow • hypotonic solutions – water flows out of solution into cells • hypertonic solutions – water flows into solutions from the cells • increased water movement into the potato slices will increase its weight

10. Potatoes and Turgidity • graphing the weight change vs. time can show you the rate of osmosis • graphing the final weight change vs. sucrose molarity tells you what [concen.] is isotonic • i.e. where the line crosses the 0 axis

11. go to this website and try the self quizzes at the ends of the exercises: http://www.phschool.com/science/biology_place/labbench/lab1/intro.html