Understanding Diffusion and Osmosis in Biology

1. Lab 5a Diffusion and Osmosis

2. Remember the Membrane?

3. Terms • Solutions: molecules (solutes) dissolved in a liquid (solvent) • All molecules are constantly in motion • Random motion causes mixing • Brownian Motion: random tendency of ALL molecules to move due to their inherent kinetic energy. • Concentration is the amount of solute in a solvent • Concentration gradient: more solute in one region of a solvent than in another

4. Passive transport: Diffusion • Due to: random motion and collision of molecules • Movement “down” a concentration gradient (from area of high concentration to area of low concentration) • Occurs until a dynamic equilibrium is reached

5. Diffusion – solid in water

6. What factors Affect Diffusion Rates? • Distance • Molecule size • Temperature • Gradient • Electrical force

7. What can/can’t diffuse through the cell membrane?

8. Osmosis = Water Movement • Water molecules diffuse across membrane toward solution with more solutes • Volume increases on the side with more solutes • Can think of it like diffusion of water: water moves from an area in which it is more concentrated (less solute) to area where it is less concentrated (more solute)

9. Osmosis

10. How Osmosis Works • More solute molecules = lower concentration of water molecules • Key to osmosis: membrane must be freely permeable to water, selectively permeable to solutes. (i.e. some solutes must be impermeable. Otherwise, diffusion would occur)

11. Osmosis • Osmosis is the net “diffusion” of water across a membrane. Osmosis only occurs when solutes cannot cross a selectively permeable membrane (no diffusion) so the solvent, water, crosses instead

12. Lab Exercise 5a - Demonstrations • Activity 0: Brownian motion demo – view in scope up front • Activity 1: Diffusion in a solid - you measure • Activity 3: Diffusion through non-living membranes - I’ll do tests, you observe • Activity 5: Diffusion through living membranes using eggs and in blood cells • Activity 6: Filtration

13. Activity 0 – Brownian Movement • Observe the demonstration setup of india ink molecules under the microscope • Note how the “very large” ink molecules appear to vibrate quickly • The fluid may also appear to be flowing in one direction or another as well. This is NOT Brownian movement, ONLY the vibration is Brownian motion

14. Activity 1 – Diffusion in a solid • Each group will get: • Agar (a water based solid) in a Petri dish • Two bottles • one of methylene blue (same dye we used for the cheek cells) MW = 320 • one of potassium permanganate, MW = 158 • Place a drop of each off-center in the dish • After approximately one hour, measure the size of each dot to determine the diffusion rate of each in mm/min • Which will have a faster rate?

15. Activity 2 – Diffusion in a liquid • Temperature effects demo • How fast will dye diffuse in hot vs cold water?

16. Activity 3 – Diffusion through membranes • Demonstration • I will set up a beaker full of iodine • I will place a “cassette” of starch solution in the iodine • One of two things will happen: iodine will enter the sac, or starch will leave

17. Activity 3. Starch + iodine = purple Purple will be inside or outside bag

18. Activity 5: Osmosis in eggs • Eggs are very large single cells, allow us to measure osmosis easily • When shell is decalcified, water easily passes through • Eggs have a relative concentration of 14% • We will place eggs into solutions of different solute concentrations (hypertonic, isotonic, hypotonic) • What will happen to the egg if the solution is hypertonic and why? What about hypotonic?

19. Activity 5: Osmosis in eggs • Get 2 eggs and weigh them in the weigh boats • Get 2 plastic trays and label them 1 and 2 • Fill 1 halfway with dH2O • Fill 2 halfway with 30% sucrose • Put one egg in each container (noting which one you put in which) • After 20 minutes, gently remove each egg and weigh them • Replace and repeat at 40 min and 60 minutes • Fill in the table “Data from Expt 1” on page 58 • Graph each

20. Activity 5 Osmosis in blood • Take a slide : • put one drop of physiological saline (0.9%) on it • Add one drop of blood to it • Coverslip and look at under high power • On a second slide: • place a drop of 5% saline • add one drop of blood, • coverslip and observe under high power • While slide is still on the scope • Add a drop of dH2O to this slide right next to the coverslip. • Fold up a Kimwipe and blot near the liquid on the other side of the coverslip. • Watch what happens to the cells

21. Osmosis in cells

22. Osmosis • Isotonic  cell ok • Hypotonic  Swelling or hemolysis (burst). Like in the bathtub • Hypertonic  crenation (shrinkage)

23. Activity 6 – Filtration • Follow instructions in Lab book page 60 • You will pour a solution containing starch, charcoal and copper sulfate into a funnel lined with filter paper (held over a beaker) • Count the number of drops that fall through in a 10 sec period (after the steady stream stops) • When funnel is half empty, again count the number of drops in a 10 second period • At the end, note what passed and what was retained • If filtrate is blue, copper sulfate passed through • Sample 2 ml and add Lugol’s solution (purple = starch) • Look for charcoal

24. Clean Up • Clean off balance, wipe off table (egg mess) • Put everything back where you got it from • Return everything up to front including putting eggs back in the vinegar

25. Now What? • To do now: • Start egg experiment (5) • Stars solid diffusion demo(1) • Then: • Make and observe blood slides (5) • Observe Brownian motion demo (0) • Observe liquid diffusion demo (2) • Don’t forget about the egg! • After about an hour: • Measure solid dots (1) • Observe and record dialysis; (3)

26. Due Next Thursday • Rates of diffusion for two dyes in a solid • Table on Page 58 (egg weights) • Review Sheet pages 63 – 66 • Cut some questions (3, 5, 8, 11) • Add: • for the starch iodine: which one moved and how do you know? • For the dyes in water: which diffused faster hot or cold. Why?