Slide 1

NY State Biology Lab Diffusion Through a Membrane By Hilary Beirne Eastchester UFSD Eastchester NY. (Make sure speakers are on) Slide 2 DiffusionDiffusion is the movement of a substance (liquid or gas) from an area of higher concentration to one of lower concentration. WHAT IS DIFFUSION? BACKGROUND WHAT IS DIFFUSION? Slide 3 What does semi-permeable mean? A membrane is said to be semi-permeable or selectively permeable when only certain molecules diffuse through it Slide 4 So what determines which molecules will diffuse across a cell membrane and which ones will not? ANSWER Molecular size and the pore size in the cell membrane Slide 5 How to fill the dialysis tubing cell with the starch and glucose solution. (Clip on image to view hints to filling dialysis tubing) Slide 6 Molecules in this lab ranked largest to smallest Largest Starch Glucose Water Pores in dialysis tubing Lugols solution Iodine. (Starch turns blue-black in the presence of iodine solution). Smallest Slide 7 Page 2 of handout Predict what will happen to the substances inside and outside of the Cell Students should be able to predict that glucose will move out of the cell and that starch is too large a molecule to move out of the cell. Slide 8 Initial State The dialysis tubing is cloudy white (glucose and starch).dialysis tubing The beaker solution is amber (iodine and water).beaker solution The dialysis tubing solution should test positive for glucose. The beaker solution should test negative for glucose. Slide 9 Final State The iodine diffuses into the tubing cell because the liquid inside the tubing cell was a bluish black color and the color in the beaker became less amber. The solution in the beaker should test positive for glucose indicating that glucose diffused out of the tubing Slide 10 From the cell results we can concluded. The iodine diffused from a region of high concentration in the beaker to a region of low concentration in the dialysis tubing. The glucose diffused from a region of high concentration in the cell to a region of low concentration in the beaker. Starch molecules which are a lot larger than glucose, water, and iodine molecules could not diffuse through the dialysis tubing membrane. Slide 11 Test to see if Glucose diffused out of the cell into the beaker (Page 4) Remove 10 drops from the beaker and place in a test tube containing Benedicts solution (glucose indicator- blue color ) and then place into a beaker of hot water. If the solution changes from blue to a brick red color, this will indicate that glucose molecules diffuse through the pores in the dialysis tubing into the beaker. Before heatingAfter heating Blue Brick red Slide 12 Chemical Testing for Starch & Glucose page 3 Handout Obtain six test tubes and place 10 drops of water in 2, 10 drops of starch in another 2 and 10 drops of glucose in the remaining 2 test tubes. Added 10 drops of Benedict's solution to three of the six test tubes (as per the image on the left below. Place the three test tubes in a hot water bath and a brick red color will indicated a positive reaction (test) for glucose. Test results in center image below. Add 10 drops of Iodine to the remaining three test tubes to obtain the test results in the iodine image below on the right Benedicts Before heating Water-Starch-Glucose Benedicts Results After heating Water-Starch-Glucose Iodine Results Water-Starch-Glucose Slide 13 Chemical Test Results (Page 3) Indicator Solution Used Distilled WaterStarchGlucose Blue-colored Glucose Indicator (Benedicts Sol) Blue (no change) Negative result Blue (no change) Negative result Orange (turned in heat) Positive result Amber-colored Starch Indicator (Iodine Sol) Dark Amber/Brown (no change) Negative result Blue/ Black Positive result Dark Amber/Brown (no change) Negative result Material Tested Slide 14 Questions (page 4) Carefully examine the cell and the beaker you put aside earlier. Record any changes, including color changes, you observe in the cell and the beaker Slide 15 Label the contents and note color changes (page 4) Slide 16 1. What is the best explanation for the color change that occurred inside the cell? Answer 1) Iodine diffuses into the cells causing the cell to change color to a blue/black color 2. Did any starch diffuse out of the cell? Explain how you can tell. Answer 2) The contents of the beaker did not change color (to blue/black) Question 1 & 2 (page 4) Slide 17 Question 3 (page 4) 3. Did any glucose diffuse out of the cell? Explain how you can tell. Slide 18 Question 4 (page 5) 4. Which substance(s) diffused through the membrane? Slide 19 Question 5 (page 5) 5. Which substance(s) did not diffuse through the membrane? Slide 20 Question 7 (page 5) 6. Explain why some substances were able to pass through the membrane while others were not able to. Slide 21 Question 7 (page 5) 7. In the initial State diagram below; Starch Indicator Solution is indicated with the letter I because it contains iodine. Using the letters S for starch and G for glucose, indicate the areas where each of these molecules are located in both diagrams. Be sure you indicate the location of iodine molecules in the Final State diagram too.