Name Date Period Passive Transport Lab: Diffusion and Osmosis OBJECTIVE: Apply your understanding of the processes of diffusion and osmosis to explain observational data. PART A: Starch and Iodine MATERIALS 25 cm piece of 2.5-cm dialysis tubing (pre-soaked) 250 ml beaker Small funnel tap water 1% starch solution Lugol s Iodine (IKI) solution Paper towel Scale PROCEDURE Figure 1. Experimental Setup. 1% starch solution (http://www.mdk12.org/assessments/high_school/look_like/2005/biology/images/6_q.gif) 1. Prepare the Beaker A. Fill a 250 ml beaker approximately two-thirds full with warm tap water (just estimate). B. Add enough Lugol s iodine solution to the water to make it turn dark yellow. (WARNING: Iodine stains and will severely damage your eyes!) 2. Prepare the Bag A. Obtain a piece of soaked dialysis tubing. Tie off one end of the tubing by making a tight knot. B. Using a funnel, slowly add 15 ml of the 1% starch solution to the bag. Remove all remaining air from the bag. Leave about 2 inches of empty space in the bag, and then tie off the open end of the tube by making another knot. C. Get the mass of the bag. Enter these data and other information in the DATA TABLE on page 2. D. Run the bag under tap water to remove any excess starch from the outside of the bag. THIS IS IMPORTANT! 1
3. Fully immerse the bag in the Iodine solution. (See Figure 1 on the previous page.) Allow the bag to incubate in the beaker for ~ 10-20 minutes, at room temperature. 4. While the bag is incubating, answer the questions below: QUESTION 1: You are studying how molecules move in and out of cells. The bag full of starch is supposed to represent one animal cell. What part of the cell is the dialysis tubing supposed to be? QUESTION 2: Starch is inside the cell. Draw a picture of one starch molecule in the space below. Is starch a large molecule or a small molecule (relative to something like water)? QUESTION 3: Lugol s Iodine is a molecule made of only three atoms: I 2 K. Compared to starch, is Lugol s Iodine a big or a small molecule? QUESTION 4: Do you think starch molecules will be able to pass through the cell membrane (dialysis tubing) and get out of the cell (the bag)? Why or why not? QUESTION 5: Do you think the Iodine molecules (I 2 K) will be able to pass through the cell membrane (dialysis tubing) and get into the cell (the bag)? Why or why not? COLLECT DATA After the incubation period, pull the bag out of the beaker and dry it using a paper towel. Get the mass of the bag and look at the color of the solution in the bag. When exposed to starch, iodine changes from brown/yellow to purple/blue. Any areas of this darker color indicate that the starch and the iodine molecules have moved and come in contact with each other. Look at the color of the solution in the beaker. Enter all data in the table below. DATA TABLE. Starch bag in iodine data. Color of solution in the bag ( cell ) Originally Color of solution in beaker Mass of the bag ( cell ) After Incubation 2
ANALYZE THE DATA: QUESTION 1: Based on the color of the starch solution, did Iodine molecules move into the bag? Explain. QUESTION 2: Based on the color of the solution in the beaker, did starch molecules move out of the bag? Explain. QUESTION 3: Based on the mass of the bag, did water molecules move into the bag? Explain. QUESTION 4: What is the name of the process by which iodine or starch molecules move from an area of high concentration to an area of lower concentration? QUESTION 5: What is the name of the process by which water molecules move from an area of high concentration to an area of lower concentration? QUESTION 6: Why did the molecules mentioned above move in the direction they moved? QUESTION 7: Why were certain molecules able to passively move through the cell membrane, while others were not? (HINT: There is one, simple reason.) QUESTION 8: In real cells, the cell membrane is selectively permeable. Did the dialysis tubing act like a selectively permeable membrane? Explain. 3
PART B: Plants and their environment 1.) Get a piece of paper towel and bring it to your lab station (desk). 2.) Draw a line down the middle of the paper towel. Label one half of the paper towel Pure Water and the other half Salt Water. 3.) From the small beakers, get one piece of celery that has been sitting in a solution of pure water (100% water), and put in on the half of the paper towel labeled Pure Water. 4.) Get one piece of celery that has been sitting in salt water (10% NaCl, 90% water), and put in on the half of the paper towel labeled Salt Water. 5.) Look at and test the rigidity of the celery (try to snap it in half by bending it). Record your observations about the appearance and rigidity of the two pieces of celery in the data table below: DATA TABLE. Celery Observations. Celery in Pure Water Celery in Salt Water Appearance Texture/Rigidity 6.) Dispose of the celery, clean up, and answer the analysis questions below. ANALYSIS QUESTIONS: QUESTION 1: Celery tissue is made of millions of plant cells. Based on your data, what did the cells in the pure water celery tissue probably look like, and what did the cells in the salt water celery tissue look like? Make a simple sketch below. PURE WATER SALT WATER 4
QUESTION 2: Think about the texture/rigidity of the celery that was in salt water. A.) Was the celery hard or wilted? B.) Based on your answer to part A, what molecule most likely moved out of the celery s cells? C.) What is the name of the process described in part B? D.) Why would the molecule in part B move out of the cells? QUESTION 3: If you put the salt water celery in a beaker of pure water, what do you think would eventually happen to the appearance and rigidity of the celery? Explain what molecule would move to cause this change, and why that molecule would move in that direction. You may use well labeled drawings to support your answer. QUESTION 4: If water moved into an animal cell, it would swell and could eventually explode. Why wasn t the celery tissue destroyed by the cells swelling/exploding? What do plant cells have to protect them against this? You may use well labeled drawings to support your answer. 5