Name: There are two things that will determine which particles will pass through and which will not:

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1 18 Diffusion and Osmosis in Living Systems Name: Problem: How do substances move into and out of cells? Introduction: In order for cells to carry on their life processes, they must take in materials and give off materials. These materials must pass through the outer boundary of the cell to get in or out. The cell membrane is therefore a very important part of the cell because all things must enter and leave through it. It is believed that the membrane of a cell has holes or pores through which things may pass. If a membrane lets things pass through it, we say the membrane is permeable. A membrane that lets only certain things pass through it is said to be semi-permeable or selectively permeable. A membrane that does not allow any material to pass through it is called an impermeable membrane. There are two things that will determine which particles will pass through and which will not: 1. The size of the particles 2. The size of the opening or pores in the membrane. Substances that are dissolved in liquids produce solutions. These dissolved substances move into and out of the cells of living things. This movement is called diffusion; it is one of the main methods used by living things to maintain a steady state. Diffusion throughout a cell involves the movement of substances from a region of greater concentration to a region of lesser concentration. It is by this process that cells obtain the chemicals they need to carry on their work and to rid themselves of waste products. Molecules are always moving around in all directions. As they move, they bump into one another. If they bump up against a membrane, they may be bumped through the holes in the membrane, providing that they are smaller than the openings. Simply stated, diffusion means that molecules move from an area where there are a lot of similar molecules to an area where there is a smaller number of the same kind of molecules. Diffusion may or may not involve the movement of molecules through a membrane. Materials: dialysis tubing (20 cm lengths) Benedict s solution two Test tubes 80% glucose solution two -250 ml beakers iodine solution funnel 1 % starch solution 500 ml beaker eyedropper kettle water

2 Procedure: In this experiment, the dialysis tubing represents the cell membrane. Though it is nonliving, it has several properties in common with a living membrane. Part One: Preliminary tests 1) Clean two 250-mL beakers and two test tubes. 2) Obtain approximately 20 ml of starch solution from the teacher and place it in one of the 250-mL beakers. In the second beaker place about 10 ml of sugar solution. 3) Place about 2 mm of starch solution in a test tube (about a pinky width.) Add a drop of iodine to the starch solution. Describe the reaction in your table of observations. Dispose of the test tube contents down the sink and rinse the test tube. Keep the remaining starch solution in the beaker for part two below. 4) Place about 2 mm of glucose solution in a test tube and then add 10 drops of Benedict s solution to the test tube. Fill the 500 ml beaker with boiling water (be careful!!) and place the tube in boiling water bath for three minutes. Describe the reaction in your table of observations. Dispose of the test tube contents down the sink and rinse the test tube. Keep the remaining sugar solution in the beaker for part two below. These two tests will be used to determine the presence of starch and glucose in the diffusion experiment. Part Two: The Diffusion Experiment 1) Take a piece of dialysis tubing that has been soaking in water. At one end tie a knot. Rub the other end between the thumb and index finger until it opens. 2) Place the funnel in the open end of the dialysis tubing and fill the tube about a quarter full with starch solution and then add glucose solution until the dialysis tubing is half full. Over a sink, twist the open end to make it easier to tie a knot in the dialysis tubing (it will look like a sausage). Gently rinse the outside of the dialysis tubing with water to wash away any of the sugar or starch solution that may have spilled out while tying the dialysis tubing.

3 3) Place the bag in a 500-mL beaker and add tap water until the bag is covered. Add enough iodine to the beaker to turn the contents a definite orange. 4) Observe the bag and beaker contents for approximately 15 minutes. The results of the preliminary test should help you understand what is happening. 5) After 15 minutes, place 5 ml (about a pinky width) of beaker fluid in a test tube. Add enough Benedict s solution to turn the contents blue. As done previously, heat the test tube in boiling water bath for about three minutes. Observe the reaction. 6) Complete the second chart (indicating which materials have diffused and a brief supporting reason). Observations Part 1: Preliminary Tests a) Describe the reaction between the iodine and the starch solution. b) Describe the reaction between the Benedict s solution and the glucose solution. Part 2: The Diffusion Experiment ( /4) Substance Iodine Starch Glucose Water Results from Diffusion Experiment Diffused Through Experimental Evidence Membrane?

4 Conclusion: ( /14) 1) Suggest a reason for the failure of some of the molecules to diffuse through the membrane. ( /1) 2) A membrane that allows some molecules to pass but not others is called ( /1) 3) In what way is the dialysis membrane like the cell membrane? ( /1) 4) In each case, the molecules moved from a concentration to a concentration. ( /2) 5) What does this experiment tell you about the size of the iodine particles compared to the size of starch particles? Refer to evidence from the experiment to answer this question. ( /2) 6) Why do you think it was important to rinse off the outside of the dialysis tubing? (See step 2 in the diffusion experiment) ( /2) 7) What are two materials that would normally enter a real living cell? ( /2) 8) What would happen if the membrane of a living cell became impermeable? Explain your answer. ( /2)

5 9) When we added iodine to the onion cells in our onion cell lab, the cytoplasm changed to a yellowish-brown colour. Why did this colour change take place? ( /1)