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PT zdev 1100 Lab Biological Membranes Biological Membranes Review the concepts of diffusion, osmosis and selectively permeable membranes before doing this lab. Chapter 4, Sections 4.1-4.12, and Figure 4.29 will be of use. This lab takes a week to complete; give yourself plenty of time to get it done. Exercise III can be completed while you are waiting for Exercises I and II. The purpose of this lab is to investigate the plasma membrane's important function as a diffusion barrier between the interior of the cell and the surrounding environment. The contents of a cell are typically very different from the contents of the surrounding fluids. Many vital substances are present inside the cell in much higher concentrations than in the cell's environment. Since the natural trend is for materials to diffuse into areas of low concentration, the role of the cell membrane in confining materials to the inside of the cell (not to mention excluding the entry of some chemicals) is pretty important. Diffusion (the passive movement of molecules from a region of high concentration toward a region of lower concentration) would cause vital chemicals like amino acids and sugars to drift away. However, as important as it is to contain the materials of the cell, it is just as important to maintain and control exchanges with the outside world. The cell must be able to get food materials and oxygen from the outside, and it must be able to export materials and rid itself of wastes. In addition, the movement of water through the cell membrane is very important. Biological chemistry is aquatic chemistry — it takes place in a water environment — and many of the chemical processes in cells use or produce water. For some types of cells, the water content in the cell is important for the physical integrity of the cell and the organ the cell is part of. For example, the ability of a leaf to remain expanded is due to the pressure of water inside its cells — turgor pressure. (Turgor means stiffness.) Everyone knows what happens to leaves when a plant doesn't get enough water. Water moves through cell membranes via a special version of diffusion called "osmosis." Osmosis is the diffusion of a solvent (in the case of cells, water) through a selectively permeable membrane. Objectives This lab's purpose is to explore the role of the cell membrane as a selectively permeable diffusion barrier. Materials · At least 2 raw eggs. Make sure the eggs are not cracked. NOTE: In case you break your eggs, you might want to prepare more than 2. IMPORTANT: Make sure that all of the eggs you use are relatively equal in size. · 1 fresh (raw), clean potato · 150 raisins · 1 bottle of clear vinegar (you'll probably use it all) · Approx. 1 cup of corn syrup · 1 liter bottle of bottled water (distilled is best; you want it as pure as possible) · Table salt · Measuring tape (as used for sewing) · Small kitchen scale (weighing device) · 1 medium sized bowl, such as a small mixer bowl. · 2 cups, glasses or jars, big enough to comfortably accommodate one egg each with none of the egg protruding above the top of the vessel · 2 small bowls · 3 cups (in addition to those noted above), big enough to contain more than one cup of water · Potato peeler. Please peel carefully :^) · Sharp knife capable of cleanly cutting your potato. Please handle your knife with respect. · 1 measuring cup · Plastic wrap capable of sealing the top of your bowl · Paper towels · Aluminum foil Preliminary Activity: On the Data Sheet answer the following questions. 1. Define simple diffusion. 2. Define osmosis. Be careful here. Osmosis is often defined in terms of solute concentration, but what moves in osmosis is the solvent, so you will make more sense of the comparison between diffusion and osmosis if you define it in terms of solvent (water) concentration. 3. What two significant aspects of osmosis (which is a special case of diffusion) make it different from simple diffusion? To answer this question, you need to carefully compare the definitions of diffusion and osmosis. Exercise I Use your data sheet to record your findings. 1. Most cells are microscopic in size, but a chicken's egg is a very large cell. While it has special features associated with its particular role in the life cycle of the chicken, it serves as a good large model of cellular behavior. (Note that the yolk of the egg is not a nucleus; it's a concentrated food source for the embryo the egg is designed to support. 2. Measure the circumference of each of your eggs around its largest diameter. Record on the data sheet. 3. Use your scale to weigh each egg. Record these numbers on the data sheet. As you proceed with this exercise, take care to maintain the ability to distinguish the eggs from each other. You will not be able to do this by marking on the surface of the egg. The best way is to keep track of each egg's position in your bowl. 4. Gently place your eggs inside the bowl, being careful not to crack or chip them. You can't continue with an egg that has been damaged. 5. Pour enough vinegar into the bowl to completely cover the eggs. Cover the bowl with plastic wrap. 6. Observe and record anything interesting about the appearance of the vinegar-covered eggs. 7. The eggs are to remain in the vinegar for 72 hours (3 days). Don't refrigerate. 8. After 72 hours, carefully remove the eggs from the vinegar bath. Handle the eggs very gently. Measure each egg's circumference and weight, then record these data on the data sheet. 9. What is the most striking difference you observe in the eggs? 10. Record all of your observations in the appropriate space on the data sheet. DO NOT discard or damage the eggs; you need them for the next part of the lab. 11. Answer the following questions about this experiment. Record your answers on the data sheet. a. Read the label of your vinegar bottle. Identify the active ingredient of vinegar. You want to find the name of the chemical. b. What is an egg's shell made of? You may have to go looking for this. c. What happened to the shell of the egg? d. What did you observe when you first poured the vinegar over the eggs that might indicate that a chemical reaction was happening? e. In general terms, what do you think was happening? f. Why do you suppose the egg retained its integrity? Be specific. Exercise II Use the second part of the data sheet to record your findings. 1. You will need your two cups (or glasses or jars). They must be very clean. 2. In the first cup, pour a couple of inches of corn syrup. 3. In the second cup, pour a couple of inches of pure water (distilled if available). 4. Carefully place one of your eggs from Exercise I in each of your cups. Be sure to keep track of which egg is which. If the fluid in either cup doesn't completely cover the egg, carefully add enough of the appropriate fluid so the egg is completely submerged. Cover each cup with plastic wrap. 5. Copy the appropriate circumferences and weights of your two eggs on from Data Sheet 1 to the indicated spaces on Data Sheet 2, being sure to associate each with the appropriate cup. 6. Your eggs will remain in this second bath for another 72 hours (three days). Don't refrigerate. 7. After the three days, answer the following questions and record your answers on Data Sheet 2. a. Still handling your eggs very carefully, weigh and measure each of them, recording the data in the appropriate places on you Data Sheet. b. Describe the appearance of each egg. How has each changed in appearance since the beginning of Exercise II? c. You will need to think a bit about this. Before you answer, consider the following: The molecules in corn syrup are fairly large, but water molecules are small. The fluid inside of an egg, like every living cell, is almost entirely composed of water; the corn syrup has a much lower concentration of water. Explain what happened to each egg. Be specific and think about the subject of this lab before you answer. d. If you carefully rinsed your two eggs and reversed their positions (placed each in the other egg's cup), what would you expect to happen, and why? Be specific and use the technical language you should be learning as you study the subject of this lab. Exercise III: Turgor Pressure Use the third part of the Data Sheet to record your findings. Osmotic balance is important for all kinds of cells, but the cells of plants have a special kind of dependence on water and the power it can exert. This special relationship creates a phenomenon called turgor pressure. Investigate the meaning of the word turgor and/or turgid. Record what you find in the space provided on the Data Sheet. As you perform these simple experiments, consider why this would be important to a plant (and not to an animal) Part A: Osmosis and Turgor 1. Add one heaping tablespoon of salt to one cup of water, mixing thoroughly. Pour the salt solution into one small bowl. 2. Place one cup of your bottled/distilled water into a second small bowl. 3. Peel your potato, and slice it into several quarter-inch thick slices. Please be careful; we are not examining blood. Spend a moment examining and feeling the slices, particularly noting firmness and stiffness. There is room on the Data Sheet for your observations. 4. Before you go further, record the answers to the following questions on the Data Sheet. a. If you put slices of potato (which is composed of plant cells) into the salt water, what do you predict will happen to them and why? b. If you put slices of potato into the fresh/distilled water, what do you predict will happen to them and why? 5. Divide your potato slices into two roughly equivalent groups. Use your scale to weigh each stack of potato slices. Record these weights on your data sheet. 6. Place one stack of your freshly cut and weighed potato slices into each bowl. Make sure you've connected the appropriate initial weight with the correct treatment. The potato slices should remain in their solutions, at room temperature, for at least thirty minutes. 7. After your potato slices have bathed for the requisite time, pour off the solutions, briefly blot the slices on paper towel, and reweigh