Laboratory 3: Diffusion and Osmosis
Background
So far we have learned about the methodology of science, how scientists make measurements, and the significance of understanding the chemical basis for life. We have not, however, applied any of these ideas to a living system! Biology, as you all know, is the study of LIFE! It is here, at last, that we first learn about one of the most basic processes that makes life possible. Diffusion is the net movement of molecules from where they are very dense (a.k.a. “concentrated”) to where they are not very dense, until it is even everywhere. Once the molecules are evenly distributed (equal everywhere) this is called equilibrium.
It is important to note that this is not a completely random process. Molecules are just about always in motion and because they are always moving, they are always bumping into each other. What happens is that the more of these molecules (say, for example, we are talking about perfume molecules) exist in one area, the more likely they are to collide with each other. When they do, they end up sailing off into another direction. As they move away, the chance that they will bump (or be bumped) is diminished. As a rule, the greater the concentration of molecules in one region, then the greater the number of collisions. Some molecules are propelled into the concentrated area and others are propelled away from the more concentrated area. Because of this, over time, the movement of molecules is always away from the “source” – where they are the most concentrated to where they are the least concentrated.
Diffusion can only occur when there is not equilibrium. A condition known as a concentration gradient must exist in order for molecules in a direction. In other words, if all the molecules are already evenly distributed, there will be no net movement. This does not mean, though, that the molecules stop moving. It means that even though they continue to bump each other, the molecules will never “reassemble” to become highly concentrated again. It is the process of diffusion that allows gases and other small molecules (like water) to enter and leave our cells. Because living things are open, dynamic systems, it is critical that there be mechanisms to regulate what enters and exits our cells; diffusion is one such process.
The direction of diffusion is always from where there were originally more molecules to where there are fewer. Think of a huge crowd dispersing from a concert or some big-time sporting event. Some people leave earlier than others and some linger. Some get to their cars just to realize they forgot something and have to go back in. While the vast numbers of people is headed in the direction of home, some (at least for the time being) are headed back in. The net movement (the direction in which most people – or molecules – are headed) is the movement of those leaving minus the movement of those returning.
Imagine your friend opens up a bottle of perfume. As you know from experience, it does not take long before you (and everyone else in the room) are able to smell the contents. Even though you cannot actually “see” this, you know it is working. This is diffusion in action. The molecules move away from where they are the most concentrated (the bottle) to where they are not (everywhere else in the room) until they are even everywhere (remember that is called equilibrium).
This process (of molecules moving away from where they are the most concentrated) occurs as long as there is a difference in concentration across space. Remember, as soon as the molecules are evenly distributed (the same number of molecules leave the bottle as enter it) there can be no more net movement of molecules and equilibrium has been reached.
A practical example of this is the diffusion of gasses in our lungs and tissues. While we inhale a mixture of oxygen (O2), carbon dioxide (CO2), and other gases, we don’t absorb these gases in equal amounts into our blood stream. If our blood contains a greater concentration of carbon dioxide and a lesser concentration of oxygen when it arrives in the lungs, the net movement of oxygen will be into the blood (where it is less concentrated) and the net movement of carbon dioxide will be out of the blood and into the lungs where it is less concentrated. The difference between the perfume example and the gas exchange example is the presence of membranes (the blood vessel and lung tissue).
Selectively permeable membranes, present in all living things, separate the contents of cells from the outside world. Do not picture the membrane as a solid wall that is used simply to contain a cell. It is far more dynamic than that: its job is to decide what is allowed in and what is allowed out of a cell (hence the descriptor “selectively”). Membranes can be created synthetically (in your Lab Kit there is a length of dialysis tubing which is created for research purposes and you will use to complete this lab). Created membranes are typically only permeable to water and other tiny molecules. In a living system, however, membranes are permeable to all kinds of things including water, salts, and other molecules (with the aid of protein channels embedded there).
The diffusion of water across a selectively permeable membrane is called osmosis. In the image to the left, an example of the movement of molecules from inside to outside of a cell is shown. On the left side, the blue circles represent water; on the right, there is a solution of water and another molecule that is too large to diffuse through. This can be seen by observing a cell in the presence of a highly concentrated solution (like salty water). Because it is more concentrated outside (there is less water outside than inside the cell) the water that is inside the cell moves out.
In this case, the cell (which is less concentrated that the solution it is in) is called hypotonic with regard to that solution. The solution (which is more concentrated than the cell) is considered hypertonic with regard to that cell.
Eventually the cell will lose water until the concentration of the cell matches its surroundings (the salty water); the cell shrivels, or plasmolyzes (see image below). The red arrows point to the position of the cell membranes that have withdrawn from their respective cell walls.
Follow the directions below to complete the lab, then record the data and answer the questions on “Lab 3 Answer Sheet” under “Laboratory” in Blackboard.
Procedure:
You will be testing the results of diffusion and osmosis by measuring the change in volume of three dialysis tubing bags with three different concentrations of molasses solutions. Volume change will be measured using the water displacement method.
Materials:
· 10 clear plastic cups (10 oz)
· 3 - 6” lengths of dialysis tubing (included in your Lab Kit)
· 6 - 3” pieces of coarse thread or string
· 1 bottle of molasses (small is fine)
· beaker (included in Lab Kit)
· graduated cylinder (included in Lab Kit)
· marking pen
Diffusion at Different Concentrations
1. Cut your dialysis tubing to size using scissors and place them in a cup of tap water.
2. Label 6 of your plastic cups. One should be “100%”, two “50%”, two “25%” and the other “Displacement”
3. Dilute your molasses. Fill the cup labeled “50%” 1/3 full with molasses (using a measuring cup or the graduated cylinder will be too messy and difficult. It is perfectly acceptable to “eyeball” these volumes—just get as close a possible to the estimated amounts). Add the same amount of water to the cup and stir until the molasses is dissolved. Fill the cup labeled “25%” about ¼ of the way with molasses. Add three times the amount of water as you added molasses. Stir until the solution is mixed well and the molasses is dissolved (this solution will be very watery!).
4. Fill the cup labeled “Displacement” with exactly 200 mL of water (using your graduated cylinder). Your cup should be about 2/3 full. If it is too full, remove all the water and add an amount that allows it to reach the 2/3 mark. It is very important that this volume be written down (Under “Initial volume” on your answer sheet)! Once the water in your “Displacement” cup is at its correct level, draw a line on the outside of the cup at the water level. Label this line with a “*” so you know what the original volume was.
5. Remove one of the pieces of dialysis tubing and tightly tie one end using the string. Open the other end of the tube by rubbing your fingers together over the end of the tubing to loosen it. Fill the bag about half-way full with the 25% solution. Carefully tie the other end with a piece of string. Set it into the clean “25%” cup.
6. Remove another piece of tubing and fill it the same way using the 50% solution. Place it into the clean “50%” cup.
7. Remove the last piece of tubing and fill it about half-way with pure (100%) molasses. This may get a bit messy! If you spill molasses on the outside of the bag that is okay…just rinse it with water before starting the experiment. Place it into the cup labeled “100%”.
8. Determine the volume of each bag. Place the 25% bag into the “Displacement” cup. The water in the cup should rise. Once the bag is submerged (try not to force it under water) draw a line on the cup at the new water level and label it “25% before”. Do the same for the other 2 bags, drawing lines and labeling the level of water once the bag is submerged. It is possible that the volume of each bag is very similar. Rotate the cup so that the lines do not overlap.
9. Place each bag into its respective cup (25% into the 25% cup, 50% into the 50% cup, 100% into the 100% cup) and add enough tap water (room temperature is perfect) to cover the bags. Leave them for 30 minutes.
10. While you are waiting, determine the initial volume of each bag. To do this, empty your “Displacement” cup. Refill it with the exact volume of water that you initially measured. Fill your graduated cylinder to the 25 mL mark. Pour water into the cup until it reaches “25% before” mark (You may need to fill the graduated cylinder a couple of times to reach the line.) Calculate how much water you poured by subtracting the amount of water left in the graduated cylinder from 25, and adding it to however many 25 mL amounts you already added. This is the initial volume of the bag. Record this number on your answer sheet under “Initial Volume 25%”. Follow this procedure for the other 2 bags, 50% and 100%.
11. Once the experiment has run for 30 minutes, determine if the volume has changed for each bag. Repeat the displacement method by duplicating steps 8 (label the lines “25% after”, “50% after”, and “100% after”) and 10, emptying and refilling the Displacement cup after each volume check. Record the volumes on the answer sheet under “After Volume…”.
12. Be sure your data tables are filled in and answer the questions on your Answer Sheet.
Composed by Dandelian Tucker
June 2004
MSJC