Experiment 2: Osmosis – Direction and Concentration Gradients

Experiment 1: Cell Structure and Function

The structure of a cell dictates the majority of its function. You will view a selection of slides that exhibit unique structures that contribute to tissues function.

Procedure

1. Examine the onion root tip digital slide images on the following pages. Then, respond to the Post-Lab Questions.

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Experiment 2: Osmosis – Direction and Concentration Gradients

In this experiment, we will investigate the effect of solute concentration on osmosis. A semi-permeable membrane (dialysis tubing) and sucrose will create an osmotic environment similar to that of a cell. This selective permeability allows us to examine the net movement of water across the membrane. You will begin the experiment with a 30% sucrose solution, and perform a set of dilutions to create lower concentration solutions.

Procedure

1. Use the permanent marker to label the three 250 mL beakers as 1, 2, and 3.
2. Cut four strips of dialysis tubing, each 15.0 cm long. Fill Beaker 3 with 100 mL of water and submerge the four pieces of dialysis tubing in the water for at least 10 minutes.
3. While the dialysis tubing soaks, reconstitute the sucrose powder according to the instructions provided on the bottle´s label (your kit contains 60 g of sucrose in a chemical bottle). This will create 200 mL of a 30% stock sucrose solution.
4. Use the permanent marker to label the 100 mL beakers with the concentrations 30%, 15% and 3%. Use Table 2 and the 10 mL graduated cylinder to create additional sucrose solutions that are concentrated respectively. Rinse and dry the graduated cylinder between uses. Set these solutions aside.

1. Use the 100 mL graduated cylinder to measure and pour 150 mL of the remaining stock sucrose solution into Beaker 1. Rinse and dry the graduated cylinder.
2. Use the 100 mL graduated cylinder to measure and pour 20 mL of the remaining stock sucrose solution into Beaker 2. Rinse and dry the graduated cylinder.
3. Use the 100 mL graduated cylinder to measure and pour 180 mL of water to into Beaker 2. Swirl the beaker to thoroughly mix the sucrose solution and water. This creates 200 mL of a 3% sucrose solution. Rinse and dry the graduated cylinder.
4. After the tubing has been submerged in water for at least 10 minutes, remove one piece of tubing from the beaker. Use your thumb and pointer finger to rub the tubing between your fingers; this will open the tubing. Close one end of the tubing by folding over 3.0 cm of one end (this will become the bottom). Fold it again and secure with a yellow rubber band.
5. Tie a knot in the remaining dialysis tubing just above or just below the rubber band. This will create a seal and ensures that solution will not leak out of the tube later in the experiment.
6. To test that no solution can leak out, add a few drops of water to the tubing and look for water leakage. If any water leaks, tighten the rubber band and/or the knot in the tubing. Make sure you pour the water out of the tubing before continuing to the next step.
7. Repeat Steps 8 – 10 with the three remaining dialysis tubes, using each of the three remaining rubber band colors.
8. Use the 10 mL graduated cylinder to measure 10 mL of the 30% sucrose solution created in Step 4. Record the actual initial volume of solution in the 10 mL graduated cylinder that will be poured into the yellow dialysis tubing in Table 3. Then, pour the 30% sucrose solution into the dialysis bag with the yellow rubber band. Seal the top of this tubing with the remaining yellow rubber band. Rinse and dry the graduated cylinder.
   Note: Be sure to hold the graduated cylinder over the dialysis tubing opening for several seconds after the fluid has been transferred into the tubing. You may also want to gently shake the cylinder to ensure all 10 mL are transferred. This technique should be applied to Steps 13 – 15 as well!
9. Use the 10 mL graduated cylinder to measure and pour 10 mL of the 15% sucrose solution in the bag with the red rubber band. Record the initial volume of solution poured into the bag in Table 3. Seal the top of the dialysis tubing with the remaining red rubber band. Rinse and dry the graduated cylinder.
10. Use the 10 mL graduated cylinder to measure and pour 10 mL of the 3% sucrose solution in the bag with the blue rubber band. Record the initial volume of solution poured into the bag in Table 3. Seal the dialysis tubing with the remaining blue rubber band. Rinse and dry the graduated cylinder.
11. Use the 10 mL graduated cylinder to measure and pour 10 mL of 3% sucrose solution in the bag with the green rubber band. Record the initial volume of solution poured into the bag in Table 3. Seal the dialysis tubing with the remaining green rubber band. Rinse and dry the graduated cylinder.
12. Place the yellow, red, and blue banded tubing in Beaker 2 (3% sucrose solution). Place the green banded tubing in Beaker 1 (30% sucrose solution). See Figure 8. Record the % sucrose of the beaker that each tubing is placed into in Table 3.
    Hint: Covering the beakers with a sheet of Saran wrap or aluminum foil helps keep the top of the dialysis tubing from drying out.

1. Hypothesize whether water will flow in or out of each dialysis bag. Include your hypotheses, along with supporting scientific reasoning in the Hypotheses section at the end of this procedure.
2. Allow the bags to sit for one hour.
3. After allowing the tubing to sit for one hour, remove them from the beakers.
4. Carefully open the tubing. The top of the tubing may need to be cut off/removed as it tends to dry out over the course of an hour. Measure the solution volumes of each dialysis bag using the 10 mL and 100 mL graduated cylinder. Make sure to empty and dry the cylinder completely between each sample.
5. Complete Table 3 by calculating the net displacement of water from each piece of dialysis tubing.
6. Take a picture of your results. Include a note with your name and date on an index card in the picture. Insert picture here.

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