Osmosis Lab – Extension

Your lab group will be assigned to one of the following extensions to complete in class. Please prepare by reading through the lab write up and come to class ready to complete your assigned section.

Extension 1 - Diffusion

Diffusion is the random movement of molecules (solute) from an area of higher concentration to an area of lower concentration. Eventually, a dynamic equilibrium will be reached. This happens continuously through semipermeable membranes of cells and requires no energy – it is passive transport.

In this lab, you will work with glucose – a small monosaccharide, starch – a large polysaccharide, and IKI – a chemical indicator that turns blue in the presence of starch.

Materials:

  • 250 mL beaker
  • 1 green dialysis clip
  • 1 clear beaker clip
  • 30 cm piece of dialysis tubing
  • 25 mL graduated cylinder
  • about 200 mL distilled water
  • 4 mL IKI
  • 15 mL 15% glucose/1% starch solution
  • 1 glucose test strip
  • 10 mL graduated cylinder

Procedure:

  1. Obtain a 30 cm piece of dialysis tubing that has been soaking in distilled water. Fold one end over and use the green dialysis clip to close it off. Rub the other end between your fingers to separate the sides and open the tube.
  2. Measure 15 mL of the 15% glucose/1% starch solution in a graduated cylinder. Your teacher will test the solution for the presence of glucose using the glucose test strip. Record the results in Table 1. Rinse graduated cylinder for later use.
  3. Place 15 mL of the 15% glucose/1% starch solution in the bag. Twist the other end of the bag and knot it leaving space for expansion. Record the color of the solution in Table 1.
  4. Fill a 250 mL beaker about 2/3 full with distilled water. Add 4 mL IKI to the distilled water and record the color in Table 1. Your teacher will test the solution for the presence of glucose using the glucose test strip. Record the results in Table 1.
  5. Immerse the dialysis bag in the beaker of water plus IKI and suspend it using the beaker clip.
  6. Allow your setup to stand for approximately 30 minutes or until you see a distinct color change in the bag or the beaker. Record the final color of the solution in the bag and the beaker in Table 1.
  7. Test the liquid in the beaker for the presence of glucose using the glucose test strips. Record the results in Table 1.

Table 1: Diffusion
Location / Initial Contents / Initial Solution color / Final Solution Color / Initial Presence of Glucose Y/N / Final Presence of Glucose Y/N
Bag
Beaker
  1. Which substance(s) are entering the bag and which are leaving the bag? What experimental evidence supports your answer?
  2. Explain the results you obtained. Include concentration differences and membrane pore size in your discussion.
  3. Based on your observations, rank the following by relative size, beginning with the smallest: glucose molecules, water molecules, IKI molecules, membrane pores, starch molecules.
  4. What results would you expect if the experiment started out with a glucose and IKI solution in the bag and only starch and water outside the bag? Why?

Extension 2 - Osmosis Part 2

In this lab, you will determine which solution is most isotonic to the potato cells.

Materials:

  • 6 250 mL beakers (labeled 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 M sucrose solution)
  • Sucrose solutions of different concentrations
  • Potato pieces – 2 per beaker
  • Knife
  • Scale
  • Paper towels

Procedure:

  1. Place 150 mL of the appropriate solution in each of the labeled beakers
  2. Cut 12 0.5”x0.5”x1.5” potato sticks
  3. Blot the outside of the potato sticks dry
  4. Mass two of the potato sticks and record in table 2 as the initial mass for the 0.0M solution.
  5. Repeat for each of the 5 additional pairs of potato sticks.
  6. Place potato sticks in their appropriate beaker.
  7. Record the time you placed the potato sticks in their beaker.
  8. Return at lunch or 6th period/afterschool to remove the sticks and check the final mass of the sticks.
  9. Determine the % change in mass and record.
  10. Plot % change in mass data on a graph and place a best fit line

Table 2 – Percent change in mass of potato in different sucrose solutions
Solution / Initial mass / Final mass / % change in mass
0.0 M sucrose solution
0.2 M sucrose solution
0.4 M sucrose solution
0.6 M sucrose solution
0.8 M sucrose solution
1.0 M sucrose solution

Extension 3 – Plasmolysis

For this extension, you will be using the aquatic plants Elodea. You will create a wet mount slide of Elodea to observe under the microscope. Then you will submerge the Elodea in a 1M salt solution to observe changes in the cells over time.

Materials:

  • Leaves from Elodea in distilled water
  • Leaves from Elodea in a salt solution
  • 1M NaCl solution
  • microscope
  • 6 slides
  • 6 coverslips
  • distilled water
  • Paper towel
  • Pipette
  • Pencil
  • Colored pencils

Procedure:

  1. Pluck a leaf from Elodea soaking in distilled water and prepare a wet mount slideusing distilled water.
  2. Pluck an Elodea leaf and place it on your slide.
  3. Place 1-2 drops of distilled water on the slide.
  4. Place one end of the coverslip on the slide and slowly drag and lower it onto your sample. Try to eliminate air bubbles under your cover slip.
  5. Observe the Elodea and draw and color, label, and describe the cells in Table 3.
  6. Pluck a leaf from Elodea soaking in the salt solution and prepare a wet mount slide using the salt solution.
  7. Observe the Elodea and draw and color, label, and describe the cells in Table 3.
  8. Each group member should perform the steps 6 –8 individually.
  9. Pluck a leaf from Elodea soaking in distilled water and prepare a wet mount slide using distilled water.
  10. Fill a pipette with the salt solution and place a drop on the left side of the coverslip while using a folded piece of paper towel to wick the NaCl solution across the slide.

  1. Watch as the cells respond to the change in the extracellular environment.
  2. Describe what occurred below.
  3. You can also try reintroducing distilled water by using the wicking method and observe changes that occur.

Table 3 – Observation of Elodea Cells
Solution / Drawing / Description
Distilled water
1M NaCl solution