Laboratory 6: Osmosis

Objectives

After completing this lab, you should be able to:

• Discuss the flow of water across various types of membranes.

• Relate how factors such as charge and size influence transport across membranes.

• Differentiate between active and passive transport and explain why each is used in cells.

• Correctly define and use the terms hypertonic, hypotonic, and isotonic.

Key Terms

Introduction

Water is essential to all living organisms. Both prokaryotic and eukaryotic cells are largely composed of water, and most biochemical reactions occur in aqueous environments. Without water, cells could not function.

Water acts as a universal solvent, dissolving many substances (though not all, such as lipids). When molecules like sugars or salts dissolve in water, they are called solutes.

Cell membranes are selectively permeable, allowing some materials to pass while restricting others—primarily based on size and charge. Small, uncharged molecules pass freely, while large or charged ones require assistance.

Passive vs. Active Transport

• Passive Transport: Molecules move without energy input (e.g., diffusion, osmosis).

• Facilitated Diffusion: Uses membrane proteins to assist passive transport.

• Active Transport: Requires ATP to move molecules against a concentration gradient.

Diffusion and Osmosis

• Diffusion: Movement of molecules from a high to low concentration region until equilibrium.

• Osmosis: Special type of diffusion referring to water movement across a selectively permeable membrane.

If a cell’s external environment has:

• Higher solute concentration: It’s hypertonic → water leaves the cell.

• Lower solute concentration: It’s hypotonic → water enters the cell.

• Equal solute concentration: It’s isotonic → no net water movement.

Examples in Biology

• Plasmolysis: In plant cells, water loss causes the plasma membrane to pull away from the cell wall, leading to wilting.

• Hemolysis: In animal cells (like red blood cells), too much water entering causes cells to swell and burst.

• Human Example: After swimming in freshwater (hypotonic), cells absorb water; in seawater (hypertonic), water leaves cells, leading to dehydration and wrinkling.

Materials

• 7 microscope slides and coverslips

• Purple onion skin

• Dialysis tubing (10 pieces, 15–20 cm)

• Beakers: one 250 mL and five 100 mL

• Microscope

• 18 small (10 mL) test tubes

• NaCl, glucose, and sucrose solutions

• Physiological saline

• DI water bottle

• Pipettes and pipette pumps

• Ruler, tape, scissors, string (10 pieces, 10–15 cm)

• Test tube holder, Sharpie or grease pencil

• Electronic balance and weigh boat

Procedures

Experiment 1: Plasmolysis in Onion Cells

1. Label seven slides for salt concentrations: 1.00M, 0.50M, 0.21M, 0.17M, 0.13M, 0.10M, 0.07M.

2. Place one drop of the corresponding salt solution on each slide.

3. Tear thin pieces of red onion epidermis—only one cell layer thick.

4. Place one onion piece into each salt solution drop.

5. Let the slides sit for 10 minutes.

6. Add a coverslip using the proper wet-mount technique.

7. Observe under the microscope and record plasmolysis in Table 2.

Observation Question:
When plasmolysis occurs, the cell interior is __________ and the environment is __________.
(Use hypertonic, hypotonic, or isotonic.)

Experiment 2: Hemolysis in Blood Cells

1. Prepare controls:

   • Negative Control: 2 mL physiological saline + 2 drops of blood → mix gently.

   • Positive Control: 2 mL DI water + 2 drops of blood → mix gently.

   • Hold each tube up to printed text:

     • If text is readable → clear (hemolysis).

     • If not → cloudy (no hemolysis).

2. Prepare two test series (glucose and NaCl) using concentrations listed in Table 3.

   • Add 20 drops of solution + 1 drop of blood in each test tube.

   • Let stand for 5 minutes.

3. Record appearance of each test tube in Table 3 (clear vs. cloudy).

Observation Question:
When hemolysis occurs, the cell interior is __________ and the external environment is __________.
(Use hypertonic, hypotonic, or isotonic.)

Experiment 3: Rate of Osmosis

1. Cut five 15 cm pieces of dialysis tubing and soak them in DI water (5–10 minutes).

2. Label five 100 mL beakers 1–5 and fill as shown:

Table 1. Experimental Set-Up for Osmosis

3. Tie one end of each tubing piece.

4. Fill each bag with the correct solution (Table 1). Tie off securely.

5. Record the initial weight (time 0) of each bag (Table 4).

6. Submerge each bag in its designated beaker.

7. Every 5 minutes for 30 minutes, remove each bag, gently dry, weigh, and record its weight.

8. After 30 minutes, calculate change in weight:

    

   $$\text{Change in Weight} = \text{Weight at time x} – \text{Initial Weight}$$Change in Weight=Weight at time x−Initial Weight

   Record in Table 5.

Experiment 4: Starch–Iodine Demonstration

(Instructor Demonstration)

1. A dialysis bag filled with starch solution is placed in a beaker of water + iodine.

2. Let stand for 30–45 minutes.

3. Observe color change (starch turns deep purple when it reacts with iodine).

Data and Observations

Table 2. Plasmolysis in Onion Cells at Varying Salt Concentrations

Table 3. Hemolysis in Blood Cells

Table 4. Raw Weight Data – Osmosis Bags

Table 5. Change in Weight – Osmosis Bags

Review Questions

1. Define diffusion and osmosis.

2. What factors influence (speed up, slow down, or stop) molecular movement? Explain.

3. Differentiate between passive, facilitated, and active transport.

4. Describe the conditions inside and outside a cell during plasmolysis using the terms hypertonic, hypotonic, and isotonic.

5. Describe the conditions inside and outside a cell during hemolysis using the same terms.

6. Does hemolysis occur in plant cells? Why or why not?

7. Using your data from Table 5, create an x–y scatter graph (change in weight vs. time) using Excel or another program.

   • Add trendlines and slope equations.

   • The slope represents the rate of osmosis (Δweight/min).

8. Interpret your graph — what does it show about rate and direction of osmosis in different solutions?

9. For the starch–iodine demonstration, explain the locations of starch, water, and iodine before and after the experiment, and why movement occurred or didn’t.

Notes

Use this section for sketches, raw data notes, and instructor comments.

Licenses and Attribution

CC Licensed Content, Original:

Other Licensed Content Included:

• Adapted with permission from BioScience I Laboratory Manual by Jamie Colson-Moon and Denise Bristol. Per request from these original authors, this content is now licensed (CC BY-NC-SA).