Diffusion and Osmosis: Experimental Procedures and Analysis

Diffusion and Osmosis in Living Membranes

Introduction to Diffusion and Osmosis

Diffusion and osmosis are fundamental processes that govern the movement of substances across cell membranes. These processes are essential for maintaining cellular homeostasis and are widely studied in biology and chemistry. In laboratory settings, experiments using artificial membranes can help illustrate these concepts.

• Diffusion: The movement of molecules from an area of higher concentration to an area of lower concentration.

• Osmosis: The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

• Application: Understanding these processes is crucial for studying transport mechanisms in living cells.

Experimental Setup: Simulating Membrane Transport

In this experiment, artificial sacs made from dialysis tubing are used to simulate cell membranes. The sacs are filled with various solutions and placed in beakers containing different external solutions to observe diffusion and osmosis.

• Materials: Dialysis tubing, beakers, graduated cylinder, marker, glucose solution, NaCl solution, boiled starch, distilled water.

• Procedure Overview:

  1. Prepare dialysis sacs with specific solutions.

  2. Place sacs in beakers containing external solutions.

  3. Incubate for 45 minutes.

  4. Test for the presence of solutes in both the sac and the beaker after incubation.

• Purpose: To determine which substances can diffuse through the membrane and to observe osmosis.

Testing for Diffusion of Solutes

After incubation, chemical tests are performed to detect the presence of glucose, NaCl, and starch in the sac and beaker. The results indicate whether these substances have diffused across the membrane.

Table: Chemical Tests for Solutes

Test for Glucose (Exp. 1-3)	Test for NaCl (Exp. 4)	Test for Starch (Exp. 5)
Procedure: 1. Put 5ml of beaker fluid into a test tube. 2. Put 5ml of sac fluid into a second test tube. 3. Add ~5ml of Benedict's solution to each tube. 4. Place tubes in boiling water for ~1 minute. Positive Result: Green, yellow, or rusty precipitate forms. Glucose is present. Negative Result: Solution remains original blue color. No glucose is present.	Procedure: 1. Add 2-3 drops of silver nitrate directly to the beaker and to the sac solution. 2. Observe immediately. Positive Result: White/cloudiness develops. NaCl is present. Negative Result: No white/cloudiness develops. No NaCl is present.	Procedure: 1. Add 2-3 drops of Lugol's iodine directly to the beaker and to the sac solution. 2. Observe immediately. Positive Result: Appearance of a black color. Starch is present. Negative Result: No black color. No starch present.

Experimental Data Table Structure

The following table is used to record the results of the experiment, including the initial and final weights of the sacs, and the results of chemical tests for solute diffusion.

Contents of sac at start (g)	40% glucose DH2O	10% glucose DH2O	10% glucose	10% NaCl DH2O	Boiled Starch DH2O
Weight of sac after incubation (g)
Change in weight of sac during time in beaker (g)
TEST for solute using:	Benedict's solution	Benedict's solution	Benedict's solution	Silver Nitrate	Lugol's Iodine Solution
Presence of solute in sac after 45 minutes? (yes/no)
Presence of solute in beaker after 45 minutes? (yes/no)

Clean Up Procedures

• Place dialysis sacs into labeled beaker (do not throw away).

• Rinse beakers with tap water and return clean beakers to your station.

• Empty sacs into sink, rinse, and discard.

Microscopic Investigation: Diffusion in Red Blood Cells

In a related experiment, red blood cells are suspended in isotonic, hypotonic, and hypertonic solutions to observe the effects of osmosis on cell shape under a microscope.

• Isotonic solution: No net movement of water; cells retain normal shape.

• Hypotonic solution: Water enters cells; cells may swell and burst (lyse).

• Hypertonic solution: Water leaves cells; cells shrink (crenate).

• Compare observed cell shapes to reference images for analysis.

Key Equations and Concepts

• Osmosis Equation: The net movement of water ($ext{H}_2 ext{O}$) across a membrane is determined by the difference in solute concentration.

• Diffusion Rate: $J = -D \frac{dC}{dx}$ where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Osmotic Pressure: $\Pi = iMRT$ where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.

Example Application

If a dialysis sac containing 40% glucose is placed in distilled water, glucose molecules may diffuse out if the membrane is permeable to glucose, while water will move into the sac by osmosis, increasing its weight.

Additional info: The experiment described is a classic biology/chemistry lab for understanding membrane transport, not directly College Algebra, but involves quantitative analysis and data recording relevant to scientific study.