Diffusion and Osmosis Laboratory Notes

Laboratory Objectives

  • Review transport mechanisms moving substances across cell membranes.

  • Investigate the variables affecting substance movement rates.

  • Learn to calculate and graph diffusion and osmosis rates.

Introduction to Transport Mechanisms

  • All organisms consist of cells. Understanding cellular processes is vital for life.

  • Cells obtain materials from their surroundings and expel waste.

  • Passive Transport:

  • No energy expenditure by cells.

  • Molecules move randomly, driven by concentration gradients.

  • Active Transport:

  • Energy required to move substances across membranes.

Key Concepts: Diffusion and Osmosis

Diffusion

  • Definition: Net movement from a region of high concentration to low concentration.

  • Example: Ammonia odor diffusion in a room.

  • Diffusion Rate Influencing Factors:

  • Molecule Size: Smaller molecules diffuse faster.

  • Temperature: Higher temperatures increase molecular movement and diffusion rates.

Diagram Explanation

  • Diffusion in Liquids: Solid solute dissolving in a liquid solvent (e.g., sugar in water).

Osmosis

  • Definition: Diffusion of water across a selectively permeable membrane.

  • The plasma membrane is selectively permeable to water.

  • Movement Direction:

  • Water moves depending on concentration gradients inside vs. outside the cell.

  • Example: Water enters a cell placed in distilled (pure) water because the surrounding water concentration is higher than inside the cell.

Laboratory Activities

Activity 1: Rate of Diffusion

  • Objective: Measure diffusion of dye in semisolid agar (model for cellular material).

  • Materials:

  • Two Petri dishes of 1% agar

  • Dye solution

  • Metric ruler

  • Sharpie pen

Procedure Steps:

  1. Create wells in Agar using a straw.

  2. Label dishes (e.g., Room temp, Cold temp).

  3. Fill wells with different dyes, cover, and time the diffusion.

  4. Measure the distance traveled by the dye after 60 minutes.

  5. Calculate the diffusion rate (distance traveled in mm/elapsed time in hr).

  6. Compare diffusion rates between temperatures.

Activity 2: Rate of Osmosis

  • Objective: Demonstrate osmosis using dialysis tubing to measure water movement.

  • Materials:

  • Dialysis tubing

  • Beakers

  • Funnel

  • Sucrose solution (10% and 40%)

Procedure Steps:

  1. Prepare dialysis tubing by soaking and tying one end.

  2. Fill with water or sucrose solution.

  3. Place in tap water; record weight changes every five minutes.

  4. Calculate percent change in weight:

  • % Change = (change in weight / initial weight) x 100

  1. Graph results of weight changes for visual representation.

Observations and Data Recording

Table Examples

  • Dye diffusion at room temperature

  • Record time, distance traveled, and calculate rate of diffusion (mm/hr).

  • Osmosis into dialysis bags with varying sucrose concentrations for weight changes.

Key Observations:

  1. Dye Diffusion:

  • Faster in warm temperature compared to cold.

  • Methylene blue may diffuse slower due to larger molecular size.

  1. Weight Changes in Dialysis Bags:

  • Significant weight gain indicates water movement into a higher solute concentration area.

  • Bags with higher sucrose concentration gain more weight (hypotonic solutions).

Analysis Points

  • Discuss why diffusion is faster in smaller cells and the impact of temperature.

  • Evaluate the significance of selective permeability in osmosis and biological systems.

Conclusion

Understanding the mechanisms of diffusion and osmosis is crucial for insights into cell biology and physiological processes. The laboratory activities illustrate the principles in a practical context, enhancing comprehension of cellular transport.