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Diffusion and Osmosis

Learning Objectives

At the end of this lab, students should know...

What is diffusion?

Diffusion is the net movement of particles from an area of high concentration to one of lower concentration.

How is movement of molecules into and out of cells regulated?

Movement of molecules into and out of cells is mediated by the semi-permeable cell membrane.

What is osmosis?

Water moving across cell membranes by diffusion is referred to as osmosis.

How do plant cell walls affect osmosis?

In plant cells, the additional cell wall means the amount of water that can enter a cell is limited by turgor pressure.

How does diffusion affect cell size and physiology?

Increased diffusion rates at larger cell sizes means unicellular organisms are typically very small. In multicellular organisms, physiology may be adapted for greater surface area in organs where diffusion is necessary.

List of Materials

  • Petri plates (3/group)
  • Spatula (Flat) /blade (1/group)
  • Ruler (1/group)
  • Hydrochloric Acid Solution 0.1 M (300 mL/group)
    1 L
  • Stopwatch (1/group)
  • Vernier Caliper (1/group)
  • Gloves (1box/station)
  • Large forceps (1/group)
  • Dialysis Tubing (20 cm pieces each) (4/group)
    20 cm X 20 pieces
  • Starch solution (1%) (10mL/group)
    50 mL
  • Sodium Chloride (NaCl) solution, 1M, (10mL/group)
    50 mL
  • Sucrose/dextrose solution, 1M, (10mL/group)
    50 mL
  • Benedict's Solution (2 mL/group)
    10 mL
  • Iodine Solution, 0.08 M, (2%) (2 mL/group)
    10 mL
  • Paper towels
    1 roll/station
  • Dehydrated agar powder
    Dependent on the lab size
  • Phenolphthalein solution (1gm/ in 100 mL 95 % Ethanol)
    Dependent on the lab size
  • Sodium Hydroxide solution, (NaOH), 0.1M (20-25 mL)
    Dependent on the lab size
  • Thermometer
    Dependent on the lab size
  • Glass tray (atleast 2.5 inches deep)
    Dependent on the lab size
  • Hot plate
    Dependent on the lab size
  • Magnetic stirrer
    Dependent on the lab size
  • Magnetic bar
    Dependent on the lab size
  • Weighing balance
    Dependent on the lab size

Lab Prep

  1. Preparation of Solutions for the Agar Cube Experiment
    • IMPORTANT: Wear gloves, goggles, and appropriate personal protective equipment – chemicals can be hazardous at high concentrations.
    • For the diffusion indicator solution, weigh out 1 g of phenolphthalein and add it to a beaker containing 100 mL of 95% ethanol.
    • To make the basic solution for the agar, measure 80 mL of water into a beaker, and then add 0.4 g of NaOH.
    • Stir until the solute is dissolved then bring the total volume of the solution to 100 milliliters to make a 0.1 molar solution.
    • To make the diffusing acid, measure out 25 mL of HCl and add it to 475 mL of water. NOTE: Plan to make around 500 mL of this HCl solution per work group.
  2. Preparation of Agar with Phenolphthalein
    • IMPORTANT: Agar needs time to cool, so prepare several hours in advance of the class.
    • To prepare the agar, first weigh out 20 g of dehydrated agar powder and then add this to 1 L of water.
    • Heat the solution to near boiling or until it becomes fully clear.
    • Then, cool the agar to 55 °C and add 10 mL of the phenolphthalein ethanol solution.
    • Slowly add the 0.1 molar sodium hydroxide solution to the agar until it becomes vibrant pink.
    • Take a clean tray with deep sides and pour the agar to a depth of 3 cm.
    • Place the tray into the refrigerator for a few hours.
  3. Preparation of Dialysis Tubing and Solutions
    • To prepare the 1% starch solution, weigh out 10 g of soluble starch and add it to 500 mL of distilled water set on a magnetic stir plate.
    • When the solute has dissolved, bring the final volume of the solution up to 1 L with more water.
    • To prepare 1M NaCl, dissolve 58.4 g of NaCl in 500 mL of distilled water, in a 1 L container. Then, bring the volume up to 1 L with distilled water.
    • To prepare 1M dextrose, add 180.2 g of dextrose powder to 500 mL of distilled water in a 1 L beaker. Once dissolved, bring the volume up to 1 L with distilled water.
    • Cut the dialysis tubing into 20 cm pieces.
    • Place the pieces to soak in distilled water; make sure they are kept moist at all times. NOTE: Excess tubing can be stored at 4 °C in 20% ethanol to prevent bacterial growth.

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