Diffusion

© 2009 Barbara J. Shaw Ph.D., Science A to Z

Permission is granted to make and distribute copies of this lesson plan for educational use only.

Background material retrieved on February 4, 2009 from:

umassk12.net/nano/materials/Diffusion.doc

Background

Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing or a state of equilibrium.

Diffusion is part of transport phenomena. Of the mass transport mechanisms, molecular diffusion is known as a slower one. Molecular diffusion is generally superimposed on, and often masked by, other transport phenomena such as convection, which tend to be much faster. However, the slowness of diffusion can be the reason for its importance: diffusion is often encountered in chemistry, physics and biology as a step in a sequence of events, and the velocity of the whole chain of events is that of the slowest step. For example, the rate at which a chemical reaction progresses can be entirely limited by the rate of diffusion of reactants/products to/from the place where the reaction occurs.

The speed of diffusion can be approximately illustrated as follows (at room temperature)

  • In gas: 100 mm in one minute;
  • In liquid: 0.5 mm in one minute;
  • In solid: 0.0001 mm in one minute.

Transport due to diffusion is slower over long length scales: the time it takes for diffusion to transport matter is proportional to the square of the distance. Conversely, diffusion can be quite fast over small length scales; inside a living cell, chemicals are almost entirely transported by diffusion.

The above numbers should be treated only as an illustration of the slowness of diffusion. Great differences exist in the diffusion speed between particular systems, particularly in the solid state. For example:

  • Hydrogen gas in solid iron at 10 °C - diffusion coefficient of 1.66x10-13 m2/s;
  • Aluminum in solid copper at 20 °C - diffusion coefficient of 1.3x10-34 m2/s.

When diffusion speed is proportional to the square root of the diffusion coefficient, then the hydrogen in iron diffuses over 10 orders of magnitude faster than does aluminum in copper.

In cell biology, diffusion is a main form of transport for necessary materials such as amino acids within cells. Diffusion of water is classified as osmosis.

Metabolism and respiration rely in part upon diffusion in addition to bulk or active processes. For example, in the alveoli of mammalian lungs, due to differences in partial pressures across the alveolar-capillary membrane, oxygen diffuses into the blood and carbon dioxide diffuses out. Lungs contain a large surface area to facilitate this gas exchange process.

Activities

Activity 1: Is diffusion fast?

Materials:

  • Petri dishes
  • Containers to mix gelatin, food dye, and tempura paint.
  • Gelatin (plain - no colors or flavors)
  • Food color.
  • Tempera paint (diluted about 50/50)
  • 2 pipettes
  • Razor blade (scalpel or craft knife)
  • Clear metric ruler

Directions:

Depending on your class, you can instruct them to do some or all of the following in preparation of this experiment.

  • Dissolve gelatin at double strength (e.g., use 2 packets in 1 cup water) x the number cups needed for your class
  • Heat the water to dissolve (e.g., microwave the mixture for 1.5 minutes)
  • Pour gelatin into Petri dishes (2 for each team of students)
  • Allow the gelatin to cool overnight
  • With the razorblade, cut a 1 cm circle from the center of the gelatin and remove.
  • Dissolve tempera paint in water in a different container so that the color is strong but still translucent
  • Using the pipette, add food coloring into the center hole of the gelatin, being careful not to get food coloring solution on the top of the gel
  • Dilute 50% tempera paint and 50% water
  • Put the name of your teams members on a sheet of paper and place the Petri dish on top of the paper
  • Using the pipette, add tempera paint solution into the center hole of the gelatin, being careful not to get food coloring solution on the top of the gel
  • Set aside each large Petri dish on the sheet of paper in a level place that will not be disturbed for several days.

Collect Data

  • Each day for one week, each person in the team measures from the edge of the hole holding the food coloring or tempera paint solution to the leading edge of the color in the gelatin
  • Record these data and find the average distance the color has traveled
  • On the last day at the end of the experiment, pour out the food dye and tempera paint solutions into containers that have been provided.
  • Use a ruler to measure the distance of penetration into the gelatin discs by looking at the bottom of the dishes (The edges of the gelatin discs and the diffusion front will be clearly visible. Both edges will be "fuzzy." Measure from the center of the “fuzzy” region for both edges.)
  • Then place the Petri dishes containing the gelatin discs into a container that has been provided.

Analyze Data

  • The rate of diffusion is the length divided by the time.
  • Compare the diffusion rate of the different dyes.

Questions

  • Are the results expected?
  • Which dyes penetrated better? Does that make sense?
  • Conversely, does fast diffusion mean greater or poorer retention?
  • How could diffusion and retention be optimized?
  • Is this the intuitive result?

Activity 2: Why are the cells of Eukaryote organisms about the same size?

Phenolphthalein is an indicator of a base. In an acid or neutral, it is clear in solution. In a base, however, phenolphthalein turns pink.

Materials

  • 9x9” pan
  • Container to mix gelatin and phenolphthalein
  • Agar (or gelatine,plain - no colors or flavors)
  • 10 ml phenolphthalein
  • Razor blade (scalpel or craft knife)
  • Clear metric ruler
  • Ammonia (dilute 25% ammonia 75% water)
  • 1 clear plastic cup per team
  • Cheesecloth strips, large enough to hold 2x2x2cm gelatin, lower into the cup, and lift it out again.
  • Clock with second hand or one stop watch per team

Directions

  • Dissolve gelatin at double strength (e.g., use 4 packets in 2 cup water)
  • Heat the water to dissolve (e.g., microwave the mixture for 1.5 minutes)
  • Add phenolphthalein
  • Pour gelatin into 9x9” pan to 2cm high
  • Allow the gelatin to cool overnight
  • Slice into one 2x2x2cm square and 1x1x1cm square for each team of students
  • Cut cheesecloth into strips ~ 2½cm wide and 25cm long
  • Pour ammonia into cup ~2½cm deep

Collect data

  • Measure the width, length and depth of each cube
  • Find the surface area of the cube and record
  • Find the volume of each cube and record
  • Find the ration of surface area/volume and record
  • With the 1x1x1cm cube, place on the cheesecloth and dip into the ammonia solution for 60 seconds, then remove
  • With the scalpel, slice the cube directly down the middle, into two pieces. Measure the pink from the edge of the cube to the leading edge of the color
  • Record measurement
  • With the 2x2x2cm cube, place on the cheesecloth and dip into the ammonia solution for 60 seconds, then remove
  • With the scalpel, slice the cube directly down the middle, into two pieces. Measure the pink from the edge of the cube to the leading edge of the color
  • Record measurement

Analyze Data

  • Graph the results of these data.

Questions

  • What would you expect if you repeated the experiment with a cube 3x3x3?
  • How does the surface area to volume ratio change?
  • Why does the cube turn pink when you put it into the ammonia solution?

Activity 3: Semi-permeable membrane

Iodine is a known indicator for starch.

Materials

  • 1 baggie per team
  • 1 plastic spoon
  • cornstarch
  • 1 dropper bottle with iodine
  • 1 clear cup with water

Directions

  • Fill a plastic baggie with a teaspoon of cornstarch and a half a cup of water tie bag. (This may already have been done for you)
  • Fill the cup halfway with water and add ten drops of iodine.
  • Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine water mixture.
  • Wait fifteen minutes and record your observations in the data table

Questions

  • What happened to the cornstarch water?