Diffusion is defined as the net movement of molecules or ions from a region of high concentration to a region of lower concentration. Diffusion continues until a state of equilibrium is reached, which means that the molecules are randomly distributed throughout the system. Diffusion is considered a form of passive transport because no energy is required in the process. Diffusion can occur in a gas, a liquid, or a solid medium. Diffusion also occurs across the selectively permeable membranes of cells.
All molecules possess kinetic energy which provides the force for movement. Molecules are in constant motion and as they move, they collide with each other. The more molecules in an environment, the higher the concentration of molecules, the higher the frequency of molecular collisions and the faster the speed of diffusion.
How might these factors influence the rate of diffusion?
- Concentration gradient
- Medium molecules diffuse in (gas, liquid, solid)
- Molecular weight of the molecule
Molecular Weight and Diffusion Rate
Molecular weight is an indication of the mass and size of a molecule. The purpose of this experiment is to determine the relationship between molecular weight and the rate of diffusion through a semisolid gel. You will investigate two dyes, methylene blue and potassium permanganate.
- Petri dish of agar semi-solid gel
- Methylene blue solution
- Potassium permanganate solution
- Small straws
- Small plastic metric ruler
- Obtain a Petri dish of agar
- Take the plastic straw and gently stick down into the agar. Lift up withdrawing a small plastic plug of agar. Repeat.
- Place a single drop of each dye into the agar well. (Figure 4.1).
- After 20 minutes, place a small, clear metric ruler underneath the Petri dish to measure the distance (diameter) that the dye has moved. Enter the data in Table
|Molecular weight (grams/mole)
|Diameter after 20 minutes (millimeters)
|Diameter after 40 minutes (millimeters)
Describe the relationship between molecular weight and speed of diffusion
Diffusion across a selectively permeable membrane
Cells acquire the molecules and ions they need from their surrounding extracellular fluid. In living cells, the ability of a molecule to cross the cell membrane is influenced by its size, charge, lipid solubility, and other characteristics. Small molecules such as water, oxygen, amino acids, and ions easy cross the membrane by passive transport processes that do not require energy (diffusion and osmosis). Other molecules do not easily fit through the lipid bilayer and the cell must expend energy to bring them across.
You will investigate two molecules, starch and iodine, for their ability to cross a selectively permeable membrane. A colorimetric test is employed to assess the movement of these molecules. Dialysis tubing is a transparent material with microscopic pores that allow only small molecules to pass. It provides a model of the cell membrane and has many uses in industry and medicine.
- Dialysis tubing
- Starch solution
- Iodine (IKI)
- Obtain a piece of dialysis tubing that has been pre-cut by the instructor. Thoroughly wet the tubing and open the ends. Tie a knot in one end.
- Add approximately 2ml (~2cm) starch solution to the dialysis bag. Tie a knot at the top of the tubing. Rinse the bag briefly with tap water to remove any traces of starch.
- Fill the beaker approximately 1⁄2 full with tap water.
- Add iodine (IKI) to the beaker of water until a deep yellow color is obtained.
- Submerge the dialysis bag in the water and incubate at room temperature until a color change is observed (~15 minutes).
- Did iodine diffuse across the selectively permeable membrane? How do you know?
- Which is the smaller molecule, iodine, or starch?
- Is diffusion a passive, or an active transport process (choose one)?