LAB. 6 -- DIFFUSION AND SELECTIVE PERMEABILITY
All molecules display random thermal motion, or kinetic energy. This is why dissolved molecules move
around in solution. Kinetic energy causes molecules to diffuse outward from regions of high concentration to
regions of lower concentrations. This random movement continues until the distribution of molecules
becomes homogenous. If a membrane is permeable to a substance, the substance will diffuse across the
membrane, until it is at equal concentrations on both sides of the membrane. If, on the other hand, a
membrane is not permeable to a substance, one side of the membrane will have a higher concentration of the
substance than the other side. Membranes that are permeable to some substances, but not others, are referred
to as selectively permeable.
BROWNIAN MOTION
Background: Heat causes random movement of molecules. Although we cannot directly see the
movement of molecules, we can infer them by observing the movement of small particles after
collisions with moving molecules. This motion of particles is called Brownian motion. In this
exercise, we will use India ink, a small soluble particle, to demonstrate Brownian movement.
Procedure:
1. Place a small drop of India ink onto a microscope slide and make observations at 400 X
magnification.
2. Leave the microscope on and observe any changes in motion as the temperature rises.
EFFECT OF MOLECULAR SIZE ON DIFFUSION
Background: Concentration, heat, and pressure contribute to the overall free energy of a
substance. Diffusion is best defined as the net movement of molecules from an area of high free
energy to an area of low free energy. Molecules diffuse from an area of low concentration, heat,
and pressure. Since biological systems are relatively constant in temperature an pressure,
concentration is the most important in the direction of diffusion in living organisms. The rate of
diffusion is determined by the steepness of the gradient and other characteristics of the specific
molecule in question, such as its molecular weight, polarity, and solubility.
Procedure:
1. Obtain 2 agar plates, containing 3 well each.
2. Add potassium permaganate (158 Daltons), malachite green (929 Daltons), and methylene blue
(37.4 Daltons) each to one well in both plates.
3. Leave one plate at room temperature and place the other in the 37C incubator.
4. After 2 hours, measure and record the diameter of the zone of diffusion with a metric ruler.
SELECTIVE PERMEABILITY
Background: Biological membranes are selectively permeable, differentiating between molecules
based on their size, shape, and charge. This exercise will use dialysis tubing to demonstrate
selective permeability based on size.
Procedure:
1. Prepare and fill a dialysis tube with a glucose / starch solution as shown by the instructor.
2. Place the dialysis bag in a beaker of water.
3. Immediately remove 2 samples of 10 drops each from the beaker and test for starch and glucose
(as indicated below). Repeat every 5 minutes.
4. To detect starch, use the iodine test. To detect glucose, add 4 drops of Benedict's solution and
heat, as demonstrated by your instructor.