BIOMEDICAL SCEINCE 100 LAB - DIFFUSION AND OSMOSIS

Introduction

Brownian motion = constant random movements of molecules
Animation: Brownian Motion

Diffusion = net movement of molecules from regions of higher concentration to regions of lower concentration
Animation: How Diffusion Works
* if there is a gradient (concentration difference between two regions), diffusion eliminates the gradient as the molecules become more “spread out”

Selectively Permeable Membrane = a membrane with pores of a size that permit the passage of some molecules while restricting the passage of other molecules
Illustration: Selectively Permeable Membrane.

Osmosis = passage of water through a membrane that is permeable to water but not to solutes (dissolved particles)
* water moves according to the law of diffusion - from the region of higher water concentration to the region of lower water concentration
* this means that water will move from the region of lower solute concentration to the region of higher solute concentration
Animation: 0smosis

Procedure
    I. SIMPLE DIFFUSION
  1. Make three equidistant "wells" in a plate of diffusion agar by pushing a straw or borer into the agar then removing the pieces. If you accidentally tear the agar, make another well or try again with a different plate. Label the top of your plate with a group or individual name.
  2. Drop potassium dichromate solution into one well until the well is approximately three-fourths full. Do not allow the liquid to overflow onto the agar surface. Repeat with potassium permanganate in the second well and with methylene blue in the third well.
  3. Place the plate on a warming tray.
  4. Periodically check the plate for evidence of diffusion. Once diffusion is apparent, rank the different dyes from greatest diameter of diffusion (1) to least diameter of diffusion (3) on your Response Form.
  5. When told to do so, dispose of used plates in the Biohazard receptacle.
    II. OSMOSIS IN SIMULATED CELLS
  1. Obtain four (4) strips of dialysis tubing and carry them to your table within a small beaker containing water. The dialysis tubing has microscopic pores that permit the passage of water but not of sugar.
  2. Take one piece of the tubing and, holding both ends of the tubing up, "roll" one end to create an opening. Put 10 ml. of a 20% sucrose solution into the bag then twist and clamp the ends together. Pass a wooden stick through the ends of the clamp, and “hang” the bag in a beaker. Label the beaker “#1”. Prepare three more bags labeled #2. through #4., similar to the first one.
  3. Weigh each bag to the nearest 0.1 g. and record their weights on scratch paper. Transfer the the weights to the Initial Weights column in Table One of the Osmosis Spreadsheet.
  4. The colored fluids are sucrose (sugar) solutions of the following concentrations: 0%, 10%, 20%, 40% Since the sum of the sugar and the water must equal 100%, the water percentages are 100%, 90%, 80%, and 60%. You will not be told the percentages in each solution yet.
  5. Pour enough colored fluid into beaker #1 to immerse the bag.  Repeat with the remaining different fluids/beakers. This is time zero (t=0).
  6. At 20 minute intervals, remove each bag from its beaker (along with the stick and clamp), gently touch the bottom with a dry paper towel, and reweigh it.  Record the weights on scratch paper then transfer them to Table One of the spreadsheet in the appropriate time column.
  7. Note the weight changes in Table Two and determine the sugar percentage of each of the external solutions. Record final results on your Response Form.
  8. Cleanup: Dispose of used fluids (sink) and dialysis tubing. Rinse beakers and return clamps, sticks, etc. to your tray.
    III. OSMOSIS IN RED BLOOD CELLS
  1. Label three microscope slides as follows: “5.0”, “0.9”, and “0.0”.
  2. Place a small amount of blood on each slide by removing the pipet or stick from the blood bottle and touching the slide with it (do not "pump" blood onto the slide).
  3. Add a drop of 5.0% sodium chloride (saline) solution and a cover slip to the slide labelled “5.0”.
  4. Add a drop of 0.9% sodium chloride (saline) solution and a cover slip to the slide labelled “0.9”.
  5. Add a drop of dH20 (0.0% sodium chloride) and a cover slip to the slide labelled “0.0”.
  6. Use a microscope (40X objective lens) to observe the sizes and shapes of the red blood cells in the following sequence: 0.9% first, 0.0% second, 5.0% last. Movements of the cells are due to microcurrents in the solutions and are of no significance.
  7. Sketch or describe the appearance of the cells in the spaces provided on your Response Form. The cells immersed in 0.9% will be of "normal" appearance.
  8. Save your slides until given instructions from a staff member. When told to do so, dispose of used slides and any other potentially contaminated supplies in the Biohazard receptacle.