Cell Transport Lab
Cells make use of both passive and active processes to transport substances across their membranes. Passive
transport processes are those that require no metabolic energy (ATP) from the cell but rely solely on the
physical properties of the substances themselves (kinetic motion of molecules aka Brownian Motion).
Active transport processes require energy expenditure (ATP) by the cell to move substances whose physical
properties prevent their independent motion through the cell membrane. In this lab, we will observe largescale examples of passive transport processes that are used by cells forming the tissues and organs of the
body.
When you finish this exercise you should be able to:
1. Discuss the nature of diffusion and osmosis
2. Use the terms hypertonic, hypotonic, and isotonic to compare solutions
Materials:
Milk (dropper bottle), Monocular microscope, Microscope slides and cover slips, Petri dishes, Dialysis
tubing (8-10 cm, presoaked), String, Sucrose solutions (15% and 40%), Syringe (or pipette) to fill dialysis
bags, Laboratory balance, Beakers (75-200 ml), Distilled water
Procedure:
Part A Diffusion and Osmosis *Note because this activity calls for an extended waiting period, you
should set it up immediately and do the other activities while you wait.
1. Obtain three 8-10 cm pieces of presoaked dialysis tubing from the beaker on the supply table. Tie one end
shut with string, forming a watertight bag. Leave about 10 cm of string dangling freely.
2. Hold the untied portion of the dialysis tubing under running water and rub the end of the tube between
your fingers until it opens.
3. With a syringe, carefully fill each of the three bags with 15% sucrose solution. Try to avoid spilling any
solution on the outside of the bag. Tie off the open end of each tube with another piece of string.
4. Quickly measure the mass of each tube on a laboratory balance. Wipe any fluid off of the balance after
weighing each of the tubes. Record the data. Make a table for this data.
5. Place each dialysis bag in a beaker. Bag #1 needs to be in an isotonic solution. Bag #2 needs to be in a
hypertonic solution, and Bag #3 needs to be in a hypotonic solution. Leave a piece of string dangling out of
the beaker as you would with the string on a tea bag.
6. After 25 minutes, remove the tubes from the beakers, roll each one gently back and forth 4 times to
“remove excess outside liquid clinging to the bag”. Immediately take the mass again and record on your data
table.
Part B The physical basis of passive transport (Brownian motion)
1. Prepare a wet-mount slide with a drop of milk
2. Allow the slide to sit undisturbed on the microscope stage for 2 minutes prior to turning on the light to
focus.
3. Observe the slide under HPO (40x objective). You should be able to see the components of the milk
moving in short, irregular distances and appear to be vibrating and bouncing off of each other. THIS IS
BROWNIAN MOTION. The largest spheres are the protein globules, the smaller ones are the fat globules
and the smallest that give a gray cast to the area between the large and small visible spheres are the water
molecules.
Because small particles of mater are always bouncing around, Brownian motion causes matter to spread
(diffuse) to areas where there is more room to bounce around. The movement is random because when
molecules collide the have tendencies to move into another direction, however, eventually particles tend to
move toward an area of lower particle concentration.
Questions: Answer each of these in complete sentences.
1. What is Brownian motion, and how did the milk demonstrate this motion?
2. Draw your results from part A. Label each set-up isotonic, hypertonic, or hypotonic, and define these 3
terms. Make sure you label what fluid is in the dialysis tubing and in the beaker. Use arrows to show the
movement of water in each set up.
3. Summarize your results from part A. Explain how the water and solutes moved in each of your 3 set-ups.
4. List another example of Brownian motion that you have seen before.
5. Define filtration and give an example of where in the body this type of movement takes place.