Biology: Chemistry Meets Biology- Osmosis & Diffusion Lab
Names: _____________________________________
Date: ______________
INTRODUCTION:
Individual cells exist in a fluid environment of about 75% water. This environment makes it
easier for materials like nutrients, glucose, “food”, water, and oxygen/carbon dioxide to move
into and out of a cell. The movement of molecules from an area of HIGH concentration to an
area of LOW concentration is known as ________________.
A. What are two factors that determine the rate diffusion will occur?
1.
2.
B. The diffusion of water molecules through a selectively permeable membrane is ___________.
C. What is a selectively permeable membrane? (Use your books)
CONCEPT:
Cell membranes are selectively permeable membranes.
OBJECTIVES:
The student will be able to conclude from their data that cell membranes
are selectively permeable and that water & solutes move from areas of
high concentration to areas of low concentration within cells.
MATERIALS
Plastic cups
Balance
Potato Slices
Carrot Slices
5% NaCl Solution
Diet&Reg Soda
Pure Water
Red Onion 20% NaCl Solution
Egg
Vinegar
PART I: Osmotic changes in mass
1. Obtain four cups and label them 5% NaCl, 20% NaCl, Coke, Diet Coke, & Pure water.
2. Cut carrot and potato into 5 pieces of approximately the same size. After zeroing your balance,
weigh each item & record its mass on the PART I data table in the BEFORE column, and place it in the
appropriately labeled cub.
3. Fill in the DATA TABLE below and hypothesize what will happen to the vegetables mass.
4. Add enough solution to cover the items in each cup. All cups should have approximately the same
amount of solution added.
5. Write your name on a piece of paper and place the experimental cups in the designated area.
6. The following class period - One cup at a time, remove the item, pat it dry with a paper towel, and
determines its mass. Record the results on your PART I data table in the column labeled AFTER.
Determine how much the item’s mass changed.
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PART II: Microscopic View
7. Prepare a red onion cell slide as instructed and obtain droppers for solutions from your teacher.
8. Focus on the onion at 40X (ask for help if you don not know how to use a microscope) and sketch
the onion cells in the boxes below. Pay close attention to the cell walls and the red portion of each cell.
The red portion is the cell’s vacuole.
9. Carefully add 1 or 2 drops of 20% NaCl solution to the side of the cover slip. Watch the red onion
cells for several minutes. Draw a picture of what you see.
10. Remove the slide and with an eyedropper carefully rinse the NaCl solution from the slide. Refocus
on the onion cells and add 1 to 2 drops of pure water. Again, watch the onion cells and draw a picture
of what you see in the PART II space provided.
PART I: Osmosis & Diffusion Data Table
Vegetable
Solution
Hypothesis
Mass
Before (g)
Mass After (g)
Measure 3x
Change in
Mass (∆g)
PART II: Red Onion Cell Observations:
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Red Onion Cells
Red Onion (20% NaCl)
Red Onion (Pure H2O)
Diffusion & Osmosis QUESTIONS: You may need to look ahead to the vocabulary in the Egg Lab
below.
1. What effect did each solution have on the vegetable? Using your knowledge of osmosis
and diffusion explain these results using appropriate vocabulary terms.
2. Using the concept of osmosis and diffusion, explain why the vacuole (the red part of the
onion) shriveled up when you added 20%NaCl. Use appropriate vocabulary.
3. When you are thirsty, is it best to quench your thirst with pop, Gatorade, or water? Use the
results from the lab to explain.
4. Compare the processes that occurred in these plant parts to what would occur in animals? What would
be the same & what would be different?
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Part III: Egg Semi-Permeability & Selective Permeability of Cell Membranes
In our final part of the experiment an egg will be the model for a cell. First, we will remove the shell to
expose the cell membrane. Vinegar is an acid that reacts with the calcium in the shell to decompose the
shell. What evidence tells us a reaction occurred? Once the shell is removed, the concepts of diffusion
and osmosis can be illustrated. The egg must be transferred from the beaker with vinegar to a beaker
containing corn syrup, salt water, & distilled water very carefully. Over several days we will examine
the circumference, mass, and texture of the egg.
Objectives:
o To understand one way that materials enter and leave a cell
o To understand the function of the cell membrane
o To understand the principles of diffusion and osmosis
o To record detailed observations of diffusion and osmosis
Vocabulary:
plasma membrane
selective permeability
osmosis
hypertonic
isotonic
dynamic equilibrium fluid mosaic model
phospholipids
concentration
diffusion
hypotonic
lyse
concentration gradient
transport proteins
cell wall
cytoskeleton
crenate
Materials: (* ~ means approximately)
o (3) 250 ml beakers
o ~250 ml corn syrup
o ~250 ml water approximately
o ~250 ml vinegar
o ~12" piece of string ruler paper and pencil to record observations
o egg (be careful handling the egg!)
Procedure:
1. Place an egg into one of the beakers. Be careful lowering the egg in.
2. Fill beaker with enough vinegar to cover the egg.
3. Record observations immediately. What do you notice? Continue to record observations over
the next three days. Be sure to note how the egg feels when you touch it each day, but don't
squeeze too hard! Develop you own Date Table.
4. Once the shell is removed (i.e. the egg is soft), gently rinse off the egg. Measure the
circumference of the egg by wrapping the string gently around the middle of the egg. Hold your
finger on the spot where the end of the string meets the rest of the string. Measure the distance
between the end of the string and your finger with the ruler. Record measurement. Also record
how the egg feels at this point. Transfer the egg to a new beaker.
5. Again, fill this beaker with corn syrup to cover the egg.
6. Record the physical properties of the syrup (i.e. is it thick (viscous)? slow to pour? Sticky? etc.).
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7. Each day for the next 3-4 days, record observations. First note if the syrup has changed. Also,
note how the egg feels. Compare these observations to those made in step 4 and step 6?
8. Next, gently take the egg out of the beaker and rinse it off. Measure the circumference as
directed in Step 4. Record measurement. Place the egg into a new beaker and fill this one almost
to the top with water.
9. In several hours (it's okay to wait until the next class period), come back and record
observations again. How does the egg feel this time? How does it compare to the observation in
step 7 and step 4?
10. Again, measure the egg as directed above. Record measurement. How does this measurement
compare to the measurements in step 4 and step 8?
Egg Semi-Permeability & Selective Permeability of Cell Membranes QUESTIONS:
1. Define the vocabulary words given above on a separate sheet of paper.
(Define on a separate sheet of paper)
2. Why was the egg’s shell removed first? Why does an egg have multiple membranes?
3. How did the syrup change (see steps 6 and 7)? Explain why this change occurred?
4. How did the egg change from the beginning of the experiment to the middle to the end? Explain why
using the vocabulary above.
5. What do you think would happen if the egg were placed in orange juice instead of corn syrup? Why?
6. What do you think would happen if you repeated step 8 a couple of times except you used new corn
syrup each time instead of the water? Why?
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7. What would happen to a plant if it were placed in a high concentration of salt water? Why? What
would happen to a ship wrecked person if they drank seawater?
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