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OSMOSIS: INTRODUCTION and INSTRUCTIONS
Part B: Osmosis
Another process that is closely related to diffusion and is sometimes referred to as
“Specialized Diffusion” is osmosis. Osmosis is the diffusion of water molecules through
a selectively permeable membrane (one which only allows certain materials to pass
through).
In the process of osmosis, water molecules pass through a selectively permeable
membrane from an area where there are many water molecules into an area where
there are fewer water molecules.
The cells in plants and animals are surrounded by membranes that are selectively
permeable. In addition, a plant cell’s membrane is further enclosed by a fairly rigid
structure called a cell wall; animal cells do not have a cell wall surrounding their cell
membranes.
The selectively permeable cell membrane of plant and animal cells contains pores
(small openings) that can control what enters and leaves the cells.
Plant and animal cells are exposed to many changes in their external environment.
They must keep a stable internal environment in order to carry out the normal cellular
activities necessary for life functions. The cells of plants and animals must maintain a
balance or “steady state” within each cell to stay alive. This internal balance is called
homeostasis. All cells are surrounded by water solutions. A solution in which the
concentration of dissolved substances (solutes) is the same inside and outside the cell
is called an isotonic solution (iso means the same). A solution in which the
concentration of dissolved substances (solutes) is higher outside the cell than inside the
cell is called a hypertonic solution (hyper means greater). A solution in which the
concentration of dissolved substances (solutes) is lower outside the cell than inside the
cell is called a hypotonic solution (hypo means less).
Because the membranes of both plant and animal cells can tell different substances
apart, they can act as “gate-keepers” for the cells. They can slow down or even stop
certain materials from entering or leaving the cell. They can also allow certain
substances to pass through freely. When substances move in and out of cells without
having to use any energy other than their own kinetic energy, the process is called
passive transport. Osmosis and diffusion are both examples of passive transport.
Passive transport allows cells to get water, oxygen and other small molecules that they
need. It also allows the cell to get rid of wastes such as carbon dioxide.
Because the human body is made up of 60% water, osmosis is an important process.
This process helps humans and all organisms to maintain their “normal” internal
environment even when their surroundings are constantly changing.
1. READ the INTRODUCTION
2. Materials:
All Groups
Groups 1 & 2
Groups 3 & 4
Groups 5 & 6 Groups 7 & 8
1. Metric Rulers
2. Plastic cups
3. Dialysis Tubing (8cm)
4.Graduated Cylinder
5. Dental Floss (4 8cm
pieces)
6. Scales
7. Gloves
8. Water
1. Corn Starch
2. Lugol’s
Solution
3. Plastic
Spoons
1.Phenolphthalein
2. Ammonia
1. Vinegar
2. Universal
Indicator
Solution
1. Distilled
Water
2. 20%
Sucrose
Solution
PROCEDURE:
1. All groups: Moisten your fingers to open the dialysis tubing. Tie a knot at one end of
the tubing approximately 1cm from the end (knot 1) using dental floss; approximately
1cm above the knot tie another knot (knot 2) using dental floss. DO NOT PULL THE
DENTAL FLOSS TOO TIGHTLY---IT MAY TEAR THROUGH THE
TUBING. MAKE SURE YOU HAVE SECURE KNOTS (NO LEAKAGE).
Keep the dialysis tube moist but avoid getting water inside the tube.
Knot 2
Knot 1
2. Groups 1 & 2: Fill the dialysis tube half full with corn starch.
Groups 3 & 4: Fill the dialysis tube half full with phenolphthalein (~40 drops)
Groups 5 & 6: Fill the dialysis tube half full with Universal Indicator (~40 drops)
Groups 7 & 8: Fill the dialysis tube half full with distilled water (~40 drops)
3. ALL Groups: Approximately 1cm from the open end of the tubing tie one knot using
dental floss and using the same CAREFUL technique used in Step 1 (Knot 1).
Approximately 1cm below Knot 1 tie the tubing again (Knot 2).
Dental Floss
Knot 1
Knot 2
Open space
Tubing Fill
Original knots
4. All Groups: Carefully rinse off the outside of the tube to remove anything that may
have spilled on it. Gently pat it dry with a paper towel.
5. All Groups: Using the scales, find the mass of the filled dialysis tube. Record this
information on Handout 1 – Part B on Data Table A.
6. Groups 1 & 2: Put 100ml of water in a plastic cup. Add 5 drops of Lugol’s solution to
the water.
7. Groups 3 & 4: Put 75ml of water in a plastic cup. Add 25ml of ammonia to the
water.
8. Groups 5 & 6: Put 75ml of water in a plastic cup. Add 25ml of vinegar to the water.
9. Groups 7 & 8: Put 100ml of 20% sucrose solution in a plastic cup.
10. All Groups: Place your dialysis tube in the plastic cup with the solution you made.
Make sure it is fully covered. Wait 15 minutes; observe and record your results on
Handout 1 – Part B, Data Table B.
11. ALL Groups: Remove the dialysis tube, gently pat it dry and find the mass using
the scales. Record this data Handout 1 – Part B, Table A.
12. ALL Groups: Compare your data with the other groups that tested different
substances. Record the results from the other groups in the correct Tables A and B.