SNC2D/2P Tissues, Organs, and Systems of Living Things/Tissues, Organs, and Systems Teacher Demo/Student Activity: Potato Osmosis Topics Timing osmosis
selectively permeable membranes
preparation: 30 min
demonstration: 30–45 min
Specific Expectations SNC2D A.1.1 formulate scientific question about observed relationships, ideas, problems, and/or issues,
make predictions, and/or formulate hypotheses to focus inquiries or research
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
B3.3 explain the links between specialized cells, tissues, organs, and systems in plants and
animals (e.g., muscle cells and nerve cells form the tissue found in the heart, which is a
component of the circulatory system; granium and thylakoid structures act as solar collectors
in the chloroplast to produce carbohydrates for plant growth)
SNC2P A.1.1 formulate scientific question about observed relationships, ideas, problems, and/or issues,
make predictions, and/or formulate hypotheses to focus inquiries or research
A1.10 draw conclusions based on inquiry results and research findings, and justify their
conclusions
B3.4 explain the general function of some of the systems in the human body (e.g., the function of
the circulatory system is to transport materials through the body; the function of the digestive
system is to absorb nutrients; the function of the respiratory system is to bring oxygen into
and remove carbon dioxide from the body)
Introduction Osmosis is the process by which the concentrations of two solutions, separated by a selectively
permeable membrane, equalize by the movement of a solvent such as water. This demonstration
shows that the ability of plant cells to absorb water is decreased by heating. This observation can
be explained by the hypothesis that heat damages the selectively permeable membranes
surrounding the cells, and that this reduces that cells’ ability to carry out osmosis.
Materials lab coat or apron
protective gloves
potato
water
2 tsp sodium chloride, NaCl(s)
anhydrous copper(II) sulfate,
CuSO4(s), 1 tsp (optional)
knife
cutting board
two 500 mL beakers
hot plate
spoon
Petri dish
Safety Considerations •
•
•
Provide MSDS sheets for all chemicals used.
Handle knives with care. You should always cut away from your hands and body.
Boiling water and steam can cause serious burns. Avoid splashing the boiling water and/or
placing your skin in the steam. After the potato is boiled, it will be hot. Handle it with care.
Hazardous Materials Identification System Rating (0-minimal 1-slight 2-moderate 3-serious 4-severe)
copper(II) sulfate anhydrous
Health
2
Fire Hazard
0
Reactivity
0
Procedure Wear appropriate PPE: lab coat or apron and protective gloves.
Prepare the following materials before class:
1.
Cut the potato in half. Place each half in a separate beaker and cover with water.
2.
Boil one half of the potato for about 25 min until it is soft. Remove it from the water and
allow it to cool to room temperature.
During class:
3.
Remove the ends from both potato halves so that there is a flat surface for the potato to sit
on (Fig.1).
4.
Spoon out a hollow in both the cooked and uncooked potatoes.
5.
Predict/Explain
Remind students that the cells of a fresh potato are alive. Ask students what they think
happened to the potato’s cells when it was boiled. Have them explain their reasoning in
one or two short sentences.
6.
Place each piece of potato, hollow side up, in a Petri dish.
7.
Add a teaspoon of salt into each hollow (Fig.2).
8.
Add 15–25 mL of water to the Petri dishes.
Fig. 1 First cut the potato in half,
boil one half, then cut off the end.
9.
Fig.2 Leave the potato halves sitting in water,
with salt in the hollows, for at least 45 min.
Predict/Explain
Have the students predict what the water will do, and whether or not fresh and boiled
potato will behave similarly. Have the students justify their predictions.
10.
11.
Observe
After 45–60 min, compare the boiled and raw potato halves and record results. (Continue
with other activities/work during this time.)
Explain
Ask students to explain their observations.
Disposal The table salt and potatoes can be disposed of in the trash, or in an organic/compost recycling
container, and the water can be poured down the sink. The copper(II) sulfate, if used (see
“Teaching Suggestions/Hints”), should be disposed of with other inorganic waste. Follow
disposal procedures that are consistent with school board protocol and appropriate for your
municipality.
What happens? A small pool of water will accumulate in the raw potato, but not in the cooked potato.
How does it work? The concentration of salt in the water in the Petri dish is low, while the concentration of salt in
the potato hollows is very high. To balance out the concentration of the solutions on both sides of
the potato, water moves up to the cavity in the potato (through the living cells of the potato). The
observation that water does not pass though a boiled potato in the same way suggests that there is
something special about the cell membranes around living cells. Living cells selectively allow
water to pass through, which boiled cells do not. This is indirect evidence of the presence of a
selectively permeable membrane that is destroyed by boiling.
Teaching Suggestions/Hints 1.
2.
Since the process of osmosis is quite slow, the set-up of the demonstration and making
final observations should be separated by a lesson and other activities.
To make the demonstration more dramatic, anhydrous copper(II) sulfate, CuSO4(s), can be
used in place of table salt. The copper(II) sulfate will additionally undergo a colour change
(from white to blue) as osmosis occurs. However, the copper(II) sulfate should not be
disposed of down the sink or into the regular trash bin.
Next Steps Once the students have an understanding of osmosis, relate this principle to absorption of water
in the large intestine and/or filtering of the blood by the kidneys.
Additional Resources 1.
2.
Explanations of how substances get into (and out of) cells with reference to diffusion,
permeability of membranes, and osmosis are provided at:
http://www.biotopics.co.uk/life/osmsis.html
This is an animated demonstration of osmosis, specifically for isotonic, hypotonic, and
hypertonic solutions: http://www.youtube.com/watch?v=7-QJ-UUX0iY&feature=related