ACTIVITY: OSMOSIS AND DIFFUSION,
IMPORTANCE OF CELL SIZE
'Does Size Matter???"
Setup and Instructions
In this activity, students are asked to ponder some questions about the movement of water into
and out of cells, and, how does the size of a cell affect this process???
Materials: Potatoes (depends on size of class how many you choose to do, about 3 large
potatoes of any type per set of 3 treaments. Such a set works well for a group of 3-4 students
working together. Other materials: Table salt, Large beakers or plastic containers (1 liter),
balance, paper towels.
Starting questions for discussion:
What types of molecules move passively across membranes in response to these principles?
(What types of molecules cannot move passively and how does cell transport them?)
Is 'being small' a strange state in the living world, or a common one?
How is the 'lifestyle' of a single cell in the environment different from that of a cell that is part of a
multicellular organism?
How are bacterial cells different from eukaryotic cells?
How does cell size affect the rate of diffusion? Is this important for cells?
How big is a typical prokaryotic cell compared to a typical eukaryotic cell?
If a bacterial (prokaryotic cell) is unusually large, how does it manage to carry out its funcitons?
Do such cells exist? (See provided readings, 'Giant Bacteria Inhabit Fish Guts,' and 'Scientists
Marvel at Discovery of New Giant Organism,).
In this experiment, we will use potato pieces to test our hypotheses about water movement in
small vs. large cells. Each piece of potato tissue will act as a representation of a cell.
I encourage you to discuss the experimental materials with your students and walk them through
the following with lots of prompting. As how they would set up an experiment to test these
questions. Ask them to succinctly state the questions and the hypotheses. Ask them to plan the
Ruth A. Gyure, Western CT State U., WISTR, July 2006
protocol for the experiment, decide on replication, etc. And of course, very important - what are
their hypotheses, and predictions for the particular experiment they will do???
QUESTIONS?
1. Does diffusion occur faster if there is a bigger difference in salt concentration between
the cell and the environment?
2. Does diffusion occur at a more rapid rate in small cells compared to larger ones? (Keep
in mind we have to compare similar mass of cells, not number of cells)
HYPOTHESES?
1. Diffusion will occur faster when there is a larger difference in salt concentration between cell
and environment.
(Null hypothesis, or alternative hypothesis: Diffusion will not occur faster when….)
2. Diffusion will occur faster in a groups of small cells, compared to one larger cell of the same
mass.
(Null hypothesis, or alternative hypothesis: Diffusion will not occur faster…..)
EXPERIMENTS TO TEST HYPOTHESIS?
1. To test the first hypothesis, we can place potato pieces in solutions with different salt
concentrations. For example, one treatment would be 'equal concentrations,' which means our
solution should be at typical physiological salt concentration or 0.85%. Another treatment could
be a very high salt concentration (10%), and another concentration would be very low (the lowest
we can test is 0% salt or no salt). Set up 3 large beakers of water with the three salt
concentrations. 0% salt is distilled water. 0.85% would be 0.85g in 100ml, or, 8.5g in 1 liter, and
10% is a lot of salt - 100g in a liter (you may need to warm it up to get it to dissolve). Simply place
equal sized piece of peeled potato into each treatment.
2. To test the second hypothesis, we need to compare an equal mass of 'cells' with one large cell
compared to many small ones. In order to do this, one can peel a potato and cut it in half. Keep
one piece intact, but cut the other into about 8 equal sized pieces (cut it in half, each of those in
half again, etc). Weight the one large piece, then weight the group of small pieces, record.
Note - both hypotheses are tested at the same time by simply doing EXPERIMENT 2!
3. Let the potato pieces sit in their solutions are room temperature for at least 1 h (2h is better).
4. Remove pieces from beaker and quickly pat dry on paper towels. Weigh the pieces or groups
of pieces as before, record. Be sure to work quickly so that pieces do not begin to dry out in a
way that will change the weights as they were coming out of the beaker.
5. Analysis: If you have at least three groups of students in class, you can replicate the results 3
times for each treatment (note there are 6 treatments). Data can be plugged into the excel
worksheet provided, or students can put their results on the board and work out the means by
hand calculator. There will be obvious differences. You may choose to either give the students a
cutoff for differences to be 'significant' (in general for this experiment the difference between
percents should be greater than 3-5). OR, if appropriate for your leve, have them do a real
statistical analysis. Instructions are provided on the worksheet and in supplementary handout on
'Using Microsofdt Excel' provided as part of the antibiotic disk activity.
6. Get back to the starting questions. Does size matter? Of course! Bacteria rely heavily on
intimate association with the environment and diffusion and other forms of transport. Bacteria
often live in groups so they can coordinate their metabolic activities (somewhat mimicking a
Ruth A. Gyure, Western CT State U., WISTR, July 2006
homeostatic multicellular organism), and, if a bacterial cell is unusually large, it needs some
adaptation to facilitate increased necessary rates of diffusion - see articles that describe the
internal membranes and vacuole in the giant bacteria, providing increased internal surface area!
Hopefully, by doing this exercise, students will be able to come up with these sorts of
observations and conclusions and have a better appreciation for what it means to be small, and
the importance of understanding osmosis and diffusion.
PLEASE SEE SUPPLEMENTS:
Osmosis and Diffusion DATA WORKSHEET, INSTRUCTOR VERSION
Osmosis and Diffusion DATA WORKSHEET, INSTRUCTOR VERSION
ARTICLE: Scientists marvel at discovery of new giant organism
ARTICLE: Giant Bacterium Inhabits Fish Guts
Photo credits:
Bacillus on head of pin: Starr, Cecie and Ralph Taggart. Biology: The Unity and Diversity of Life.
Wadsworth Publishing Co. Belmont, CA 1995.
Blood cells and diffusion: Gould, James and William Keeton. Biological Science. 6th Ed. W.W.
Norton and Co. New York.. 1996.
Ruth A. Gyure, Western CT State U., WISTR, July 2006