A STUDY OF OSMOSIS
Water passes through aquaporins in cell membranes from an area of high water concentration (low
solute concentration) to an area of low water concentration (high solute concentration). This process is
called osmosis. It requires no cellular energy to be used, and occurs due to the random, continuous
motion of all molecules.
If a cell is placed in an environment in which the concentration of water is less / concentration of
solutes is greater than in the cell (hypertonic), water will flow from the cytoplasm and/or water vacuole
through the membrane into the environment. (Some solute moves into the cell but this does not account
for the dramatic changes that you see or weigh) The cytoplasm and/or vacuole of the cell shrinks. In an
animal cell, the entire cell shrivels. In a plant cell, the cytoplasm shrinks away from the cell wall. A cell
placed in a hypotonic environment, with a higher concentration of water / lower concentration of solutes,
will gain water by osmosis. A cell placed in an isotonic environment, containing the same concentration
of water and solute as in the cell, will not gain weight, as water moves equally in both directions.
Sports drinks have become a multi-million dollar industry, with advertisements aimed at everyone
from Olympic level athletes to weekend warriors. How scientific are the claims advanced by these
manufacturers? How quickly and how effectively do these drinks actually rehydrate your body's cells?
Perhaps this lab can help you answer that question.
Purpose:
to observe and measure the process of osmosis in living cells, qualitatively and
quantitatively.
Materials:
red onion cell, slides, coverslip, compound microscope, 4% salt solution, distilled water,
apples, beakers, balance, paper towels
Procedure:
Part A Qualitative Observations of Osmosis
1. Make a wet mount of a small, thin piece of purple epidermis from a red onion bulb's leaf scales.
2. Photograph and label what you observe under high power. Be sure to include: cell wall, cell
membrane, nucleus, central vacuole (takes up most of the space in the cell and contains water and a
reddish/ purple pigment called anthocyanin, central vacuole membrane).
3. Lift off the coverslip and place 2 drops of 4% salt solution directly on the sample.
4. Photograph your cells. Note the color and position/shape of the vacuole.
Label central vacuole membrane.
5. Lift the coverslip and add 3-5 drops of distilled water to the slide.
6. Photograph the cells. Again, note the color and shape of the vacuole.
Results: Part A
onion cell (before)
onion cell – after 4% salt / 96% water
onion cell – after distilled water (100% water)
Part B How Much Water Is In Those Apples??
Remember those apples from the Sweet Lab? What would you hypothesize makes one apple sweeter than
another? The amount of sugars? What is the concentration (%) of sugar and water inside of an apple
cell? How would you set up an experiment in order to determine this?
Think about what happens to apple cells (or any cell, like the onion cells in part A) when placed in
solutions of different concentrations. If they are placed in a hypotonic solution (for ex.100% H2O), water
will enter the cells and they will gain weight. If they are placed in a hypertonic solution (for ex. 70%
H2O and 30% solute = 30 g NaCl + 70 ml H2O), water will leave the cell and they will lose weight. If
they are placed in an isotonic solution (? % H2O and ? % solute), the weight will remain the same as
equal amounts of water move into and out of the cell.
(http://www.williamsclass.com/SeventhScienceWork/CellsOrganization.htm)
Hints:
1. cut apples into 16 pieces (discard the core/seeds)
2. make 200 ml of different concentration salt or sugar solutions.
3. apples must sit overnight in their beakers
4, Calculate the % change in mass for each of the groups of apple pieces.
% change = final mass - initial mass
---------------------------- X 100
initial mass
5. you can combine your efforts with another group so that your data collection can be more complete, in
order to be as exact as possible.
Your procedure?
Your Results:
*design your own data table
Your Graph:
Design and construct a graph that best represents your data and answers the question that you were trying
to solve. Use a linear fit (best fit) to help you determine the exact % water (or sugar) in your apple.
Your Conclusion:
1. what is the % solute (sugars) in your apples? explain how you determined this.
2. how does this finding correspond to your sweet levels from the sweetness lab?
Discussion Questions:
1. Explain the changes in the size of the vacuole and relative color of the pigment following the addition
of the salt solution and then the distilled water. Use your pictures with arrows and labels showing the
movement of water.
2. Animal cells exposed to distilled water swell and burst but plant cells do not. Based on your
knowledge of cell structure, why might this occur?
3. How do organisms that live in fresh or salt water cope with an environment that differs from that of
their cells? Use your class notes, textbook, Haiku Osmoregulation article and/or this website (or others) to
help you answer this. http://en.wikipedia.org/wiki/Osmoregulation
4. The solute concentration of contact lens saline solutions must be isotonic to the cells of your eyes.
Why?
5. How do sports drinks need to be formulated so that they are able to effectively rehydrate your cells?
Why might Coke or lemonade not be good at quenching your thirst?
Teaching Notes:
http://www.ehow.com/facts_7190808_much-sugar-apple-have_.html
Size

Apples average about 3 grams of sugar per ounce. According to the California Apple Commission, a medium
apple weighing 5.5 oz. has 16 grams of sugar. The Washington Apple Commission states that a large, 8-ounce apple
contains 25 grams of sugar.
Variety

Some apple varieties are sweeter than others. Red and Golden Delicious have 11 to 15 percent sugar. Granny
Smith apples have 12 to 18 percent sugar. The sugar content of Gala apples is 14 to 16 percent. Fuji is one of the
sweetest varieties with 16 to 18 percent sugar.

Carbohydrates

Sugar accounts for about 73 percent of the carbohydrates in an apple. Fiber makes up most of the
remaining carbohydrates. A medium apple contains about 5 grams of fiber.