Name: ______________________________________________ Date: ______________ Block: __________
Osmosis in Plant Cells
Background Information:
Cells lose or gain water due to the difference in concentration between the cytoplasm and the solution
surrounding the cell. The movement of water in and out of a cell is governed by the laws of diffusion: water
flows from a region of higher water (low solute) concentration to a region of lower water (higher solute)
concentration.
When a cell is in a concentrated solution (like salt water), it will experience a loss of water. Saltwater contains
a higher concentration of dissolved materials than the cell and therefore a lower concentration of water.
Consequently, water will flow out of the cell from the region of higher water concentration to the region of lower
water concentration in an attempt to reach equilibrium.
When a cell is in a dilute solution (like fresh water), it will experience a gain of water. Fresh water contains a
lower concentration of dissolved materials than the cell and therefore a higher concentration of water.
Consequently, water will flow into the cell from the region of higher water concentration to the region of lower
water concentration.
When a cell is in a balanced solution (like Blood), it will experience neither a net gain nor loss of water. A
balanced solution contains an equal concentration of dissolved materials as the inside of the cell and therefore
an equal concentration of water. Consequently, water will flow equally into and out of the cell.
When a plant cell shrinks because of loss of water, we call that plasmolysis. Because plant cells have cell
walls, the whole cell will not collapse when it first loses water; the cell wall will remain as rigid support. When a
plant cell is put in water that has a lot of materials dissolved in it, like salty water, the cytoplasm will shrink as it
loses water through osmosis. Water diffuses out of the cell and into the more concentrated solution
surrounding the cell. See figure 1 below. During plasmolysis the cell membrane pulls away from the cell wall as
the cytoplasm loses water. In the lab exercise, you will examine this process (as well as other osmotic
processes) by observing the effects of varying salt concentrations on plant cells.
By the way, because animal cells do not have the support of a cell wall, they usually cannot survive treatment
with a very concentrated solution, like the salty water we use in this lab. They shrivel too much to recover.
Figure 1:
Part One: Observing osmosis in celery exposed to differing concentrations of solute.
Day 1 Procedures:
1. Prepare and label 6 medicine cups with a group name and the color of each solution.
2. Each colored solution corresponds with a different molar concentration of sucrose solution (0.0M, 0.2M,
0.4M, 0.6M 0.8M, and 1.0M)
3. Cut a stalk of celery into 6 equal-sized pieces and mass each piece before placing it into each of the cups.
Record qualitative observations (flexibility, toughness etc.). Record your masses in the DAY 1 chart below.
4. Fill each cup will the correct color solution. Pour in just enough solution to cover the celery piece. Do not
waste.
5. Predict (hypothesize) which molarities will cause the celery to increase in mass or which molarities will
cause a decrease in celery mass. Form this into alternative hypotheses.
Null Hypothesis: ________________________________________________________________________
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Alternative Hypothesis (If/Then/Because): ___________________________________________________
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Alternative Hypothesis (If/Then/Because): ___________________________________________________
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Day 2
1. Again test the flexibility and take the mass of celery. Record in the chart below.
Table 1: Percent Change in Mass of Celery Pieces Soaked in Various Molar Concentrations of
Sucrose Solution
Day 1
Solution
0M
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M
Description
Day 2
Mass in
grams
Description
Mass in
grams
% change
in mass
Gain or
Lose H20
Class Averages for % change in mass (g):
0M
0.2 M
0.4 M
0.6 M
0.8 M
1M
Celery
Label, title and graph your results and class average results.
Plot your data points and the class average data points on the graph. Think about where your X axis would go
before drawing! (Hint: You have negative values.)
Your molar concentration for celery: _________
Class molar concentration for celery: __________
1. Thoroughly explain how you know the approximate molar concentration of the celery.
2. Propose an additional experiment to determine a more precise measurement of the molar
concentration of celery.
Part Two: Observing plasmolysis in red onion cells.
Procedure
1. Prepare a wet mount of red onion epidermis. Locate a good section of leaf under low power and then
observe under high power. Sketch a single red onion cell in the space below and describe the appearance
in the plant cell.
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2. Add 2 to 3 drops of 15% NaCl to one edge of the cover slip. Draw the salt solution across the slide by
touching a piece of paper towel to the fluid under the opposite edge of the cover slip. Observe the plant
cells in the microscope while you draw the salt water across the slide. Sketch a single red onion cell in the
space below and describe what has happened to the plant cell.
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3. Remove the cover slip and flood the red onion tissue with fresh water. Observe under high power. Describe
and explain what has happened.
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Further Analysis
1. In the winter, icy roads are often salted to remove the ice and make them less slippery. Grasses and other
herbaceous plants often die near the side of these roads. What causes this to happen?
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2. When a person is given fluid intravenously (an I.V.) in the hospital, the fluid is typically a saline solution
balanced – with the correct amount of dissolved materials – to human body tissues. Explain why this is
necessary. ____________________________________________________________________________
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Many freshwater one-celled organisms, like Paramecium, have contractile vacuoles. These structures
collect and pump out excess water that accumulates in the cell. Explain why these organisms need such a
structure. _____________________________________________________________________________
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3. Explain why contractile vacuoles would be of no value to one –celled organisms living in salt water.
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Popcorn sold at movie theatres is very salty, causing people to become thirsty and to buy soft drinks.
Explain why salty popcorn causes this thirst. _________________________________________________
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4. Explain why soft-bodied invertebrates, like slugs, die when you pour salt on them.
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5. Why can’t many land and freshwater plants live in the ocean? ___________________________________
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6. How does testing the molar concentrations of the celery help you understand osmotic properties in salt
water and fresh water environments? _______________________________________________________
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