Osmosis in Plant Cells: Research Grade

Name: ______Date: ______Block: ______

Osmosis in Plant Cells: Research Grade

Background Information:

Cells can 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 concentration to a region of lower 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.

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 cell and therefore an equal concentration of water. Consequently, water will flow equally into and out of the cell and that will balance out.

When a plant cell shrinks because of loss of water, we call that plasmolysis. Since plant cells have cell walls, the whole cell will not collapse when it first loses water. The cell wall will still 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: Understanding osmosis using the selectively permeable membrane of an egg.

Environment the Egg is Soaked in for 48 Hours
Soaking in Syrup / Soaking in Distilled Water
Initial Mass without shell (g):
Final Mass after being soaked in varying environments (g):
Observations: / Observations: / Observations:

Use the following terms to describe what is happening in the egg demonstration. Remember to include your data to help support your description. Highlight or underline each of the terms as you use them. (Diffusion, Osmosis, Selectively Permeable, Equilibrium, Hypertonic Solution, Hypotonic Solution and Turgor Pressure)

______

______

Part Two: Observing osmosis in celery exposed to differing concentrations of solute.

Procedures:

1.  Prepare and label your cups. Each cup should have 20 mL of a molar concentration solution (amount of substance concentration) of each of the following solutions: 0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, and 1 M solutions.

2.  Obtain 6 pieces of celery and mass each piece before placing it into each of the solutions. Also record any observations (flexibility, toughness etc.). Record your masses in the DAY 1 chart below.

3.  Predict (hypothesis) which molarities will show that the celery has an increase in mass and which molarities will have a decrease in mass. Explain your reasoning!

Hypothesis (If/Then/Because): ______

______

Null Hypothesis: ______

______

Day 2

1.  Again test the flexibility and mass of celery. Record in chart.

Data Chart Celery:

Day 1 / Day 2
Solution / Description / Mass in grams / Description / Mass in grams / % change in mass / Gain or Lose H20
0 M
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M

Class Averages for % change in mass (g):

0 M / 0.2 M / 0.4 M / 0.6 M / 0.8 M / 1 M
Celery

Label, title and graph your results and class average results.

Your molar concentration for celery: ______Class molar concentration for celery: ______

Part Three: 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.

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.

3.  Explain what would happen if you were to flood the red onion tissue with fresh water?

______

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?

______

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. ______

______

3.  What if the unthinkable happened at the hospital! A patient was given an I.V. bag with distilled water in it rather than saline solution. Describe what would happen to their red blood cells and explain why this would occur. ______

______

______

4.  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. ______

______

______

5.  Explain why contractile vacuoles would be of no value to one –celled organisms living in salt water.

______

______

6.  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. ______

______

______

7.  Explain why soft-bodied invertebrates, like slugs, die when you pour salt on them.

______

8.  Why can’t many land and freshwater plants live in the ocean? ______

______

9.  How does testing the molar concentrations of the celery help you understand osmosis properties in salt water and fresh water environments? ______

______

______