Osmosis in Plant Cells

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: Egg “cell” in different environments

Table 1: Understanding osmosis using the selectively permeable membrane of an egg.

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)

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Part 2: Observing osmosis in celery exposed to differing concentrations of solute.

Research Question: Does changing the concentration of solute in a solution impact plant cell size?

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, take the mass of celery. Record in the chart below.

Table 2: ______

Graph your results and class average results:

One the grid provided below, plot your data points, draw a line of best fit, then, plot the class average data points and draw a line of best fit. Think about where your X axis would go before drawing! (Hint: You have negative values.)

Graph 1: ______

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

1.  Thoroughly explain the science behind 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 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.  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. ______

______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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