Photo Credit: By Frank Vincentz [GFDL or CC-BY-SA-3.0], via Wikimedia Commons
Students will microscopically observe various subcellular components and determine the effects of different salt solutions on Elodea plant cells.
At the high-school level, students should understand that the eukaryotic cell is a highly structured unit composed of several subcellular components. These subcellular components are specialized parts, each of which performs a unique function necessary for the survival of the cell. Most of these structures are difficult to see in living cells not only because they are small, but also because they are colorless.
In this lesson, students will microscopically detect the presence of chloroplasts, cell walls, and cell membranes of the common aquarium plant, Elodea. Students will then determine the effects of different salt solutions on the Elodea plant cell structures.
By adding salt water to the Elodea cell environment, students will observe the process of plasmolysis, shrinkage of the cell contents due to water loss. However, you should not focus on terminology, but rather what happens during plasmolysis. By conducting this lab, students will be able to develop an operational understanding of plasmolysis, as well as reinforce their understanding of osmosis and diffusion.
Research indicates that it may be easier for students to understand that the cell is the basic unit of structure (which they can observe) than that the cell is the basic unit of function (which has to be inferred from experiments). (Benchmarks for Science Literacy, p. 342.) It is important to emphasize that cellular processes have an impact on the entire organism.
By the end of 8th grade, students should know that within cells, many of the basic functions of organisms are carried out and that the way in which cells function is similar in all living organisms. (Benchmarks for Science Literacy, p. 112.) Students should also be familiar with the use of a microscope.
This lesson should follow some discussion about the various subcellular structures in animal and plant cells. It does not introduce the cell membrane, cell wall, and chloroplasts to students; rather, it demonstrates the presence of these cell components and the process of plasmolysis.
The student sheet can be distributed to students prior to the lab. As homework, students can enter the relevant information into their lab notebooks and use these notebooks in lab rather than the worksheet.
- Elodea can be purchased from Carolina Biological Supply (www.carolina.com/onlinecatalog) or by calling customer support at 1-800-334-5551. A pack of 25 Elodea is approximately $9.00. Note: Carolina Biological Supply does not ship to Canada, South Carolina, or Washington.
- To make a 5% salt solution, weigh 5 grams of NaCl and place in a graduated cylinder. Bring volume up to 100 mL with distilled water.
- To make a 10% salt solution, weigh 10 grams of NaCl and place in graduated cylinder. Bring volume up to 100 ml with distilled water.
- What is the major function of a cell membrane?
- Why did the first site depict the cell membrane as a gate?
- Describe the structure of a cell membrane.
- How is the structure of the cell membrane related to its function?
Use these questions to discuss the necessity for the cell membrane to act as a physical barrier marking the boundary of the cytoplasm and protecting the contents of the cell. In this discussion, lead students to an exploration of the nature of the interaction of the cell with its surroundings.
The analogy of a gate is an effective one because it allows students to understand that anything going into or out of the cell must first cross the cell membrane. Thus, the cell membrane plays a critical role in the acquisition of gases (such as oxygen necessary for cellular respiration), nutrients, and water. Similarly, the cell membrane also allows things to leave the cell such as carbon dioxide gas, water, and wastes.
Have students go to the Plant Cell Wall website.
- What is the major function of the cell wall?
- Describe the structure of the cell wall and its composition.
- How is the structure of the cell wall related to its function?
Discuss with students the differences and similarities between the cell membrane and the cell wall. Surrounding every cell is some sort of covering that keeps what's inside the cell inside and prevents harmful particles in the external environment from diffusing into the cell. Both the cell membrane and the cell wall serve this function. All cells have a cell membrane, and certain cells (plant and bacterial) also have a cell wall.
The cell membrane's main function is to regulate the movement of materials into and out of the cell. However, not everything can just pass through the cell membrane—only certain materials. Thus, scientists say that the cell membrane is selectively permeable, which means that only selective (certain) substances can permeate (go through) the membrane. The cell wall is a structure that surrounds the cell membrane and provides strength and rigidity to cells. Unlike the cell membrane, the cell wall is not selectively permeable; things can easily pass through the wall.
Introduce the activity to students by saying: "As we have discussed, one of the functions of the cell membrane is to control the flow of materials into and out of the cell. In this investigation, you will observe the effects of placing plant cells in solutions of various salt concentrations."
Ask students to predict what will happen when an Elodea plant cell is placed in water vs. varying concentrations of salt solutions. Do not confirm right or wrong answers at this point.
Guiding questions that may be used are:
- What do you think will happen to the plant cell when it is placed in water?
- What will be the effect on the cell membrane? The cell wall? Why?
- What do you think will happen to the plant cell when it is placed in salt water?
- What will be the effect on the cell membrane? The cell wall? Why?
- Will increasing the concentration of salt solution have a different effect on the cell? Why or why not?
Distribute copies of the Plasmolysis in Elodea Plant Cells student sheet to all students. While the drawings and observations can be made directly on the sheet, it is recommended that students enter this information in lab notebooks. Review the directions for the lab, focusing on the techniques of slide preparation. Demonstrate how to make a wet mount slide and review the use of the microscope.
- Why do you think we will observe the Elodea plant cell in tap water first? (The tap water is the experimental control. It will allow students to observe Elodea plant cells in their normal freshwater environment. Thus, when the salt water solutions are added, any changes that occur to the plant cells can be attributed to the salt water solutions only.)
Encourage students to make careful sketches of their observations using colored pencils and to attempt to label as many structures as they can identify. The students will likely need help in identifying suitable regions of the Elodea to observe. You can prepare a typical slide and project it to help students do this.
The post-laboratory discussion should center on the students' explanations of the changes observed in the Elodea cells. Discuss what happens to the cell wall as the salt concentration increases. Students should have been able to distinguish between the cell walls and the cell membrane more clearly as more water leaves the cell and the cytoplasm shrinks.
Ask students the following questions to guide the post-lab discussion. As students answer the questions, sketch an Elodea plant cell on the board, filling in the cell with the subcellular structures being discussed.
- What color were the Elodea cells? (They were colorless except for green bodies.)
- What were the green bodies inside the Elodea cells? (Chloroplasts.)
- Where were these green bodies mostly located? (They were mostly located at the edges of the cell.)
- Describe the shape of these chloroplasts. (Ovals.)
- Why are these chloroplasts green? (They are green because of the presence of chlorophyll, the light-absorbing pigment necessary for photosynthesis.)
- Were the chloroplasts stationary or moving around the cell? (They should have been moving.)
- Why were the chloroplasts moving around? (The cytoplasm within the cell is constantly moving, thereby moving the various subcellular structures within the cell as well. This is known as cytoplasmic streaming.)
- Did anyone notice a large space inside the cell?
- What is this large space? (The central vacuole.)
- What is the function of the central vacuole? (The central vacuole is an organelle in plant cells that stores nutrients and water for the cell. It can take in and release water depending on the cell's needs. Animal cells do not have a central vacuole; they have many small vacuoles, which contain proteins, carbohydrates, water, and nutrients.)
- Describe what happened to the Elodea cells in the presence of 5% salt solution. (You should draw Elodea in 5% salt solution on the board.)
- Describe what happened to the Elodea cells in the presence of 10% salt solution. (You should draw Elodea in 10 % salt solution on the board.)
- Why did the cells shrink? (Higher salt concentration caused the diffusion of more water from within the cell to outside the cell.)
- Why didn't the salt from the outside just move inside the cell instead of the water moving out of the cell? What does this tell you about the cell membrane? (The cell membrane is selectively permeable. It allows the movement of water but not salt.)
- What structure did the shrinking of the cell allow you to observe? (The cell wall.)
- Why didn't the cell wall shrink? (It is rigid and provides support to the Elodea plant cell. Also, it allows salt to go through so it does not shrink, unlike the cell membrane.)
- Predict what would happen if we used a 20% salt solution.
- Describe what happened when you flushed out the salt solution with salt water.
- From your observations, is Elodea a freshwater or saltwater plant?
See the Elodea Plasmolysis website for pictures of Elodea cells in various salt solutions. You can review these with the class and ask students to compare these with what they saw in their observations.
The following key concepts should be discussed with students:
- When plant cells are surrounded with salt water, the water inside the plant moves from where there is more water (less salt) through the cell wall and membrane to the outside where there is less water (more salt). This process of water movement from a high concentration of water to a lesser concentration of water is called osmosis.
- When the water movement is out from a cell, we call this plasmolysis.
- Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out of the cell and into a high salt concentration solution.
- During plasmolysis, the cell membrane pulls away from the cell wall. This does not happen in low salt concentration because of the rigid cell wall. Plant cells maintain their normal size and shape in a low salt concentration solution.
- Plasmolysis is a reversible process
Ask students to use what they have observed to answer the following questions:
- Elodea normally lives in fresh water. What changes would you observe in the cells of an Elodea plant that was suddenly moved from fresh water to salt water? Why? How would plasmolysis affect the entire plant? (Plasmolysis would occur because the high concentration of salt outside the cells would cause the diffusion of water from within the cell to outside the cells. This would result in the entire plant becoming very flaccid.)
- If you wish to restore flabby, wilted green vegetables or carrot sticks to crispy "freshness," would you soak them in salt water or in plain water? Explain your answer. (Vegetables should be soaked in plain water. Because of the high concentration of water outside the cells, water will flow into the cells. As the cells and central vacuole fill up with water, cells will become rigid.)
Have students answer the Conclusions questions of the worksheet in their lab notebook. These questions can be used as an assessment tool to check student understanding.
A website with an active description about osmosis and the movement of water in and out of cells is How Substances Get Into and Out of Cells. Students can read the information on this site and view the animations until the section entitled "Demonstration of Osmosis Using Visking Tubing," which is not relevant to high school students.
From this activity, students could proceed to Diffusion, Osmosis and Active Transport, a site that can enhance their understanding of dynamic equilibrium within cells, diffusion, and osmosis. There is an interactive on this page that allows students to observe diffusion, osmosis, and active transport.