Italicized font represents information to be shared orally or physically completed with the students at this time.

The non-italicized font represents additional information included to support the teacher’s understanding of the content being introduced within the CELL.

ANALYZE IT

Note: Questions marked with a triangle (∆) are included to enrich students’ understanding. These questions do not appear in students’ SDRs but should be used as additional discussion points during the PostLab.

• ∆ How would you describe the solute contents of the liquids you used to mount the onion or Elodea specimens? The liquids each had a different concentration of salt. The tap water contained very little sodium chloride (salt). Although no salt was added to the sample labeled tap water, small amounts of sodium or chloride may have been present initially. As the 2%, 4%, and 8% salt solutions were created, the amount of salt in the water increased. Instruct students to answer question #14 in their Investigation Four Data Record.

• ∆ Instruct students to describe what occurred in the Elodea leaf when placed in liquids with successively higher concentrations of salt. What did you observe? When observing the Elodea in the tap water, students should have been able to identify the cytoplasm, chloroplasts, nuclei and combination cell wall/cell membrane. The chloroplasts were highly visible and contributed the green color to the Elodea cells and helped to mark the boundary of the cytoplasm and cell membrane. The students observed the cytoplasm with its cellular contents as a small green circular structure within the cell wall. Students may have observed that as the concentration of the salt solution increased, the green structure within each cell wall became smaller. What students observed during this event was the cell membrane and cytoplasm being pulled away from the cell wall as water traveled out of the cell through the cell membrane and the cell wall. As water flowed out of the cell, the turgor pressure within the cytoplasm decreased, the cell membrane was pulled away from the cell wall, and the chloroplasts, cytoplasm, and nucleus were contained within a smaller area of the cell. The boundaries between the cell membrane and cell wall were easily visible as this occurred.

• Instruct students to describe what occurred in the onion. What did you observe? Students should have observed the cytoplasm, nuclei, and combination cell membrane/cell wall within the red onion specimen. As the specimen was prepared from the bulb, students would not have observed chloroplasts. Students may also have observed the presence of a purple pigment contained within the cell membranes of the specimen. Students observed that as the concentration of salt solution increased, more small areas of light purple became visible. The light purple areas in the cells indicated the boundaries of the cell membranes and cytoplasm after the cell membranes pulled away from the cell walls. Increasing concentrations of salt resulted in more and more water passing through the cell membranes and the cell walls, causing the cells to shrink. Because the onion cells did not contain chloroplasts, the change in cell structure may not have been as easy to see as it was in the Elodea leaf.

• What conclusions can you draw from your experiments concerning the permeability of the Elodea and onion cell to water and sodium chloride? Are the membranes permeable to water? To sodium chloride? The cell membranes were permeable to water. Observations that support this conclusion include the changes in cell (cytoplasm) size as solutions with comparatively higher osmolarities (less water concentration) were added to the Elodea and onion specimens. The change in size occurred as water diffused from the cytoplasm through the cell membrane into the external environment. Had the membrane been impermeable to water, students would not have observed a change in the size of the cell membrane and cytoplasm. The cell membranes were impermeable to salt. Had the cell membranes been permeable to salt, salt would have diffused from the external environment into the cell cytoplasms, in an attempt to equalize the osmolarity (the concentration of sodium chloride) on both size the membrane. This would have caused an increase in osmotic pressure inside the cells. Because of the opposing pressure of the cell walls, students would have observed this event as the maintenance of cell membrane and cytoplasmic size. However, the students should indicate that they did not see the maintenance of cytoplasmic size, but rather a decrease. Thus, students should conclude that in the absence of membrane permeability to the sodium chloride, diffusion of water out of the cells was necessary to equalize the osmolarity (sodium chloride concentration) on both sides of the membrane, resulting in a decrease in cytoplasmic size. Instruct students to answer question #15 in their Investigation Four Data Record.

Ask students to compare their observations of the onion cell and the cells in the Elodea leaf.

• Which was easier to observe, the cells in the onion or the Elodea leaf? The cells in the Elodea leaf were likely easier to observe because the onion cells did not contain chloroplasts. The changes in the Elodea leaves were easier to observe because of the green chloroplasts made the pulling away of the cell membrane very noticeable.

• The color of the Elodea leaf likely made it easier for you to view the effects of osmosis. Does the Elodea leaf contain color due to the staining of the slide or to the natural characteristics of the cells? The color of the Elodea leaf is not due to staining, since students did not stain their slides as they were preparing them. In this case, the color of the cells is due to the natural characteristics of the Elodea leaf cells.

Instruct students to complete the Analysis Questions in their SDRs then discuss them as a class. Use the suggested responses below to guide students’ answers.

Use what you know about osmosis to answer questions 9, 10, and 11. 8.

• Why did the cytoplasm and cell membranes shrink when the cells were placed in the salt solutions? The cell membranes of the cells in salt solution pulled away from the cell walls as the cells shrank. Because the solutions used to make the slides contained salt, there was a difference in the concentration of salt outside the cell (in the solution) and inside the cell. In other words, the salt solutions contained less water per salt compared to the internal environment of the onion and Elodea cells. In order to balance this concentration, water from the cell flowed through the cell membrane and cell wall into the salt solution, causing the salt solution to become more dilute and to be less concentrated.

• Why didn’t the cells shrink as they were placed in the tap water? The cells did not shrink because the difference in concentration of salt was not great enough to cause the movement of water. Because the concentrations outside the cell (in the tap water) and inside the cell were similar, no water movement needed to occur.

• Compare the reaction of the cells to the 2% solution to the reaction to an 8% solution. Explain your observations. When the specimens were placed in the 2% solution, the cell membranes pulled slightly away from the cell walls. When the specimens were placed in an 8% solution, the cell membranes pulled away from the cell walls to a much greater degree. This difference came as a result of the difference in concentrations of salt. The smaller the difference in concentration, the less water that had to move through the cell membrane and cell wall into the surrounding solution.

GET FOCUSED

Instruct students to complete the Focus Questions in their SDRs then discuss them as a class. Use the suggested responses below to guide students’ answers.

• Is the cell membrane of a plant cell permeable to salt? The cell membrane of a plant cell is not permeable to salt. This was evidenced by the shrinking of cells. To balance the concentration of salt within and outside the cell, water flowed from the cells into the surrounding environment. If the cell membrane was permeable to salt, water and salt could have both flowed back and forth through the cell membrane until both environments contained equal concentrations of salt.

• Are the cell wall and the cell membrane of a plant cell permeable to water? The cell wall and cell membrane are permeable to water. This was also evidenced by the shrinking of cells. To balance the concentration of salt inside and outside the cells, water flowed from the cells into the surrounding environment. If the cell membrane were not permeable to water, the cells would not have shrunk.

• How does the structure of the cell wall and cell membrane affect the movement of substances in and out of a cell? The structure of the cell wall and cell membrane determines whether or not each structure is permeable to salt and water. The permeability of each structure determines whether or not salt moves in or out, which in turn affects whether or not water moves in and out of the cell.