When Elodea leaves were placed in 10% NaCl, what was the result?
A.
The cell membrane did not change at all because of the protection of the cell wall.
B.
The cells burst because the solution was hypotonic.
C.
The cell membrane shrank, but the central vacuole remained full.
D.
The cell membrane shrank away from the cell wall because the solution was hypertonic.
E.
The cell became turgid.
The Correct Answer and Explanation is:
The correct answer is:
D. The cell membrane shrank away from the cell wall because the solution was hypertonic.
Explanation:
Elodea is a common aquatic plant used in laboratory experiments to observe cellular processes such as osmosis. When Elodea leaves are placed in a 10% sodium chloride (NaCl) solution, which is hypertonic relative to the cell’s internal environment, the following occurs:
- Osmosis and Hypertonic Solutions: Osmosis is the movement of water across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. In a hypertonic solution like 10% NaCl, the solute concentration outside the cell is much higher than inside the cell. This creates a gradient where water inside the cell moves out to balance the concentration of solutes.
- Effect on Elodea Cells: As water exits the Elodea cells due to osmosis, the volume of the cytoplasm decreases. The cell membrane, which is semi-permeable and flexible, begins to pull away from the rigid cell wall. This process is known as plasmolysis. The cell wall of Elodea cells provides structural support, but it does not prevent the shrinking of the cell membrane or the loss of water from the cell.
- Observation of Plasmolysis: Under a microscope, this results in the appearance of the cell membrane detaching from the cell wall. The central vacuole, which is responsible for storing water and maintaining turgor pressure, shrinks as it loses water, causing the cell membrane to collapse inward.
- Contrast with Hypotonic Solutions: In a hypotonic solution, where the solute concentration outside the cell is lower than inside, water would enter the cell, causing it to swell and potentially burst (a condition known as cytolysis). However, in this case, the solution is hypertonic, leading to water loss and the observable shrinkage of the cell membrane.
Therefore, in a 10% NaCl solution, the Elodea cells experience plasmolysis due to the hypertonic nature of the solution, causing the cell membrane to shrink away from the cell wall.