The Beaker with a Leaky Membrane
A student places a single-celled organism in a beaker containing a 5% salt solution. The organism's cytoplasm is 1% salt. The cell membrane is selectively permeable, but after 10 minutes the student observes that the cell has shrunk and its membrane has pulled away from the cell wall. The student then places a second identical organism in a beaker of distilled water. After 10 minutes, this cell is swollen and firm. The student concludes that the membrane is not a barrier and that salt moves into the cell. The teacher points out that the cell wall is made of cellulose and that the membrane is a phospholipid bilayer with embedded proteins.
1. Evaluate the student's conclusion that the membrane is not a barrier and that salt moves into the cell. Use evidence from both beakers to justify your answer.
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The student's conclusion is wrong. In the 5% salt solution, the cell shrank because water left the cell by osmosis. The cytoplasm (1% salt) had a higher water potential than the surrounding 5% salt solution, so water moved out through the selectively permeable membrane. The membrane pulling away from the cell wall is plasmolysis, which shows the membrane is a barrier that allows water but not salt to pass freely. If salt had moved in, the cell would not have shrunk; it would have gained salt and possibly water. In distilled water, the cell became firm because water entered by osmosis; the cell wall prevented bursting. The cellulose wall is fully permeable, but the membrane is not. The membrane's phospholipid bilayer and proteins make it selectively permeable. The student confused the movement of water with the movement of salt.
2. Explain why the cell in distilled water did not burst, while the cell in the 5% salt solution shrank. Include the role of the cell wall and the membrane in your answer.
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In distilled water, the water potential outside is higher than inside the cell, so water enters by osmosis. The cell wall is strong and made of cellulose; it resists stretching and creates turgor pressure that stops more water entering. The membrane is pressed against the wall, and the cell becomes turgid but does not burst. In the 5% salt solution, the water potential outside is lower than inside, so water leaves the cell by osmosis. The membrane shrinks away from the cell wall because the wall is rigid and does not shrink with the membrane. This is plasmolysis. The cell wall does not prevent water loss because it is fully permeable; it only prevents bursting when water enters. Thus the wall protects against bursting but not against shrinking.