Why the Fertiliser Burned the Seedlings
A class sets up three trays of 10-day-old tomato seedlings. Tray A is watered with 200 mL of 2% urea solution instead of water. Tray B gets 200 mL of distilled water. Tray C gets 200 mL of 0.9% salt solution. Next morning Tray A seedlings are wilted with brown leaf margins; Tray B is firm; Tray C is slightly wilted. The students peel epidermis from Tray A and Tray B stems and mount them in the same solutions. Under the microscope, Tray A cells show the protoplast shrunken away from the cell wall; Tray B cells are full. They also mount human cheek cells in distilled water and in 0.9% salt solution. The teacher asks them to explain the fertiliser damage using the onion-peel osmosis idea, not just 'the plant dried up'.
1. Explain, in terms of water potential and osmosis, why 2% urea wilted the seedlings while distilled water did not.
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The 2% urea solution has a higher solute concentration, and therefore a lower water potential, than the cell sap of the root hair cells. Water moves by osmosis from the higher water potential (the soil solution) to the lower water potential (the cell) only when the outside is more dilute. Here the gradient is reversed: water leaves the root hair cells into the soil solution.
As cells lose water, the vacuole shrinks, turgor pressure falls, and the shoot loses support - the visible wilting. The brown leaf margins are consistent with salt or solute injury at the leaf edges, where water loss is greatest. Distilled water is hypotonic to the cell sap, so water enters the cells, turgor rises and the seedlings stay firm. The key comparison is not 'wet versus dry' but the direction of the water-potential gradient across the root membrane.
2. The students say the shrunken cells are dead. Evaluate this using the plasmolysis observations and predict what happens if the peel is returned to distilled water.
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Shrinkage of the protoplast away from the cell wall is plasmolysis, and it does not by itself prove death. Plasmolysis is a physical response of a living, selectively permeable membrane to water loss; a dead cell loses membrane selectivity and will not show a clean, reversible plasmolysis pattern.
If the peel is returned to distilled water and the cells are still alive, water re-enters by osmosis. The protoplast swells back towards the wall - deplasmolysis - and turgor is restored. If the cells were already dead, the membrane would no longer control water movement and no orderly recovery would occur. To distinguish the two, the students should look for cytoplasmic streaming, an intact nucleus and a clean plasma membrane, or use a vital stain. The claim of death is therefore not supported by shrinkage alone.
3. Why did cheek cells in distilled water swell and burst while tomato cells did not? What does this tell you about the role of the cell wall?
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Cheek cells are animal cells and have only a flexible plasma membrane. In distilled water they are in a hypotonic solution, so water enters and the cell swells until the membrane ruptures - lysis. Tomato cells are plant cells and have a rigid cellulose cell wall outside the membrane. As water enters, turgor pressure rises, but the wall resists stretching and limits further water entry, so the cell becomes turgid rather than bursting.
This shows that the wall is a mechanical constraint, not a permeability barrier: osmosis is still controlled by the plasma membrane, but the wall sets the limit. It also explains why 0.9% salt is roughly isotonic for cheek cells but not necessarily for plant cells, whose sap has a different solute concentration. The comparison is a useful reminder that the same osmotic principle produces different outcomes depending on cell structure.