The onion strip that 'never recovered'
Two strips of epidermis peeled from the same onion scale leaf are mounted, one in distilled water and one in 5% salt solution. After five minutes under a compound microscope (10× eyepiece, 45× objective), the water mount shows cells whose purple pigment fills the cell right up to the wall, while in the salt mount the coloured contents have shrunk into rounded masses with clear spaces between them and the walls. A third strip is soaked in the salt solution for 20 minutes, then mounted in distilled water and examined after only ten seconds; it still shows shrunken contents. The student writes in the record: 'The salt solution killed the cells, because they never recovered.' He also plans to mount cheek cells from his mouth in the same salt solution. A printed photograph of the same slide at the same magnification shows one epidermal cell 45 mm across.
1. Explain the appearance of the salt mount and judge whether the record's conclusion (that the cells were killed) is supported.
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Explanation: the salt solution has a lower water potential than the cell sap. Water leaves the vacuole and cytoplasm by exosmosis, the protoplast shrinks, and the living protoplast pulls away from the rigid cellulose wall - plasmolysis. The clear space seen is the gap between the shrunken protoplast and the wall.
Is the cell dead? Shrinking alone proves nothing about death; healthy cells plasmolysed in a strong solution look exactly like this. The student's evidence for death is the 'no recovery' observation, and it is not valid:
- Deplasmolysis takes minutes, not ten seconds. The membrane must first take water back, and the strip was mounted for only ten seconds, so the result is inconclusive rather than negative.
- A true sign of death would be loss of pigment into the surrounding liquid (the tonoplast and plasma membrane no longer being selectively permeable) and no recovery even after 10–15 minutes in distilled water.
Judgement: the experiment should be repeated with a wait of several minutes and the bathing fluid checked for purple colour. The record must say 'inconclusive', not 'killed'.
2. Predict what will happen to the cheek cells in the same 5% salt solution and explain why they will not look like the onion cells.
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Prediction: water will leave the cheek cells by osmosis, and each cell will shrivel into a smaller, irregularly wrinkled (crenated) shape.
Why the appearance differs:
- An animal cell has no cell wall - only a flexible plasma membrane. When water is lost, the membrane simply follows the shrinking cytoplasm, so there is no rigid outline left behind and no visible gap.
- A plant cell has a rigid cellulose wall that keeps its shape; only the protoplast shrinks inside it, which is why a clear space appears.
The point being tested: the underlying process (water moving down its potential gradient through a selectively permeable membrane) is identical in both cells. The difference in what the observer sees is structural, not a difference in the mechanism. A common wrong answer is that cheek cells 'do not do osmosis' - they do.
3. Calculate the real diameter of the photographed cell and comment on whether the value is reasonable.
Read the worked answer
Total magnification = eyepiece × objective = 10 × 45 = 450×.
Real size = image size ÷ magnification = 45 mm ÷ 450 = 0.1 mm = 100 µm.
Check for reasonableness: a typical onion epidermal cell is a few tenths of a millimetre - about 100–400 µm - so 100 µm sits comfortably in range. If the same 45 mm image had been produced at 100× instead of 450×, the answer would have been 450 µm, which is unusually large and would signal an error. This is why the magnification must always be recorded with the drawing.
Useful scale check: with a 10× eyepiece the field of view is roughly 1.5–2 mm, so only about 15–20 such cells fit across the field. The student should therefore record 'about 100 µm (2 significant figures)', since the image measurement itself is uncertain by at least a millimetre.