During a trial at a vegetable-pickling factory, a technician placed a thin piece of red onion epidermis into concentrated brine. Before adding the brine, th...
Question 1 Report
During a trial at a vegetable-pickling factory, a technician placed a thin piece of red onion epidermis into concentrated brine. Before adding the brine, the onion cells had large vacuoles and their cell contents pressed against the cell wall. After 12 minutes, the technician viewed the same cells with a microscope. Fig. 1 shows the change in one cell. Table 1 gives measurements of the width of the cell contents from several cells. This investigation helps the factory control the salt solution used to preserve food.
Fig. 1 shows an onion cell before and after immersion in brine.
(a) Name the process that causes water to move out of the onion cell. [1]
(b) Describe two changes visible in the cell after it has been in the brine. [2]
(c) Explain why water moves from the cell into the salt solution. [3]
(d) Calculate the percentage decrease in the mean width of the cell contents. Show your working. [2]
(e) Suggest why the technician measured cells from several different pieces of onion. [2]
Table 1
| Condition | Mean width of cell contents / µm |
|---|---|
| Before brine was added | 80 |
| After 12 minutes in brine | 68 |
Answer Details
(a) Water leaves the onion cells by osmosis. [1 mark]
(b) Any two visible changes are: the cell membrane or cytoplasm moves away from the cell wall; the vacuole becomes smaller; or the cell contents shrink and become more concentrated. [2 marks]
(c) Brine is more concentrated, so it has a lower water concentration than the cell contents. Water crosses the partially permeable cell membrane from the higher water concentration inside the cell to the lower water concentration in the brine. [3 marks]
(d)
\[\text{decrease}=80\,\mu\text{m}-68\,\mu\text{m}=12\,\mu\text{m}\]
\[\text{percentage decrease}=\frac{12}{80}\times100=15\%\]
The percentage decrease is 15%. [2 marks]
(e) Measuring cells from several onion pieces makes the results more reliable, reduces the effect of an unusual cell or unusual piece of onion, and allows a mean to be calculated. Any two earn credit. [2 marks]
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