A student photographed onion epidermis cells down a microscope and printed the micrograph. Fig. 6.1 shows three of the cells drawn in a row with an unlabell...
Question 1 Report
A student photographed onion epidermis cells down a microscope and printed the micrograph. Fig. 6.1 shows three of the cells drawn in a row with an unlabelled dimension line under the first cell and a millimetre ruler printed below. She measured the image length of each cell against the ruler and recorded the values in Table 6.1, then began to calculate the actual lengths using the printed magnification of times 400.
Table 6.1
| Cell | Image length / mm | Magnification | Actual length / micrometres |
|---|---|---|---|
| A | 60 | times 400 | |
| B | 48 | times 400 | |
| C | 72 | times 400 |
(a) Measure, in mm, the length of the dimension line under the first cell in Fig. 6.1 using the ruler [1]
(b) State the magnification equation rearranged to give the actual size [1]
(c) Calculate the actual length of cell A in micrometres. Show your working [2]
(d) Complete the table by calculating the actual lengths of cells B and C in micrometres [2]
(e) Calculate the mean actual length of the three cells. Show your working [2]
(f) Explain why the same magnification applies to every cell in one micrograph [2]
(g) Suggest why cell sizes are recorded in micrometres rather than millimetres [1]
Answer Details
(a) Read the length of the dimension line under the first cell against the mm ruler; accept the candidate's correct ruler reading. [1]
(b) The magnification equation rearranged to give the real size is actual size = image size / magnification. [1]
(c) Actual length of cell A \(=\dfrac{\text{image length}}{\text{magnification}}=\dfrac{60}{400}=0.15\text{ mm}\) [1] \(=0.15\times1000=150\) micrometres [1].
(d) Completed for the other cells: cell B \(=\dfrac{48}{400}=0.12\text{ mm}=120\) micrometres [1]; cell C \(=\dfrac{72}{400}=0.18\text{ mm}=180\) micrometres [1].
| Cell | Image length / mm | Magnification | Actual length / micrometres |
|---|---|---|---|
| A | 60 | ×400 | 150 |
| B | 48 | ×400 | 120 |
| C | 72 | ×400 | 180 |
(e) Mean actual length \(=\dfrac{150+120+180}{3}=\dfrac{450}{3}=150\) micrometres. Working [1]; answer 150 micrometres [1].
(f) The whole micrograph was enlarged by the same amount [1], so every cell and structure in that one image shares the same magnification [1].
(g) Cells are very small, so micrometres give convenient whole numbers instead of tiny decimals of a millimetre (150 micrometres rather than 0.15 mm). [1]
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