Fig. 1.1 shows part of a clear nail-varnish impression that a student peeled from the lower surface of a leaf and viewed under a microscope. The student cou...
Question 1 Report
Fig. 1.1 shows part of a clear nail-varnish impression that a student peeled from the lower surface of a leaf and viewed under a microscope. The student counted the number of stomata in five separate fields of view on both the upper and the lower surfaces of the leaf. The counts are shown in the table below.
| Field of view | 1 | 2 | 3 | 4 | 5 | Mean |
|---|---|---|---|---|---|---|
| Lower surface | 18 | 21 | 19 | 22 | 20 | |
| Upper surface | 4 | 3 | 5 | 4 | 4 |
(a) Name the piece of apparatus used to view the impressions. [1]
(b) An eyepiece scale was used. One guard cell of a stoma reached from the 10 mark to the 28 mark on the scale. Measure the length of this guard cell in scale units. [1]
(c) The paired guard cell reached from the 9 mark to the 26 mark. Measure the length of this guard cell in scale units. [1]
(d) The eyepiece scale was calibrated so that 1 unit = 4 micrometres. Calculate the real length of the guard cell in part (b) in micrometres. Show your working. [2]
(e) Complete the table by calculating the mean number of stomata per field of view for each surface. [2]
(f) Describe the difference in the number of stomata between the two surfaces of the leaf. [2]
(g) Each field of view has a diameter of 0.6 mm. Using your mean for the lower surface, calculate the number of stomata per mm squared. Use area = 3.14 x radius x radius. Show your working. [3]
(h) Explain why the lower surface of a leaf usually has more stomata than the upper surface. [3]
(i) Draw a large labelled diagram of a single stoma, showing the two guard cells and the pore. [3]
(j) Suggest why the student counted stomata in five fields of view rather than only one. [2]
Answer Details
Labelled answer diagram:
Stomata are the tiny pores that let gases in and out of a leaf, each guarded by a pair of guard cells. Counting them in several fields of view and finding a mean lets you compare the two leaf surfaces reliably.
(a) The impressions are viewed with a (light) microscope [1].
(b) Guard-cell length \( = 28 - 10 = 18 \) scale units [1].
(c) Paired guard-cell length \( = 26 - 9 = 17 \) scale units [1].
(d) Real length \( = 18 \times 4 = 72 \) micrometres; working [1], answer [1].
(e) Means: lower \( = \dfrac{18+21+19+22+20}{5} = 20 \); upper \( = \dfrac{4+3+5+4+4}{5} = 4 \). Completed table:
| Field of view | 1 | 2 | 3 | 4 | 5 | Mean |
|---|---|---|---|---|---|---|
| Lower surface | 18 | 21 | 19 | 22 | 20 | 20 |
| Upper surface | 4 | 3 | 5 | 4 | 4 | 4 |
Both means correct [2]; one correct [1].
(f) The lower surface has far more stomata than the upper surface [1], about five times as many (20 compared with 4) [1].
(g) Area of one field \( = 3.14 \times 0.3 \times 0.3 = 0.283 \) mm\( ^2 \) [1]; stomata per mm\( ^2 = \dfrac{20}{0.283} \) [1] \( = 71 \) per mm\( ^2 \) (accept 70 to 71) [1].
(h) The lower surface is shaded and cooler, away from direct sun [1], so placing stomata there reduces water loss by transpiration [1] while still allowing gas exchange for photosynthesis and respiration [1].
(i) A single stoma is drawn as a pair of sausage- or kidney-shaped guard cells enclosing a central pore:
Marks: two guard cells drawn as a pair [1]; a clear pore between them [1]; correct labels, drawn large with clean single lines [1].
(j) Counting five fields of view lets a reliable mean be calculated [1] and reduces the effect of any unusual or anomalous field [1].
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