Fig. 2.1 is a micrograph of a pair of guard cells forming a stoma in a leaf epidermis. A scale bar is printed beside the image and an unlabelled dimension l...
Question 1 Report
Fig. 2.1 is a micrograph of a pair of guard cells forming a stoma in a leaf epidermis. A scale bar is printed beside the image and an unlabelled dimension line has been drawn across the length of one guard cell. The pore between the two guard cells is labelled. A student was asked to use the scale bar to work out the real length of a guard cell and to identify the structures around the pore.
(a) Measure, in mm, the length of the scale bar in Fig. 2.1 [1]
(b) Measure, in mm, the length of the dimension line drawn across the guard cell [1]
(c) Use your two measurements to calculate the actual length of the guard cell. Show your working [2]
(d) Name the structures that control the size of the pore [1]
(e) State one function of a stoma in a leaf [1]
Answer Details
A scale bar lets you turn a measurement on the image into a real length: the printed bar represents a known real distance, so the guard cell's real length is found by proportion.
(a) Length of the scale bar [1] Measure the printed scale bar with a ruler; a typical reading is \(20\ \text{mm}\) (accept an honest, correct reading). [1]
(b) Length of the dimension line [1] Measure the dimension line drawn across the guard cell; a typical reading is \(32\ \text{mm}\) (accept an honest, correct reading). [1]
(c) Actual length of the guard cell [2] The scale bar represents 50 micrometres, so scale each measurement by the same ratio: \[\text{actual length}=\frac{\text{dimension line}}{\text{scale bar}}\times 50\ \mu\text{m}=\frac{32}{20}\times 50=80\ \mu\text{m}\] correct method [1]; answer about \(80\ \mu\text{m}\) using your own readings [1].
(d) Structures controlling the pore [1] The guard cells. [1]
(e) One function of a stoma [1] It allows gas exchange (letting carbon dioxide in and oxygen and water vapour out) and helps control water loss. [1]
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