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Date November 2020 Marks available 2 Reference code 20N.3.HL.TZ0.1
Level Higher level Paper Paper 3 Time zone TZ0 / no time zone
Command term Calculate Question number 1 Adapted from N/A

Question

The stomatal density is the number of stomata per area of leaf epidermis. A study was done on the stomatal density of the lower epidermis of leaves from Protium decandrum, a tree found in the Amazon forest.

The image shows the epidermis stripped from the lower surface of a leaf.

[Source: Camargo, M.A.B. and Marenco, R.A., 2011. Density, size and distribution of stomata in 35
rainforest tree species in Central Amazonia. Acta Amazonica, 41(2), pp.205–212. Image courtesy of Miguel from
Camargo & Marenco (2011).]

Calculate the magnification of the image, showing your working.

 

. . . . . . . . . . . . . . . . . . . . x

[2]
a.

The mean stomatal density for the lower epidermis of P. decandrum was around 600 per mm2. Predict how the stomatal density for the upper epidermis would compare.

[1]
b.

Markscheme

Calculation: size of bar ÷ 15 μm (1.5 cm ÷ 15 μm or 15 000 μm ÷ 15 μm);

Answer: 1000 x;

First marking point is for division by 15 μm;

Second marking point is for the correct answer; accept 930 and 1070 x.

a.

(upper surface/epidermis usually has) fewer stomata/lower stomatal density/no stomata/OWTTE

Do not accept a numerical value only.

b.

Examiners report

Most candidates managed to calculate the magnification correctly by dividing their scale bar measurement by the provided scale, others inverted the division or displayed incomprehensible calculations; some calculations were difficult to follow, with no apparent methodology or structure; there were too many unit conversion errors, sometimes because candidates did not use units within their calculation (e.g. measuring in centimetres, but calculating as millimetres); a limited number measured the complete image width but didn't adjust the scale bar proportionally.

a.

Most predicted that the upper epidermis would have a lower density, although some stated the reverse.

b.

Syllabus sections

Core » Topic 1: Cell biology » 1.1 Introduction to cells
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