A-LevelBiologyCell structure9700/33 · Feb/Mar 2018 · Q219 Marks
P1 is a slide of a stained transverse section through a plant leaf. You are not expected to be familiar with this specimen. You are required to: • use the eyepiece graticule to measure the depth of the leaf and the depth of a vascular bundle at the mid-rib • use these measurements to draw a plan diagram of part of the leaf. The eyepiece graticule in the microscope can be used to measure different tissues. Select the widest part of the leaf (mid-rib) on P1, shown by Y in Fig. 2.1. [Figure 2.1]
- (a)(i)Use the eyepiece graticule in the microscope to measure: • the depth of the leaf at the mid-rib • the depth of the vascular bundle at the mid-rib. depth of leaf eyepiece graticule units depth of vascular bundle eyepiece graticule units[1]
- (a)(ii)Use the measurements from (a)(i) to help you to draw a large plan diagram of the section of the leaf shown by the shaded area in Fig. 2.2. Use one ruled label line and label to identify the vascular bundle. [Figure 2.2] You are expected to draw the correct shape and proportions of the different tissues.[5]
- (a)(iii)Observe the upper epidermis of the leaf and the layer of cells below the upper epidermis. Fig. 2.2 identifies the upper and lower surfaces of the leaf. Select three epidermal cells and one cell in the layer below that touches these cells. Make a large drawing of this group of four cells. Each cell must touch at least one of the other cells. Do not draw the cuticle. Use one ruled label line and label to identify the cell wall of one cell.[5]
- (b)(i)Determine the simplest whole number ratio of the total length of L1 and L2 (the leaf tissue) to the length of L3 (the air space). Show all the steps in your working. simplest whole number ratio[4]
- (b)(ii)Observe the leaf on slide P1 and the leaf shown in Fig. 2.3. Identify one feature to reduce water loss that can be observed for both leaves. Describe how this feature reduces water loss. feature description[1]
- (b)(iii)Observe the leaf on P1 and the leaf in Fig. 2.3 and identify differences between them. Record the observable differences in Table 2.1. [Table 2.1][3]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.
- [1] depth given to whole or half numbers (eyepiece graticule units) + vascular bundle in correct proportion to depth of leaf ;
- [1] drawing at the appropriate size + no shading + no cells ;
- [2] only area shaded in Fig. 2.2 drawn ;
- [3] correct position of vascular bundle relative to whole depth of leaf;
- [4] draws at least two layers of tissue in vascular bundle;
- [5] label line and label to vascular bundle ;
- [1] lines should be continuous, thin and sharp + drawn to occupy most of the space provided;
- [2] draws only four cells + each cell touching at least one of the other cells ;
- [3] two lines drawn around each cell + three lines where cells touch ;
- [4] cell in epidermal layer in correct proportion to the cell beneath ;
- [5] label line and label to cell wall;
- [1] L1, L2 and L3 measured using same units ;
- [2] correct values for L1, L2 and L3;
- [3] simplest ratio shown as larger number to smaller number ;
- [4] answer to lowest common denominator;
- [1] any one feature to reduce water loss + description ; e.g. trichomes + trap layer of moist air leaves folded + stops water diffusing away
- [1] any three observable differences between P1 and Fig. 2.3:
- feature: folding, P1: folds at ends, Fig. 2.3: folded up completely ;
- feature: relative size of xylem vessel elements, P1: smaller, Fig. 2.3: larger ;
- feature: quantity of trichomes, P1: lots / more, Fig. 2.3: fewer ;
- AVP
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