A-LevelBiologyTransport in plants9700/22 · May/June 2019 · Q18 Marks

Fig. 1.1 is a photomicrograph of a low power image of part of the common sunflower, Helianthus annuus. Fig. 1.1 is a transverse section. [Figure 1.1]

(a)State, with a reason, whether Fig. 1.1 shows a section through the root or the stem of H. annuus.[1]
(b)Cell type A in Fig. 1.1 has a large central vacuole. Suggest, with reasons, the role of the tissue formed by this type of cell.[2]
(c)(i)Name structure B.[1]
(c)(ii)The actual length of structure B is 650 μm. State the actual length of structure B in mm.[1]
(d)When structure B is observed at a higher magnification, more detail of xylem vessel elements and phloem sieve tube elements can be seen. Outline the differences in the structure of a xylem vessel element and a phloem sieve tube element.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] stem / not a root, because, transport tissue / vascular bundles / vascular tissue(s) / xylem and phloem / xylem / phloem, around the edges / not in the centre / AW ;
A stem because, pith / parenchyma tissue, is in the centre
[2] any two from:
(hydrostatic) support / described ; R if incorrect context e.g. support because of thick cell walls A packing tissue
(cells), turgid / store water ;
storage; in context of substances other than water e.g. sucrose /starch/waste
[1] vascular bundle ;
[1] 0.65 (mm) ;
[3] mp1-3 must have statements for both xylem and phloem
mp4-7 need statements for either xylem or phloem
three from:
1 xylem: no cytoplasm / hollow / no contents; phloem: (peripheral / little / some) cytoplasm ; I protoplasm R full of cytoplasm / AW
2 xylem: no organelles / hollow / no contents; phloem: (a few) organelles ; A examples of organelles mitochondria or ER I has SER/RER/ribosomes R has nucleus
3 xylem: lignified R idea of lignin within element; phloem: no lignin / (only) cellulose ;
4 xylem: no end wall(s) / no sieve plate(s) A end walls have broken down; or phloem: sieve plate(s) / perforated end wall(s) ; A (end walls) have sieve pores
5 xylem: (side walls) contain pits; or phloem: no pits (in side walls);
6 xylem: no plasmodesmata; or phloem: plasmodesmata (to companion cells);
7 xylem: thick(er), cell wall / walled; or phloem: thin(ner), cell wall / walled;

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