A-LevelBiologyGas exchange9700/21 · May/June 2024 · Q412 Marks

Fig. 4.1 is a scanning electron micrograph showing the tissue that lines the bronchi in the gas exchange system. Fig. 4.2 is a transmission electron micrograph of a horizontal section made at the position indicated by the two arrows in Fig. 4.1. [Figure 4.1] [Figure 4.2]

(a)(i)Name the cells labelled A and B in Fig. 4.1. A B[2]
(a)(ii)Describe how the tissue shown in Fig. 4.1 is adapted to its function in the gas exchange system.[3]
(b)(i)The structures labelled X in Fig. 4.1 have a characteristic internal appearance, as seen in Fig. 4.2. Describe the internal appearance of the structures labelled X.[2]
(b)(ii)Explain how Fig. 4.2 shows that each of the structures labelled X are intracellular.[1]
(c)Stem cells are found in the lining of the bronchi. Describe the function of centrioles and explain how they are involved in the cell cycle of a stem cell.[4]

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 (May/June 2024). The mark or mark type is shown in brackets.

[2] A - ciliated epithelial (cell); R ciliated epithelium
[2] B - goblet (cell);
[3] 1 goblet cells / B, secrete / produce / release, mucus / mucin ;
[3] 2 mucus, covers / AW, the (ciliated) epithelium to trap (named) particles ; or idea that mucus prevents (named) pathogens reaching, epithelial cells / alveoli / gas exchange surface; I lungs A acts as a barrier to (named) pathogens
[3] 3 cilia / ciliated epithelial cells, move mucus (and trapped material), upwards / towards mouth / towards throat or pharynx / away from alveoli / away from gas exchange surface / away from lungs / out of the airways ; I 'out of the lungs' / 'to be swallowed' unqualified / out of the respiratory system / out of the gas exchange system
[2] 9+2, pattern / arrangement / structure ;
[2] composed of microtubules ;
[2] AVP; e.g. ref. to dynein 'arms' / AW
[2] outer 9 are, pairs of microtubules / doublets, and central two are single
[1] idea that each cilium / structure X, is surrounded by cell (surface) membrane (so contents are inside the, cell / cytoplasm);
[4] must have either mp1 or mp2 or both to gain max 4, otherwise max 3
[4] 1 (centrioles) make / organise, microtubules; A microtubular organising centre / MTOC
[4] 2 to form, the spindle / spindle fibres ;
[4] 3 (during each cell cycle) each centriole, replicates / duplicates / forms two centrioles ;
[4] 4 during, S phase / G2 phase ; A before mitosis mp4 is linked to mp3
[4] 5 centriole pairs / centrioles, move to the (opposite) poles (of cell during prophase of mitosis); R if incorrect phase R 'poles of nucleus'
[4] 6 centrioles, lengthen / shorten, the spindle fibres / microtubules ; I ‘contract'