A-LevelBiologyGas exchange9700/22 · May/June 2022 · Q56 Marks

Fig. 5.1 is a photomicrograph of a transverse section through part of the bronchus of the human gas exchange system. The image is of a good resolution for a light microscope. The bronchial epithelium is a single layer of cells lining the lumen of the bronchus. [Figure 5.1]

(a)The luminal surface shown in Fig. 5.1 is not clearly defined and appears slightly blurred. State why the luminal surface of the bronchial epithelium appears slightly blurred, even though the resolution of the image is good.[1]
(b)Some cells of the bronchial epithelium shown in Fig. 5.1 appear darker than others. For example, cell D appears darker than cell L. With reference to the bronchial epithelium shown in Fig. 5.1, explain why some cells, such as cell D, appear darker and other cells, such as cell L, appear lighter.[2]
(c)In Fig. 5.1 the tissue in box B is cartilage. The tissue shown in box A is different from the tissue in box B. Outline the differences in the structure and function of tissue A compared with tissue B.[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 (May/June 2022). The mark or mark type is shown in brackets.

[1] any one from: I too small presence of cilia; A ciliated, cells / epithelial cells / epithelium
[1] AVP; e.g. suggestion that mucus on surface makes detail not easily seen
[2] epithelium has goblet cells and ciliated epithelial cells ; A ciliated cells A two different cell types present I ref. to cell division
[2] ref. to staining; e.g. use of stains gives different, shades / colours differential staining one cell type more neutral / acidic different parts of cells stain differently
[2] idea of cells have different densities; e.g. idea of more structures / larger nucleus, gives greater density / AW R no cell structures for lighter cell I ref. to number of cilia (sectioning may include) more of nucleus so, affects light penetration / makes cell darker ora
[2] AVP; idea of different components in cells appearing, darker/lighter
[3] any three from: A | B / cartilage max two for structure smooth muscle / composed of smooth muscle cells or no chondrocytes vs no smooth muscle cells or has chondrocytes ;
[3] cells close together vs cells, (more) isolated / in gaps / in lacunae ;
[3] cells, spindle shaped / elongated / AW vs cells (more) rounded / AW ;
[3] AVP structure e.g. not surrounded by, perichondrium / AW contains blood vessels vs surrounded by, perichondrium / AW no blood vessels ;
[3] function contractile / contracts (and relaxes) I recoil vs non-contractile; A bronchi walls contract R bronchi contract
[3] changes diameter of airways A controls size of lumen vs protective / support; A described e.g. prevents collapse of, bronchus or keeps bronchus open if incorrectly quotes A as the cartilage, use right hand column to allow one mark max (ecf)