A-LevelBiologyGas exchange9700/21 · Oct/Nov 2020 · Q513 Marks

(a) Fig. 5.1 is a photomicrograph of a section through the lungs showing a bronchus and some alveoli. [Figure 5.1]

(a)(i)State the function of tissue X labelled in Fig. 5.1.[1]
(a)(ii)Describe how the distribution of tissue X in the trachea differs from that shown in Fig. 5.1.[1]
(a)(iii)Describe how the epithelial tissue, Y, is adapted for its function.[2]
(b)(i)Fig. 5.2 is a photograph of two African elephants, Loxodonta africana. [Figure 5.2] Describe the difference in surface area to volume ratio between the adult elephant and baby elephant shown in Fig. 5.2.[1]
(b)(ii)Suggest why animals such as elephants require a gas exchange system.[3]
(b)(iii)The feet of elephants are protected by structures under the skin known as cushions. The cushions are made up of a large number of cells surrounded by connective tissue containing many fibres of collagen. The fibres help to maintain the structure of the cushion. The collagen fibres are made of collagen molecules. Describe the structure of a collagen molecule.[4]
(b)(iv)The cushion in the foot is very strong and is able to resist extremely large forces acting on it due to the large mass of the elephant. Suggest how the structure of a collagen fibre can help the cushion resist these large forces.[1]

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 (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] any one from : (when the air pressure lowers inside bronchus) holds, bronchus / airway open; A supports, bronchus / airway; prevents, bronchus / airway, from collapsing;
[1] idea that (cartilage arranged as) incomplete / C-shaped, rings in the trachea ; idea of more regular distribution (of incomplete rings) along the length of trachea;
[2] max 1 if cilia or mucus linked to incorrect cell (ciliated epithelial cells have) cilia, to, waft / sweep / AW, mucus, away / up the trachea / AW ; (goblet cells that) secrete mucus to trap, dust / bacteria / pathogens;
[1] the larger elephant has a smaller surface area to volume ratio ; ora
[3] any three from: are multicellular / have many cells ; need a large quantity of / enough, oxygen to supply, every cell / all parts of body ; active so high demand for, oxygen / removal of CO2 ; diffusion across body surface is not fast enough (to meet needs) ; AW long diffusion distance (so diffusion is not fast enough to meet needs);
[4] any four from: three polypeptide chains ; (three polypeptides) form triple helix; A coil around each other R ref. to alpha helix hydrogen bonds hold the three. polypeptides / strands. together; the three polypeptides, lie close together / form a tight coil / are tightly wound ; every third amino acid (in the polypeptide chain) is glycine / many glycine-proline alanine repeats / many gly-pro-ala repeats; glycine is found on the inside of each, polypeptide / strand; AVP; e.g. detail hydrogen bonds, between peptide bond NH (of a glycine) and C=O group in adjacent polypeptide
[1] allow reference to collagen fibrils instead of collagen molecules, in correct context (collagen molecules form fibrils, which form fibres) any one from: (many) collagen molecules lie parallel ; no weak points because strong / covalent, (cross) links between molecules ; (ends of) molecules (in, fibril / fibre), are staggered ;