Collagen is a major component of the cartilage found in some of the structures of the human gas exchange system. Cells that synthesise and secrete the components of cartilage are known as chondrocytes. Fig. 4.1 is a transmission electron micrograph of a chondrocyte. Fig. 4.2 shows part of the primary structure of a collagen polypeptide. Fig. 4.3 shows the molecular structure of the amino acid glycine (gly).

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [2] much / AW, rough endoplasmic reticulum / rough ER/RER for, polypeptide / protein / collagen, synthesis; A for translation many mitochondria provide, energy / ATP, for, polypeptide / protein / collagen, synthesis ; large nucleus indicates, active cell / (much) transcription ;
2. [2] any two from in trachea; in, bronchus / bronchi; if only one correct structure named, allow one qualification mark trachea C-shaped / incomplete, rings ; surround smooth muscle ; bronchus plates / irregular ;
3. [2] any two from keep airways open ; provides support ; allow flexibility; allow described e.g. bending neck, swallowing food rings allow, lengthening / widening, during, breathing in / inspiration / inhalation ;
4. [1] peptide (bond);
5. [3] any three from collagen, structural / fibrous, protein or collagen gives strength / flexibility; glycine / gly, small / smallest, amino acid; A has H as, R-group / side chain glycine / gly, regular / every third amino acid ; (so) triple helix tightly packed / three polypeptides closely associated / AW ; ref. to (peptide bond ) NH of gly can form hydrogen bond with (peptide bond) C=O of adjacent amino acid (in other polypeptide);