A-LevelBiologyBiological molecules9700/23 · Oct/Nov 2025 · Q214 Marks

Collagen is a fibrous protein found in many tissues in animals. Fig. 2.1 shows the composition of a collagen fibre.

(a)(i)Describe the arrangement of the 3 polypeptides in each molecule of collagen.[2]
(a)(ii)With reference to Fig. 2.1, explain how the molecules of collagen are arranged and held together in a collagen fibril.[3]
(b)(i)Suggest two properties of collagen that contribute to the function of cartilage in the trachea in the gas exchange system.[2]
(b)(ii)State the function of elastic fibres in the alveoli in the lungs.[1]
(c)(i)Complete Table 2.1 to show the amino acids coded by the DNA nucleotide sequence in Table 2.1.[1]
(c)(ii)The sequence of amino acids that you have worked out is representative of the whole of the collagen polypeptide. Explain how the sequence of amino acids makes the polypeptide suitable as a component of a collagen molecule.[3]
(c)(iii)Two mutations, P and Q, can have an effect on the primary structure of the polypeptide. • Mutation P is a deletion of the first nucleotide pair in the DNA nucleotide sequence shown in Table 2.1. • Mutation Q is a substitution of G with T as the first base in the DNA nucleotide sequence shown in Table 2.1. State the effects of the mutations, P and Q, on the primary structure of the polypeptide. mutation P mutation Q[2]

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 2025). The mark or mark type is shown in brackets.

[1] any two from
[1] each polypeptide is a (left-handed), helix / helical shape ; R alpha helix
[1] (three) polypeptides are tightly, coiled / wound (around each other) / AW ;
[1] (three polypeptides form a) triple helix ;
[1] I bonding
[1] any three from
[1] collagen molecules arranged in parallel;
[1] staggered / ends are not aligned / AW ;
[1] molecules held together by – max 2
[1] covalent, bonds / links ; I hydrogen bonds R if part of a list that includes other bonds in proteins
[1] between R-groups of amino acids on different molecules; A side chains
[1] AVP; e.g. covalent bonds between lysine / hydroxylysine (residues)
[1] provides (high tensile) strength to, help keep trachea open / support trachea; A provides strength to prevent trachea collapsing
[1] allows flexibility for, breathing / change of diameter of trachea / bending of trachea/AW ;
[1] any one from:
[1] (inhalation) allow, stretching / expansion, without, rupture / AW ; A prevent overstretching
[1] elastic fibres recoil to help, expel air / exhalation / expiration; R to empty
[1] (gly)-pro-met-gly-pro-arg-gly-pro-pro-gly ;
[1] any three from:
[1] glycine is every third amino acid; allow gly for glycine
[1] glycine, allows tight folding for the polypeptides / makes it easy for polypeptides to fit closely together / AW ; A compactness A glycine allows the turning points (of the polypeptide)
[1] glycine has smallest R-group ; A glycine is the smallest (amino acid) / glycine has H as R-group or side chain
[1] NH in glycine can form hydrogen bonds (with carboxyl group on another amino acid residue)
[1] (so) allows many hydrogen bonds between polypeptides ;
[1] AVP; ala/pro, also has a small R-group / AW
[1] P deletion of first nucleotide pair changes the, sequence / order, of amino acids (in primary structure) ; I shorter
[1] A (GTC CAA TGG GTC CCC GAG GTC CCC CAG GT_) val gln trp val pro glu val pro gln (val)
[1] Q substitution in first triplet (GGT to TGT in non-transcribed strand) cys (instead of gly) is first amino acid, and no change to the rest (of, sequence / order, of amino acids) ; A only first amino acid is different