A molecule of collagen consists of three identical polypeptides that form a triple helix. The amino acid glycine forms one third of the amino acids in a collagen molecule. Fig. 5.1A shows a polypeptide molecule during protein synthesis. A molecule of glycine is shown just before it is added to the polypeptide. [Figure 5.1]

Mark scheme

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

1. [2] I presence or absence of water
2. diagram shows bond between C (of lysine) and N (of glycine) and double bond to O shown correctly;
3. glycine drawn correctly;
4. R if R is used instead of H on glycine
5. [1] condensation; A dehydration (reaction)
6. [2] I high tensile strength
7. any two from: allows close packing of, triple helix / three polypeptides ; A chains I tight coiling A 'binds more tightly'
8. glycine, has smallest R group / R group of glycine is H / is smallest amino acid ; A glycine is small
9. glycine, is every third amino acid in the, polypeptide / chain ;
10. [4] I collagen is insoluble / collagen is strong
11. any four from: collagen, has high tensile strength / does not stretch / AW ;
12. 1 A withstands large pulling forces
13. 2 any ref. to use of collagen in the body ; e.g. (walls of) arteries or veins / tendons / cartilage / skin / basement membranes / around alveoli / bones / teeth I hair / R if any ref. to elastic(icity)
14. three from 3 triple helix has many hydrogen bonds between, polypeptides / chains ;
15. mp4, 5, 6 and 7 – must be clear that answers are about molecules, but accept triple helix for molecule
16. 4 collagen molecules form, fibrils / fibres ;
17. 5 strong / covalent, (cross) links between molecules ;
18. 6 (ends of) molecules (in fibril / fibre) are staggered ;
19. 7 AVP ; e.g. (many) collagen molecules lie parallel
20. [5] induced fit
21. 1 active site changes shape; A active site moulds round substrate A 'active site changes to fit the substrate (more closely)'
22. 2 (so) active site becomes (fully) complementary shape to collagen ;
23. any four from
24. 3 formation of, enzyme-substrate complex / ESC;
25. 4 lowering of activation energy ;
26. 5 breakage of peptide bond ;
27. 6 active site returns to, pre-ESC / original, shape and can be reused;
28. 7 AVP ; e.g. ref. to, binding site / catalytic site (of active site) e.g. suggestion of how activation energy lowered, e.g. strain put on bond / alternative pathway
29. [4] description
30. 1 general description of effect of pH on activity ; e.g. as pH increases the activity increases and then decreases e.g. as pH increases activity reaches a peak e.g. use of data with unit for relative activity to describe increase and decrease (see next page)
31. 2 optimum pH is 7 ;
32. explanation any three from:
33. 3 partial denaturation in, acid and alkaline conditions / AW ; A 'starts to denature' as alternative to partial denaturation small changes either side of optimum
34. 4 (ionisable) R-groups in, active / catalytic, site affected ; large changes either side of optimum
35. 5 hydrogen bonds / ionic bonds, break / disrupted;
36. 6 active site (shape) / (enzyme) tertiary structure, changes, so substrate / collagen, no longer fits into active site / fewer ESC complexes formed; A fits less well in context of partial denaturation
37. 7 AVP ; e.g. detail of R groups