(a) Describe how tropomyosin and myosin are each involved in the sliding filament model of muscle contraction.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. max 2 of:
2. 1 tropomyosin / it, covers / uncovers, myosin binding sites on actin ; R inhibits R active site
3. 2 when calcium ions bind to troponin, tropomyosin / it, moves / changes shape;
4. 3 allows myosin to, bind to actin / form cross-bridges ; ora
5. max 4 of:
6. 1 ATP hydrolysis / ATP → ADP + Pi ;
7. 2 (causes myosin) head to, pivot / rotate / tilt / stand up ;
8. 3 myosin / head, binds to actin / forms cross-bridges with actin; R active site
9. 4 ADP and Pi detach;
10. 5 (myosin) head, swings back / returns to previous position ;
11. 6 actin is moved / power stroke occurs ;
12. 7 (new) ATP binds ;
13. 8 myosin / head, detaches from actin / cross-bridges break;
14. A mps in any order apart from 1, 4 and 7 which must be linked to correct action
15. max 2 of:
16. 1 to, supply / provide, (enough / plenty of) glucose ;
17. 2 for glycolysis ;
18. 3 as little ATP is produced by anaerobic respiration;
19. 4 as few capillaries are present (to supply glucose directly) ;
20. max 2 of:
21. 1 to, supply / provide, (enough / plenty of) oxygen ;
22. 2 aerobic respiration / oxidative phosphorylation;
23. 3 to remove, carbon dioxide / lactate; A lactic acid
24. 4 to, avoid fatigue or promote, stamina / endurance (for exercise / work);