Describe the ultrastructure of a striated muscle fibre.

Mark scheme

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

1. [1] fibres are multinucleate;
2. [1] cell surface membrane is sarcolemma ;
3. [1] sarcoplasm has many mitochondria ;
4. [1] sarcoplasmic reticulum membranes have protein pumps ;
5. [1] transverse system tubules / T-system ;
6. [1] ref. to myofibrils ;
7. [1] thick filament/myosin, attached to M line ;
8. [1] thin filament/actin, attached to Z line ;
9. [1] interdigitation of filaments causes striated appearance ;
10. [1] description of one of, A/H/I, bands;
11. [1] sarcomere is the distance between M lines;
12. [1] myosin is a fibrous protein with globular protein head ;
13. [1] actin is a chain of globular protein molecules ;
14. [1] tropomyosin / troponin, attached to actin ;
15. [1] allow marks from an annotated diagram
16. [1] acid-growth (hypothesis);
17. [1] auxin stimulates proton pumps ;
18. [1] (in) cell surface membrane ;
19. [1] protons / H+, pumped into cell wall ;
20. [1] using energy / by active transport;
21. [1] pH of cell wall decreases / cell wall becomes (more) acidic ;
22. [1] pH-dependent enzymes activated;
23. [1] ref. to expansins;
24. [1] bonds between cellulose microfibrils broken ;
25. [1] idea that cell wall, 'loosens' / becomes more elastic / able to stretch;
26. [1] (more) water enters cell / turgor pressure increases ;
27. [1] (so) cell (wall) expands ;