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Date	November 2019	Marks available	3	Reference code	19N.2.HL.TZ0.5
Level	Higher level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Explain	Question number	5	Adapted from	N/A

Question

This light micrograph shows skeletal muscle.

[Source: adapted from https://en.wikipedia.org/wiki/Skeletal_muscle#/media/File:Skeletal_
muscle_%E6%A8%AA%E7%BA%B9%E8%82%8C1.JPG, Urana/ 乌拉跨氪]

Identify the dark structure indicated by I.

[1]

a.i.

Identify the protein producing the thick filament in the dark band indicated by II.

[1]

a.ii.

Identify the structure indicated by III.

[1]

a.iii.

Discuss whether the tissue shown in the micrograph consists of cells or not.

[2]

Explain how calcium is involved in muscle contraction.

[3]

Markscheme

nucleus ✔

a.i.

myosin ✔

a.ii.

muscle fibre/muscle cell ✔

Reject myofibril because it would be much narrower – diameter

a.iii.

a. «muscle fibres are» multinucleate/contain many nuclei «whereas cells are expected to have only one/so muscle fibers are an exception to the cell theory» ✔

b. one cell membrane/sarcolemma enclosing a whole muscle fibre «as expected for cells» ✔

c. very large/much larger/longer/than most cells ✔

d. muscle fibres formed by fusion of cells/are syncytia ✔

a. action potential/nerve impulse causes release of calcium ✔

b. from sarcoplasmic reticulum/specialized endoplasmic reticulum ✔

c. binds to troponin ✔

d. causes tropomyosin to move/be removed «from binding sites» ✔

e. exposes myosin-binding sites on actin/allows myosin «heads» to bind to actin ✔