The structure of skeletal muscle, the sliding filament theory of contraction, the role of ATP and calcium, and the role of the skeleton and joints.

What this dot point is asking

WJEC wants you to describe the structure of skeletal muscle and the sarcomere, explain the sliding filament theory of contraction and the roles of ATP and calcium ions, and describe how the skeleton and joints produce movement.

Muscle structure

The sliding filament theory

A nerve impulse releases calcium ions from the sarcoplasmic reticulum into the sarcoplasm. Calcium binds troponin, which moves tropomyosin aside to expose the myosin binding sites on actin. Myosin heads bind to form cross-bridges and flex (the power stroke), pulling the actin inward; ATP then binds each myosin head to detach it, and its hydrolysis re-cocks the head ready for the next cycle. The cycle repeats many times per second as long as calcium and ATP are present. When stimulation stops, calcium is pumped back, tropomyosin re-covers the sites, and the muscle relaxes.

The skeleton and joints

The skeleton supports and protects the body and gives anchorage for muscles, with the rigid bones acting as levers. Muscles work in antagonistic pairs across a joint: as one (the flexor, such as the biceps) contracts and bends the joint, the other (the extensor, such as the triceps) relaxes, and to straighten the joint they swap roles. Tendons attach muscle to bone and ligaments join bone to bone, holding the joint together while allowing movement.

Examples in context

Example 1. Rigor mortis. After death, respiration stops so no ATP is made. Without ATP the myosin heads cannot detach from actin, so the cross-bridges lock and the muscles stiffen. This grim observation is direct evidence that ATP is required for muscle relaxation, not just contraction.

Example 2. The biceps and triceps at the elbow. To bend the arm the biceps contracts while the triceps relaxes; to straighten it the triceps contracts while the biceps relaxes. This everyday antagonistic pair is the standard WJEC example of how muscles, working only by pulling, can move a joint in both directions.

Try this

Q1. Name the protein on actin that calcium ions bind to during contraction. [1 mark]

• Cue. Troponin.

Q2. Explain why muscles must work in antagonistic pairs. [2 marks]

• Cue. Muscles can only pull (contract), not push, so one muscle moves the joint one way and its partner moves it back.

Q3. A sarcomere shortens from $2.5$ to $2.0 \text{ micrometres}$. Calculate the percentage shortening. [2 marks]

• Cue. $\frac{2.5 - 2.0}{2.5} \times 100 = \frac{0.5}{2.5} \times 100 = 20\%$.