Striated muscle and the sliding filament model
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 3 practice questions, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Filaments that slide
- Muscle structure
- The sarcomere
- A past-paper example
Read the transcript of the teaching part
Last time
Before we start: last time, in Receptors, neurones and the Venus fly trap, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Filaments that slide
When you bend your arm, no filament shortens. The thin and thick filaments inside your muscle slide past each other, pulling the ends closer together. Calcium, a protein switch and a molecular motor drive the whole thing, and the structure of the sarcomere shows you exactly what is happening.
Muscle structure
Striated muscle is made of long cells called muscle fibres. Each fibre is multinucleate, because many cells fused during development. A fibre contains many myofibrils. The cell membrane is the sarcolemma, and the specialised endoplasmic reticulum is the sarcoplasmic reticulum, which stores calcium ions. Many mitochondria lie close to the myofibrils to supply A T P.
The sarcomere
A myofibril is divided into repeating units called sarcomeres. The ends of a sarcomere are the Z lines. Thin actin filaments are attached to the Z lines. Thick myosin filaments lie in the centre. The dark A band is the length of the myosin filaments, and the light I band contains only actin. The H zone, in the middle of the A band, contains only myosin, and the M line holds the myosin filaments together.
A past-paper example
February March 2023, Paper 42, question 9. A diagram shows a relaxed sarcomere. State what happens to the A band and the I band when the sarcomere contracts. The A band stays the same length, because the myosin filaments do not change length. The I band gets shorter, as actin slides in between the myosin. The H zone also gets shorter, and the sarcomere as a whole is shorter.
Chapter 15: Coordination
- Stomata, auxin and gibberellin 6 min
- Receptors, neurones and the Venus fly trap 6 min
- Resting potential, action potentials and myelin 6 min
- Cholinergic synapses, drugs and toxins 6 min
- The endocrine system compared with the nervous system 6 min
- Striated muscle and the sliding filament model 5 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.