Muscles and how they contract
Answers are for parents and teachers.
1
A muscle fibre holds 1200 myofibrils. How many myofibrils are in a bundle of 15 such fibres?
15 × 1200 = 18 000 myofibrils.
2
A relaxed sarcomere is 2.4 µm long and shortens to 1.8 µm. By what percentage of its length does it shorten?
2.4 − 1.8 = 0.6 µm; 0.6 ÷ 2.4 = 0.25, so 25 %.
3
Explain why a muscle that runs short of ATP cannot relax.
ATP is needed to detach the cross bridges from the actin filaments. Without it the bridges stay attached and the muscle stays contracted.
4
Put the steps in order: acetylcholine is released; cholinesterase breaks it down; the impulse reaches the nerve ending; sodium ions enter the fibre; the sarcomere shortens.
The impulse reaches the nerve ending; acetylcholine is released; sodium ions enter the fibre; the sarcomere shortens; cholinesterase breaks acetylcholine down.
5
During contraction, does the length of the A-band (the thick myosin filaments) change? Explain with the sliding-filament idea.
No. The thick filaments keep their length. The thin actin filaments slide in between them and the Z-lines come closer, so the sarcomere shortens.