A student measures four sarcomeres of the same model muscle…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student measures four sarcomeres of the same model muscle. Each has an A band of u. The I band and the H zone are in u.
| Sarcomere | I band | H zone |
|---|---|---|
| 1 | 6 | 2 |
| 2 | 9 | 5 |
| 3 | 4 | 0 |
| 4 | 2 | 0 |
- Find the length of a thin filament in sarcomeres 1 and 2, as (sarcomere H zone) .
- In sarcomere 4 the same formula gives a different answer. What is it?
- Why is that answer wrong?
Need a reminder? The key ideaShow
The sarcomere lies between two Z lines. The A band is as long as the thick filaments. The I band holds only thin filaments. The H zone is the part of the A band that the thin filaments do not reach. In the model, the thick filament is u and each thin filament is u.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. A band u in every sarcomere.
Plan. Find each sarcomere, apply the formula, and then think about the case with no H zone.
Step 1. Sarcomere 1. u. Thin filament: u.
Step 2. Sarcomere 2. u. Thin filament: u.
Step 3. Sarcomere 4. u, and the formula gives u.
Step 4. Why it is wrong. With no H zone, the two thin filaments meet and overlap in the middle. Together they are longer than the sarcomere. Here they overlap by u. The formula assumes that they do not overlap.
Answer: u; u; u, which is wrong because the thin filaments overlap in the middle.
Check. Sarcomere 3 is exactly u, so the thin filaments just meet. The formula gives u there, which is right.
What if? The thin filament has the same length of u in all four sarcomeres. The filaments do not shorten when the muscle contracts. They slide.
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