Identify sarcomere ultrastructure
Use diagrams and micrographs to identify Z-discs, actin, myosin and the A band, I band and H zone.
Section 15.1 · Control and coordination in mammals
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which structures mark the ends of one sarcomere?
What you will learn
- Identify sarcomere structures in diagrams and electron micrographs
- Relate band positions to thick and thin filament arrangement
Sarcomere landmarks
| Feature | Meaning |
|---|---|
| Z-disc to Z-disc | One sarcomere |
| A band | Length of thick myosin filaments; remains the same during contraction |
| I band and H zone | Regions with less overlap; become narrower during contraction |
| Filaments | Actin and myosin slide; neither filament shortens |
Build the mechanism
A sarcomere is the region between adjacent Z-discs. Thin actin filaments attach to Z-discs and extend towards the centre. Thick myosin filaments lie centrally; their length corresponds to the A band. The I band contains thin filaments without thick-filament overlap, and the H zone contains thick filaments without thin-filament overlap. During contraction, overlap changes, while the A-band width remains constant.
Keep the sequence precise
Electron micrographs show cross-sections or longitudinal views at different positions. Use filament arrangement and landmarks rather than colour. The I band and H zone are regions, not separate filaments.
Pause and explain
During contraction the Z-discs move closer, but the A band retains the same width. Explain this observation.
Worked example
Which band corresponds to the length of the thick myosin filaments?
Show the worked solution
- Locate the thick filaments in the central region.
- The A band spans their length.
Answer The A band corresponds to the length of the thick myosin filaments.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe where actin and myosin lie in a sarcomere.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Actin attaches to Z-discs and extends inward.
- Myosin lies centrally and overlaps actin.
Thin actin filaments attach to the Z-discs and extend toward the centre, where they overlap with centrally located thick myosin filaments.
Look for these in your work
- Mechanism stated
- Steps linked in order
A micrograph shows a central region containing thick filaments but no thin-filament overlap. Identify the region.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- The region is within the A band.
- The thick-filament-only central part is the H zone.
It is the H zone, the central region of the A band where thick myosin filaments are not overlapped by thin actin.
Look for these in your work
- Evidence interpreted
- Biological direction explained
During contraction the Z-discs move closer, but the A band retains the same width. Explain this observation.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- The thick myosin filaments keep their length.
- Sliding changes overlap rather than filament length.
The A band represents the length of thick myosin filaments, which do not shorten. Contraction occurs as actin slides past myosin, increasing overlap and bringing Z-discs closer.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Saying the A band shortens when the sarcomere contracts
- Calling the I band a type of filament
What can you explain now?
Which structures mark the ends of one sarcomere?
Compare with the explanation
Adjacent Z-discs
A sarcomere extends from one Z-disc to the next.
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Make it stick
Which sarcomere bands change width during contraction, and which stays the same?
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