The big idea: Striated (skeletal) muscle looks striped under the microscope because it is built from millions of identical repeating units lined up end to end.
One of these repeating units is a sarcomere — the functional contractile unit of muscle.
A sarcomere is defined as the region between two adjacent Z-discs. When the muscle contracts, every sarcomere shortens a little, and together that shortens the whole fibre.
One sarcomere lies between two Z-discs. Thin ACTIN filaments are anchored to the Z-discs; thick MYOSIN filaments sit in the centre, held in register by the M-line. The light I band is actin only, the dark A band is the full length of myosin, and the H zone is the central myosin-only region.
Interactive diagram
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- Sarcomere
- The functional contractile unit of striated muscle — the region between two adjacent Z-discs.
- Z-disc
- The protein boundary at each end of a sarcomere; the actin filaments are anchored to it.
- Thin filament (actin)
- A thin protein filament anchored to the Z-discs and projecting inward toward the centre of the sarcomere.
- Thick filament (myosin)
- A thick protein filament that carries protruding heads (cross-bridges); it sits in the centre of the sarcomere.
- M-line
- The line down the middle of the sarcomere that links the myosin filaments and holds them in register.
- Titin
- A giant elastic protein that anchors myosin to the Z-disc, keeps it centred, and provides passive recoil (elasticity).
Thin = actin, thick = myosin: Thin filaments are actin — anchored to the Z-discs at the ends.
Thick filaments are myosin — in the centre, with heads that stick out.
Memory hook: myosin = the middle, massive (thick) filament with the moving heads.
A sarcomere is built from just a few proteins, each in a fixed place. Once you know where each one sits, the striped banding you see under the electron microscope (EM) follows automatically — the bands are simply named after which filaments lie there and whether they overlap.
What's inside one sarcomere
- Z-discs at each end form the boundary; actin (thin) filaments are anchored to them and reach inward.
- Myosin (thick) filaments sit in the centre, carrying protruding heads (cross-bridges) that point toward the actin.
- The M-line runs down the middle and links the myosin filaments, keeping them lined up.
- Titin, a giant elastic protein, runs from each Z-disc to the myosin, anchoring it and acting like a molecular spring.
| Component | What it is | Where it sits | Its job |
|---|---|---|---|
| Actin (thin filament) | A thin protein filament | Anchored to the Z-discs, projecting inward toward the centre | The filament the myosin heads pull on during contraction |
| Myosin (thick filament) | A thick protein filament with protruding HEADS (cross-bridges) | In the centre of the sarcomere, held in register by the M-line | Its heads grab and pull the actin to shorten the sarcomere |
| Titin | A giant ELASTIC protein (a molecular spring) | Runs from the Z-disc to the myosin filament | Anchors myosin to the Z-disc, keeps it centred, and provides recoil / elasticity |
| Z-disc | A protein plate / boundary line | At each END of the sarcomere | Anchors the actin filaments; marks where one sarcomere meets the next |
| M-line | A protein line down the middle | The centre of the sarcomere | Links the myosin filaments and holds them in register |
Titin — the molecular spring: Titin is the largest protein in the body, and it does three jobs at once:
Anchors each myosin filament to the Z-disc, so myosin can't drift out of place.
Centres the myosin in the middle of the sarcomere (keeps it in register with the M-line).
Provides recoil — like a stretched spring, titin pulls the sarcomere back to its resting length after a stretch and resists overstretching. This passive elasticity is why a relaxed muscle springs back rather than staying floppy.
Now the banding. The bands are just labels for regions of the sarcomere, based on what lies there:
Reading the bands (light vs dark)
- I band — actin only (no myosin), so it looks light. It spans a Z-disc.
- A band — the full length of the myosin filament. Because myosin is present (and overlaps actin at its edges), it looks dark. The A band is as wide as one whole myosin filament.
- H zone — the myosin-only region in the middle of the A band, where no actin reaches. It's a paler stripe inside the dark A band.
- M line — the centre line that links the myosin filaments (sits in the middle of the H zone).
- Z disc — the boundary of the sarcomere, in the middle of the I band, where actin is anchored.
Reading the bands: I band = actin only (light, across each Z-disc) · A band = the full myosin length (dark) · H zone = myosin only in the middle (no actin overlap) · M line = centre · Z-disc = sarcomere boundary.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
| Region | What lies there | Appearance under EM | Quick definition |
|---|---|---|---|
| I band | Actin only (no myosin) | Light | The light band; spans a Z-disc |
| A band | The full length of the myosin filament (incl. the actin–myosin overlap) | Dark | As wide as one whole myosin filament |
| H zone | Myosin only (the middle of the A band, no actin overlap) | Paler centre of the dark A band | Myosin with no actin |
| M line | The centre, linking the myosin filaments | A line in the middle of the H zone | Centre / register of the myosin |
| Z disc | Where actin is anchored | A dark line in the middle of the I band | The sarcomere boundary |
The logic in one line: Light = actin only (I band), dark = myosin present (A band), and the myosin-only middle = H zone.
Every band name is just a statement about which filament is there — that's all you need to remember to label a sarcomere diagram correctly.
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How this is tested: The classic task is Annotate / Label a sarcomere electron micrograph or diagram, and Outline the structure of a sarcomere.
Score points for: the sarcomere = between two Z-discs; actin (thin) anchored to the Z-discs; myosin (thick) in the centre with heads; the M-line; titin as the elastic anchor; and the bands — I (actin only, light), A (full myosin length, dark), H zone (myosin only).
A common trap is mixing up the A band (full myosin length) with the H zone (myosin only, the central part) — keep those distinct.
IB-style question — outline the structure of a sarcomere
Outline the structure of a sarcomere, including the filaments present and the banding seen under the electron microscope. [6]
How to score all six marks
- Define the unit. A sarcomere is the functional contractile unit, lying between two adjacent Z-discs.
- Thin filaments. Actin (thin) filaments are anchored to the Z-discs and project inward.
- Thick filaments + M-line. Myosin (thick) filaments, carrying protruding heads, sit in the centre, held in register by the M-line.
- Titin. The elastic protein titin anchors myosin to the Z-disc and provides recoil / elasticity.
- Light vs dark bands. The I band is actin only (light); the A band is the full length of the myosin filament (dark).
- H zone. The H zone is the myosin-only region in the middle of the A band (no actin overlap). (Award 1 mark per distinct point, up to 6.)
Final answer
A sarcomere is the contractile unit between two Z-discs. Actin (thin) is anchored to the Z-discs; myosin (thick), with protruding heads, sits in the centre on the M-line; titin (elastic) anchors myosin to the Z-disc and gives recoil. The I band is actin only (light), the A band is the full myosin length (dark), and the H zone is the central myosin-only region.
✓ Why this scores full marks: It defines the sarcomere by its Z-disc boundaries, places each named protein (actin, myosin + heads, M-line, titin) correctly, and then maps the three bands to which filament lies there.
A weak answer just lists 'actin and myosin' without saying where they are anchored or what the bands mean — and loses the labelling marks.
| Region | What lies there | Appearance under EM | Quick definition |
|---|---|---|---|
| I band | Actin only (no myosin) | Light | The light band; spans a Z-disc |
| A band | The full length of the myosin filament (incl. the actin–myosin overlap) | Dark | As wide as one whole myosin filament |
| H zone | Myosin only (the middle of the A band, no actin overlap) | Paler centre of the dark A band | Myosin with no actin |
| M line | The centre, linking the myosin filaments | A line in the middle of the H zone | Centre / register of the myosin |
| Z disc | Where actin is anchored | A dark line in the middle of the I band | The sarcomere boundary |