How this is tested: The classic task is to Annotate / Label a sarcomere electron micrograph or diagram, and to 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, and the M-line.
- Titin as the elastic anchor.
- The bands: I (actin only, light), A (full myosin length, dark), H zone (myosin only).
A common trap: 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 |
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