The Structure of Myofibrils (A-level Biology)
The Structure of Myofibrils
Ultrastructure of a Myofibril
- Myofibrils are composed of two types of myofilaments (protein structures).
- The thicker myofilaments are made up of myosin, and the thinner myofilaments are made up of actin.
- Myosin and actin form repeating units called sarcomeres. These line up in the muscle fibre so that:
- Thicker myosin filaments line up to form dark bands called A-bands. A-bands also contain some overlapping actin filaments.
- Thinner actin filaments line up to form lighter bands called I-bands.
Myofilament Structure
- Actin molecules are comprised of two protein strands that twist to form a simple helix. The helix is surrounded by a protein called tropomyosin, that helps the actin filaments move past each other.
- Myosin molecules have long fibrous tails with a hinged globular head, allowing the head to move.
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