Biology Topic 7.1



  • Joint are able to move through mmuscles: extensors and flexors. Flexors contract to pull muscle in, and extensors contract to cause extension of joint. These two types work as antagonistic pairs.
  • Synovial joints are bones that are seperated by a cavity filled with synovial fluid when they move. Bones are held to bone with ligaments. Tendons hold muscle to bone and cartalidge stops bones rubbing against each other and acts as a shock absorber.
  • A fibrous capsule encloses joint. 
  • The difference between a ligament and tendon is that ligaments are made of elastic fibres, and tendons are made of non-elastic muscle fibres.
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  • Muscles are made up of muscle fibres. 
  • A muscle fibre is a muscle cell surrounded by cell surface membrane, which are multinucleate (multiple nuclei) as one could not control the metabolic reactions of a long cell (several cells fusing together to make an elongated muscle fibre.)
  • Muscle cells are striped.
  • The muscle fibres are bound together by connective tissue continuous with the tendon.
  • Inside a muscle fibre, there is cytoplasm that has mitochondria and other organelles. 
  • Each muscle fibre also contains myofibrils, which contains multiple contractile units called sarcomeres. 
  • A sarcomere is made up of actin and myosin which are thin filaments that bring about contractions through them sliding over each other.
  • The filaments overlap which makes muscle cells seem striped. 
  • Just actin is a light band. Both together is a dark band. Where there is only myosin, there is a intermediate colour band.when the myosin and actin ,ove over each other the sarcomere shortens, causing contraction.
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Sliding Filament Theory

1. A nerve impulse arrives at the neuromuscular junction.

2. The sarcoplasmic reticulum releases calcium ions, which diffuse through the sarcoplasm.

3. The calcium ions attaches to a troponin molecule which causes it to move.

4. This causes the tropomyosin to move on the actin and reveal the myosin head binding site on the actin.

5. The myosin head binds to this binding site, forming a cross bridge. This uses ATP which then releases ADP and a phosphate ion.

6. The myosin then changes shape which leads to the myosin head nodding foreward and it pulls the actin over itself.

7. ATP then attaches to the binding site which causes it to dettatch. Then the ATPase on the myosin head hydrolyses the ATP into ADP and a phosphate ion. 

8. The hydrolysis causes the myosin head to change shape again and leaves it in an upright position.

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When the muscles relaxes

  • It is no longer stimulated by nerve impulses.
  • Calcium ions are actively pumped out of the muscle sarcoplasm, using ATP. 
  • The troponin and tropomyosin move back, blocking the myosin-binding sites on the actin
  • If there is no ATP, the crossbridge stays, and the muscle is stuck. This is what happens in rigor mortis.
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