This document offers a selection of summary notes on the chapter: Run for your Life. This is based on textbook A-Level Biology A (2015) Salters-Nuffield
Synovial membrane Secretes synovial fluid Cartilage Absorbs synovial fluid to
which acts as lubricant acts a shock absorber
Tendon (inelastic) Joins muscle to bone Pad of cartilage Additional protection
Ligament Joins bone to bone Fibrous capsule Encloses joints
Antagonistic muscles are a pair of muscles which work together to move a bone; one is a flexor and one is an extensor.
→ A muscle that bends a joint when it contracts is called a flexor
𝐼𝑚𝑎𝑔𝑒 mm → μm [x1000]
→ A muscle that straightens a joint when it contracts is called a extensor 𝑀=
𝐴𝑐𝑡𝑢𝑎𝑙 cm → μm [x10,000]
Explain why muscles occur in antagonistic pairs. [2 marks]
Muscles can only work in one direction; thus to create opposite forces there must be an extensor and flexor
• Muscle cells = muscle fibres; have a cell surface membrane (sarcolemma)
• Muscle fibres = multinucleate; high metabolism → many proteins → many nuclei to carry out metabolic processes
• Muscle fibres contain mitochondria; ATP energy from respiration is produced and used for contraction
• ATP is hydrolysed into ADP and Pi to produce energy
• Transverse (T) tubules = bits of infolding sarcolemma (stick to SP); carry electrical signals throughout the fibre
• A bundle of muscle fibres are bound together by connective tissue, which is continuous with the tendons
• Muscle fibre → myofibrils → sarcomeres (repeating contractile units)
→ Sarcomeres a made up of 2 protein filaments; actin (light thin I band) and myosin (dark thick A band)
→ When the muscle contracts the length of myosin and actin remain the same and slide past each other – the
sarcomere shortens as the Z lines move closer together
→ Sarcomeres store Ca2+ in sarcoplasm; these ions are taken into the SR via ATP using carrier proteins
Muscle Contraction:
1) An electrical signal called an action potential is sent from brain to muscle along the motor neurone (nerve cell)
2) The motor neurone meets the muscle fibre is the neuromuscular junction
3) The action potential depolarises (alters charge) of the sarcolemma; depolarisation
4) The depolarisation spreads down the t tubules to the sarcoplasmic reticulum
5) The sarcoplasmic reticulum releases Ca2+ into sarcoplasm
Sliding Filament Theory Hydrolysed ATP → myosin in high energy state
1) Troponin and tropomyosin are found between actin (they help myofilaments move past each other)
2) Myosin binding sites blocked by myosin; myosin head cannot bind (ADP + Pi)
3) Ca2+ binds to troponin; troponin changes shape; causes tropomyosin to move; exposes myosin binding sites
4) When myosin head binds to actin, ADP and Pi on myosin head are released
5) Myosin changes shape causing myosin head to nod forward; attached actin moves over myosin
6) ATP binds to myosin head, causing it to detach from actin
7) ATPase (on myosin head) hydrolyses ATP into ADP + Pi
8) Hydrolysis causes change in shape of myosin head; returns to upright position
9) ATP also provides energy to break actin-myosin cross bridge
10) Myosin head reattaches to different binding site further along actin filament; new cross bridge forms
11) Many cross bridges form and break very rapidly which shortens the sarcomere, causing the muscle to contract
12) The cycle will continue as long as Ca2+ are present and bound to troponin
How does the sliding filament theory explain Rigor Mortis (permanent contraction)? [3 marks]
After death, ATP is no longer produced as respiration stops (no O 2 or glucose available for mitochondria)
No ATP can bind to myosin head to release myosin from actin filament; ATP required to break cross bridge
Hence muscle stays in a permanent contracted state
When a muscle relaxes, it is no longer stimulated by nerve impulses; Ca2+ are actively pumped out of the muscle sarcoplasm, using
ATP. The troponin and tropomyosin move back, blocking the myosin binding sites on the actin. In absence of ATP the cross bridges
remain attached, which is what happens in Rigor Mortis (any contracted muscles become rigid).
, A Level Notes RFL: Muscle Contraction Biology
State what happens to the lengths of the following when the muscle contracts:
A band (all of myosin) Stays the same
H zone (only myosin – the gap in actin) Shortens
I band (only actin) Shortens
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