A-Level Biology AQA Notes

Skeletal Muscles

Muscles act in antagonistic pairs against an incompressible skeleton to allow movement
Skeletal muscle is made up of fibres called myofibrils, which in turn are made up of many repeating units, called sarcomeres
Myofibrils are made up of two types of protein filaments, the thinner actin and the thicker myosin

	Slow-Twitch Muscle	Fast-Twitch Muscle
Type of Activity	Endurance	Burst of activity
Contraction Details	Contracts slowly and for longer Fatigues slowly	Contracts quickly and then relaxes rapidly
Mitochondria Density	High	Low
Type of Respiration	Aerobic	Anaerobic
Concentration of Myoglobin	High Concentration	Low Concentration
Glycogen & Phosphocreatine Stores	Small	Large
Muscle Colour	Dark	Light

Muscle Contraction

The sliding filament theory describes how muscle contraction occurs
An action potential travels into the muscle fibre via T tubules, causing release of calcium ions from the sarcoplasmic reticulum. The calcium ions bind to the tropomyosin molecules and cause them to move, exposing the myosin binding site on the actin filament. Myosin attaches to actin forming a actin-myosin cross-bridge. ATPases hydrolyse ATP to detach the myosin head, allowing reattachment at a further site. This cycle continues, causing sarcomeres to shorten.

When nervous stimulation stops, Ca^2+ ions are actively transported back into the sarcoplasmic reticulum using energy from ATP hydrolysis. This allows tropomyosin to block the actin filament from binding to myosin and muscle contraction stops.
ATP can be generation via aerobic or anaerobic respiration
Phosphocreatine generates ATP quickly by adding phosphate to a molecule of ADP released by the contracting muscle