Muscles - A2

Question 1

Describe the structure of a skeletal muscle.

Answer 1

Compound of many muscle fibres bound together by connective tissue. Each muscle fibre is surrounded by a thin membrane called sarcolemma, containing nuclei. Cell contains many microfibrils, surrounded by sarcoplasmic reticulum. Microfibrils made up of myofilaments (actin). These are arranged into contractile units called sarcomeres.

Question 2

Describe the sliding filament theory of muscle action.

Answer 2

• calcium ions are released from sarcoplasmic reticulum
• calcium ions bind to troponin and cause tropomyosin to move, exposing the myosin head binding sites on the actin filament
• myosin heads bind to actin binding sites forming cross bridges
• each myosin head contains a molecule of ATP which hydrolyses into ADP + Pi by ATPase. This energy released causes nod action of myosin head, pulling the actin filament a short distance.
• a new ATP attaches to myosin head, breaking cross bridge and separating it from actin
• The process repeats moving the actin along a bit more each time

Question 3

Describe and explain the role of ATP and phosphocreatine in providing energy during contraction.

Answer 3

• phosphocreatine is store in muscle fibres for when ATP in muscle fibres runs out (4 seconds roughly)
• it generates ATP from ADP rapidly in aerobic conditions
• provides energy for muscle contraction
• phosphate is given to ADP hence phosphocreatine becomes creatine

Question 4

Describe and explain slow twitch muscle fibres.

Answer 4

• have lots of mitochondria
• respire aerobically to regenerate ATP
• high concentration of myoglobin for a very high affinity to oxygen which is stationary and stays in muscle
• closely associated with large number of capillaries to provide good oxygen supply
• less extensive sarcoplasmic reticulum as less calcium ions required at one time
• less glycogen as glucose broke fully down via aerobic respiration

Question 5

Describe and explain fast twitch muscle fibres.

Answer 5

• produce rapid, strong contractions but only sustained over short period of time
• thicker myosin, stronger heads
• have store of oxygen and glucose (x19 more ATP)
• anaerobic respiration - not much oxygen supply
• use phosphocreatine to regenerate ATP
• have fewer mitochondria
• lower concentration of myoglobin