13.10 Sliding filament model

Question 1

what happens when muscles contract

Answer 1

• during contraction, myosin pulls actin inwards towards centre of sarcomere
• light band becomes narrower
• z-lines move closer and shorten sarcomere
• h-zone become narrower
• dark bands don’t change, just overlap actin by greater amount
• simultaneous contraction = enough force to pull on bone and cause movement

Question 2

describe the structure of myosin

Answer 2

• globular heads which are hinged = can move back and forwards
• binding site on head for each of actin and ATP
• tails of several hundred myosin molecules aligned to form the filament

Question 3

describe the structure of actin

Answer 3

• filaments have binding sites for myosin heads (actin-myosin binding sites)
• binding sites often blocked by tropomyosin which is held in place by troponin
• when relaxed, active sites are blocked so myosin can’t bind and filaments can’t slide
• when contraction is stimulated, binding with myosin forms actin-myosin bridges
• myosin heads change flex In unison to pull actin filament along myosin filament and then the myosin detaches and reattaches further along (up to 100/s)