5.5.11 muscle contraction Flashcards
define creatine phosphate
compound in muscle that acts as a store of phosphates & can supply phosphates to make ATP rapidly
what are myofibrils
contractile units of skeletal muscle
2 types of protein filaments within myofibrils
- thin filament = align to make light band & held together by z-line (actin)
- thick filaments = make up dark band (myosin)
what is the H zone
where the thick and thin filaments don’t overlap (just thick)
what is a sarcomere
the distance between the two Z-lines
what are the thick/thin filaments surrounded by
sarcoplasmic reticulum
describe the thin filaments
- consist of actin
- each filament contains 2 chains of actin subunits twisted around each other
- molecule of tropomyosin wound around actin which is attached to troponin
- each troponin complex consists of 3 polypeptides = 1 binds to actin, 1 binds to tropomyosin & 1 binds to calcium when available
- at rest, tropomyosin & troponin cover binding sites where thick filaments can bind
describe the thick filaments
- consists of bundle of myosin molecules
- each myosin molecule has 2 protruding heads (stick out at each end of molecule)
- heads are mobile & bind to actin when binding sites exposed
what is the sliding filament hypothesis
- during contraction, the light band & H zone get shower
- Z lines move closer together
- sarcomere becomes shorter
- thick/thin filaments slide past one another during contraction
describe the mechanism of contraction
- sliding action caused by movement of myosin heads
- tropomyosin is moved aside when muscle is stimulated = exposes binding sites on actin
- myosin heads attach to actin & move = causes actin to slide past myosin
I-band
thin filament (actin)
A-band
actin/myosin overlap (thick & thin filaments)
H zone
just myosin (thick filament)
Z-line
end of sarcomere
outline how contraction is controlled (6 steps)
- when muscle is stimulated, action potential passes along sarcolemma & down transverse tubules into muscle fibre
- action potential carried to sarcoplasmic reticulum (stores calcium ions) & causes release of calcium ions into sarcoplasm
- calcium ions bind to troponin which alters shape & pulls tropomyosin aside = exposes binding sites on actin
- myosin heads bind to actin, forming cross-bridges between filaments
- myosin heads move, pulling actin filament past myosin filament
- myosin heads detach from actin & can bind further up actin filament
how does ATP supply the energy for contraction (4 steps)
- myosin head attaches to actin filament, forming cross-bridge
- myosin head moves, causing actin filament to slide past myosin filament = power stroke
- during power stroke, ADP & Pi are released from myosin head - after power stroke, new ATP molecule attaches to myosin head, breaking cross-bridge
- myosin head returns to original position as ATP is hydrolysed, releasing energy to make this movement occur
- myosin head can now make new cross-bridge further along actin filament
how is the supply of ATP maintained: aerobic respiration
- in mitochondria of muscle tissue
- bohr effect helps release more oxygen from haemoglobin in blood
- (however) at intense exercise, ATP production rate limited by delivery of oxygen to muscles
how is the supply of ATP maintained: anaerobic respiration in sarcoplasm of muscle tissue
- anaerobic respiration can release little more ATP than
respiratory substrates - (however) leads to production of lactate which is toxic, so can only last few seconds before lactic acid build-up begins to fatigue
how is the supply of ATP maintained: creatine phosphate in sarcoplasm
- reserve store of phosphate groups
- phosphate can be transferred from creatine phosphate to ADP molecules = creates ATP molecules very rapidly
- enzyme creatine phosphotransferase is involved
- supply of creatine phosphate is sufficient to support muscular contraction for a further 2-4 seconds
