muscles PART 2 [sliding filament model and sources of ATP]

What does myosin consist of?

-myosin tails

-myosin heads

what does myosin head have?

-binding site for actin

-binding site for ATP

what does actin filaments consist of?

-tropomyosin and troponin [proteins]

-binding sites for myosin heads

labelled diagram of sliding filament model

describe the sliding filament model of muscle contraction

-a nerve impulse causes the release of calcium ions from the sarcoplasmic reticulum [1]

-calcium ions bind to troponin, causing it to change shape [2]

-this causes tropomyosin to move aside, exposing the binding sites on actin [2]

-myosin head bind to actin, forming actin-myosin cross-bridge [3]

-myosin head performs a power stroke (tilts), pulling the actin filaments past myosin and releasing ADP [4]

-ATP binds to myosin head, causing it to detach from actin [5] [break actin-myosin cross-bridge]

-ATP is hydrolysed by ATPase to ADP and phosphate, releasing energy [6]

-this energy returns the myosin head to its original position [6]

-the myosin head reattaches further along the actin filament

-The cycle repeats, causing muscle contraction

what happens after muscle contraction?

-calcium ions are actively transported back into the sarcoplasmic reticulum

-this requires ATP

-troponin returns to its original shape

-tropomyosin moves back, blocking the binding sites on actin

-the actin filaments slide back to their relaxed position

-the Sarcomere returns to its original length

-muscle stops contracting and relaxes

explain why glucose is required for the contraction of skeletal muscle [hint: mention why ATP is required]

-glucose is required for respiration to produce ATP

-ATP is required to detach myosin from actin [break actin-myosin cross-bridge]

-ATP is hydrolysed to ADP and phosphate to return the myosin head to its original position

-ATP is required to actively transport calcium ions back into the sarcoplasmic reticulum

what does a lack of ATP lead to in muscles and why?

-muscles cannot relax

-ATP is required to detach myosin from actin

-Without ATP, actin-myosin cross-bridges remain attached

why do muscles fail to relax after contraction if the sarcoplasmic reticulum is damaged?

-calcium ions are not reabsorbed into the sarcoplasmic reticulum

-calcium ions remain bound to troponin

-binding sites on actin remain exposed

-actin-myosin cross-bridges remain attached→ muscle cannot relax

Sources of ATP for muscle contraction

-aerobic respiration

-anaerobic respiration

-creatine phosphate (phosphocreatine)

using aerobic respiration

-occurs if oxygen is available

-requires oxygen for oxidative phosphorylation in mitochondria

-produces more ATP

-cannot occur during intense exercise when oxygen supply is limited

-suitable for long periods of low-intensity exercise

using anaerobic respiration

-required during intense exercise when oxygen is limited

-ATP is produced by glycolysis

-pyruvate is converted to lactate

-build up of lactate decreases pH, causing muscle fatigue

using creatine phosphate

-found in the sarcoplasm of muscle fibres

-creatine phosphate donates a phosphate group to ADP to form ATP

-produces ATP quickly

-runs out quickly

-doesn’t require oxygen

-used during short bursts of intense exercise