Sliding filament theory

Question 1

Process of muscle contraction

Answer 1

1- Ca2+ ions diffuse into myofibrils from sarcoplasmic reticulum + bind to tropomyosin causing it to move up, exposing the binding sites on actin
2- Myosin heads attach to actin filaments forming an actinmyosin cross bridge. ATP-> ADP+ Pi, Pi is released
3- Myosin head bends pulling actin filaments along, ADP is released - known as power stroke (makes sarcomere shorter so muscle contracts)
4- ATP binds to myosin head causing it to detach from actin binding site (Z lines move closer together + sarcomere shortens so muscle cells contract
5- Hydrolysis of ATP to ADP + Pi by ATPase provides energy needed to re-cock myosin head to original position ready to form a new cross bridge. Ca2+ ions are actively transported back to sarcoplasmic reticulum + tropomyosin returns to original shape + covers binding sites

Question 2

What happens to banding pattern in sarcomere when it contracts

Answer 2

• A band stays the same length as myosin filament doesn’t shorten
• I band + H zone get shorter as actin filaments are pulled over myosin filaments
• Z lines get closer together as sarcomere gets shorter overall

Question 3

How is ATP constantly generated

Answer 3

Aerobic respiration- most ATP is generated via oxidative phosphorylation, only happens when there’s enough O2 (good for long periods of exercise)
Anaerobic respiration- ATP is made rapidly by glycolysis in sarcoplasm (useful for short periods of intense exercise, lactate can cause muscle fatigue)
Phosphocreatine- stored in muscle cells + rapidly provides Pi to add to ADP to produce ATP. Used in short bursts of vigorous exercise as runs out quickly. Anaerobic + doesn’t form any lactate, creatine can be broken down into creatinine + removed by kidneys - high levels may indicate kidney damage

Question 4

Compare actin + myosin structure

Answer 4

Actin:
- Has binding sites for myosin heads called actin-myosin binding sites
- Tropomyosin forms a long chain which coils around the actin filaments - helps filaments move past each other but blocks actin-myosin binding sites when muscle is relaxed
Myosin:
- Has many globular heads that are hinged so they can move back + forth using energy from ATP
- Each head has a binding site for actin + ATP

Question 5

What must happen for muscle contraction to occur

Answer 5

• Action potential from motor neurone has to depolarise sarcolemma , then wave of depolarisation travels down T- tubules to sarcoplasmic reticulum. Depolarisation triggers sarcoplasmic reticulum to release Ca2+ ions into sarcoplasm
• Influx of Ca2+ ions triggers muscle contraction
• Ca2+ ions are needed to move tropomyosin + expose actin- myosin binding sites so myosin heads can bind to actin. No Ca2+, no muscle contraction as filaments can’t slide past one another