Which bands change length on contraction?

The I band and H zone get shorter; the A band stays the same.

What does tropomyosin do in relaxed muscle?

It covers the myosin-binding sites on actin so cross-bridges cannot form.

What does troponin do?

It binds calcium ions and changes shape, which moves tropomyosin away from the binding sites.

Where do the calcium ions for contraction come from?

The sarcoplasmic reticulum, released when the action potential spreads down the T-tubules.

What happens in the power stroke?

The myosin head changes angle, pulling actin towards the centre of the sarcomere, and releases ADP and Pi.

What is the role of ATP binding to myosin?

It breaks the cross-bridge, detaching the head from actin.

What is the role of ATPase?

It hydrolyses ATP to ADP and Pi, releasing the energy that returns the myosin head to its original position.

How does muscle relax?

Calcium ions are actively pumped back into the sarcoplasmic reticulum, tropomyosin covers the binding sites again and heads detach.

Why do muscles stiffen after death?

No ATP is made, so myosin heads cannot detach and calcium is not pumped away, leaving cross-bridges in place (rigor mortis).

How would you calculate the percentage shortening of a sarcomere?

(Decrease in length ÷ original length) × 100.

Muscle contractionEdexcel A-Level Biology A: Flashcards

Card 1 of 120 of 12 known

Question

What is the sliding filament theory?

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

What is the sliding filament theory?: Contraction occurs as actin filaments slide between myosin filaments, shortening the sarcomere, without the filaments themselves shortening.
Which bands change length on contraction?: The I band and H zone get shorter; the A band stays the same.
What does tropomyosin do in relaxed muscle?: It covers the myosin-binding sites on actin so cross-bridges cannot form.
What does troponin do?: It binds calcium ions and changes shape, which moves tropomyosin away from the binding sites.
Where do the calcium ions for contraction come from?: The sarcoplasmic reticulum, released when the action potential spreads down the T-tubules.
What is attached to a myosin head that is ready to bind actin?: ADP and inorganic phosphate.
What happens in the power stroke?: The myosin head changes angle, pulling actin towards the centre of the sarcomere, and releases ADP and Pi.
What is the role of ATP binding to myosin?: It breaks the cross-bridge, detaching the head from actin.
What is the role of ATPase?: It hydrolyses ATP to ADP and Pi, releasing the energy that returns the myosin head to its original position.
How does muscle relax?: Calcium ions are actively pumped back into the sarcoplasmic reticulum, tropomyosin covers the binding sites again and heads detach.
Why do muscles stiffen after death?: No ATP is made, so myosin heads cannot detach and calcium is not pumped away, leaving cross-bridges in place (rigor mortis).
How would you calculate the percentage shortening of a sarcomere?: (Decrease in length ÷ original length) × 100.

Exam questions on Muscle contraction

A student studies a skeletal muscle under a microscope and measures one sarcomere. When the muscle is relaxed the sarcomere is 2.4 μm long, and each thick filament is 1.6 μm long. When the muscle is contracted the same sarcomere is 1.8 μm long.
Explain why the H zone becomes narrower when the sarcomere contracts.2 marks
Researchers study two isolated skeletal muscle fibres supplied with plenty of ATP. Fibre P is treated with a chemical that prevents calcium ions being released from the sarcoplasmic reticulum. Fibre Q is treated with a chemical that blocks the ATP-binding site of the myosin heads. Both fibres are then stimulated electrically.
Explain how calcium ions allow myosin heads to bind to actin.2 marks
In relaxed skeletal muscle, each myosin head carries ADP and an inorganic phosphate group and is held in a high-energy position. The head is part of a protein that has ATPase activity. Muscle contraction occurs when myosin heads bind to actin and move it, and this cycle needs a supply of ATP.
Explain the role of ATP and ATPase in the cycle of movement of a myosin head.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).