4th, 3

What happens to the Z-lines during sarcomere contraction?

The Z-lines move closer together.

What happens to the H-zone during sarcomere contraction?

The H-zone decreases.

What happens to the I-band during sarcomere contraction?

The I-band decreases.

What happens to the A-band during sarcomere contraction?

The A-band stays the same length.

What are the three proteins that make up the thin filament?

Actin, tropomyosin, and troponin.

What is G-actin?

A small globular actin protein.

What is F-actin?

A long strand formed by many G-actin molecules joined together.

What does each G-actin molecule contain?

A binding site for myosin.

What is tropomyosin?

A long, thin protein that wraps around F-actin.

What is the function of tropomyosin?

It wraps around F-actin and covers the myosin-binding sites on actin.

What is troponin?

A small globular protein attached to tropomyosin.

What do troponin and tropomyosin together form?

The troponin-tropomyosin complex.

What is the function of the troponin-tropomyosin complex?

It blocks the myosin-binding sites on actin at rest.

What does troponin contain a binding site for?

Calcium (Ca²⁺).

What happens when calcium binds to troponin?

Troponin moves tropomyosin away, exposing the myosin-binding sites on actin.

What is the thick filament made of?

Hundreds of myosin proteins.

What does the myosin head contain?

A binding site for actin.

At rest, which direction are the myosin heads angled?

Away from the M-line.

Why can’t actin and myosin bind together when the muscle is at rest?

Tropomyosin blocks the myosin-binding sites on actin.

What is the key ion needed for muscle contraction?

Calcium (Ca²⁺).

What is the first step in muscle contraction?

Calcium binds to troponin.

What happens after calcium binds to troponin?

Tropomyosin shifts and exposes the myosin-binding sites on actin.

What happens after the binding sites on actin are exposed?

Myosin heads bind to actin, forming cross-bridges.

What is a cross-bridge?

The attachment of a myosin head to actin.

What is the power stroke?

The movement of the myosin heads that pulls actin toward the M-line.

What happens to the myosin heads during the power stroke?

They flip toward the M-line.

After the power stroke, which direction are the myosin heads angled?

Toward the M-line.

What slides over what during muscle contraction?

Actin slides over myosin.

What causes a muscle contraction?

Actin sliding over myosin, which shortens the sarcomere.