Sliding Filament Theory

Question 1

What two proteins are actin associated with?

Answer 1

Tropomyosin and troponin.

Question 2

What happens when a nerve impulse arrives at a neuromuscular junction?

Answer 2

Calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum.

Question 3

What is the sarcoplasmic reticulum?

Answer 3

A specialised type of endoplasmic reticulum: a system of membrane-bound sacs around the myofibrils.

Question 4

How is the movement of the protein filaments on actin triggered?

Answer 4

The calcium ions diffuse through the sarcoplasm (the name given to cytoplasm in a muscle cell).

Question 5

What are the first staged of the SFT?

Answer 5

• Ca²⁺ attaches to the troponin molecule, causing it to move.
• As a result, the tropomyosin on the actin filament shifts its position, exposing myosin binding sites on the actin filaments.
• Myosin heads bind with myosin binding sites on the actin filament, forming cross-bridges.

Question 6

What are the later stages of the SFT?

Answer 6

• When the myosin head binds to the actin, ADP and P_i on the myosin head are released.
• The myosin changes shape, causing the myosin head to detach.
• An ATPase on the myosin head hydrolysises the ATP, forming ADP and P_i.
• THis hydrolysis causes a change in the shape of the myosin head. It returns to its upright position. This enables the cycle to start again.

Question 7

What happens when a muscle relaxes?

Answer 7

The muscle is no longer being stimulated. Ca2+ is actively being pumped out of the muscle sarcoplasm, using ATP. The troponin and tropomyosin move back, once again blocking the myosin binding sites.

In the absence of ATP, the cross-bridges remain attached. This is what happens in rigor mortis. Any contracted muscles remain stiff.