13.10 Sliding Filament Model

Question 1

Summary of sliding filament model

Answer 1

• model for muscle contraction
• during muscle contraction, myosin and actin filaments slide over one another to make the sarcomeres contract — the myofilaments themselves don’t contract
• the simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract
• sarcomeres return to their original length as the muscle relaxes

A-band stays the same length
I band gets shorter
H zones get shorter
Z-lines get closer together — the sarcomeres get shorter

Question 2

Describe myosin filaments

Answer 2

• have globular heads that are hinged, so they can move back and forth
• each myosin head has a binding site for actin and a binding site for ATP

Question 3

Describe actin filaments

Answer 3

• have binding sites for myosin heads, called actin-myosin binding sites
• two other proteins, tropomyosin and troponin are found between actin filaments
• these proteins are attached to each other (troponin holds tropomyosin in place) and they help myofilaments move past each other

Question 4

Describe the state of myosin heads in a resting state

Answer 4

• for myosin and actin filaments to slide past each other, the myosin head needs to be bind to the actin-myosin binding site on the actin filament
• in a resting (unstimulated) muscle the actin-myosin binding site is blocked by tropomyosin
  This means myofilaments can’t slide past each other because the myosin heads can’t bind to the actin filaments

Question 5

4 steps of muscle contraction

Answer 5

• arrival of an action potential
• movement of the actin filament
• breaking of the cross bridge
• return to resting state

Question 6

Describe the arrival of an action potential

Answer 6

When an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-tubules and to the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm.
This influx of calcium ions into the sarcoplasm triggers muscle contraction.

Calcium ions bind to troponin, causing it to change shape.
This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament. This exposes the binding site, which allows the myosin head to bind.
The bond formed when a myosin head binds to an actin filament is called the actin-myosin cross bridge.

Question 7

Describe the movement of the actin filament

Answer 7

Calcium ions also activate the enzyme ATPase, which breaks down ATP (into ADP and Pi) to provide the energy needed for muscle contraction.

The energy released from ATP moves the myosin head to the side, which pulls the actin filament along in a kind of rowing action.

Question 8

Describe breaking of the cross bridge

Answer 8

• ATP also provides the energy to break the actin-myosin cross bridge, so the myosin head detaches from the actin filament after its removed.
• the myosin head then returns to its stating position, and reattached to a different brining site further along the actin filamen.
  A new actin-myosin cross bridge is formed and the cycle is repeated.
  This pulls actin filament along - which shortens sarcomere, causing the muscle to contract

The cycle will continue as long as calcium ions are present and bound to troponin

Question 9

Describe the myosin head returning to its resting state

Answer 9

• when the muscle stops being stimulated, calcium ions leave their binding site on the troponin molecules and are moved by active transport back into the sarcoplasmic reticulum (this needs ATP too)

The troponin molecules return to their original shape, pulling the attached tropomyosin molecules with them. This means the tropomyosin molecules block the actin-myosin binding sites again.

Muscle no longer contracts, actin filament slide back to relaxed position, which lengthens the sarcomere

Question 10

Where does the ATP needed for muscle contraction come from? (3 ways)

Answer 10

1) aerobic respiration
2) anaerobic respiration
3) ATP-creatine phosphate (ATP-CP) system
ATP is made by phosphorylating ADP— adding a phosphate group taken from CP
CP runs out after a few seconds so its used during short bursts of vigorous exercise, and it the ATP-CP system is anaerobic and its alactic (it doesn’t form any lactate)

ADP + CP —> ATP + C