Contraction Of Skeletal Muscle

Question 1

Do muscles pull or push

Answer 1

They can only pull

Question 2

What do antagonistic Pairs do

Answer 2

These pairs pull in opposite directions and when one is contracted the other is relaxed

Question 3

What happens to the sarcomere when a muscle contracts

Answer 3

The I band becomes narrower

The Z lines move closer together

The H zone becomes narrower

The A band remains the same width

Question 4

Why does the A band remain the same width

Answer 4

As the width of this band is determined by the length of myosin filaments, it follows that the myosin filaments have not become shorter.

This discounts the theory that muscle contraction is due to filaments themselves shortening

Question 5

What is a myosin

Answer 5

Made up of two types of proteins; a fibrous protein arranged into filament made up of several hundred molecules ( tail ). A globular protein formed init two bulbous structures at one end (the head)

Question 6

What is actin.

Answer 6

A globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand

Question 7

What is tropomyosin

Answer 7

Forms long thin threads that are wound around actin filaments

Question 8

By what mechanism do the filaments slide past one another

Answer 8

The bulbous heads of the myosin filaments form cross bridges with the actin filaments.

They do this by attaching themselves to binding sites on the action filaments, and then flexing in unison pulling the actin filaments along the myosin filaments

They then become detached and, using ATP as a source of energy, return to their original angle and reattach themselves further along the actin filaments

Question 9

Describe muscle stimulation

Answer 9

An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob

The calcium ions cause the synaptic vesicles fo fuse with the presynaptic membrane and release their acetylcholine into the synaptic cleft

Acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell surface membrane, causing it to depolarise

Question 10

Describe the muscle contraction

Answer 10

1. Sarcolemma depolarised
2. Sarcoplasmic reticulum releases calcium ions, which move through T tubules
3. Calcium ions bind to troponin I’m actin filaments. This causes troponin to move out of the way to reveal binding sites for the myosin head to form cross bridges
4. Myosin head binds to actin to form cross links
5. Myosin head changes position pulling actin over the myosin (adp is released )
6. ATP is hydrolysed to reposition myosin head to allow the crossbridge cycle to begin again

Question 11

Describe muscle relaxation

Answer 11

When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP

this reabsorption of the calcium ions allows tropomyosin to block the actin filament again

Myosin heads are now unable to bind to actin filaments and contraction ceases, that is, the muscle relaxes

In this state force from antagonistic muscles can pull actin filaments out from between myosin

Question 12

What is the energy release from muscle contraction needed for

Answer 12

The movement of myosin heads

The reabsorption of calcium ions into the endoplasmic reticulum by active transport

Question 13

Where is most atp regenerated

Answer 13

From adp during the respiration of pyruvate in the mitochondria, which are particularly plentiful in the muscle.

Question 14

How is rapidly generating ATP anaerobically?

Answer 14

Particularly achieved using a chemical called phosphocreatine and partly by more glycolysis

Question 15

What does phosphocreatine do

Answer 15

It can’t supply energy directly to the muscle, so instead it refer ages ATP, which can.

Question 16

Where is phosphocreatine store and what does it act as

Answer 16

In the muscle

And acts as a reserve supply of phosphate, which is available immediately to combine ADP and so reform ATP.

Question 17

How is the phosphocreatine store replenished

Answer 17

Using phosphate from atp when the muscle is relaxed

Question 18

Explain how the shape of the myosin molecule is adapted to its role in muscle contraction

Answer 18

Myosin is made of two proteins. The fibrous protein is long and thin in shape, which enables it to combine with others to form a long thick filament along which the actin filament can move. The globular protein forms two bulbous structures at the end of a filament. This shape allows it to exactly fit recesses in the actin molecule, to which it can become attached. It’s shape also means it can be moved at an angle. This allows it to change its angle when attached to actin and so move it along, causing the muscle to contact

Question 19

Trained sprinters have high levels of phosphocreatine in the muscles. Explain the advantage of this.

Answer 19

Phosphocreatine store the phosphate that is used to generate atp from adp in anaerobic conditions. A sprinters muscles often work so strenuously that the oxygen supply cannot meet the demand. The supply of atp from mitochondria during aerobic respiration therefore ceases. Sprinters with the most phosphocreatine have an advantage because atp can be supplied to their muscles for longer and so they perform better

Question 20

Dead cells can no longer produce ATP. Soon after death, muscles contract, making the body stiff - a state known as rigor mortis. From your knowledge of muscle contraction, explain the reasons why rigor mortis occurs after death

Answer 20

One role of atp in muscle contraction is to attach to myosin heads, thereby causing them to detach from the actin filament and making the muscle relax. As no atp is produced after death, there is none to attach to the myosin, which therefore remains attached to actin, leaving the muscle in a contracted state