chapter 13 p6

Question 1

There are two types of muscle fibres found in your body.

Answer 1

Slow-twitch and fast-twitch muscles:

Different muscles in the body have different proportions of each fibre.

Question 2

Properties of slow-twitch fibres

Answer 2

fibres contract slowly
provide less powerful contractions but over a longer period
used for endurance activities as they do not tire easily
gain their energy from aerobic respiration

Question 3

structural Properties of slow-twitch fibres

Answer 3

rich in myoglobin, a bright red protein which stores oxygen - this makes the fibres appear red
rich supply of blood vessels and mitochondria.
Slow-twitch fibres are found in large proportions in muscles which help to maintain posture such as those in the back and calf muscles which have to contract continuously to keep the body upright.

Question 4

Properties of fast-twitch fibres:

Answer 4

fibres contract very quickly
produce powerful contractions but only for short periods
used for short bursts of speed and power as they tire easily
gain their energy from anaerobic respiration

Question 5

structural Properties of fast-twitch fibres:

Answer 5

pale coloured as they have low levels of myoglobin and blood vessels
contain more, and thicker, myosin filaments
store creatine phosphate - a molecule that can rapidly generate ATP from ADP in anaerobic conditions.
Fast-twitch fibres are found in high proportions in muscles which need short bursts of intense activity, such as biceps and eyes.

Question 6

Sliding filament model

Answer 6

In order for skeletal muscle fibres to contract and cause movement, the actin and myosin filaments within the myofibrils have to slide past each other.
Muscle contraction is usually described using the sliding filament model.
During contraction the myosin filaments pull the actin filaments inwards towards the centre of the sarcomere

Question 7

What does the pulling of the actin filament inwards result in:

Answer 7

the light band becoming narrower
the Z lines moving closer together, shortening the sarcomere
the H-zone becoming narrower.
- The dark band remains the same width, as the myosin filaments themselves have not shortened, but now overlap the actin filaments by a greater amount.

Question 8

relaxed vs contracted sarcomeres

Answer 8

Question 9

What does the simultaneous contraction of lots of sarcomeres result in:

Answer 9

that the myofibrils and muscle fibres contract.
This results in enough force to pull on a bone and cause movement.
When sarcomeres return to their original length the muscle relaxes.

Question 10

diagram of relaxed vs contracted muscles

Answer 10

Question 11

Structure of myosin:

Answer 11

Myosin filaments have globular heads that are hinged which allows them to move back and forwards.
On the head is a binding site for each of actin and ATP.
The tails of several hundred myosin molecules are aligned together to form the myosin filament.

Question 12

Structure of myosin diagram

Answer 12

Question 13

Structure of actin p1

Answer 13

Actin filaments have binding sites for myosin heads called actin-myosin binding sites
However, these binding sites are often blocked by the presence of another protein called tropomyosin which is held in place by the protein troponin.
When a muscle is in a resting state (relaxed) the actin-myosin sites are blocked by tropomyosin.
The myosin heads can therefore not bind to the actin, and the filaments cannot slide past each other.

Question 14

Structure of actin p2

Answer 14

When a muscle is stimulated to contract, the myosin heads form bonds with actin filaments known as actin-myosin cross-bridges.
The myosin heads then flex (change angle) in unison, pulling the actin filament along the myosin filament.
The myosin then detaches from the actin and its head returns to its original angle, using ATP.
The myosin then reattaches further along the actin filament and the process occurs again.
This is repeated up to 100 times per second.

Question 15

Structure of actin diagram

Answer 15

Question 16

Neuromuscular junction:

Answer 16

Muscle contraction is triggered when an action potential arrives at a neuromuscular junction - this is the point where a motor neurone and a skeletal muscle fibre meet
There are many neuromuscular junctions along the length of a muscle to ensure that all the muscle fibres contract simultaneously.
If only one existed, the muscle fibres would not contract together therefore the contraction of the muscle would not be as powerful.
It would also be much slower, as a wave of contraction would have to travel across the muscle to stimulate the individual fibres to contract.

Question 17

Motor Units and Force Regulation inNeuromuscular junction:
All the muscle fibres supplied by a …

Answer 17

single motor neurone are known as a motor unit - the fibres act as a single unit.
If a strong force is needed, a large number of motor units are stimulated, whereas only a small number are stimulated if a small force is required.
When an action potential reaches the neuromuscular junction, it stimulates calcium ion channels to open.

Question 18

Action Potential Transmission in neuromuscular junction

Answer 18

• Calcium ions then diffuse from the synapse into the synaptic knob, where they cause synaptic vesicles to fuse with the presynaptic membrane.
• Acetylcholine is released into the synaptic cleft by exocytosis and diffuses across the synapse.
• It binds to receptors on the postsynaptic membrane (the sarcolemma), opening sodium ion channels, and resulting in depolarisation.
• Acetylcholine is then broken down by acetylcholinesterase into choline and ethanoic acid.
• This prevents the muscle being overstimulated.
• Choline and ethanoic acid diffuse back into the neurone, where they are recombined into acetyicholine, using the energy provided by mitochondria.

Question 19

Neuromuscular junction diagram

Answer 19

Question 20

Sarcoplasm 1

Answer 20

The depolarisation of the sarcolemma travels deep into the muscle fibre by spreading through the T-tubules.
These are in contact with sarcoplasmic reticulum.
The sarcoplasmic reticulum contains stored calcium ions which it actively absorbs from the sarcoplasm.
When the action potential reaches the sarcoplasmic reticulum it stimulates calcium ion channels to open.
The calcium ions diffuse down their concentration gradient flooding the sarcoplasm with calcium ions

Question 21

sarcoplasm 2: Actin-Myosin Cross-Bridge Formation:

Answer 21

The calcium ions bind to troponin causing it to change shape.
This pulls on the tropomyosin moving it away from the actin-myosin binding sites on the actin filament.
Now that the binding sites have been exposed the myosin head binds to the actin filament forming an actin-myosin cross-bridge.
Once attached to the actin filament the myosin head flexes, pulling the actin filament along.
The molecule of ADP bound to the myosin head is released.
An ATP molecule can now bind to the myosin head This causes the head to detach from the actin filament.
The calcium ions present in the sarcoplasm also activate the ATPase activity of the myosin.

Question 22

sarcoplasm 3 Muscle Contraction Cycle:

Answer 22

This hydrolyses the ATP to ADP and phosphate, releasing energy which the myosin head uses to return to its original position.
The myosin head can now attach itself to another actin-myosin binding site further along the actin filament and the cycle repeats.
The cycle continues as long as the muscle remains stimulated.
During The period of stimulation many actin-myosin bridges form and rest rapidly, pulling the actin filament along.
This shortens the sarcomere and causes the muscle to contract.

Question 23

Energy supply during muscle contraction:

Answer 23

Muscle contraction requires large quantities of energy.
This is provided by the hydrolysis of ATP into ADP and phosphate.
The energy is required for the movement of the myosin heads and to enable the sarcoplasmic reticulum to actively reabsorb calcium ions from the sarcoplasm.
The three main ways ATP is generated: Aerobic respiration, Anaerobic respiration and Creatine phosphate

Question 24

Aerobic respiration:

Answer 24

Most of the ATP used by muscle cells is regenerated from ADP during oxidative phosphorylation.
This chemical reaction takes place inside the mitochondria which are plentiful in the muscle.
However, this can only occur in the presence of oxygen.
Aerobic respiration is therefore used for long periods of low-intensity exercise.

Question 25

Anaerobic respiration:

Answer 25

In a very active muscle, oxygen is used up more quickly than the blood supply can replace it. Therefore, ATP has to be generated anaerobically.
ATP is made by glycolysis but, as no oxygen is present.
The pyruvate which is also produced is converted into lactate (lactic acid).
This can quickly build up in the muscles resulting in muscle fatigue.
Anaerobic respiration is used for short periods of high-intensity exercise, such as sprinting.

Question 26

Creatine phosphate:

Answer 26

Another way the body can generate ATP is by using the chemical creatine phosphate which is stored in muscle.
To form ATP, ADP has to be phosphorylated - a phosphate group has to be added.
Creatine phosphate acts as a reserve supply of phosphate, which is available immediately to combine with ADP, reforming ATP.
This system generates ATP rapidly, but the store of phosphate is used up quickly.
As a result this is used for short bursts of vigorous exercise, such as a tennis serve.
When the muscle is relaxed, the creatine phosphate store is replenished using phosphate from ATP.