6.3 Skeletal muscles

Question 1

what are the muscle fibres called

Answer 1

myofibrils

Question 2

name the 3 types of muscles in the body and where they are located

Answer 2

cardiac - exclusively found in heart

Smooth - walls of blood vessels and intestines

Skeletal - attached to incompressible skeleton by tendons

Question 3

What does the phrase antagonstic pair of muscles mean

Answer 3

muscles can only pull, so they work in pairs to move bones around joints

Pairs pull in opposite directions - agonist contracts while antagonist is relaxed

Question 4

Describe the gross structure of the skeletal muscle

Answer 4

muscle cells fused together - form myofibrils-(bundles of parallel muscle fibres)

Arrangement ensure - no point of weakness between cells

bundle- surrounded by endomycium - loose connective tissues with many capillaries

Question 5

Describe the microscopic structure of skeletal muscle

Answer 5

myofibrils

Sacroplasm

Sacrolemma

Question 6

myofibrils

Answer 6

site of contraction

made up of shared nuclei and cytoplasm with lots of mitochondria and endoplasmic reticulum

Question 7

Sacroplasm

Answer 7

cytoplasm (sarcoplasm)

Question 8

Sacrolemma

Answer 8

folds inwards towards sacroplasm to form transverse (T) tubules

Cell surface membrane = sarcolemma
Cytoplasm = sarcoplasm

Question 9

Draw a diagram to show the ultrastrcutre of myofibril

Answer 9

Z line - bondarary between sarcomeres

I band - only actin

A band - overlap of actin and myosin

H zone - only myosin

Question 10

How does each band appear under an optical microscope

Answer 10

I band - light

A band - dark

Question 11

How is muscle contraction stimulated?

Answer 11

Neuromuscular junction: Action potential = voltage-gated Ca²⁺ channels open.
Vesicles move towards & fuse with presynaptic membrane.
Exocytosis of acetylcholine (ACh), which diffuses across synaptic cleft.
ACh binds to receptors on Na⁺ channel proteins on skeletal muscle cell membrane.
Influx of Na⁺ = depolarisation.

Question 12

Explain the role of Ca²⁺ ions in muscle contraction.

Answer 12

1.Action potential moves through T-tubules in the sarcoplasm = Ca²⁺ channels in sarcoplasmic reticulum open.

2- Ca²⁺ binds to troponin, triggering conformational change in tropomyosin.

3- Exposes binding sites on actin filaments so actinomyosin bridges can form.

Question 13

Outline the ‘sliding filament theory’.

Answer 13

Myosin head with ADP attached forms cross bridge with actin.
Power stroke: Myosin head changes shape & loses ADP, pulling actin over myosin.
ATP attaches to myosin head, causing it to detach from actin.
ATPase hydrolyses ATP → ADP + Pi so myosin head can return to original position.
Myosin head re-attaches to actin further along filament.

Question 14

How does sliding filament action cause a myofibril to shorten?

Answer 14

Myosin heads flex in opposite directions = actin filaments are pulled towards each other.
Distance between adjacent sarcomere Z-lines shortens.
Sliding filament action occurs up to 100 times per second in multiple sarcomeres.

Question 15

State 4 pieces of evidence that support the sliding filament theory.

Answer 15

H-zone narrows.
I-band narrows.
Z-lines get closer (sarcomere shortens).
A-band remains same width (proves that myosin filaments do not shorten).

Question 16

What happens during muscle relaxation?

Answer 16

Ca²⁺ is actively transported back into endoplasmic reticulum.
Tropomyosin once again blocks actin binding site.

Question 17

Explain the role of phosphocreatine in muscle contraction.

Answer 17

Phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited (e.g. during vigorous exercise).

Question 18

How could a student calculate the length of one sarcomere?

Answer 18

View thin slice of muscle under optical microscope.
Calibrate eyepiece graticule.
Measure distance from middle of one light band to middle of another.

Question 19

Where are slow and fast-twitch muscle fibres found in the body?

Answer 19

Slow-twitch: Sites of sustained contraction (e.g. calf muscle).
Fast-twitch: Sites of short-term, rapid, powerful contraction (e.g. biceps)

Question 20

Explain the role of slow and fast-twitch muscle fibres.

Answer 20

Slow-twitch: Long-duration contraction; well-adapted to aerobic respiration to prevent lactate buildup.
Fast-twitch: Powerful short-term contraction; well-adapted to anaerobic respiration

Question 21

Explain the structure and properties of slow-twitch muscle fibres.

Answer 21

Glycogen store: Many terminal ends can be hydrolysed to release glucose for respiration.

Contain myoglobin: Higher affinity for oxygen than haemoglobin at lower partial pressures.

Many mitochondria: Aerobic respiration produces more ATP.

Surrounded by many blood vessels: High supply of oxygen & glucose.

Question 22

Explain the structure and properties of fast-twitch muscle fibres.

Answer 22

Large store of phosphocreatine.
More myosin filaments.
Thicker myosin filaments.
High concentration of enzymes involved in anaerobic respiration.
Extensive sarcoplasmic reticulum: Rapid uptake & release of Ca²⁺.

Question 23

What is a motor unit?

Answer 23

One motor neuron supplies several muscle fibres, which act simultaneously as one functional unit.

Question 24

what are microfibrils made up of

Answer 24

actin and myosin

Question 25

actin

Answer 25

is a globular protein long chain thinner and consists of two stands twisted around one another

Question 26

myosin

Answer 26

made up of two types of proteins: 1- fibrous protein arranged into a filament 2- globular protein - two bulbous structure at one end thicker and consists of long rod- shaped tails with bulbous heads that project to the side

Question 27

tropomyosin

Answer 27

forms a fibrous strand around the actin filament

Question 28

what is the neuromuscular junction

Answer 28

point where a motor neuron meets a skeletal muscle fibre

Question 29

contraction of muscle involves

Answer 29

the sliding filament mechanism

Question 30

muscle relaxation

Answer 30

when nervous stimulation end Ca2+ ions AT back into endoplasmic reticulum - using energy from hydrolysis of ATP reabsorption of Ca ions allows tropomyosin to block the actin filament again Myosin head now unable to bind to actin filament contraction end

Question 31

energy for muscle contraction

Answer 31

hydrolysis of ATP for... movement of myosin head Reabsorption of ca ions into endoplasmic reticulum by AT

Question 32

explain the band pattern shown in the diagram

Answer 32

light I band only actin H zone/ band only myosin darkest overlapping region actin and myosin

Question 33

Another group of scientists suggested that a decrease in the force of muscle contraction is caused by an increase in the concentration of inorganic phosphate, Pi, in muscle tissues. Their hypothesis is that an increase in the concentration of Pi prevents the release of calcium ions within muscle tissues. Explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contraction.

Answer 33

less tropomyosin moved from binding site fewer actinomyosin bridges formed Myosin head doesn't move less ATP hydrolyase

Question 34

In muscles, pyruvate is converted to lactate during prolonged exercise. Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration

Answer 34

regenerates/ produces NAD so glycolysis continues

Question 35

Name structures C, D and E.

Answer 35

C- M line/ myosin filament D - mitochondrion E - myofibril

Question 36

Give the name of the structure shown between points A and B.

Answer 36

sacromere

Question 37

The image shows glycogen granules present in skeletal muscle. Explain their role in skeletal muscle.

Answer 37

as a store of glucose to be hydrolysed to glucose for respiration/ to provide ATP

Question 38

During vigorous exercise, the pH of skeletal muscle tissue falls. This fall in pH leads to a reduction in the ability of calcium ions to stimulate muscle contraction. Suggest how.

Answer 38

1- low ph changes shape of calcium ion receptors 2- fewer calcium ions bind to tropomyosin 3- fewer tropomyosin molecules move away 4- fewer binding sites on actin revealed 5- fewer cross - bridges can form 6- fewer myosin heads can bind

Question 39

Describe the roles of calcium ions and ATP in the contraction of a myofibril. 5

Answer 39

1-Cacium ions diffuse into myofibrils from sacroplasmic reticium 2- causes movement of tropomyosin on actin 3- Movement causes exposure of the binding site on the actin 4- myosin heads attach to binding sites on actin 5- hydrolysis of ATP on myosin head causes myosin heads to bend 6- bending pulling actin molecules 7- attachment of a new ATP molecule to each myosin head causes myosin heads to detach from actin sites

Question 40

What is the role of ATP in myofibril contraction?

Answer 40

1- To break actinomyosin bridges 2.    to bend the myosin head 3- change shape of myosin head 4.      So actin filaments are moved inwards 4.      For active transport of calcium ions into sacroplasm reticlum

Question 41

 Describe and explain how taking creatine supplements (lines 5–6) and ‘carbohydrate loading’ (lines 6–7) can improve performance of different types of muscle fibres during different types of exercise.

Answer 41

1- Fast skeletal muscle fibres used during intense exercise; 2.      Slow skeletal muscle fibres used during long-term exercise; 3.      Creatine used to form phosphocreatine; 4.      (Phosphocreatine) combines with ADP to form ATP; 5.      (Carbohydrate/glucose) stored as glycogen    Glycogen hydrolysed to glucose OR Glycogenolysis; 7.      Glucose for respiration;

Question 42

An increase in muscle activity causes an increase in heart rate (lines 12–1 describe and explain how

Answer 42

ncrease in CO2 detected by chemoreceptors; Accept increase in acidity/H+ or decrease in pH for increase in CO2. Ignore location of chemoreceptors. 2.      Send (more) impulses to cardiac centre OR Send (more) impulses to the medulla; 2 and 3 Reject reference to ‘an/one impulse’ once only. 2 and 3 Reject ‘signals’, ‘messages’ for ‘impulses’ once only. 2 and 3 Accept ‘action potentials’ for impulses. 3.      More impulses (from centre/medulla) along sympathetic pathway/neurones/nerves OR Fewer impulses (from centre/medulla) along parasympathetic/vagus pathway/neurones /nerves; 4.      (To) SAN;

Question 43

ATP is an energy source used in many cell processes. Give two ways in which ATP is a suitable energy source for cells to use.

Answer 43

  1.      Releases relatively small amount of energy / little energy lost as heat; Key concept is that little danger of thermal death of cells 2.      Releases energy instantaneously; Key concept is that energy is readily available 3.      Phosphorylates other compounds, making them more reactive; 4.      Can be rapidly re-synthesised; 5.      Is not lost from / does not leave cells.