Skeletal Muscles

Question 1

How do muscles act?

Answer 1

Muscles act in antagonistic pairs against an incompressible skeleton.

Question 2

Describe the gross structure of skeletal muscles.

Answer 2

• Sarcolemma → T-tubules
• Sarcoplasm → Mitochondira and myofibrils
• Sarcoplasmic reticulum (SR) → Protein pumps

Question 3

Describe the structure of myofibrils.

Answer 3

• Myosin (thick filaments)
• Actin (thin filaments)
• H-band (only myosin)
• A-band (myosin and actin) CONSTANT
• I-band (only actin)
• Sarcomere (section of myofibril between two Z-lines)
• Z-lines (attachment for actin)
• M-lines (attachment for myosin)

Question 4

Describe the sliding filament model?

Answer 4

1. Ca2+ ions released into sarcoplasm
2. Calcium ions bind to troponin molecules, stimulating them to change shape
3. Tropomyosin moves position exposing the myosin binding sites
4. heads of the myosin molecules bind with these exposed sites forming cross bridges
5. cross- bride causes tension, sliding the actin filament along the myosin ADP and Pi released
6. ATP binds to the myosin heads cuasing myosin head to change shape, no longer complementary to actin and detaches
7. ATP hydrolase hydrolyses ATP into ADP and Pi which causes the myosin heads to move back to their original positions

Question 5

Outline the roles of ATP and phosphocreatine.

Answer 5

• ATP is needed for the return movement of myosin heads that causes the actin filaments to slide
• The return of calcium ions back into the sarcoplasmic reticulum occurs via active transport
• The mitochondria present in the muscles fibres are able to aerobically respire and produce ATP but this is slow
• Phosphocreatine is a molecule stored by muscles that can be used for the rapid production of ATP
• A phosphate ion from phosphocreatine is transferred to ADP
  ADP + phosphocreatine → ATP + creatine
• It allows for muscles to continue contracting for a short period of time in times where aerobic respiration cant create enough ATP to meet the demand

Question 6

Fast skeletal muscle fibres:

Answer 6

• Fast muscle fibres contract rapidly
• The myosin heads bind and unbind from the actin-binding sites five times faster than slow muscle fibres
• Require large amounts of calcium ions present to stimulate contraction
• They rely on anaerobic respiration for ATP supply
• They are suited to short bursts of high-intensity activity as they fatigue quickly due to the lactate produced from anaerobic respiration
• Fast muscle fibres have fewer capillaries
• Slow supply of oxygen and glucose for aerobic respiration
• Low amounts of myoglobin are present in fast muscle fibres
• Myoglobin functions as a store of oxygen in muscles and increases the rate of oxygen absorption from the capillaries
• Paler in colour than slow muscle fibres

Question 7

Slow skeletal muscle fibres:

Answer 7

• Slow muscle fibres contract more slowly and are suited to sustained activities
• They rely on aerobic respiration for ATP
• They fatigue less quickly due to less lactate production, making them ideal for endurance
• These muscles have to contract for long periods of time in order to keep the skeleton erect when standing or sitting
• Slow muscle fibres have a denser network of capillaries
• This means they have a short diffusion distance and a good supply of oxygen and glucose for aerobic respiration
• High amounts of myoglobin, haemoglobin and mitochondria are present
• This increases the rate of oxygen supply, oxygen absorption and aerobic respiration
• Slow muscle fibres appear a dark red

Question 8

why are there so many mitochondria in the myofibrils

Answer 8

many mitochondria= more ATP
for muscle contraction

Question 9

In contraction the sarcomere bands..

Answer 9

• I band and H zone decrease in width
• A band will never change bc myosin is constant
  -Z lines closer together

Question 10

slow twitch fibres structure: structure and properties

Answer 10

• structure: contains large stores of myoglobin (protein that stores lots of O2), a rich blood supply and many mitochondria
• properties: contract slower and can respire aerobically for longer periods of time due to bloody supply and myoglobin store

Question 11

fast twitch fibres structure and properties

Answer 11

• Structure: thicker muscle and more myosin filaments, large store of glycogen and phosphocreatine to help generate ATP,
• Properties: contract faster to provide short burst of powerful contractions, (these muscles are adapted for exercise)
• muscles fatigue quickly due to build up of lactic acid