3.6.3 - CONTRACTION OF SKELETAL MUSCLES

Question 1

explain the role of glycogen granules in skeletal muscle (2)

Answer 1

1. to be hydrolysed to glucose
2. for respiration

Question 2

describe the roles of calcium ions and ATP in the contraction of a myofibril (5)

Answer 2

1. calcium ions diffuse into myofibrils from sarcoplasmic reticulum
2. calcium ions cause movement of tropomyosin on actin
3. this movement causes exposure of binding sites on the actin
4. myosin heads attach to binding sites on actin
5. hydrolysis of ATP on myosin heads causes myosin heads to bend
6. bending/pulling actin molecules
7. attachment of a new ATP molecule to each myosin head cause myosin heads to detach from actin sites

Question 3

explain how a decrease in the concentration of calcium ions within muscle tissue could cause a decrease in the force of muscle contraction (3)

Answer 3

1. less/no tropomyosin removed from actin binding sites
2. so fewer/no actinomyosin/cross-bridges bridges formed/actin and myosin do not bind
3. myosin head does not pull actin filaments

Question 4

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 (2)

Answer 4

1. regenerates/produces NAD OR oxidises reduced NAD
2. so glycolysis continues

Question 5

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 (3)

Answer 5

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 OR fewer myosin heads can bind

Question 6

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 (2)

Answer 6

1. releases relatively small amount of energy/little energy is lost as heat
2. releases energy instantaneously

Question 7

what is the role of ATP in myofibril contraction? (2)

Answer 7

1. reaction with ATP breaks/allows binding of myosin to actin/actinomyosin bridge.
2. provides energy to move myosin head

Question 8

explain why both slow and fast muscle fibres contain ATPase. (2)

Answer 8

1. spitting/hydrolysis of ATP
2. muscle contraction enquires energy/ATP
3. use of ATP by myosin

Question 9

what is the role of phosphocreatine (PC) in providing energy during muscle contraction? (2)

Answer 9

1. phosphocreatine provides phosphate/phosphorylates
2. to make ATP

Question 10

people who have McArdle’s disease produce less ATP than healthy people.

as a result, they are not able to maintain strong muscle contraction during exercise.

use your knowledge of the sliding filament theory to suggest why (3)

Answer 10

ATP is needed for:
1. attachment/cross bridges between actin and myosin
2. power strok/movement of myosin heads/pulling of actin
3. detachment of myosin heads
4. myosin heads move back to original position/recovery stroke

Question 11

describe the part played by tropomyosin in myofibril contraction (2)

Answer 11

1. moves out of the way when calcium ions bind
2. allowing myosin to bind to actin / cross bridge formation

Question 12

describe the part played by myosin in myofibril contraction (2)

Answer 12

1. head of myosin binds to actin and pulls actin/slide past
2. myosin detaches from actin and moves further along
3. this sues ATP

Question 13

describe the part played by ATP in myofibril contraction (2)

Answer 13

1. releases myosin from actin
2. causes myosin head to move

Question 14

explain how muscles maintain posture (3)

Answer 14

1. antagonistic muscles
2. working at/across joints
3. both contract to keep joint/the body at a certain angle
4. isometric contraction
5. only a few fibres contract to avoid fatigue/sow muscle fibres use

Question 15

describe how calcium ions cause the myofibril to start contracting (2)

Answer 15

1. Ca2+ binds to part of the actin
2. causes tropomyosin to be displaced
3. uncovers binding sites on actin

Question 16

describe the events that occur within a myofibril which enable it to contract (3)

Answer 16

1. myosin heads bind to actin / cross bridge formation
2. myosin heads swivel
3. causing actin to slide relative to myosin
4. energy is provided for this by the hydrolysis of ATP

Question 17

cross-channel swimmers may suffer from muscle fatigue during which the contraction mechanism is disrupted.

one factor thought to contribute to muscle fatigue is a decrease in the availability of calcium ions within muscle fibres.

explain how a decrease in the availability of calcium ions could disrupt the contraction mechanism in muscles. (3)
_______________________

Answer 17

1. cannot move tropomyosin from binding sites on actin
2. myosin heads do not bind
3. energy not released from ATp

Question 18

calcium ions are involved in myofibril contraction.

describe how (3)

Answer 18

1. rise in Ca2+
2. leads to movement of tropomyosin
3. exposes binding sits on actin filament
4. allows cross-bridge formation
5. activate ATPase on myosin