Explain how skeletal muscle contracts.

Active skeletal muscle requires a good supply of oxygen. Outline the mechanism of ventilation in the lungs.

Remember, up to TWO “quality of construction” marks per essay.

a. sliding filament model / filaments/actin and myosin slide past each other;
b. action potential/depolarisation/nerve impulse arrives at end of motor neurone;
c. neurotransmitter/acetylcholine released causing action potential (in muscle fibre);
d. sarcoplasmic reticulum releases calcium ions;
e. calcium ions cause binding sites on actin/for myosin to be exposed;
f. myosin heads bind to sites on actin/form cross-bridges;
g. myosin (head) moves actin filament using energy from ATP;
h. actin moved towards the centre of sarcomere/M line/M band;
i. sarcomeres shortened;
j. (binding of) ATP causes release of myosin head from actin;
k. conversion of ATP to ADP and Pi causes myosin heads to change angle;
l. cycle (of events) repeated (during muscle contraction);
Accept the above points in annotated diagrams.

Remember, up to TWO “quality of construction” marks per essay.

during inhalation:
a. external intercostal muscles contract moving rib cage up and out;
b. diaphragm contracts becoming lower/flatter;
c. increase in volume and decrease in pressure (of thorax);
d. air flows into lungs as atmospheric pressure is higher;

during exhalation:
e. internal intercostal muscles contract so ribs move in and down;
f. diaphragm relaxes and returns to domed shape;
g. decrease in volume and (therefore) increase in pressure (of thorax);
h. air moves out until pressure in lungs falls/is equal to atmospheric pressure;
i. abdominal muscles can be used to make a stronger/forced exhalation;

Contraction of muscles

Perhaps because Question 6 tended to attract many of the weaker candidates, accounts of muscle contraction were mostly very poor. Some candidates missed the point and wrote about reflex arcs instead. The way in which ATP releases its energy and how this energy is then used was very rarely correct. Diagrams helped with some answers but only where there was full annotation.  

Ventilation

Stronger candidates wrote full and accurate accounts and often scored full marks but others wrote error-strewn and confused accounts. A popular misconception was that the gas breathed in is oxygen and the gas breathed out is carbon dioxide.