Questions on muscles and movement: the sliding-filament model

Question 1

Describe how the release of calcium ions from the sarcoplasmic reticulum causes the thin filaments of a sarcomere to slide inwards.

Accepted Answer

B1 calcium ions bind to troponin, which changes shape B1 troponin pulls tropomyosin aside, exposing the myosin binding sites on the actin, which were covered before B1 a myosin head carrying ADP and phosphate attaches to an exposed binding site and forms a cross-bridge B1 the head bends in a power stroke, releasing the ADP and phosphate and pulling the thin filament about 10 nm towards the centre of the sarcomere A1 ATP binds to the head so that it detaches, and hydrolysis of that ATP returns it to its upright position to attach further along; heads work out of step so the filament is never released

Question 2

Explain the three ways ATP is used by a contracting and relaxing muscle fibre, and explain why the muscles of a body stiffen after death.

Accepted Answer

B1 ATP binding to the myosin head causes it to detach from the actin at the end of a power stroke B1 hydrolysis of that ATP by the head's own ATPase activity returns the head to its upright position, ready to attach further along the thin filament B1 ATP is used to pump calcium ions back into the sarcoplasmic reticulum during relaxation, so tropomyosin can slide back over the binding sites A1 after death respiration stops and ATP runs out, so the heads cannot detach and calcium cannot be pumped back, and the cross-bridges stay formed until the proteins themselves break down

Question 3

Compare slow twitch muscle fibres with fast twitch muscle fibres, linking each structural difference to the way the fibre works.

Accepted Answer

B1 slow twitch fibres contract more slowly but in a sustained way, whereas fast twitch fibres contract rapidly and powerfully for a short time B1 slow twitch fibres have many mitochondria and respire aerobically, whereas fast twitch fibres have few and rely on anaerobic glycolysis with a large glycogen store B1 slow twitch fibres have a dense capillary supply and much myoglobin, so oxygen is delivered and stored, whereas fast twitch fibres have a sparse supply and little myoglobin and are paler A1 slow twitch fibres therefore resist fatigue and dominate postural muscles and the calf muscles of distance runners, whereas fast twitch fibres accumulate lactate and tire quickly, and dominate the leg muscles of sprinters

Question 4

A relaxed muscle fibre is 3.0 cm long and its myofibrils are made of sarcomeres each 2.4 micrometres long, joined end to end. Calculate the number of sarcomeres in one myofibril of this fibre, and calculate the length of the fibre in centimetres when every sarcomere has shortened to 2.0 micrometres.

Accepted Answer

M1 convert to the same unit: 3.0 cm is 30 000 micrometres M1 number of sarcomeres is the fibre length divided by the length of one sarcomere, so 30 000 ÷ 2.4 A1 12 500 sarcomeres in series A1 12 500 × 2.0 = 25 000 micrometres, which is 2.5 cm

Question 5

A sprinter's leg muscles fail after about 40 seconds of maximal effort. A student says this is because the muscles have used up all their ATP. Suggest why that explanation is wrong, and suggest what does limit the muscles over that time.

Accepted Answer

B1 the ATP already in the fibres lasts only about two seconds, so a muscle working for 40 seconds must be regenerating ATP throughout rather than running on a store of it B1 phosphocreatine in the sarcoplasm transfers its phosphate group directly to ADP to regenerate ATP anaerobically, but it is spent within about ten seconds B1 most of the rest of the effort is supplied by anaerobic glycolysis, because oxygen cannot be delivered fast enough for aerobic respiration to meet the demand A1 anaerobic glycolysis yields only two ATP per glucose and the lactate it produces accumulates, so what fails is the rate at which ATP can be resupplied rather than the quantity of ATP present

Question 6

Explain how an action potential arriving at a neuromuscular junction leads to the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.

Accepted Answer

B1 acetylcholine released at the neuromuscular junction binds to receptors on the sarcolemma and depolarises it B1 the depolarisation spreads across the sarcolemma and down the transverse tubules, which are folds of the sarcolemma running deep into the fibre B1 this carries the depolarisation to the sarcoplasmic reticulum, the specialised endoplasmic reticulum that surrounds the myofibrils and stores calcium ions B1 the depolarisation triggers the sarcoplasmic reticulum to release its stored calcium ions into the sarcoplasm

Question 7

Compare thick filaments with thin filaments in a sarcomere, referring to what each is made of, where each is anchored, and what happens to their length during contraction.

Accepted Answer

B1 thick filaments are made of myosin, with tails bundled together and heads projecting outwards, whereas thin filaments are made of actin wound with tropomyosin and studded with troponin B1 thick filaments sit in the middle of the sarcomere, unattached to the Z lines, whereas thin filaments are anchored to the Z lines at each end B1 during contraction the thin filaments slide inward past the thick filaments, whereas the thick filaments stay in a fixed position relative to the centre of the sarcomere B1 neither type of filament changes in length during contraction; only the degree of overlap between them changes

Question 8

A myofibril contains sarcomeres each 2.5 micrometres long when relaxed. During a contraction each sarcomere shortens by 15%. Calculate the length of one contracted sarcomere, and calculate the number of sarcomeres needed, joined end to end, to make a contracted myofibril 1.7 mm long.

Accepted Answer

M1 contracted sarcomere length = 2.5 × (1 − 0.15) = 2.5 × 0.85 A1 2.1 micrometres to two significant figures, keeping the unrounded 2.125 for the next step M1 convert 1.7 mm to micrometres, 1700 µm, and divide by the contracted sarcomere length: 1700 ÷ 2.125 A1 800 sarcomeres in the contracted myofibril

Question 9

A drug blocks the calcium pumps in the sarcoplasmic reticulum, so calcium ions cannot be pumped back in after release. Suggest the effect of this drug on a muscle fibre's ability to relax after a contraction, and suggest what would eventually happen to its ATP concentration if contraction continued unchecked.

Accepted Answer

B1 with the pumps blocked, calcium ions would stay in the sarcoplasm rather than being returned to the sarcoplasmic reticulum, so calcium would remain bound to troponin B1 tropomyosin would not move back over the myosin binding sites on actin, so cross-bridges could keep forming and the fibre could not relax B1 the cross-bridge cycle would continue running for as long as ATP was available, with each cycle using ATP B1 ATP would be used up faster than in a normal resting muscle, and once it ran out the myosin heads could not detach from actin, so the muscle would become permanently contracted, similar to rigor mortis

Question 10

State what happens to the width of the A band, the width of the I band and the width of the H zone when a sarcomere contracts.

Accepted Answer

B1 the A band is unchanged B1 the I band gets shorter B1 the H zone gets shorter, and in a strong contraction it can disappear altogether

Question 11

Describe the structural organisation of a skeletal muscle fibre, from the whole fibre down to a single sarcomere.

Accepted Answer

B1 a muscle fibre is a single, multinucleate cell containing hundreds of myofibrils B1 each myofibril is a chain of sarcomeres joined end to end at Z lines B1 the sarcomere, running from one Z line to the next, is the functional unit of contraction, containing the thick and thin filaments

Question 12

Explain why the A band does not change in width during contraction, while the I band and the H zone both get shorter.

Accepted Answer

B1 the A band is the length of the thick filaments, which neither shorten nor move relative to the centre of the sarcomere during contraction, so its width stays the same B1 the I band contains only thin filaments, and as they slide further into the A band, less of their length lies outside it, so the I band narrows B1 the H zone is the part of the A band not yet reached by thin filaments; as they slide further in, this unreached region shrinks, so the H zone narrows too

Question 13

Explain why phosphocreatine can regenerate ATP more rapidly than aerobic respiration can, but only for a short time.

Accepted Answer

B1 phosphocreatine regenerates ATP by transferring its phosphate group directly to ADP in a single step, whereas aerobic respiration requires a longer sequence of reactions including the delivery of oxygen, so phosphocreatine responds faster B1 the store of phosphocreatine in the sarcoplasm is small, so it is used up within about ten seconds of maximal effort B1 aerobic respiration continues for as long as oxygen and substrate keep being delivered, so it sustains ATP production for much longer once it is underway

Question 14

Name the protein that makes up the thick filaments of a sarcomere, and name the two proteins found on the thin filaments in addition to actin.

Accepted Answer

B1 myosin makes up the thick filaments B1 tropomyosin and troponin are found on the thin filaments, in addition to actin

Question 15

State what the term multinucleate means as applied to a muscle fibre, and state the name given to a muscle fibre's cell surface membrane.

Accepted Answer

B1 multinucleate means the cell contains many nuclei rather than just one B1 the cell surface membrane of a muscle fibre is called the sarcolemma