3.6.3 Skeletal muscle Questions | AQA A-level Biology

Specification route: 3.6.3
Question bank: 18 questions
Course stage: Year 13 / A-level only

Sample questions

QUESTION 1 · 3.6.3 · LEVEL 4

According to the sliding filament theory, what triggers the exposure of the myosin binding sites on the actin filament?

Calcium ions bind to troponin, causing the tropomyosin molecule to change shape and move away from the binding sites.
ATP binds to the actin filament, forcing it to physically untwist and expose the sites.
An action potential jumps directly from the sarcolemma onto the actin filament.
Acetylcholine diffuses across the sarcoplasm and chemically dissolves the tropomyosin.

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An ion released from the sarcoplasmic reticulum is the key to unlocking the filament.

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Answer: Calcium ions bind to troponin, causing the tropomyosin molecule to change shape and move away from the binding sites.

The sliding filament theory of muscle contraction. Action potentials trigger the release of $Ca^{2+}$ from the sarcoplasmic reticulum. $Ca^{2+}$ binds to troponin, which moves tropomyosin, exposing the binding sites on actin.

QUESTION 2 · 3.6.3 · LEVEL 4

During muscle contraction, what is the specific role of ATP binding to the myosin head?

It causes the myosin head to detach from the actin filament, breaking the cross-bridge.
It provides the power stroke that physically pulls the actin filament along.
It actively pumps calcium ions into the sarcoplasm to initiate contraction.
It binds troponin and tropomyosin together to maintain contraction.

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When a person dies and stops producing ATP, their muscles lock up (rigor mortis) because the heads cannot detach.

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Answer: It causes the myosin head to detach from the actin filament, breaking the cross-bridge.

The role of ATP in muscle contraction. ATP binding to the myosin head causes it to detach from the actin filament. The subsequent hydrolysis of ATP (to ADP and Pi) provides the energy to 'cock' the myosin head back to its original position.

QUESTION 3 · 3.6.3 · LEVEL 2

Which characteristic is typical of slow-twitch muscle fibres?

They contain a high concentration of myoglobin, many mitochondria, and a rich capillary supply for aerobic respiration.
They contract extremely rapidly and fatigue very quickly.
They have a massive store of phosphocreatine and very few mitochondria.
They rely primarily on anaerobic respiration to generate ATP for explosive movements.

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These fibres are adapted for endurance activities, like a marathon.

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Answer: They contain a high concentration of myoglobin, many mitochondria, and a rich capillary supply for aerobic respiration.

The role of slow and fast skeletal muscle fibres. Slow-twitch fibres are adapted for endurance and aerobic respiration, possessing high levels of myoglobin (giving them a red colour), many mitochondria, and a dense capillary network.

QUESTION 4 · 3.6.3 · LEVEL 3

In fast-twitch muscle fibres, what is the role of phosphocreatine?

It acts as a rapidly available reserve of phosphate to quickly regenerate ATP from ADP in anaerobic conditions.
It acts as the final electron acceptor in the electron transfer chain during aerobic respiration.
It is an enzyme that directly hydrolyses the cross-bridges between actin and myosin.
It actively transports calcium ions back into the sarcoplasmic reticulum.

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Explosive muscle contractions use up ATP faster than glycolysis can replace it.

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Answer: It acts as a rapidly available reserve of phosphate to quickly regenerate ATP from ADP in anaerobic conditions.

The role of phosphocreatine (PCr) in providing a supply of ATP. PCr donates a phosphate group to ADP to rapidly form ATP ($ADP + PCr \rightarrow ATP + Creatine$), essential for short, explosive contractions in fast-twitch fibres.

QUESTION 5 · 3.6.3 · LEVEL 3

A student suggests that the neuromuscular junction is identical to a cholinergic synapse between two neurones. Which of the following is a key difference?

Neuromuscular junctions are only ever excitatory, whereas cholinergic synapses between neurones can be excitatory or inhibitory.
Neuromuscular junctions use adrenaline as a neurotransmitter, whereas cholinergic synapses use acetylcholine.
Neuromuscular junctions do not possess any acetylcholinesterase.
Neuromuscular junctions trigger an action potential that travels backwards up the motor neurone.

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Muscles only receive signals to contract; they don't receive signals to actively 'not contract' via synapses.

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Answer: Neuromuscular junctions are only ever excitatory, whereas cholinergic synapses between neurones can be excitatory or inhibitory.

Comparison of transmission across a cholinergic synapse and across a neuromuscular junction. NMJs only link neurones to muscles and are always excitatory. Cholinergic synapses link neurones to neurones (or effectors) and can be excitatory or inhibitory.

QUESTION 6 · 3.6.3 · LEVEL 4

When a skeletal muscle contracts, what changes are observed in the banding pattern of a sarcomere under an electron microscope?

The I band becomes narrower, the Z lines move closer together, and the H zone becomes narrower, but the A band remains the same width.
The A band becomes narrower, the I band remains the same width, and the Z lines move further apart.
All bands and zones become narrower as the actin and myosin filaments themselves physically shorten.
The H zone disappears completely, while the I band and A band double in width.

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The dark A band represents the length of the myosin filaments, which do not change length.

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Answer: The I band becomes narrower, the Z lines move closer together, and the H zone becomes narrower, but the A band remains the same width.

According to the sliding filament theory, actin slides over myosin. Therefore, the I band (only actin) and H zone (only myosin) narrow as the overlap increases. The A band (length of myosin) stays the same, and the whole sarcomere (Z line to Z line) shortens.

QUESTION 7 · 3.6.3 · LEVEL 2

In a resting skeletal muscle, why are the myosin heads unable to bind to the actin filament?

The protein tropomyosin blocks the myosin-binding sites on the actin filament.
There is no ATP available in a resting muscle to form the cross-bridges.
The sarcoplasmic reticulum has absorbed all the actin molecules.
The myosin heads are permanently denatured until an action potential arrives.

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A regulatory protein acts as a physical barrier.

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Answer: The protein tropomyosin blocks the myosin-binding sites on the actin filament.

In a resting muscle, tropomyosin wraps around the actin filament, physically blocking the myosin-binding sites. Calcium ions are required to move tropomyosin and expose the sites.

QUESTION 8 · 3.6.3 · LEVEL 2

Which of the following adaptations is characteristic of fast-twitch skeletal muscle fibres?

A high concentration of glycogen and enzymes for anaerobic respiration, allowing for rapid, powerful contractions.
A massive capillary network and high myoglobin content for sustained aerobic respiration.
A high concentration of mitochondria to produce ATP constantly over long periods.
The complete absence of phosphocreatine within the sarcoplasm.

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These fibres are used for explosive movements like sprinting, which rely on energy generated without oxygen.

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Answer: A high concentration of glycogen and enzymes for anaerobic respiration, allowing for rapid, powerful contractions.

The role of slow and fast skeletal muscle fibres. Fast-twitch fibres are adapted for intense, short bursts of activity. They have thicker/more myosin, high glycogen stores, anaerobic enzymes, and high phosphocreatine stores.

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