AQA A-level Biology: Muscle Contraction (7402) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions follow section 3.6.3 (A-level only) of the AQA AS and A-level Biology specification for first teaching 2015 (7402). Explanations are in the study guide and the revision notes. See also the AQA A-level Biology hub, the printable checklist and the free diagnostics. Data in the questions are supplied for practice.

Questions

1. Explain why the muscles that move the knee must work as an antagonistic pair. [3]

2. (a) Describe what you would see in a longitudinal section of skeletal muscle under an optical microscope. [2] (b) Describe the arrangement of thick and thin filaments in one sarcomere. [3]

3. Two electron micrographs of one myofibril are taken at ×18 000. On the first, relaxed, a sarcomere is 41.4 mm long. On the second, contracted, it is 31.5 mm long. The A band is 27 mm long on both images.

(a) Calculate the actual lengths of the relaxed and contracted sarcomere, in µm. [2] (b) Calculate the percentage decrease in sarcomere length, to 1 decimal place. [1] (c) Calculate the length of the I band within one sarcomere when relaxed and when contracted. [2] (d) State why the A band stays the same length. [1]

4. Describe how calcium ions and ATP bring about the shortening of a sarcomere. [6]

5. A sprinter’s muscle contains 4.5 mmol of ATP and 14.5 mmol of phosphocreatine per kg. During a sprint, ATP is used at 2.0 mmol per kg per second. Assume one phosphocreatine regenerates one ATP.

(a) Calculate how long the ATP store alone would last. [1] (b) Calculate how long the ATP and phosphocreatine stores last together. [2] (c) Explain why phosphocreatine suits the start of a sprint better than aerobic respiration. [2] (d) State how the phosphocreatine store is restored after the sprint. [1]

6. The table shows data supplied for two muscle fibres from the same leg muscle.

Property	Fibre P	Fibre Q
Capillaries touching each fibre	2.5	5.5
Myoglobin (mg per g)	1.4	6.3
Glycogen (mg per g)	24	10
Time to peak tension (ms)	35	90

(a) Identify the slow twitch fibre, using two pieces of evidence from the table. [3] (b) Calculate how many times more myoglobin fibre Q has than fibre P. [1] (c) Give one place in the body where you would expect to find fibre P. [1]

7. Five volunteers squeezed a hand grip as many times as they could in each of four consecutive 30 second blocks, with no rest. The table shows the number of squeezes.

Volunteer	Block 1	Block 2	Block 3	Block 4
1	62	58	49	41
2	70	63	55	48
3	55	52	44	38
4	66	60	50	40
5	59	55	47	39

(a) Calculate the mean number of squeezes in block 1 and in block 4. [2] (b) Calculate the percentage decrease in the mean from block 1 to block 4, to 1 decimal place. [2] (c) Explain why the number of squeezes falls. [2]

8. Explain why a body becomes stiff soon after death. [3]

9. Explain the roles of calcium ions and tropomyosin when a muscle relaxes after stimulation stops. [4]

10. Explain why the muscles of the back suit slow twitch fibres, while the muscles that move the eye suit fast twitch fibres. [4]

11. A sample of 250 fibres was counted from a leg muscle of three people.

Person	Fibres counted	Fibres that were fast twitch
Sprinter	250	175
Marathon runner	250	45
Untrained person	250	120

(a) Calculate the percentage of slow twitch fibres in each person. [3] (b) Explain how the fibre composition of the marathon runner suits her event. [2] (c) Suggest two limitations of drawing conclusions from this data. [2]

Answers

1. Muscles can only pull when they contract [1]; the quadriceps contracts to straighten the knee while the hamstring relaxes, and the reverse bends it, so one muscle cannot move a bone both ways [1]; the incompressible skeleton acts as a lever, so the second muscle is needed to return the bone [1] [3] Examiner insight: Naming which muscle contracts and which relaxes shows the explanation; “they work together” alone describes less.

2. (a) Fibres lie parallel, with several nuclei in each fibre [1]; regular light and dark bands (striations) across the fibres [1] [2] (b) Thin actin filaments are attached to the Z lines [1]; thick myosin filaments lie in the middle, in the A band [1]; thin and thick filaments overlap at the ends of the A band, with a central H zone of thick filaments only [1] [3] Examiner insight: Say which filament is where, and name the bands; “light band and dark band” alone leaves out the filaments.

3. (a) Relaxed: 41.4 ÷ 18 000 = 0.0023 mm = 2.3 µm [1]; contracted: 31.5 ÷ 18 000 = 0.00175 mm = 1.75 µm [1] [2] (b) (2.3 − 1.75) ÷ 2.3 × 100 = 23.9% [1] (c) A band = 27 ÷ 18 000 = 1.5 µm; relaxed I band = 2.3 − 1.5 = 0.8 µm [1]; contracted I band = 1.75 − 1.5 = 0.25 µm [1] [2] (d) The A band is the length of the thick filaments, which do not shorten as the thin filaments slide [1] Examiner insight: Write the unit conversion on its own line; a slip in powers of ten is easy to make and hard to spot otherwise.

4. Calcium ions are released from the sarcoplasmic reticulum [1]; they cause tropomyosin to move, exposing binding sites on actin [1]; myosin heads bind to actin, forming actinomyosin bridges [1]; the heads tilt, which drags the thin filaments inwards [1]; ATP binds to the heads, which detaches them, and ATPase hydrolyses ATP so the heads return to their original position [1]; the cycle repeats, so the Z lines move closer and the sarcomere shortens [1] [6] Examiner insight: Keep the events in order and say what each does; a list of names without functions gains less.

5. (a) 4.5 ÷ 2.0 = 2.25 s [1] (b) Total = 4.5 + 14.5 = 19.0 mmol per kg [1]; 19.0 ÷ 2.0 = 9.5 s [1] [2] (c) Phosphocreatine regenerates ATP in one step, so ATP is available very quickly [1]; it needs no oxygen, whereas aerobic respiration is slower to supply ATP at the start of exercise [1] [2] (d) Creatine is rephosphorylated using ATP from aerobic respiration during recovery [1] Examiner insight: A suggestion needs a reason; “it is quick” is stronger when you say why quick matters at the start.

6. (a) Fibre Q is slow twitch [1]; it has more capillaries (5.5 against 2.5), so more oxygen for aerobic respiration [1]; it has more myoglobin (6.3 against 1.4), or a longer time to peak tension (90 ms against 35 ms) [1] [3] (b) 6.3 ÷ 1.4 = 4.5 [1] (c) Eye muscles (or the muscles used in sprinting) [1] Examiner insight: Quote both values from the table when you compare; a bare statement that one is higher does not use the data.

7. (a) Block 1: (62 + 70 + 55 + 66 + 59) ÷ 5 = 62.4 [1]; block 4: (41 + 48 + 38 + 40 + 39) ÷ 5 = 41.2 [1] [2] (b) (62.4 − 41.2) ÷ 62.4 × 100 [1] = 34.0% [1] [2] (c) The muscle uses ATP faster than aerobic respiration can supply it, so anaerobic glycolysis makes lactate [1]; lactate lowers pH, which reduces enzyme activity, and less ATP is available for cross-bridge cycling [1] [2] Examiner insight: Divide by the first value, not the last, for a decrease; use the figures in the table when describing the trend.

8. ATP is no longer made after death [1]; without ATP, myosin heads cannot detach from actin [1]; the actinomyosin bridges stay in place, so the muscle remains contracted and stiff [1] [3] Examiner insight: The link between no new ATP and no detachment is the point being tested.

9. Nerve impulses stop, so calcium ions are no longer released [1]; calcium ions are actively transported back into the sarcoplasmic reticulum [1]; tropomyosin moves back over the binding sites on actin [1]; myosin heads cannot bind, so the bridges break and the muscle relaxes [1] [4] Examiner insight: Mention the active transport, as that is where ATP is needed; this is a common omission.

10. Back muscles keep posture for long periods, so they need fibres that resist fatigue [1]; slow twitch fibres have many mitochondria and much myoglobin and make ATP aerobically [1]; eye muscles need very rapid movements [1]; fast twitch fibres contract quickly and powerfully, using phosphocreatine and glycolysis for rapid ATP supply [1] [4] Examiner insight: Link each structure to its function; naming a muscle and a fibre type is not an explanation.

11. (a) Sprinter: slow = 250 − 175 = 75, so 75 ÷ 250 × 100 = 30% [1]; marathon runner: 250 − 45 = 205, so 82% [1]; untrained: 250 − 120 = 130, so 52% [1] [3] (b) She has a high proportion of slow twitch fibres (82%) [1]; these resist fatigue and make ATP aerobically, so they supply energy over a long race [1] [2] (c) Only one muscle was sampled in each person, which may not represent the whole body [1]; there are only three people, so no pattern can be shown [1] [2] Examiner insight: A limitation must be specific to this data; “more repeats” alone is not tied to the data.

Where marks are usually lost

• Saying the A band, or the filaments themselves, shorten.
• Writing that calcium ions bind to actin or myosin instead of moving tropomyosin.
• Not saying that ATP is needed to detach the head.
• Leaving out the active transport of calcium ions back into the sarcoplasmic reticulum.
• Calling phosphocreatine a fuel instead of a phosphate donor.
• Dividing by the later value when working out a percentage decrease.
• Mixing mm and µm in micrograph calculations.
• Comparing fibre types without quoting figures from the table.

Next steps

• Revise with the revision notes and the study guide.
• Visit the AQA A-level Biology hub and the printable checklist.
• Take the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

AQA AS and A-level Biology specification (7401, 7402), for first teaching 2015, AQA. Section 3.6.3, Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only).