Biology  by Bradford
IB Biology 2025 · Theme B · Form and function

B3.3 Muscle and motility. Practice questions with markscheme.

45 original IB-style questions on B3.3, written from the 2025 guide: 20 multiple-choice, 14 short-answer, 5 data-based, 3 extended-response part, 2 drawing, 1 labelling. Below is a 20-mark higher-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

0 statements at SL and HL, 10 additional higher level.

  1. B3.3.1HL Adaptations for movement as a universal feature of living organisms
  2. B3.3.2HL Sliding filament model of muscle contraction
  3. B3.3.3HL Role of the protein titin and antagonistic muscles in muscle relaxation
  4. B3.3.4HL Structure and function of motor units in skeletal muscle
  5. B3.3.5HL Roles of skeletons as anchorage for muscles and as levers
  6. B3.3.6HL Movement at a synovial joint
  7. B3.3.7HL Range of motion of a joint
  8. B3.3.8HL Internal and external intercostal muscles as an example of antagonistic muscle action to facilitate internal body movements
  9. B3.3.9HL Reasons for locomotion
  10. B3.3.10HL Adaptations for swimming in marine mammals

In the bank for B3.3

  • 20 multiple-choice
  • 14 short-answer
  • 5 data-based
  • 3 extended-response part
  • 2 drawing
  • 1 labelling
  • 45 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

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The practice paper

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Biology · topic quiz
Higher level · topic practice, not an exam format
30 minutes20 marks

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Covers B3.3 Muscle and motility
Name:
1.

During contraction of a sarcomere, which of the following shorten?
I. The sarcomere (Z-disc to Z-disc)
II. The individual actin and myosin filaments
III. The light band (region of actin only)

[1]
  1. I, II and III
  2. II and III only
  3. I and II only
  4. I and III only
2.

Using a named example, explain why skeletal muscles occur in antagonistic pairs.

[3]
3.

Glycerinated muscle fibres have had their membranes and stored calcium removed, but their contractile proteins remain functional. Bundles of such fibres were measured, treated with different solutions, and their lengths remeasured. The percentage shortening is shown.

Solution addedShortening / %
buffer only0
ATP only3
Ca²⁺ ions only1
ATP + Ca²⁺ ions32
ATP + Ca²⁺ + cross-bridge inhibitor2
(a)State the purpose of the "buffer only" treatment.[1]
(b)Describe the evidence that both ATP and calcium ions are required for contraction.[2]
(c)Explain the roles of calcium ions and of ATP in the shortening observed.[3]
(d)Predict, with a reason, the effect of adding ATP + Ca²⁺ to fibres in which troponin has been removed.[2]
4.

A single skeletal muscle fibre was stretched to different lengths and, at each length, stimulated to contract while it was held so that it could not shorten. The tension it developed was recorded and the sarcomere length was measured with a microscope. The graph shows tension, as a percentage of the greatest value recorded, against sarcomere length. In this muscle the thick (myosin) filaments are 1.6 µm long and each thin (actin) filament is 1.0 µm long.

1 1.5 2 2.5 3 3.5 4 0 20 40 60 80 100 Sarcomere length / µm Tension developed / % of maximum
(a)State the range of sarcomere lengths over which maximum tension is developed.[1]
(b)Explain why tension decreases as sarcomere length increases above 2.2 µm.[2]
(c)Explain why tension decreases when the sarcomere is shorter than 2.0 µm.[2]
(d)Outline the role of ATP in the interaction between actin and myosin during contraction.[3]

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Markscheme BbB-EAEAAAEAABQAOrOA

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. B3.3.2 [1]
  • D: filaments slide past each other but do not themselves shorten; the sarcomere and the light band narrow as overlap increases;
2. B3.3.3 [3 max]
  • muscles can only generate force by contracting/pulling, they cannot (actively) push/lengthen;
  • (so) a second muscle is needed to reverse the movement / restore the first to its extended state;
  • the two muscles of a pair produce opposite movements at a joint;
  • named example, e.g. biceps flexes the elbow, whereas triceps extends it;

Accept other valid antagonistic pairs.

3. B3.3.4-B3.3.5
  • (a) [1]
    • a control: shows shortening does not occur without the test substances / provides a comparison baseline;
  • (b) [2]
    • either substance alone gives (almost) no shortening (3 % / 1 %);
    • together they give (marked) shortening (32 %), so both are required;
  • (c) [3 max]
    • Ca²⁺ binds troponin, moving tropomyosin off the (myosin-)binding sites on actin;
    • (so) myosin heads can attach to actin, forming cross-bridges;
    • ATP powers the cycle: its hydrolysis re-cocks the myosin head and its binding detaches the head for the next cycle;
    • repeated cycles slide the filaments, shortening the sarcomeres/fibres;
  • (d) [2]
    • shortening would occur (even without Ca²⁺ regulation) / contraction no longer depends on Ca²⁺;
    • without troponin(-tropomyosin) the binding sites on actin are permanently exposed, OWTTE;
4. B3.3.4
  • (a) [1]
    • 2.0–2.2 µm;
  • (b) [2 max]
    • the thin filaments are pulled away from the centre so they overlap less of the thick filament;
    • fewer myosin heads can form cross-bridges with actin, so less force is generated;
    • at 3.6 µm (1.6 + 2 × 1.0) there is no overlap at all so no cross-bridges can form and tension is zero;

    Accept any two.

  • (c) [2 max]
    • the thin filaments from opposite ends of the sarcomere overlap / collide with each other in the centre, interfering with cross-bridge formation;
    • below about 1.6 µm the thick filaments are pressed against the Z discs and become compressed / distorted;
    • so fewer effective cross-bridges can form and the filaments resist further sliding;

    Accept any two.

  • (d) [3 max]
    • ATP binds to the myosin head, causing it to detach from actin;
    • hydrolysis of ATP to ADP and phosphate changes the shape of the myosin head so that it is "cocked" / moves to a new position along the actin;
    • the head binds to a new site on actin, forming a cross-bridge;
    • release of ADP and phosphate causes the power stroke, in which the head pivots and pulls the thin filament towards the centre of the sarcomere;
    • without ATP the myosin heads remain attached to actin (rigor);

    Content pivot. Accept any three.

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