IB DP Biology: Muscle and motility: sliding filaments, joints and locomotion (HL) (DP Biology) – 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 – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover section B3.3 (Muscle and motility) of the IB Biology guide, first assessment 2025, for DP Biology. The whole section is additional higher level, so every question is HL only. Read the study guide first if a topic is new, and use the revision notes to check recall. See also the IB DP Biology hub and the printable checklist.

Questions

1. A barnacle (adult) is fixed to a rock. A seagull flies along the shore.

(a) Identify which is sessile and which is motile. [1]

(b) Describe one movement shown by a sessile organism of your choice. [1]

(c) Suggest one benefit to the seagull of being motile. [1]

2. In a sample of relaxed muscle a sarcomere is 3.0 µm long. When contracted it is 2.2 µm long. The A band is 1.6 µm in both states. (Data supplied for this question.)

(a) Calculate the percentage shortening of the sarcomere. [2]

(b) Calculate the percentage change in the combined length of the I bands. [3]

(c) Explain what the constant A band shows about the mechanism of contraction. [2]

3. Outline the roles of calcium ions and ATP in the cross-bridge cycle. [5]

4. (a) State two roles of the protein titin in a sarcomere. [2]

(b) Explain why skeletal muscles occur in antagonistic pairs. [2]

5. A muscle has 8 small motor units of 25 fibres each and 5 large motor units of 400 fibres each. Assume each active fibre exerts 0.10 N.

(a) Define the term motor unit. [1]

(b) Calculate the force when all 8 small units are active. [2]

(c) Calculate the force when 4 small units and 1 large unit are active. [3]

(d) Explain why muscles that make fine movements tend to have small motor units. [2]

6. Compare exoskeletons and endoskeletons in how they support muscle action. [3]

7. A person holds a 40 N load in the hand, 35 cm from the elbow. The biceps tendon attaches 5 cm from the elbow. Treat the forearm as a lever.

(a) Calculate the force the biceps must exert to hold the arm still. [3]

(b) Explain one advantage of the muscle attaching close to the joint. [2]

8. The hip is a synovial joint between the femur and the pelvis.

(a) State the type of joint at the hip. [1]

(b) Describe the roles of cartilage, synovial fluid and ligaments at the joint. [3]

(c) Distinguish between a tendon and a ligament. [2]

9. A student used a goniometer to measure hip flexion and hip abduction in the same person. Five readings each, in degrees. Flexion: 62, 60, 65, 63, 60. Abduction: 48, 50, 47, 49, 46.

(a) Calculate the mean for each movement. [2]

(b) Calculate the percentage by which the flexion mean exceeds the abduction mean. [2]

(c) Suggest two sources of error in using a goniometer. [2]

(d) State the advantage of repeating the readings. [1]

10. Explain how the external and internal intercostal muscles act as an antagonistic pair, and how titin contributes. [6]

11. (a) State four reasons why animals show locomotion, giving a named example of each. [4]

(b) A caribou herd travels 3000 km in 30 days. Calculate the mean distance per day. [1]

(c) Suggest one cost of migration. [1]

12. Seals, whales and dolphins are adapted for swimming.

(a) Explain how streamlining, flippers and the fluke help a dolphin to swim. [4]

(b) A dolphin swims at 4.5 m per second with a tail beat of 1.5 Hz. Calculate the distance travelled per tail beat. [1]

(c) One breath replaces 9 L of the 12 L of air in the lungs. A resting human replaces about 15% per breath. Compare the two. [2]

(d) Outline how changes to the airways allow periodic breathing between dives. [2]

Answers

1. (a) The barnacle is sessile [1]; the gull is motile. (b) Any one: an oak tree turns its leaves towards the light; a root grows downwards in response to gravity; stomata open and close. [1] (c) Any one: it can search a large area for food, escape predators or find a mate. [1] Examiner insight: A movement of the whole body is not credited as sessile movement; name a part and what it does.

2. (a) Shortening = 3.0 − 2.2 = 0.8 µm [1]; 0.8 ÷ 3.0 × 100 = 26.7% [1] (b) I bands before = 3.0 − 1.6 = 1.4 µm [1]; after = 2.2 − 1.6 = 0.6 µm [1]; change = (0.6 − 1.4) ÷ 1.4 × 100 = −57.1%, a fall of 57.1% [1] (c) The thick filaments keep their length [1]; the filaments slide past each other, so the overlap increases rather than the filaments shortening [1] Examiner insight: Take the starting length as the denominator; using the contracted length gives a different figure.

3. Calcium ions leave the sarcoplasmic reticulum and enter the cytoplasm [1]; they attach to troponin, and tropomyosin shifts, uncovering binding sites on actin [1]; a myosin head binds to actin, forming a cross-bridge [1]; the power stroke pulls the thin filament, and ATP binds to the head, causing detachment [1]; ATP is hydrolysed, which resets the head for another cycle [1] Examiner insight: Give the role of each substance separately; saying “ATP provides energy” alone is too vague for these marks.

4. (a) Helps the sarcomere recoil after stretching [1]; prevents overstretching [1] (b) A muscle exerts force only while contracting, so it can pull but not push [1]; a second muscle pulling the opposite way is needed to return the bone, and its contraction stretches the first [1] Examiner insight: “Muscles relax” is not an explanation; the point is that relaxation does not itself produce the return movement.

5. (a) A motor neuron and all the muscle fibres it controls [1] (b) 8 × 25 = 200 fibres [1]; 200 × 0.10 = 20 N [1] (c) 4 × 25 = 100 fibres and 1 × 400 = 400 fibres [1]; total 500 fibres [1]; 500 × 0.10 = 50 N [1] (d) Each unit adds only a few fibres [1], so force can be changed in small steps [1] Examiner insight: Show the fibre count before multiplying; clear working lets a reader follow your method even if an arithmetic slip occurs.

6. Both give muscles a firm structure to anchor to and act as levers [1]; in an exoskeleton the muscles attach to the inside of the hard external covering (arthropods) [1]; in an endoskeleton the muscles attach to the outside of internal bones through tendons (vertebrates) [1] Examiner insight: A comparison needs a similarity or a difference stated for both types, not two separate descriptions.

7. (a) Moments balance: force × 5 = 40 × 35 [1]; force = 1400 ÷ 5 [1]; 280 N [1] (b) The hand moves 7 times as far as the insertion point [1]; so a small contraction gives a large range and speed of movement, at the cost of needing more force [1] Examiner insight: Quote the unit; a bare number for a force does not show the answer is complete.

8. (a) Ball-and-socket joint [1] (b) Cartilage covers bone ends and reduces friction and absorbs shock [1]; synovial fluid lubricates and supplies nutrients to cartilage [1]; ligaments join bone to bone, stabilising the joint and limiting excess movement [1] (c) A tendon joins muscle to bone [1]; a ligament joins bone to bone [1] Examiner insight: Name the bones involved (femur and pelvis) when describing the joint; general statements about “bones” are weaker.

9. (a) Flexion: (62 + 60 + 65 + 63 + 60) ÷ 5 = 62° [1]; abduction: (48 + 50 + 47 + 49 + 46) ÷ 5 = 48° [1] (b) (62 − 48) ÷ 48 × 100 [1]; = 29.2% [1] (c) Any two: arms not lined up with the bones [1]; the pivot not placed over the joint centre, or the starting posture differing between readings [1] (d) It allows a mean to be calculated and reduces the effect of random error [1] Examiner insight: A mean should be given with units; a reader needs to know the values are in degrees.

10. The two layers have fibres running in different directions [1]; external intercostals contract and raise the ribs up and out [1]; this increases the volume of the thorax, for inhalation [1]; internal intercostals contract and pull the ribs down and in [1]; when one layer contracts, it stretches the other [1]; the stretch stores potential energy in titin, which helps the stretched layer to recoil [1] Examiner insight: State the direction of rib movement for both layers; many answers name the muscles but omit the direction.

11. (a) Foraging for food (wolf pack hunting) [1]; escaping from danger (scallop swimming away from a starfish) [1]; searching for a mate (male moth following pheromones) [1]; migration (wildebeest following rainfall) [1] (b) 3000 ÷ 30 = 100 km per day [1] (c) Any one: uses large amounts of energy, risk of predation on route, or loss of body condition [1] Examiner insight: An example must be a named organism; “animals escape predators” repeats the reason and earns no extra credit.

12. (a) Streamlining reduces drag [1]; flippers, from modified forelimbs, are used to steer and balance [1]; the fluke is moved up and down by powerful muscles [1]; this provides thrust to push the animal forward [1] (b) 4.5 ÷ 1.5 = 3.0 m [1] (c) Dolphin: 9 ÷ 12 × 100 = 75% [1]; this is five times the human value of 15%, so the dolphin renews far more of its lung air per breath [1] (d) The nostrils are moved to the top of the head as a blowhole, so the animal can breathe at the surface with little of its body exposed [1]; a large proportion of the air is replaced in each breath, so one breath can supply oxygen for a long dive [1] Examiner insight: Distinguish the fluke (up and down) from a fish tail (side to side); stating the wrong direction is a common reason for losing credit here.

Where marks are usually lost

• Saying filaments shorten instead of sliding.
• Forgetting that the A band is constant.
• Dividing by the new value when working out percentage change.
• Leaving out units for force, length and angle.
• Treating tendons and ligaments as the same structure.
• Saying that muscles push.
• Not stating the direction of rib movement for each intercostal layer.
• Describing a reason for locomotion without a named example.
• Writing that the fluke moves side to side.

Next steps

• Review the revision notes.
• Re-read the study guide.
• Return to the IB DP Biology hub and tick off the printable checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section B3.3, Muscle and motility, additional higher level.