Biology  by Bradford
IB Biology 2025 · Theme C · Interaction and interdependence

C3.1 Integration of body systems. Practice questions with markscheme.

52 original IB-style questions on C3.1, written from the 2025 guide: 23 multiple-choice, 19 short-answer, 5 data-based, 3 extended-response part, 1 labelling, 1 drawing. Below is a 22-mark standard-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

16 statements at SL and HL, 7 additional higher level.

  1. C3.1.1SL / HL System integration
  2. C3.1.2SL / HL Cells, tissues, organs and body systems as a hierarchy of subsystems that are integrated in a multicellular living organism
  3. C3.1.3SL / HL Integration of organs in animal bodies by hormonal and nervous signalling and by transport of materials and energy
  4. C3.1.4SL / HL The brain as a central information integration organ
  5. C3.1.5SL / HL The spinal cord as an integrating centre for unconscious processes
  6. C3.1.6SL / HL Input to the spinal cord and cerebral hemispheres through sensory neurons
  7. C3.1.7SL / HL Output from the cerebral hemispheres to muscles through motor neurons
  8. C3.1.8SL / HL Nerves as bundles of nerve fibres of both sensory and motor neurons
  9. C3.1.9SL / HL Pain reflex arcs as an example of involuntary responses with skeletal muscle as the effector
  10. C3.1.10SL / HL Role of the cerebellum in coordinating skeletal muscle contraction and balance
  11. C3.1.11SL / HL Modulation of sleep patterns by melatonin secretion as a part of circadian rhythms
  12. C3.1.12SL / HL Epinephrine (adrenaline) secretion by the adrenal glands to prepare the body for vigorous activity
  13. C3.1.13SL / HL Control of the endocrine system by the hypothalamus and pituitary gland
  14. C3.1.14SL / HL Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors
  15. C3.1.15SL / HL Feedback control of ventilation rate following sensory input from chemoreceptors
  16. C3.1.16SL / HL Control of peristalsis in the digestive system by the central nervous system and enteric nervous system
  17. C3.1.17HL Observations of tropic responses in seedlings
  18. C3.1.18HL Positive phototropism as a directional growth response to lateral light in plant shoots
  19. C3.1.19HL Phytohormones as signalling chemicals controlling growth, development and response to stimuli in plants
  20. C3.1.20HL Auxin efflux carriers as an example of maintaining concentration gradients of phytohormones
  21. C3.1.21HL Promotion of cell growth by auxin
  22. C3.1.22HL Interactions between auxin and cytokinin as a means of regulating root and shoot growth
  23. C3.1.23HL Positive feedback in fruit ripening and ethylene production

In the bank for C3.1

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

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

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The practice paper

Take it on screen → Build a fresh paper Paper code BbB-EAAAAAABABQAOz_f
Biology · topic quiz
Standard level · topic practice, not an exam format
35 minutes22 marks

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Covers C3.1 Integration of body systems
Name:
1.

The heart pumps blood, yet no single heart cell can pump. What does this illustrate?

[1]
  1. Organs function independently of the cells they contain
  2. Interactions between components produce emergent properties
  3. Each individual cell performs the function of the whole organ
  4. The heart contains only one type of specialized cell
2.

Which row correctly compares nervous and hormonal signalling?

[1]
RowSpeedTransmissionDuration of effect
A.nervous fasternerves: electrical along neurons; hormones: chemicals in bloodhormonal (usually) longer-lasting
B.hormonal fasterboth travel in bloodnervous longer-lasting
C.equalboth electricalequal
D.nervous fasternerves: chemicals in blood; hormones: electricalnervous longer-lasting
  1. Row A
  2. Row B
  3. Row C
  4. Row D
3.

Which process is coordinated by the spinal cord without conscious control by the brain?

[1]
  1. Withdrawal of the hand from a hot surface before any pain is felt
  2. Deciding to pick up a cup after seeing it standing on a table
  3. Remembering the route from home to school and following it
  4. Adjusting the balance of the body and the timing of muscle contractions while running
4.

Which sequence shows the path of the impulse in the reflex shown?

[1]
spinal cord (transverse section)grey matter (shaded) · white matter (unshaded)skin of handfree nerve ending(pain receptor)dorsal root ganglionIIIIIIIVbiceps
  1. receptor → III → II → I → IV
  2. receptor → I → II → III → IV
  3. receptor → I → III → II → IV
  4. receptor → II → I → III → IV
5.

How do the hypothalamus and pituitary gland interact to control hormone secretion in the body?

[1]
  1. The pituitary gland monitors the blood and instructs the hypothalamus to secrete hormones
  2. The hypothalamus secretes all of the body's hormones directly into the blood
  3. The pituitary gland sends nerve impulses to every endocrine gland in the body
  4. The hypothalamus monitors the blood and controls hormone secretion by the pituitary gland
6.

What is the role of the cerebellum?

[1]
  1. Initiating conscious decisions in response to stimuli
  2. Detecting changes in blood CO₂ concentration
  3. Secreting hormones that control other endocrine glands
  4. Coordinating skeletal muscle contraction and balance
7.

Which sequence correctly orders the levels of organization in a multicellular animal?

[1]
  1. Cell → organ → tissue → organ system → organism
  2. Tissue → cell → organ → organ system → organism
  3. Organ → tissue → cell → organ system → organism
  4. Cell → tissue → organ → organ system → organism
8.

Distinguish between a neuron and a nerve.

[2]
9.

Outline the roles of sensory receptors, the central nervous system and effectors in producing a coordinated response.

[2]
10.

Outline the roles of the spinal cord and the brain in processing information.

[3]
11.

A student pedalled an exercise bike at a constant moderate load. Heart rate was recorded at rest, during 10 minutes of exercise, and for 10 minutes of recovery.

Time / minPhaseHeart rate / beats min⁻¹
0rest66
2exercise118
6exercise142
10exercise149
12recovery112
16recovery84
20recovery70
(a)Describe the changes in heart rate over the 20 minutes.[2]
(b)Calculate the percentage increase in heart rate from rest to the final minute of exercise.[1]
(c)Explain how the increase in heart rate during exercise is brought about.[3]
(d)Suggest why heart rate remains above the resting value for some minutes after exercise stops.[2]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-EAAAAAABABQAOz_f

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. C3.1.1 [1]
  • B: system integration produces emergent properties at each level (cells → tissues → organs → systems);
2. C3.1.3 [1]
  • A: nerve impulses are rapid, targeted and brief; hormones are slower, broadcast in blood, with longer-lasting effects;
3. C3.1.5 [1]
  • A: a withdrawal reflex is integrated in the grey matter of the spinal cord through a single interneuron, so the response begins before impulses reach the brain and is unconscious; B is a conscious decision made in the cerebral hemispheres; C depends on learning and memory in the brain; D is unconscious but is coordinated by the cerebellum, part of the brain;
4. C3.1.9 [1]
  • B: the impulse passes from the pain receptor along the sensory neuron into the spinal cord, across the interneuron in the grey matter, and out along the motor neuron to the muscle;
5. C3.1.13 [1]
  • D: the hypothalamus links the nervous and endocrine systems — it monitors the blood / receives sensory input and controls the pituitary, whose hormones control growth, water balance and other endocrine glands;
6. C3.1.10 [1]
  • D: the cerebellum coordinates/refines movement and posture; conscious control arises in the cerebral hemispheres;
7. C3.1.2 [1]
  • D: cells of one type form tissues; several tissues form an organ; organs cooperate in systems; systems integrate into the organism;
8. C3.1.8 [2 max]
  • a neuron is a single (nerve) cell (that conducts impulses), whereas a nerve is a bundle of nerve fibres/axons belonging to many neurons;
  • a nerve is enclosed in a protective sheath (of connective tissue);
  • a nerve may contain both sensory and motor fibres, whereas an individual neuron is either sensory or motor (or a relay/interneuron), OWTTE;
9. C3.1.6-C3.1.7 [2 max]
  • receptors detect stimuli (internal or external) and generate nerve impulses;
  • the CNS (brain/spinal cord) integrates/processes the information and "decides" the response;
  • effectors (muscles or glands) carry out the response (contraction/secretion), OWTTE;
10. C3.1.4-C3.1.5 [3 max]
  • the spinal cord carries information between the body and brain (in tracts of neurons);
  • the spinal cord integrates (unconscious) reflexes / rapid responses;
  • the brain integrates complex/conscious processing, learning, memory, decision-making;
  • (both are) part of the central nervous system, receiving input from sensory neurons and sending output through motor neurons, OWTTE;
11. C3.1.14
  • (a) [2]
    • rises steeply at the start of exercise, then more slowly toward a plateau (~149 beats min⁻¹);
    • falls rapidly at first in recovery, then more slowly toward (but not quite reaching) the resting value, OWTTE;
  • (b) [1]
    • 126 %; (149 − 66)/66 × 100, accept 125–126 %;
  • (c) [3 max]
    • muscles produce more CO₂ (lowering blood pH), detected by chemoreceptors;
    • the (cardiovascular centre of the) medulla increases impulses along the sympathetic nerve to the sinoatrial node/pacemaker;
    • adrenaline (from the adrenal glands) also speeds the pacemaker;
    • (so) the SA node fires more often, raising heart rate (and delivering more O₂/glucose to muscle);
  • (d) [2]
    • an oxygen debt/EPOC must be repaid, e.g. to break down lactate / restore creatine phosphate and O₂ stores;
    • CO₂ (and adrenaline) levels take time to fall, so stimulation of the pacemaker declines gradually, OWTTE;

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