IB Biology 2025 · Theme C · Interaction and interdependence

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

74 original IB-style questions on C3.1, written from the 2025 guide: 34 multiple-choice, 25 short-answer, 10 data-based, 3 extended-response parts, 1 labelling, 1 drawing. Below are two ready-made practice papers built from them, one at standard level (21 marks) and one at higher level (21 marks); switch between them above the paper. Hand one out as a class quiz or homework, or sit it 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.

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

In the bank for C3.1

34 multiple-choice
25 short-answer
10 data-based
3 extended-response parts
1 labelling
1 drawing
22 higher level only

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

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The practice paper

21 marks at SL · 21 at HL · each has its own code

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Biology · topic quiz

Standard level · topic practice, not an exam format

30 minutes21 marks

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Covers C3.1 Integration of body systems · SL

Name:

Which row correctly compares nervous and hormonal signalling?

[1]

Row	Speed	Transmission	Duration of effect
A.	nervous faster	nerves: electrical along neurons; hormones: chemicals in blood	hormonal (usually) longer-lasting
B.	hormonal faster	both travel in blood	nervous longer-lasting
C.	equal	both electrical	equal
D.	nervous faster	nerves: chemicals in blood; hormones: electrical	nervous longer-lasting

Row C
Row D
Row A
Row B

Which of the labelled structures lie entirely within the central nervous system?

[1]

I only
I and IV
III and IV
I, III and IV

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

[1]

Organs function independently of the cells they contain
Interactions between components produce emergent properties
The heart contains only one type of specialized cell
Each individual cell performs the function of the whole organ

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

[1]

The pituitary gland monitors the blood and instructs the hypothalamus to secrete hormones
The pituitary gland sends nerve impulses to every endocrine gland in the body
The hypothalamus secretes all of the body's hormones directly into the blood
The hypothalamus monitors the blood and controls hormone secretion by the pituitary gland

Melatonin secretion by the pineal gland rises in the evening and falls at dawn. What is its role?

[1]

Contributing to the control of circadian rhythms
Stimulating digestion of food during the night
Raising blood glucose concentration during sleep
Promoting muscle growth and repair during sleep

Distinguish between a neuron and a nerve.

[2]

Adrenaline (epinephrine) is secreted in response to threat. Explain how its effects prepare the body for vigorous action.

[3]

Explain how ventilation rate is matched to the body's needs during exercise.

[3]

Thirty volunteers each tasted 20 fruit-flavoured solutions, one at a time. All the solutions were colourless and contained the same concentration of sugar and no acid. After each taste the volunteer named the fruit and said whether the solution was sweet, sour or neither. Each volunteer did the test once breathing normally and once wearing a nose clip, which stops air flowing through the nose. The graph shows the percentage of correct answers.

(a)Calculate the percentage decrease in correct naming of the fruit when the nose clip was worn.[1]

(b)Deduce what the results show about the information needed to name the fruit and to recognize the solution as sweet.[2]

(c)Suggest why all the solutions were colourless.[1]

(d)Volunteers named the flavours of fruits they eat often more successfully than those of fruits they rarely eat. Suggest a reason.[1]

(e)Outline the role of the brain in recognizing a fruit from its flavour.[3]

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. C3.1.3 [1]

C: nerve impulses are rapid, targeted and brief; hormones are slower, broadcast in blood, with longer-lasting effects;

2. C3.1.9 [1]

A: the interneuron (I) lies wholly in the grey matter of the spinal cord; the sensory neuron (III) begins at a receptor in the skin, and the motor neuron (IV) has an axon running out of the spinal cord to the muscle, so both extend outside the CNS;

3. C3.1.1 [1]

B: system integration produces emergent properties at each level (cells → tissues → organs → systems);

4. 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;

5. C3.1.11 [1]

A: melatonin entrains the sleep–wake cycle to light/dark; its cycle explains jet lag (and why light exposure shifts it);

6. 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;

7. C3.1.12 [3 max]

heart rate (and stroke volume) increase, delivering more O₂/glucose to muscles;
bronchioles dilate / ventilation increases, improving gas exchange;
glycogen is broken down (in the liver), raising blood glucose (for respiration);
blood is redirected to muscles (vasodilation there) and away from the gut/skin (vasoconstriction);
pupils dilate / awareness increases, OWTTE;

8. C3.1.15 [3 max]

exercising muscles release more CO₂, lowering blood pH (carbonic acid);
chemoreceptors (in the medulla / carotid and aortic bodies) detect the fall in pH / rise in CO₂;
the (respiratory centre of the) medulla oblongata increases impulses to the diaphragm and intercostal muscles;
(so) ventilation becomes faster and deeper, removing CO₂ and supplying O₂, until pH returns to normal (negative feedback), OWTTE;

9. C3.1.4

(a) [1]
- (78 − 14) ÷ 78 × 100 = 82 %;
Accept 82.1 %.
(b) [2 max]
- recognizing sweetness needs only information from the tongue/taste receptors, since the nose clip made almost no difference (97 % and 95 %);
- naming the fruit also needs information from the nose/smell receptors, since correct naming fell from 78 % to 14 % with the clip;
- so naming a flavour depends on information from more than one type of receptor/input being combined, OWTTE;
(c) [1]
- colour is a visual input that the brain could combine with taste and smell (to recognize the fruit) / volunteers could use colour to guess the fruit, so it would affect the naming results, OWTTE;
(d) [1]
- naming depends on comparing the taste and smell with memories of eating the fruit, and memories/learning are strengthened by repeated experience, OWTTE;
(e) [3]
- sensory neurons carry impulses from taste receptors (in the tongue) and smell receptors (in the nose) to the brain;
- the brain processes information from several inputs at once / combines/integrates the inputs;
- the combined information is compared with memories (of flavours met before);
- these memories were formed by learning from earlier experiences of eating the fruit;
- the brain then initiates a (conscious) response, such as naming the fruit, OWTTE;
Content pivot. Do not accept "the tongue recognizes the fruit".

Take it online → Build a fresh paper Paper code BbB-EAEAAAABABQAYifN

Biology · topic quiz

Higher level · topic practice, not an exam format

30 minutes21 marks

Paper code: this paper was generated, so the code is its recipe. Enter it at biologybybradford.com/exam-maker to rebuild this exact paper and its markscheme.

Covers C3.1 Integration of body systems · HL

Name:

Which of the labelled structures lie entirely within the central nervous system?

[1]

I only
I and IV
III and IV
I, III and IV

What is the role of the cerebellum?

[1]

Secreting hormones that control other endocrine glands
Detecting changes in blood CO₂ concentration
Coordinating skeletal muscle contraction and balance
Initiating conscious decisions in response to stimuli

Which statement best describes phytohormones?

[1]

Enzymes that catalyse the reactions of growth in meristems
A variety of chemicals used as signals controlling growth and development
A single chemical, auxin, that controls all growth responses in plants
Proteins secreted by glands and transported only in the phloem sap

Segments were cut from young oat (Avena sativa) shoots just below the tip, so that they contained little auxin of their own, and were floated in different solutions for three hours. Which interpretation of the results is correct?

[1]

Solution	Mean increase in length / mm
Water, no auxin	0.3
Auxin in water	1.8
Acidic buffer (pH 4.5), no auxin	1.6
Auxin in a strong buffer that holds the pH at 7.0	0.4

An acidic solution causes elongation only when auxin is also present
The buffers damaged the cells, so no buffered solution allows elongation
Auxin causes elongation whatever the pH of the solution around the cells
Auxin causes elongation by lowering the pH in the walls of the cells

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

[2]

In a healthy heart, the muscle cells in the walls of the ventricles are stimulated by an electrical signal that spreads through the walls. In ventricular fibrillation, a condition that can kill within minutes, this signal breaks up into many small, disorganized waves. The table compares the ventricles in the two conditions.

Condition	Contractions of each ventricle muscle cell / min⁻¹	Timing of contraction of the cells across the ventricle wall	Blood pumped by the heart / dm³ min⁻¹
Normal heartbeat	70	almost all at the same moment	5.0
Ventricular fibrillation	300 to 500	at different moments, in no fixed order	less than 0.3

(a)Deduce, with a reason from the data, whether the heart pumps almost no blood during fibrillation because its muscle cells are unable to contract.[2]

(b)Explain, with reference to system integration, why the ventricles pump almost no blood during fibrillation.[3]

(c)Suggest why the brain begins to be damaged within minutes of the start of fibrillation, although the brain itself is not affected by the electrical signal in the heart.[2]

(a)Calculate the percentage decrease in correct naming of the fruit when the nose clip was worn.[1]

(b)Deduce what the results show about the information needed to name the fruit and to recognize the solution as sweet.[2]

(c)Suggest why all the solutions were colourless.[1]

(d)Volunteers named the flavours of fruits they eat often more successfully than those of fruits they rarely eat. Suggest a reason.[1]

(e)Outline the role of the brain in recognizing a fruit from its flavour.[3]

Show the markscheme

Markscheme BbB-EAEAAAABABQAYifN

1. C3.1.9 [1]

A: the interneuron (I) lies wholly in the grey matter of the spinal cord; the sensory neuron (III) begins at a receptor in the skin, and the motor neuron (IV) has an axon running out of the spinal cord to the muscle, so both extend outside the CNS;

2. C3.1.10 [1]

C: the cerebellum coordinates/refines movement and posture; conscious control arises in the cerebral hemispheres;

3. C3.1.19 [1]

B: plants use a variety of chemicals (auxin, cytokinin, ethylene, gibberellin etc.) as phytohormones: signalling chemicals controlling growth, development and responses to stimuli;

4. C3.1.21 [1]

D: acid alone mimics auxin (1.6 mm against 1.8 mm), and a strong buffer that holds the pH at 7.0 blocks the effect of auxin (0.4 mm, close to water at 0.3 mm), so auxin acts by acidifying the walls; the pH 4.5 buffer still allowed elongation, so the buffers did not damage the cells; (auxin stimulates secretion of hydrogen ions into the apoplast, which loosens cross-links between cellulose molecules);

5. 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;

6. C3.1.1

(a) [2 max]
- no, the muscle cells are still able to contract / the cause is not a failure of the cells themselves;
- each cell contracts more often (300 to 500 min⁻¹) than in a normal heartbeat (70 min⁻¹);
- what differs is the timing / coordination of the contractions of the cells, OWTTE;
The first mark needs the conclusion; the second needs a reason from the table.
(b) [3]
- pumping blood is a function of the ventricle/heart as a whole, not of any one cell / is an emergent property of the cells acting together;
- the contractions of the cells must be coordinated/synchronized for their forces to add together;
- during fibrillation some cells contract while neighbouring cells relax, so their effects do not combine / the wall quivers instead of squeezing, OWTTE;
- so the volume of the ventricle is not reduced / the pressure in the ventricle does not rise enough to push blood out;
- coordination (here by the spreading electrical signal) is needed for the component parts to perform the overall function together;
(c) [2]
- the brain depends on blood pumped by the heart to deliver oxygen and glucose;
- without blood flow brain cells cannot respire / cannot produce enough ATP, so they are damaged/die;
- organs/body systems are integrated/depend on each other, so failure of one system (the circulation) leads to failure of others, OWTTE;

7. C3.1.4

(a) [1]
- (78 − 14) ÷ 78 × 100 = 82 %;
Accept 82.1 %.
(b) [2 max]
- recognizing sweetness needs only information from the tongue/taste receptors, since the nose clip made almost no difference (97 % and 95 %);
- naming the fruit also needs information from the nose/smell receptors, since correct naming fell from 78 % to 14 % with the clip;
- so naming a flavour depends on information from more than one type of receptor/input being combined, OWTTE;
(c) [1]
- colour is a visual input that the brain could combine with taste and smell (to recognize the fruit) / volunteers could use colour to guess the fruit, so it would affect the naming results, OWTTE;
(d) [1]
- naming depends on comparing the taste and smell with memories of eating the fruit, and memories/learning are strengthened by repeated experience, OWTTE;
(e) [3]
- sensory neurons carry impulses from taste receptors (in the tongue) and smell receptors (in the nose) to the brain;
- the brain processes information from several inputs at once / combines/integrates the inputs;
- the combined information is compared with memories (of flavours met before);
- these memories were formed by learning from earlier experiences of eating the fruit;
- the brain then initiates a (conscious) response, such as naming the fruit, OWTTE;
Content pivot. Do not accept "the tongue recognizes the fruit".

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