IB Biology · Theme C Interaction and interdependence · Organisms
C3.1 Integration of body systems
Body systems must be coordinated, and that coordination gives emergent properties no system has alone. Nerves send fast, targeted messages; hormones in blood send slower, wider ones; blood also carries materials. At HL, plants integrate growth with phytohormones such as auxin, cytokinin and ethylene.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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C3.1.1 Parts of a living system must work together
System integration is the coordination of parts so they perform an overall function.
No part could perform that function alone.
It is necessary at every level, from organelles to organs.
Students often think each body system works in isolation. In fact systems must be coordinated to do what none could do alone.
C3.1.2 Cells build tissues, organs and systems, and new properties emerge
Cells form tissues; tissues form organs; organs form body systems; systems form the organism.
Each level is a subsystem of the one above, integrated with the others.
An emergent property arises from interaction between parts and belongs to none of them.
A cheetah is an effective predator through integration of muscular, skeletal, nervous, respiratory and circulatory systems.
Students often think the muscular system makes the cheetah a predator. In fact no single system does; predation emerges from all of them together.
Students often think an organism can only do what its cells already do. In fact each level has properties its parts lack.
C3.1.3 Nerves, hormones and blood link organs together
Nervous signalling: impulses along fixed neuron pathways; fast, precise, short-lived.
Hormonal signalling: hormones secreted into blood reach everywhere; only cells with receptors respond.
Hormonal responses start more slowly and last longer, and can affect many organs at once.
Blood transports materials: oxygen from lungs, glucose from gut, urea to kidneys, hormones to targets.
Students often think nerves carry hormones and nutrients. In fact those travel in blood; nerves carry impulses only.
Students often think blood delivers a hormone to one target organ. In fact it goes everywhere; receptors decide who responds.
C3.1.4 The brain combines inputs, learns and remembers
The brain receives information from many receptors at once.
It combines this with stored information from past experience.
Learning is a change in behaviour from experience; memory is storing and retrieving information.
The combined result decides the response, making the brain the central integrating organ.
Students often think each input is processed separately. In fact inputs are combined with memory into one coordinated response.
Students often think practised movements are stored in muscles. In fact learning and memory are in the brain; muscles store nothing.
C3.1.5 The spinal cord integrates responses we are not aware of
A conscious process involves awareness and the cerebral hemispheres.
An unconscious process, such as a reflex, needs no awareness or decision.
The spinal cord has grey matter (cell bodies, interneurons, synapses) inside white matter (myelinated fibres).
It conducts impulses to and from the brain and integrates reflexes itself.
Students often think the spinal cord is only a cable. In fact it also integrates unconscious processes such as reflexes.
Students often think everything unconscious happens outside the brain. In fact the medulla and cerebellum work unconsciously too.
C3.1.6 Sensory neurons carry messages in
A receptor cell detects a stimulus and signals to a sensory neuron.
Examples: photoreceptors in the retina, hair cells in the cochlea.
A sensory neuron carries impulses from receptors into the central nervous system.
It conducts in one direction only, towards the CNS.
Students often think every sensory neuron detects its own stimulus. In fact most sense organs use separate receptor cells; free nerve endings are the exception.
Students often think one neuron carries messages both ways. In fact sensory neurons carry in and motor neurons carry out.
C3.1.7 Motor neurons carry commands out to muscles
A motor neuron carries impulses from the CNS to an effector.
Output from the cerebral hemispheres ends at skeletal muscle fibres.
The impulse stimulates the muscle to contract.
A muscle relaxes when stimulation stops; an antagonistic muscle lengthens it.
Students often think a motor neuron can make a muscle extend. In fact it can only stimulate contraction; another muscle does the lengthening.
C3.1.8 A nerve is a bundle of many fibres
A nerve is a bundle of nerve fibres in a protective sheath of connective tissue.
Most nerves hold fibres of both sensory and motor neurons.
In section, a myelinated fibre shows a thick sheath from Schwann cells; it conducts fast.
An unmyelinated fibre has no sheath and conducts more slowly. Both occur in one nerve.
Students often think the protective sheath is myelin. In fact myelin wraps single fibres; connective tissue wraps the whole nerve.
Students often think a fibre without myelin is damaged. In fact healthy nerves normally contain both types.
C3.1.9 The pain reflex withdraws the hand before you feel it
A reflex is rapid and involuntary; a free nerve ending in the hand detects pain.
The sensory neuron reaches the spinal cord's grey matter.
One interneuron there links it to a motor neuron.
Skeletal muscle in the arm is the effector and contracts to withdraw the hand.
Other interneurons carry the signal up to the brain, so pain is felt just after the hand has moved.
Students often think the brain feels pain and then decides. In fact the spinal cord withdraws the hand; awareness comes later.
Students often think an involuntary response must use smooth muscle. In fact arm movement uses skeletal muscle, reflex or not.
C3.1.10 The cerebellum makes movement smooth and keeps balance
The cerebellum sits below and behind the cerebral hemispheres.
It coordinates skeletal muscle contraction and maintains balance and posture.
It does not start voluntary movements; the cerebral hemispheres do.
It uses information from eyes, inner ear, muscles and joints to make movements accurate.
Students often think the cerebellum starts movements. In fact the cerebral hemispheres command; the cerebellum coordinates.
Students often think the cerebellum controls heart rate and breathing. In fact the medulla does that.
C3.1.11 Melatonin from the pineal gland sets the sleep cycle
A circadian rhythm is a roughly 24-hour cycle, synchronised to day and night by light.
Melatonin is secreted by the pineal gland in the brain.
Secretion rises in the evening, peaks at night and falls by day.
Light detected by the retina suppresses it; high melatonin promotes sleepiness.
Students often think the pituitary makes melatonin. In fact the pineal gland does.
Students often think melatonin rises after falling asleep. In fact it rises in the evening dark and promotes sleep.
C3.1.12 Epinephrine readies the body for hard exercise
Epinephrine (adrenaline) comes from the adrenal glands in danger, stress or before activity.
Heart rate and stroke volume rise; bronchioles dilate; ventilation increases.
Students often think breathing speeds up because oxygen is low. In fact rising carbon dioxide lowering pH is the main stimulus.
Students often think more carbon dioxide makes blood alkaline. In fact it forms an acid and pH falls.
C3.1.16 Swallowing and egestion are voluntary; peristalsis in between is not
From 2028 this is supplied in the Biology data booklet — you need to recognise and interpret it, not reproduce it from memory.
Peristalsis is waves of smooth muscle contraction moving material along the gut.
It is involuntary and works regardless of gravity.
The enteric nervous system, a neuron network in the gut wall, coordinates it without the CNS.
The CNS voluntarily initiates swallowing and controls egestion of faeces.
Students often think the brain drives each wave of peristalsis. In fact the enteric nervous system does it.
Students often think food falls through the gut by gravity. In fact peristalsis moves it in any orientation.
C3.1.17 Recording tropisms: diagrams and angles HL
A qualitative observation is a description, such as a labelled diagram of a curved shoot.
A quantitative observation is a number, such as the angle of curvature in degrees.
Precision: how closely repeats agree; limited by instrument resolution and random error.
Accuracy: closeness to the true value; reduced by systematic error such as a misaligned protractor.
Reliability improves with many seedlings and repeats, but repeats do not remove systematic error.
Students often think agreeing repeats prove accuracy. In fact they show precision; a systematic error could shift them all.
Students often think drawings are not data. In fact annotated diagrams record features an angle cannot.
C3.1.18 Shoots grow towards light from the side HL
A tropism is a growth response whose direction is set by the stimulus direction.
Positive phototropism is shoot growth towards lateral light.
Cells on the shaded side elongate more than cells on the lit side.
It is growth, not movement of existing tissue.
Students often think the lit side grows more. In fact the shaded side elongates more, bending the shoot towards light.
Students often think the shoot moves and can move back. In fact it is new growth directed by light.
C3.1.19 Plants signal with a variety of chemicals HL
A phytohormone is a plant signalling chemical regulating growth, development or responses.
It works at very low concentrations.
Examples: auxins, cytokinins, gibberellins, abscisic acid and the gas ethylene.
Plants have no glands; phytohormones move cell to cell or in xylem and phloem.
Students often think plants have hormone glands and a circulation for hormones. In fact many tissues make them and they move locally or in sap.
Students often think auxin explains every plant response. In fact plants use a variety of phytohormones.
C3.1.20 Efflux carriers push auxin one way through a tissue HL
Auxin diffuses freely into cells but cannot diffuse out.
Auxin efflux carriers actively transport it out, on whichever side they sit.
If all cells place carriers on the same side, auxin moves in one direction.
It accumulates in one part of the plant, maintaining a concentration gradient.
Students often think auxin diffuses both ways and evens out. In fact it leaves only through efflux carriers.
Students often think efflux carriers pump auxin in. In fact efflux means out.
C3.1.21 Auxin loosens cell walls so cells elongate HL
Auxin stimulates proton pumps to secrete hydrogen ions into the apoplast.
The acidic wall has looser cross links between cellulose molecules.
The wall stretches and the cell elongates as it takes up water.
In a shoot lit from one side, auxin concentrates on the shaded side, which grows faster.
Students often think auxin makes cells divide. In fact it makes them elongate by loosening the wall.
Students often think light destroys auxin on the lit side. In fact auxin is transported to the shaded side; the total is unchanged.
C3.1.22 Root and shoot tips signal to each other HL
Shoot tips make auxin, transported to the roots.
Root tips make cytokinin, transported to the shoots, where it promotes growth.
Each organ signals its growth to the other.
So root and shoot growth stay integrated; neither outgrows what the other can support.
Students often swap the sources. In fact auxin is from shoot tips and cytokinin from root tips.
Students often think each hormone acts where it is made. In fact each is transported to the other organ.
C3.1.23 Ethylene and ripening drive each other HL
Ethylene (ethene) is a gaseous phytohormone that stimulates ripening: softening, starch to sugar, colour.
Ripening stimulates more ethylene, which spreads by diffusion to neighbouring cells and fruits.
This is positive feedback: the response increases the stimulus.
Ripening is therefore rapid and synchronised, attracting seed dispersers together.
Students often think ripening uses negative feedback. In fact ethylene and ripening amplify each other.
Students often think ethylene acts only inside its own fruit. In fact it is a gas and ripens neighbours too.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement best describes what is meant by integration in a living system?
Answer and reasoning
Coordination of the component parts so that together they perform an overall function that none performs alone. — Integration is a necessary process in living systems: coordination is needed for the component parts of a system to collectively perform an overall function.
Each component part performs its own function independently, so the parts do not need to interact. — A student who pictures body systems as separate machines picks this. Parts that do not interact cannot perform a collective function; integration is the coordination between them.
A single dominant part carries out the overall function while the other parts play no role in it. — A student who assigns each function to one 'responsible' system picks this. An overall function of a system emerges from all its parts working together, not from one part alone.
Coordination is achieved only by conscious decisions, so it applies only to organisms with a brain. — A student who equates coordination with conscious control picks this. Integration occurs at every level of living systems, including unconscious processes and organisms with no brain at all.
2 Which statement correctly distinguishes the roles of the nervous system and the endocrine system in sending messages between organs?
Answer and reasoning
Hormones are carried along nerves to their target organs, while nerve impulses are carried around the body in the blood plasma. — A student who has merged the two systems into one set of wires picks this. Hormones travel in the blood and nerve impulses travel along neurons; the routes are not interchangeable.
Both systems produce equally rapid responses that last equally long, and differ only in the chemical that carries the message. — A student who thinks of an 'adrenaline rush' as instantaneous picks this. Nervous responses take milliseconds; hormonal responses are slower to start and longer-lasting, and the nervous message is electrical, not a chemical carried in blood.
Nervous messages travel as impulses along neurons to specific targets; hormones are carried by the blood to every organ. — The nervous system sends electrical impulses along fixed pathways of neurons to particular cells. The endocrine system secretes hormones into the blood, which carries them everywhere; only cells with receptors respond.
The blood carries each hormone only to its target organ, in the same way that a nerve reaches only one target. — A student who reads 'target' as aiming picks this. The blood carries a hormone to all parts of the body; specificity comes from receptors on target cells, not from the route the hormone takes.
3 Which statement about the spinal cord and unconscious processes is correct?
Answer and reasoning
The spinal cord only conducts impulses between the body and the brain, so every kind of integration must take place in the brain itself. — A student who pictures the spinal cord as a cable picks this. The cord conducts impulses, but its grey matter also integrates reflexes; a withdrawal reflex is completed before the brain is involved.
The spinal cord integrates some unconscious responses, receiving sensory input and sending motor output without the brain deciding. — The spinal cord is an integrating centre for unconscious processes such as reflexes: its grey matter contains interneurons and synapses that link sensory input to motor output without the cerebral hemispheres deciding.
Every unconscious process is integrated in the spinal cord, because everything that the brain does is conscious. — A student who equates 'brain' with 'conscious' picks this. Many unconscious processes, such as control of heart rate and ventilation, are integrated in the medulla, which is part of the brain.
Reflexes are called unconscious only because the brain makes the decision too quickly for us to be aware of doing so. — A student who cannot imagine a response without the brain deciding picks this. A spinal reflex does not involve a decision by the brain at all; the response is produced in the spinal cord and the brain learns of it afterwards.
4 A person decides to straighten their arm. How does output from the cerebral hemispheres produce this movement?
Answer and reasoning
Motor neurons stimulate the triceps to contract and other motor neurons stimulate the biceps to lengthen actively. — A student who thinks a nerve can drive a muscle in both directions picks this. Muscles can only be stimulated to contract; the biceps is lengthened by the contraction of its antagonist, not by a signal to extend.
Sensory neurons carry the command from the cerebral hemispheres down to the triceps, causing it to contract. — A student who has swapped the neuron types picks this. Sensory neurons carry input to the CNS; output from the cerebral hemispheres to muscles is through motor neurons.
Epinephrine released into the blood stimulates the triceps to contract, because hormones control the skeletal muscles. — A student who thinks epinephrine causes muscle action picks this. Skeletal muscle contracts when stimulated by motor neurons; epinephrine prepares the body for activity but does not command a movement.
Motor neurons stimulate the triceps to contract, and the biceps relaxes because it is no longer stimulated. — Output from the cerebral hemispheres reaches muscles through motor neurons, which stimulate contraction. The triceps contracts to straighten the arm; the biceps relaxes because stimulation of it is withdrawn, and it is lengthened passively.
5 Which statement describes the role of the cerebellum in the control of body movements?
Answer and reasoning
It coordinates the contraction of skeletal muscles so that movements are smooth and accurate, and it maintains balance. — The cerebellum coordinates skeletal muscle contraction and balance, using sensory information to adjust the timing and strength of contractions commanded by the cerebral hemispheres.
It initiates voluntary movements by deciding which skeletal muscles should contract and at what moment. — A student who reads 'controls movement' as 'commands movement' picks this. Voluntary movements are initiated by the cerebral hemispheres; the cerebellum coordinates them.
It has no role in balance, which is maintained entirely by the semicircular canals within the inner ear. — A student who takes 'organ of balance' literally picks this. The inner ear detects movement of the head; the cerebellum integrates this with other input and adjusts muscle contraction to keep the body balanced.
It controls involuntary movements such as the heartbeat and breathing rather than any of the skeletal muscles. — A student who confuses the cerebellum with the medulla picks this. Heart rate and ventilation are coordinated by the medulla; the cerebellum's role is with skeletal muscle.
6 A sprinter waits for the starting signal and epinephrine is secreted into the blood. Which set of effects of epinephrine helps to facilitate intense contraction of the leg muscles during the race?
Answer and reasoning
Faster heart rate and an equal increase in blood flow to every organ, including the gut, the skin and the kidneys. — A student who reasons that more output means more blood everywhere picks this. Epinephrine redistributes blood: vessels to the gut and skin constrict while those to skeletal muscle dilate.
Direct transfer of energy from the adrenal glands to the leg muscle fibres, so that they can contract more strongly. — A student who takes 'adrenaline rush' as a burst of energy picks this. Epinephrine is a signal; the energy for contraction comes from cell respiration of glucose in the muscle.
Direct stimulation of the leg muscle fibres to contract, taking over from the motor neurons for the duration of the race. — A student who thinks the hormone commands the movement picks this. Skeletal muscle contracts only when stimulated by motor neurons; epinephrine prepares the body but does not replace nervous control.
Faster heart rate, wider bronchioles, glycogen breakdown in the liver and more blood flow to skeletal muscle. — These widespread effects of epinephrine increase the delivery of oxygen and glucose to the muscles: more cardiac output, easier ventilation, higher blood glucose and blood diverted to skeletal muscle, allowing intense cell respiration and contraction.
7 Which statement gives the location and the function of the receptors that provide sensory input for the control of heart rate?
Answer and reasoning
Baroreceptors and chemoreceptors are both in the wall of the heart, where they monitor the pressure and the chemistry of the blood as it passes through it. — A student who expects the heart to monitor itself picks this. The receptors are in the major arteries leaving the heart and in the medulla, not in the heart wall.
Baroreceptors in the aorta and carotid arteries monitor blood pressure; chemoreceptors there and in the medulla monitor pH, oxygen and carbon dioxide. — Baroreceptors are stretch receptors in the walls of the aorta and carotid arteries. Chemoreceptors in the same arteries and in the medulla monitor blood pH and the concentrations of oxygen and carbon dioxide. Both send impulses to the medulla.
Baroreceptors in the aorta monitor blood pressure and chemoreceptors in the carotid arteries monitor only the concentration of oxygen. — A student who thinks oxygen is the only chemical sensed picks this. Chemoreceptors monitor pH and carbon dioxide as well as oxygen, and they are found in the aorta and medulla as well as the carotid arteries.
Baroreceptors and chemoreceptors are both located in the medulla, which monitors the pressure and chemistry of the blood flowing through it. — A student who places all the sensors in the brain picks this. The medulla does contain chemoreceptors, but baroreceptors are in the artery walls, where stretch can be detected directly.
8 Students investigating phototropism recorded three observations of seedlings lit from one side: (1) a labelled drawing showing the shoot curved towards the lamp; (2) 'the shoots bent much more than those of the control'; (3) an angle of curvature of 32° measured with a protractor. Which classification is correct? HL
Answer and reasoning
(2) and (3) are quantitative because both describe the amount of bending; (1) is qualitative. — A student who thinks any statement about 'how much' is quantitative picks this. 'Much more' is a comparison in words with no measured value, so (2) is qualitative.
Only (3) is an observation; (1) and (2) are not data as they have no number. — A student who thinks only numbers count as data picks this. Diagrams recording the direction of curvature are qualitative data that the IB expects students to gather; they are valid observations.
(1) and (2) are qualitative observations and (3) is a quantitative observation. — A drawing and a comparative description record features without numerical measurement, so they are qualitative. A measured angle in degrees is a quantitative observation.
(1) is quantitative because a careful scale drawing records the exact shape of the shoot. — A student who equates careful with quantitative picks this. However carefully made, a drawing is a description of shape, not a numerical measurement, so it is a qualitative observation.
9 Which statement about phytohormones is correct? HL
Answer and reasoning
Plants use a variety of chemicals, including auxins, cytokinins and ethylene, as signals controlling growth, development and responses to stimuli. — Phytohormones are signalling chemicals controlling growth, development and response to stimuli in plants, and a variety of different chemicals serve this role.
Auxin is the only phytohormone, and all plant growth, development and responses to stimuli are controlled by its concentration. — A student for whom auxin stands for all plant signalling picks this. Cytokinins, ethylene and other chemicals are phytohormones with their own roles, such as cytokinin in shoot growth and ethylene in ripening.
Phytohormones are secreted by glands in the plant and carried in the xylem to all parts, just as animal hormones are carried in blood. — A student who transfers the animal model to plants picks this. Plants have no glands; phytohormones are made in tissues such as shoot and root tips and move from cell to cell or in vascular tissue.
Phytohormones diffuse freely into and out of cells, so their concentration ends up the same in every part of the plant. — A student who assumes diffusion is symmetrical picks this. Auxin, for example, enters cells freely but leaves only through efflux carriers, which allows concentration gradients to be set up.
10 Which statement describes how auxin promotes the elongation of a plant cell? HL
Answer and reasoning
It stimulates the cell to divide by mitosis, so the tissue becomes longer because it contains a larger number of cells. — A student who equates growth with division picks this. Auxin's promotion of growth in this context is by cell elongation, not by increasing cell number.
It stimulates secretion of hydrogen ions into the cell wall, where the acid dissolves the cellulose molecules so the wall can be rebuilt longer. — A student who hears 'acidify' as 'dissolve' picks this. The low pH loosens the cross links between cellulose molecules; the cellulose itself is not broken down, and the wall stretches rather than being rebuilt.
It promotes hydrogen ion secretion into the apoplast; the acidified wall has its cellulose cross links loosened and can stretch. — Auxin promotes hydrogen ion secretion into the apoplast. The acidified cell wall has its cross links between cellulose molecules loosened, which facilitates elongation as the cell takes up water.
It acidifies the cytoplasm, and the increased acidity inside the cell draws in water by osmosis so that the cell swells and lengthens. — A student who places the acid inside the cell picks this. Hydrogen ions are secreted out of the cytoplasm into the apoplast; it is the wall that is acidified and loosened, allowing the cell to elongate.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
20 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A cheetah can sprint at over 100 km per hour and catch fast-moving prey. Why is its effectiveness as a predator described as an emergent property?
Answer and reasoning
It is a property of the muscular system alone, because the muscles are what make the cheetah run so fast. — A student who matches each function to one system picks this. Muscles cannot run without nervous control, a skeleton to act on and oxygen and glucose delivered by the blood; the speed emerges from all of these together.
It arises from the integration of several body systems and is not a property of any one of them on its own. — An emergent property arises from the interaction of the parts of a system. A cheetah becomes an effective predator by integration of its body systems; the property is not possessed by any one of them.
It is a property already present in each of its muscle cells; the whole animal simply has many of them. — A student who thinks higher levels only scale up the properties of cells picks this. A single muscle cell can contract but cannot hunt; hunting is a new property that emerges at the level of the integrated organism.
Each of its body systems has its own separate ability, and the cheetah's speed is simply the sum of these separate abilities. — A student who sees the systems as working independently picks this. An emergent property is more than the sum of the parts' abilities: it depends on their coordination, which none of them has separately.
2 After a meal, glucose absorbed in the small intestine is used for cell respiration in the leg muscles. How does the blood system integrate these two organs?
Answer and reasoning
The glucose passes to the muscles directly through the tissues, since blood only carries oxygen. — A student who links blood only with oxygen picks this. Glucose cannot diffuse the length of the body through tissues; it is absorbed into the blood and delivered by the circulation, as are amino acids, urea and hormones.
Nerves running from the intestine to the leg muscles carry the glucose molecules along their fibres. — A student who thinks nerves are the body's general delivery system picks this. Nerves conduct electrical impulses; they do not transport materials. Glucose is carried in the blood plasma.
Each organ makes the glucose that it needs for itself, so no transport between them is needed. — A student who sees organs as self-sufficient picks this. Muscles do not make glucose; they depend on glucose absorbed by the intestine or released by the liver and delivered by the blood, which is exactly why the organs must be integrated.
Blood carries the glucose from capillaries in the intestine wall to the muscle cells. — The blood system integrates organs by transporting materials between them: glucose absorbed by the villi enters the blood and is carried, via the liver and the general circulation, to muscle cells that respire it.
3 A tennis player watches the ball, hears it hit the opponent's racket, senses the position of her own arm and remembers where the opponent usually aims. What is the role of her brain in producing her return shot?
Answer and reasoning
It combines the information from all these inputs with stored memory and uses the result to decide the response. — The brain is the central information integration organ: it processes information combined from several inputs (sight, hearing, body position) together with learning and memory, and produces one coordinated response.
It processes each input in its own region, and each region sends its own response to the muscles. — A student who pictures the brain as a set of independent sensory modules picks this. Separate responses from each sense would conflict; the brain integrates the inputs before a single motor output is sent.
It only supplies timing, because the movement itself is stored in the arm muscles from previous practice. — A student who takes 'muscle memory' literally picks this. Learning and memory are functions of the brain; muscles store nothing and contract only when motor neurons stimulate them.
It uses only the input she is consciously aware of, sight, since the brain cannot process input unconsciously. — A student who thinks all brain processing is conscious picks this. Much of the brain's integration, such as using the sense of arm position, happens without awareness; all the inputs contribute.
They carry impulses out from the central nervous system to the receptors that must then respond. — A student who has swapped the two neuron types picks this. Impulses out of the CNS travel along motor neurons to effectors; receptors do not respond, they detect. Sensory neurons carry input inwards only.
They detect every stimulus themselves, so the body needs no separate receptor cells. — A student who generalizes from the free nerve ending of the pain reflex picks this. In most sense organs, separate receptor cells (such as photoreceptors in the retina) detect the stimulus and signal to the sensory neuron.
They convey impulses from receptor cells to the spinal cord and the cerebral hemispheres. — Sensory neurons convey messages from receptor cells to the central nervous system, providing the input to the spinal cord and cerebral hemispheres.
They are the nerves that connect each sense organ to the brain, each made of one long fibre. — A student who treats nerve and neuron as the same thing picks this. A sensory neuron is a single cell; a nerve is a bundle of fibres from many neurons, usually including both sensory and motor fibres.
5 A transverse section of a nerve shows an outer layer of connective tissue enclosing many small circular profiles. Some profiles have a thick pale ring around a central dot; others have no ring. What does the section show?
Answer and reasoning
A bundle of nerve fibres inside a protective sheath, the ringed profiles being myelinated fibres and the rest unmyelinated. — A nerve is a bundle of nerve fibres enclosed in a protective sheath. In transverse section each myelinated fibre shows its axon (the dot) surrounded by the pale, fatty myelin sheath; unmyelinated fibres are bare axons.
Several separate nerves, because the thick pale ring around each ringed profile is a nerve's protective sheath. — A student who confuses the myelin sheath with the nerve's protective sheath picks this. The ring around a single axon is myelin; the protective sheath is the connective tissue layer around the whole bundle.
A bundle of fibres in which the profiles without a ring are damaged fibres that have lost their myelin. — A student who believes every healthy fibre is myelinated picks this. Normal nerves contain both myelinated and unmyelinated fibres; the unmyelinated ones simply conduct more slowly.
A single neuron cut across, with the outer layer being its cell membrane and the profiles its organelles. — A student who treats a nerve as one cell picks this. Each circular profile is the fibre of a different neuron; the outer layer is connective tissue enclosing hundreds or thousands of such fibres.
6 Which statement about the relationship between a nerve and a neuron is correct?
Answer and reasoning
A nerve and a neuron are the same structure; nerve is simply the word used for a neuron long enough to be seen unaided. — A student who uses the two words interchangeably picks this. A neuron is one cell; a nerve is a visible bundle of the fibres of many neurons, which is why it can be seen without a microscope.
A nerve is a bundle of nerve fibres from many neurons and usually contains fibres of both sensory and motor neurons. — Nerves are bundles of nerve fibres of both sensory and motor neurons, enclosed in a protective sheath. A neuron is a single cell whose fibre is one of many in the bundle.
A nerve contains the fibres of many neurons, but all the fibres in any one nerve are either sensory or motor. — A student who extends the reflex-arc labels to whole nerves picks this. Most nerves are mixed, carrying impulses towards the CNS in sensory fibres and away from it in motor fibres.
A nerve is a group of neurons wrapped together in a single myelin sheath, which acts as its protective sheath. — A student who merges the two kinds of sheath picks this. Myelin surrounds individual fibres; the protective sheath of the nerve is connective tissue around the whole bundle, and some fibres in it have no myelin.
7 A person touches a hot pan and withdraws their hand before feeling any pain. Which sequence describes the reflex arc that produced the withdrawal?
Answer and reasoning
free nerve ending → sensory neuron → spinal cord → cerebral hemispheres → motor neuron → skeletal muscle of the arm — A student who thinks the brain must decide picks this. The reflex is integrated in the spinal cord; impulses do also travel to the brain, but the withdrawal has already been triggered before they arrive.
free nerve ending → sensory neuron → interneuron in the spinal cord grey matter → motor neuron → smooth muscle of the arm — A student who thinks involuntary responses must use involuntary muscle picks this. The effector in the pain reflex is skeletal muscle; the arm has no smooth muscle that could move it.
free nerve ending → sensory neuron → interneuron in spinal cord grey matter → motor neuron → skeletal muscle of the arm — The pain reflex arc has a free sensory nerve ending as the receptor, a sensory neuron into the spinal cord, a single interneuron in the grey matter, a motor neuron out, and skeletal muscle as the effector.
skeletal muscle of the arm → sensory neuron → interneuron in the grey matter of the spinal cord → motor neuron → free nerve ending — A student who has swapped receptor and effector picks this. The free nerve ending detects the stimulus and so comes first; the muscle carries out the response and so comes last.
8 In the pain withdrawal reflex, the hand is pulled away before the person becomes aware of any pain. Which explanation accounts for this order of events?
Answer and reasoning
The brain feels the pain first and decides to withdraw the hand, but this decision is made too fast to be noticed. — A student who puts feeling before action picks this. If the brain decided, awareness of pain would come before or with the movement, not after it; the observed order shows the brain was not in the loop.
The spinal cord only conducts impulses, so the brain must have ordered the withdrawal and awareness of pain simply lags behind. — A student who thinks the spinal cord cannot process anything picks this. The cord's grey matter integrates the reflex itself; that is why the movement can precede the brain's involvement.
The arm muscle acts as the receptor as well as the effector, so it responds before the hand has sent any impulses at all. — A student who confuses receptor with effector picks this. The receptor is the free nerve ending in the hand; the muscle responds only when a motor neuron stimulates it, after the impulse has passed through the spinal cord.
The reflex is completed in the spinal cord, and impulses giving awareness of pain reach the brain only later. — The reflex arc passes through a single interneuron in the spinal cord, so the motor output leaves before impulses have travelled up the cord to the cerebral hemispheres, where the pain is perceived.
9 Blood melatonin concentration in a person was measured over 24 hours. It was low from 08:00 to 20:00, rose from 21:00, was highest between 02:00 and 04:00 and had fallen by 07:00. Which statement explains this pattern and its effect?
Answer and reasoning
Falling asleep stimulates the pineal gland to secrete melatonin, which is why the level rises after bedtime. — A student who reverses cause and effect picks this. Secretion begins to rise from 21:00, in the dark, before sleep; the rise promotes sleep rather than being caused by it.
The pineal gland secretes melatonin while the eyes detect little light, and high melatonin at night promotes sleep. — Melatonin is secreted by the pineal gland in a diurnal pattern: darkness allows secretion and light suppresses it. The night-time peak promotes sleep, helping to establish the cycle of sleeping and waking.
The pituitary gland secretes melatonin in response to darkness, and its high level makes the person sleepy. — A student who attributes every brain hormone to the master gland picks this. Melatonin is secreted by the pineal gland, not the pituitary; the rest of the description is correct.
Light stimulates melatonin secretion during the day; the night-time peak is melatonin that has not yet been broken down. — A student who expects light to be the stimulus picks this. Light suppresses secretion, which is why the level is low all day; the night-time peak is newly secreted melatonin, not a leftover.
10 A student who uses a bright screen until midnight finds it hard to fall asleep and wakes late the next morning. Using the role of melatonin in circadian rhythms, which explanation is best supported?
Answer and reasoning
Light from the screen stimulates melatonin secretion, and the high melatonin level keeps the student awake until it has all been used up. — A student who thinks light stimulates melatonin picks this. Melatonin promotes sleep, not wakefulness, and light suppresses it; the student stays awake because melatonin is low, not high.
No melatonin was secreted because the student stayed awake, since the pineal gland only begins to secrete it once sleep has begun. — A student who thinks sleep causes melatonin picks this. Secretion normally rises in the evening before sleep; what delayed it here was the light, not the wakefulness.
Light from the screen suppresses evening melatonin secretion, delaying the rise that promotes sleep and shifting the cycle later. — Light detected by the eyes suppresses secretion by the pineal gland. Evening light delays the normal rise of melatonin, so sleepiness comes later and the whole sleep–wake cycle, including waking, is shifted later.
Melatonin is secreted by the pituitary gland, not in response to light, so the screen could not have altered its timing. — A student who attributes melatonin to the pituitary picks this. Melatonin is secreted by the pineal gland, and its secretion is suppressed by light detected by the eyes, which is exactly why evening screen light delays sleep (C3.1.11).
11 Which statement describes how the hypothalamus and the pituitary gland control the endocrine system?
Answer and reasoning
The hypothalamus controls the pituitary gland, which secretes hormones that act on body tissues or that control other endocrine glands. — The hypothalamus links the nervous and endocrine systems and controls the pituitary. Pituitary hormones act on tissues directly or regulate other glands, so the two structures together control much of the endocrine system.
The pituitary gland controls the other endocrine glands by itself; the hypothalamus is part of the nervous system and plays no role. — A student who stops at 'master gland' picks this. The pituitary is itself controlled by the hypothalamus, which is the link between nervous input and hormonal output.
The hypothalamus controls the pituitary gland by nerve impulses only, because the hypothalamus cannot secrete any hormones. — A student who thinks a brain region cannot be endocrine picks this. The hypothalamus does produce hormones, and some of these control the pituitary's secretion.
Each endocrine gland monitors the blood and secretes its hormone independently, so no gland controls any other gland. — A student who generalizes from the pancreas picks this. Several pituitary hormones control the secretion of other glands, such as the thyroid gland and the gonads, so the endocrine system is hierarchically controlled.
12 A person stands up quickly and their blood pressure falls for a few seconds. Which sequence describes the feedback response that restores blood pressure?
Answer and reasoning
Baroreceptors detect the fall and the medulla sends impulses along the vagus nerve to reduce heart rate, because a lower rate lets the pressure recover. — A student who has the direction of the feedback reversed picks this. Reducing heart rate would lower pressure further; the vagus nerve slows the heart when pressure is too high, not too low.
The medulla sends the impulses that cause each heartbeat more frequently, because the heart cannot beat at all unless the medulla drives it. — A student who thinks the medulla generates the heartbeat picks this. The heart beats on its own; the medulla's nerve impulses only speed it up or slow it down and change its stroke volume.
Baroreceptors in the arteries send fewer impulses to the medulla, which sends sympathetic impulses that raise heart rate and stroke volume. — Lower pressure stretches the artery walls less, so baroreceptor impulses to the medulla decrease. The medulla responds by increasing sympathetic impulses to the heart, raising heart rate and stroke volume so that pressure rises: negative feedback.
Baroreceptors in the heart wall detect the fall in pressure and the heart raises its own rate without any involvement of the medulla. — A student who locates the receptors in the heart picks this. Baroreceptors are in the aorta and carotid arteries, and the response is coordinated by the medulla, not by the heart itself.
13 During a run, a student's blood carbon dioxide concentration rises, blood pH falls from 7.40 to 7.32 and ventilation rate rises from 12 to 30 breaths per minute. Which statement correctly links these changes?
Answer and reasoning
The extra carbon dioxide makes the blood more alkaline, as shown by the fall in pH, and chemoreceptors in the brainstem respond to this by increasing ventilation. — A student who confuses the direction of the pH scale picks this. A fall in pH means the blood has become more acidic, which is what carbonic acid formed from carbon dioxide causes.
Carbon dioxide is a gas and so cannot change pH; the fall in pH is due to lactic acid from the muscles, which is detected by chemoreceptors in the lungs. — A student who thinks only lactic acid acidifies blood picks this. Dissolved carbon dioxide forms carbonic acid and is the usual cause of the fall in pH, and the chemoreceptors involved are in the brainstem, not the lungs.
The student consciously decides to breathe faster on feeling out of breath, and the changes in carbon dioxide and pH are consequences of this, not causes. — A student who thinks breathing is voluntary picks this. Ventilation rate is raised automatically by the medulla in response to chemoreceptor input; the rise in carbon dioxide and fall in pH are the causes of faster breathing, not its results.
Carbon dioxide forms carbonic acid, lowering pH; brainstem chemoreceptors detect this and the medulla increases signals to the breathing muscles. — Carbon dioxide from increased cell respiration dissolves to form carbonic acid, which lowers blood pH. Chemoreceptors in the brainstem detect the change and the medulla increases signals to the diaphragm and intercostal muscles, raising ventilation rate and removing carbon dioxide.
14 A person swallows a mouthful of food. Which statement correctly describes how the movement of this food through the digestive system is controlled?
Answer and reasoning
Starting the swallow is voluntary and controlled by the CNS; peristalsis along the oesophagus and gut is involuntary and controlled by the ENS. — Initiation of swallowing (and egestion of faeces) is under voluntary control by the CNS. Between these points, peristalsis is under involuntary control by the enteric nervous system, which coordinates the passage of material through the gut.
The CNS controls the whole process, sending an impulse to the gut muscles for each wave of peristalsis until faeces are egested. — A student who assumes every muscle is driven by the CNS picks this. The gut wall has its own enteric nervous system, which controls peristalsis without instructions from the brain or spinal cord.
After swallowing, the food moves down the oesophagus and the intestine mainly under gravity, with muscle contraction only assisting. — A student who thinks food falls through the gut picks this. Peristalsis, coordinated by the ENS, moves material along the gut regardless of orientation; gravity is not required.
Swallowing is a reflex from the moment food enters the mouth, so no stage of the movement is under voluntary control. — A student who has heard swallowing called a reflex picks this. The initiation of swallowing is a voluntary act controlled by the CNS; only the peristalsis that follows is involuntary.
15 Two students each measured the angle of curvature of the same seedling shoot five times with a protractor. Student A recorded 28°, 29°, 28°, 29°, 28°. Student B recorded 22°, 35°, 30°, 26°, 37°. A later check showed that the true angle was 35° and that Student A had aligned the protractor with the edge of the pot instead of the vertical part of the stem. Which conclusion is justified? HL
Answer and reasoning
Student A's measurements are both precise and accurate, because five repeats that agree so closely must lie close to the true value. — A student who treats precise and accurate as synonyms picks this. The repeats agree with each other but not with the true value of 35°; consistent measurements can all share the same systematic error.
Student A's measurements are precise but not accurate, because a systematic error shifted all of them away from the true value. — A's repeats agree within 1°, so they are precise, but all are 6–7° below the true value because of a systematic error in aligning the protractor. Precision (agreement between repeats) does not guarantee accuracy (closeness to the true value).
Student A could have obtained an accurate mean by taking more repeats, because repetition removes systematic error. — A student who thinks repeats cure all error picks this. More repeats would reduce random scatter but every repeat would carry the same alignment error, so the mean would stay near 28°.
Student B's readings are qualitative rather than quantitative, because they vary too much between repeats to be measurements. — A student who judges the quantitative/qualitative distinction by data quality picks this. B's readings are numerical measurements in degrees, so they are quantitative; they are simply imprecise.
16 A seedling is grown in a box with a window on its left side only. After two days the shoot has curved towards the window. Which statement explains the curvature? HL
Answer and reasoning
Cells on the lit left side elongated more because the light supplied the energy for their growth, pulling the shoot over. — A student who reasons that growth happens where the light is picks this. If the lit side grew more the shoot would bend away from the light; it is the shaded side that elongates faster.
The existing tissues of the shoot bent towards the window in the way an animal turns its head, without any growth occurring. — A student who thinks of tropism as movement picks this. A tropism is a growth response; the curvature is produced by new elongation of cells over hours to days, not by movement of existing tissue.
Light destroyed all the auxin on the left side, so cells there stopped growing while the cells on the right grew normally. — A student who believes light breaks down auxin picks this. Auxin is redistributed by efflux carriers towards the shaded side, not destroyed; the lit side still grows, just more slowly.
Cells on the shaded right side elongated more than cells on the lit left side, so growth bent the shoot to the left. — Positive phototropism is a directional growth response: auxin accumulates on the shaded side, cells there elongate more, and the unequal growth bends the shoot towards the light.
17 In a column of cells in a shoot, every cell has its auxin efflux carriers concentrated in the membrane at its lower (basal) end. What happens to auxin in this tissue? HL
Answer and reasoning
Auxin diffuses in and out of every cell across all parts of its membrane, so the carriers make no difference and the auxin stays evenly spread throughout. — A student who thinks auxin leaves cells as freely as it enters picks this. Auxin cannot diffuse out of cells; it exits only through the efflux carriers, whose position therefore sets the direction of transport.
Auxin enters each cell freely by diffusion but can leave only through the basal carriers, so it is transported downwards and accumulates below. — Auxin diffuses freely into cells but not out. With every cell's efflux carriers on its lower side, auxin is pumped out of the base of each cell into the next cell down, so it moves in one direction through the tissue and becomes concentrated lower in the plant.
The efflux carriers pump auxin into each cell from the cell below, so auxin is transported upwards and accumulates at the shoot tip. — A student who reads efflux as influx picks this. Efflux carriers pump auxin out of the cell; carriers on the basal side therefore move auxin downwards, away from the shoot tip.
Auxin is carried in the xylem sap, so the position of the carriers in the cell membranes has no effect on where it accumulates. — A student who expects a bloodstream-like transport system picks this. Directional auxin transport through tissues is from cell to cell via efflux carriers, and their position determines where auxin becomes concentrated.
18 A plant's roots are damaged, reducing the number of root tips. Using the interaction between auxin and cytokinin, what is the predicted effect on the shoots? HL
Answer and reasoning
Less auxin is transported to the shoots, because root tips are where auxin is produced, so shoot growth is reduced. — A student who has swapped the sources picks this. Auxin is produced in shoot tips and transported to the roots; it is cytokinin that root tips produce and send to the shoots.
Shoot growth is unaffected, because cytokinin acts only in the roots where it is made, and auxin only in the shoots. — A student who expects each hormone to act locally picks this. Each hormone is transported to the other organ, which is precisely what allows root damage to influence shoot growth.
Shoot growth is unaffected, because auxin, the only phytohormone, is made in the undamaged shoot tips. — A student who thinks auxin is the sole plant hormone picks this. Cytokinin from the root tips is also needed for shoot growth, so damage to the roots does affect the shoots.
Less cytokinin reaches the shoots, so shoot growth is reduced to match the smaller root system. — Root tips produce cytokinin, which is transported to the shoots and promotes their growth. Fewer root tips means less cytokinin reaching the shoots, so shoot growth is restrained: this is how the interaction keeps root and shoot growth integrated.
19 The concentration of ethylene in the air around a ripening fruit was measured each day: 0.1, 0.3, 1.2 and 4.8 ppm on days 1 to 4, by which time the fruit was fully ripe. Which statement best interprets these data? HL
Answer and reasoning
Ethylene stimulates ripening and ripening stimulates more ethylene production, so the concentration rises faster and faster: positive feedback. — The concentration rises by a larger amount each day (0.2, 0.9, 3.6 ppm), an accelerating rise characteristic of positive feedback: ethylene stimulates the changes of ripening, and ripening stimulates increased ethylene production.
Ethylene production is under negative feedback, so each rise in ethylene reduces further production and ripening proceeds steadily. — A student who assumes all feedback is negative picks this. Negative feedback would hold the concentration steady; the data show an accelerating rise, which only positive feedback produces.
The mechanism is positive feedback because ripening benefits the plant; a harmful process with this pattern would be negative feedback. — A student who reads 'positive' as 'good' picks this. Positive feedback describes a response that increases the stimulus, whatever its value to the organism; the accelerating rise is what makes this positive feedback.
Ethylene acts only inside the fruit that produces it, so the rising concentration in the air is a waste product with no effect on ripening. — A student who thinks a hormone must stay inside the organism picks this. Ethylene is a gas that diffuses through the air and stimulates ripening in neighbouring fruits; it is a signal, not a waste product.
20 Why is positive feedback in ethylene production beneficial to a plant? HL
Answer and reasoning
Positive feedback is, by definition, feedback that benefits the organism, so its benefit to the plant needs no further explanation. — A student who gives 'positive' its everyday meaning picks this. The term describes a response that amplifies the stimulus; whether it is beneficial has to be explained by its effects, here rapid, synchronized ripening.
It makes ripening rapid and synchronized, so the fruits on the plant become attractive to seed-dispersing animals together. — Positive feedback ensures that once ripening starts it accelerates and, because ethylene spreads between fruits, all ripen together; a synchronized crop of ripe fruit attracts the animals that disperse the seeds.
It keeps the rate of ripening slow and steady, so the plant is not left with more ripe fruit than can be dispersed. — A student who thinks feedback always stabilizes picks this. Positive feedback accelerates ripening rather than steadying it; the benefit is that ripening is rapid and synchronized.
It confines ethylene to the fruit that makes it, so one over-ripe fruit cannot make the others on the plant ripen. — A student who thinks ethylene stays within its fruit picks this. Ethylene is a gas that diffuses between fruits, and this spread is part of what synchronizes ripening across the plant.
That was your twenty minutes. Real practice on C3.1 is past-paper questions marked against the mark scheme.
What the exam asks of C3.1
Paper 1A asks you to match structures to functions: cerebellum, medulla, pineal gland, baroreceptors, enteric nervous system. Paper 1B may give data on heart rate, ventilation or seedling curvature and ask you to describe patterns, identify variables and judge precision and accuracy. Paper 2 uses *outline*, *describe* and *explain*: explain the pain reflex arc, explain feedback control of heart rate or ventilation, compare nervous with hormonal signalling. At HL, expect *explain* on phototropism through auxin efflux carriers and wall acidification, and *outline* on ethylene positive feedback.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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